Metallography and Microstructures of Titanium

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Metallography and Microstructures of

Titanium and Its AlloysLuther M. Gammon, Robert D. Briggs, John M. Packard, Kurt W. Batson, Rodney Boyer, and Charles W. Domby,The Boeing Company

METALLOGRAPHY is a complex processwith many variables that involve compromisesbetween time, resources, and the end product orpurpose of the investigation. For example, a re-

search lab may benefit from the more time-con-suming method of vibratory polishing, while aproduction quality-control lab may not requirespecimen preparation with a vibratory polisher.

A lab for teaching also may benefit from training experience of manual polishing, or pishing may be done with semiautomatic poliing machines.

With a little forethought and planning, exclent metallographic samples can be produceda short time for light microscopy of titanium aits alloys. This article describes the fundamenof titanium metallographic sample preparatiRepresentative micrographs are also presenfor each class of titanium alloys, which incluunalloyed titanium, alpha alloys, alpha-betaloys, and beta titanium alloys. Metallograand metallographic sample preparation of tnium alloys are also described in more detaiRef 1 and 2.

Types of Titanium Alloys

Titanium is an allotropic element; that isexists in more than one crystallographic foAt room temperature, titanium has a hexago

Fig. 1 Cross section through the abrasive saw-cut edgeof a Ti-6Al-4V sample. Note there is less than 5

lm depth of disturbed material requiring removal forproper specimen preparation, seen as a thin layer at thesurface. This layer would be deeper in commercially puretitanium and more difficult to discern.

Fig. 2 This micrograph shows the impact of mountingdefects on edge retention. Note the edge round-

ing near the air bubble and the sharp edge where themounting material filled the gap. This shows the impor-tance of good-quality mounting techniques and materials.

Fig. 3 Sample holders for semiautomated polishing machines.(a) Fixed-sample holder withload applied froma centralcolumn. (b) Nonfixed specimen mover plate with load applied over one mount

Fig. 4 Mount with two specimens for manual polisor polishing on a semiautomated polisher w

non-fixed specimen mover plate

ASM Handbook, Volume 9: Metallography and Microstructures

G.F. Vander Voort, editor, p899–917

DOI: 10.1361/asmhba0003779

Copyright © 2004 ASM International®

All rights reserve

www.asminternational.oColor images cited in this article appear at end of article.



900 / Metallography and Microstructures of Nonferrous Alloys

close-packed (hcp) crystal structure, which is re-ferred to as “alpha” phase. This structure trans-forms to a body-centered cubic (bcc) crystalstructure, called “beta” phase, at 883 C (1621F).

Alloying elements generally can be classifiedas alpha or beta stabilizers. Alpha stabilizers,such as aluminum and oxygen, increase the tem-perature at which the alpha phase is stable. Beta

stabilizers, such as vanadium and molybdenum,result in stability of the beta phase at lower tem-peratures. This transformation temperature froman alpha-beta phase (or all-alpha phase) to allbeta is known as the beta transus temperature.The beta transus is defined as the lowest equilib-rium temperature at which the material is 100%beta.

Below the beta transus temperature, titaniumwill be a mixture of b if the material con-tains some beta stabilizers, or it will be all alphaif it contains no beta stabilizers. The beta transusis important, because processing and heat treat-ment are often carried out with reference to someincremental temperature above or below the beta

transus. Alloying elements that favor the alphacrystal structure and stabilize it by raising thebeta transus temperature include aluminum, gal-lium, germanium, carbon, oxygen, and nitrogen.

Two groups of elements stabilize the betacrystal structure by lowering the transformationtemperature. The beta isomorphous group con-sists of elements that are miscible in the betaphase, including molybdenum, vanadium, tan-talum, and niobium. The other group forms eu-tectoid systems with titanium, having eutectoidtemperatures as much as 333 C (600 F) belowthe transformation temperature of unalloyed ti-tanium. The eutectoid group includes manga-nese, iron, chromium, cobalt, nickel, copper, and

silicon. Two other elementsthat often are alloyedin titanium are tin and zirconium. These ele-ments have extensive solid solubilities in alphaand beta phases. Although they do not stronglypromote phase stability, they retard the rates of transformation and are useful as strengtheningagents.

Alloy Classes. Titanium alloys have generallybeen classified as alpha alloys, alpha-beta alloys,and beta alloys. Alpha alloys have essentially anall-alpha microstructure. Beta alloys are those al-loys from which a small volume of material canbe quenched into ice water from above its betatransus without martensitic decomposition of thebeta phase. Alpha-beta alloys contain a mixtureof alpha and beta phases at room temperature.

Within the alpha-beta class, an alloy that con-tains less than 2 to 3% beta, such as Ti-8Al-1Mo-1V, may also be referred to as a “near-alpha” or“super-alpha” alloy.

The principal alloying element in alpha alloysis aluminum (oxygen is the principal alloyingelement in commercially pure titanium), but cer-tain alpha alloys and most commercially pure(unalloyed) titanium contain small amounts of beta-stabilizing elements. Similarly, beta alloyscontain small amounts of alpha-stabilizing ele-ments as strengtheners in addition to the betastabilizers.

The beta alloys can be further broken downinto beta and “near-beta.” This distinction is nec-

essary, because the phase transformations thatoccur, the reaction kinetics, and the processingcould be different if the alloy is a near-beta (lean)alloy, such as Ti-10V-2Fe-3Al, or a rich beta al-loy, such as Ti-13V-11Cr-3Al.

Further information on the metallurgy, selec-tion, processing, and application of titanium al-loys is contained in Ref 3 and in Properties and

Selection: Nonferrous Alloys and Special-Pur- pose Materials, Volume 2 of the ASM Handbook

(see, for example, the articles “Wrought Tita-nium and Titanium Alloys” and “Titanium andTitanium Alloy Castings”).

Specimen Preparation

Specimen preparation comprises many de-tailed steps. The first stages of sample prepara-tion are equipment dependant, while the finalpolish step is driven by the needs of the inves-tigator. Sufficient attention must be paid to eachstep or the quality of the finished mount may becompromised.

The method chosen depends on two factors:the facilities and equipment present and the pur-

pose of the investigation. There is a large diffeence between methods used in a research envronment, where time may not be as pressing ain a production environment, or in a college instructional lab where the facilities may not be aelaborate.

Sectioning. Common methods for sectionintitanium metallographic samples include thband saw, abrasive cut-off wheel, and slow

speed wafering wheels. Band sawing titaniushould be done with slow blade speed using

Fig. 5 Etched with Kroll’s reagent for 45 s. Abusivelypolished example of a Ti-6Al-4V fastener result-

ing in a smeared and scratched surface. Excessive etchingcannot correct poor specimen preparation. Notethe severedistortion in microstructure and edge rounding. Fig. 6 A 200 cm (8 in.) wax wheel with relief grooves

Fig. 7 Ti-6Al-4V alloy with Widmanstatten alpha inbeta matrix after furnace cooling from above t

transus. Beta anneal temperature was 1040 C (1900 FSamples were etched with the oxalic tinting reagent fors after polishing by (a) four-step method for optimizing moval of deformed material, (b) four-step method for opmizing edge retention, or (c) three-step semiautomatmethod for optimizing preparation time (note the lackdetail in the dark regions). See text for description of pishing procedures. See also Fig. 58 in the article “SelectColor Images” in this Volume for color version.



Metallography and Microstructures of Titanium and Its Alloys / 9

toothed blade and high pressure applied to theworkpiece. If a high blade speed and low pres-sure are applied to the workpiece, damage in theform of cold work will be introduced into thesample, possibly preventing the true microstruc-ture from being observed. With all three cuttingmethods, sufficient amounts of coolant should beused to prevent the introduction of heat damageinto the sample. Abrasive cut-off wheels should

be a soft rubber-bonded abrasive type. The ero-sion of the rubber-bonded wheel will continuallyprovide a fresh cutting surface and prevent tita-nium debris from loading up on the blade. If adull band saw blade is used or if an abrasiveblade has loaded up with cutting debris, the sam-ple will be damaged from overheating and coldwork. Figure 1 shows the edge of a cross sectioncut with an abrasive cut-off wheel.

Mounting. The sample should also be de-greased and dried before mounting to ensure ad-equate adhesion of the mounting media. Carefulconsideration is also necessary for making aproper metallurgical mount.

The first consideration is choosing the most

appropriate mounting medium. Titanium is avery abrasion-resistant material, and it is essen-tial that the titanium be mounted correctly to pro-duce a quality metallographic sample. The se-lection of mounting material has a significantimpact on edge retention and the surface flatnessof the mount. Failure to use the proper mountingmedia may cause rounding of the interface be-

tween the mount and sample, resulting in pooredge retention. It can also cause rounding or fa-ceting of the overall mount surface. (See the ar-ticle “Mounting of Specimens” in this Volumefor more information on mounting and edge re-tention.)

In selecting a mounting material, it is recom-mended to use a mineral- or glass-filledhot-com-pression thermosetting resin. While the costs of

the filled resins are higher than the traditionalbakelite or epoxy resins, the performance of filled resins is superior, as the filler can allowclose matching of the abrasive wear resistanceof the specimen and the mount. The cost-to-ben-efit ratio makes filled resins a good choice whentransparency is not needed. When transparencyis needed or voids are present in a part with com-plex shape, it is necessary to use an alternativecold-setting material such as a clear epoxy. Thiscan be vacuum impregnated into sample voidsand irregularities such as the gap in Fig. 2.

The second consideration is the sample con-figuration, which refers to the position and num-ber of samples in a mount. The method of pol-

ishing can determine the sample configuration,as described in more detail in the article “Mount-ing of Specimens” in this Volume. In general,there are three types of polishing methods:

● Semiautomatic polishing machine with sam-ples held in a “fixed” sample holder (Fig. 3a)

● Semiautomatic polishing machine with sam-ples held in a “nonfixed” sample holder(Fig. 3b)

● Manual or hand polishing

Polishing with a fixed-sample holder in a seiautomated machine is achieved by a powerhthat moves the sample holder around the poliing platen. In this method, mounted samplesfixed in place within a rigid sample holder, acentral force loading is applied to all specimin the holder through a centrally located columIn this method, three or six mounted sampmust be symmetrically placed in the holder

order to ensure good flatness of the specimafter polishing. The mount surface thus remaflat, as the samples are held in-plane by the saple holder. This method provides optimal eretention and flatness and is the recommendsample preparation method for operators requing larger volumes of throughput. With method, each mount can contain only one spimen.

In the semiautomated nonfixed (or individforce) method, the specimens sit in a hole iholder (a thin plate), and a piston comes doand presses each specimen against the worksurface. In this case, two or more specimshould always be placed in each mount (Fig.

By centering them in each side of the mount,specimens support the mount so it will not teto rock back and forth. The result is a flatter saple with better edge retention. This is still nogood as the fixed method. A single specimshould never be mounted in the center omount. The result is usually a convex and/or feted mount surface with poor edge retention. Tmount will have the tendency to rock back aforth about the small, hard specimen, roundthe mount surface and degrading the quality

Fig. 8 Deformed grainstructurefrom drilling in solutiontreated and aged Ti-6Al-4V. Solution treatment

was at 925 C (1700 F) and aged. Polishing was the four-step method for edge retention, and it was etched with theoxalic tint etch to reveal the deformed grain structure fromdrilling. (a) Depth of the cold work as evidenced by thedisturbed microstructure to a depth of 310 lm. (b) Normalmicrostructure for comparison

Macroetchants

50 mL HCl, 50 mL H2O General-purpose etch for

b alloys

30 mL HNO3, 3 mL HF, 67mL H 2O (slow) to 10 mL

HNO3, 8 mL HF, 82 mL

H2O (fast)

Used at room temperature to55 C (130 F) for 3–5

min. Reveals grain size

and surface defects

15 mL HNO3, 10 mL HF, 75mL H 2O

Etch about 2 min. Revealsflow lines and defects

Two-stage etch(a) consisting

of: (1) 8 mL HF, 10 mL

HNO3, 82 mL H2O and(2) 18 g/L (2.4 oz/gal) of

NH4HF2 (ammonium

bifluoride) in H2O

Reveals and b segregation

(aluminum segregation)

Microetchants

1–3 mL HF, 10 mL HNO3,30 mL lactic acid Reveals hydrides inunalloyed titanium1 mL HF, 30 mL HNO3, 30

mL lactic acid

Reveals hydrides in

unalloyed titanium

Kroll’s reagent: 1–3 mL HF,

2–6 mL HNO3, H2O to1000 mL

General-purpose etch for

most alloys

10 mL HF, 5 mL HNO3, 85

mL H 2O


most alloys

1 mL HF, 2 mL HNO3, 50mL H 2O2, 47 mL H2O

Removes etchant stains formost alloys

10 mL HF, 10 mL HNO3, 30

mL lactic acid

Chemical polish and etch

most alloys

2 mL HF, 98 mL H2O Reveals case for mostalloys

98 mL saturated oxalic acid

in H2O, 2 mL HF

Reveals case (interstitia

contamination) for mos

alloys6 g NaOH, 60 mL H2O, heat

to 80 C (180 F), add 10

mL H2O2

Good -b contrast, gener

microstructures for mo

alloys

2 mL HF, 98 mL H2O, then1 mL HF, 2 mL HNO3, 97

mL H2O

General-purpose etch fornear- alloys(b)

10 mL KOH (40%), 5 mLH2O2, 20 mL H2O

Stains , transformed b

18.5 g benzalkonium

chloride, 33 mL ethanol,40 mL glycerol, 25 mLHF


Al-Zr and Ti-Si alloys

2 mL HF, 4 mL HNO3, 94

mL H2

Reveals microstructure in

aged Ti-13V-11Cr-3Al

50 mL 10% oxalic acid, 50mL 0.5% HF with H2O

Etch 12–20 s. General-purpose etch for b allo

10 s with Kroll’s, then 10–15

s with 50 mL 10% oxalic

acid, 50 mL 0.5% HF withH2O

Brings out aged structure

Ti-10V-2Fe-3Al

(a) Two-stage etch procedure: Degrease (if necessary) and clean, making sure the surface is water-break free. Immerse in solution (1) at 45–5

(110–135 F) for 2–3 min and rinse thoroughly in clean cold water. Immerse in agitated bath of solution (2) at room temperature for 1–2 min. R

thoroughly in clean cold water, rinse thoroughly in clean hot water at 90–100 C (190–210 F), blow dry with clean compressed air. Solutions

be used fresh. (b) First etchant stains phase; second etchant removes stain.

Etchant Comments Etchant Comments

Microetchants (continued)

Table 1 Etchants for examination of titanium and titanium alloys




the edge. This convex surface will have an ad-verse effect on the appearance of the microstruc-ture. A similar-sized mount with two small sam-ples in the holders will reduce the rocking effect,making it possible to prepare a flatter sample.

Manual or hand polishing is similar to thesemiautomatic nonfixed method. Two or morespecimens should always be mounted in eachsample. The only difference is that the mass of

titanium in the mount for hand preparationshould be kept to a minimum to facilitate grind-ing and maintain a uniform applied pressureacross the mount.

Grinding. The purpose of grinding is to re-move the damage caused by the sectioning pro-cess. Sectioning methods, such as slow-speedwafering, that do not introduce much damageinto the sample do not require extensive grindingand decrease sample preparation time.

Semiautomated grinding with a specimenmover plate or the fixed holder can be done withsemiautomated polishers using diamond-embed-ded platens or platens with proprietary coatingsdesigned for applied diamond suspensions.

There is a wide assortment of diamond platenson the market to be used with automated grind-ing.

For a 20 to 30 cm (8 to 12 in.) diamond-embedded platen, the following parametersshould be used. Keep in mind that there are manypossible ways to accomplish a grinding opera-tion depending on the complexity of the part,amount of material to be removed, and timeavailable. Various combinations of these stepscan be used:

● The speed should be 150 rpm (note: tita-nium machineswork best at highpressure andlow speed).

● The applied pressure should be 40 to 70 N (9

to 15 lbf) per 38 mm (1.5 in.) diam mount.● Grinding step A uses 70 lm or 220 grit dia-

mond.● Grinding step B uses 1200 grit diamond.● Always use a sufficient amount of coolant to

prevent heat damage.

With proper sectioning most ordinary samplescan be ground with a single 220 grit finish fol-lowed by a 9 lm diamond suspension on eithera grinding platen with a proprietary coating or awoven non-nap silk cloth.

Grinding by hand usually involves the use of silicon-carbide papers. The following parametersshould be observed:

● The speed should be kept to no more than 150rpm.

● Always use new paper. The maximum paperlifetime is 15 s (or perhaps up to 60 s max inone-time manual grinding). Abrasives

quickly lose their cutting ability and smearthe sample and introduce cold-work damage.

● Apply as much pressure as can be controlledwhen holding the sample to the paper. Highpressure and slow speed will produce favor-able results.

● The common grit progression sequence is 120(or 240), 320, and 600 grit. If the sectioningprocess produces a fine smooth face, it is pos-sible to start the grinding process with 320 or600 grit papers, but there must be sufficientmaterial removal to eliminate all cutting dam-age.

● Always use sufficient amounts of coolant orwater to prevent heat damage.

Polishing can be broken down into twophases, the intermediate polish and final polish.The purpose of polishing is to gradually removethe trace amounts of damage and the surfacescratches introduced during the grinding opera-tions. Again, there are numerous methods doc-umented for intermediate and final polishing thatmay fit different operations. The list below dis-cusses of a few of the procedures. An exampleof abusive polishing is shown in Fig. 5 after etch-ing. Excessive etching cannot correct poor spec-imen preparation.

Intermediate polishing is the bridge step orsteps between grinding and 1 lm final step orsteps. It can be done successfully by either the

semiautomated or hand method. The semiauto-mated method is generally recommended be-cause it is very effective with typical removalrates of 5 lm/min and as much as 25 lm/minwith minimal cold work introduced into the sam-ple. It can be utilized both as a fine grinding anda polishing step at the same time. Several sem-iautomated intermediate polishing parametershave been found effective:

● A 9 lm diamond suspension with an alcohol-based lubricant on a woven non-nap cloth

such as silk or a proprietary platen designefor diamond suspension application is used

● A 3 lm diamond suspension on a polyestecloth with an emulsified oil-based lubricant used.

● Speed should be 120 to 150 rpm.● Direction of specimen holder rotation shou

be complementary to the rotation of thplaten.

● Applied force should be 40 to 80 N (9 to 1lbf) per 38 mm (1.5 in.) diam mount.

Fig. 9 Coarse lamellar alpha revealed by differeetches in Ti-6Al-4V structure after beta annea

1040 C (1900 F) and furnace cooling. Preparation wfour-step polishing with final polish of 16 h on vibratopolisher and 10% alumina slurry. Slightly uncrossedpolized light for all three etches: (a) ammonium bifluori(ABF) tint etch, 60 s; (b) Kroll’s reagent, 15 s; (c) oxalic tetch, 60 s. See also Fig. 59 in the article “Selected CoImages” in this Volume for color version.

Table 2 Typical compositions of microetchants suitable in most applications of titanium

metallographyName Typical composition Notes Figures

Kroll’s reagent 1.5 mL HF

4 mL HNO3

94 mL H2O

. . . Fig. 5, 9–11, 13, 14, 16, 35,

37, 56

Oxalic reagent (tint etch) 20 mL HF

20 g oxalic

98 mL H2O

15 s for Ti-6Al-4V. Do not

remove etch products.

Fig. 9–11, 15, 28, 46, 50, 57,

58, 62, 64

Ammonium bifluoride

(ABF)

1 g ammonium bifluoride

(NH4FHF)

99 mL H2O

Do not remove etch

products.

Fig. 9–11, 47–49, 61

Lactic hydride reagent Mix fresh 5 mL lactic acidand 5 mL stock solution (3

mL HF, 97 mL HNO3)

Commercially pure titaniumhydrides

. . .




There are two options for intermediate hand pol-ishing; the beeswax platen and the traditionalnap cloth diamond method. The nap cloth and diamond option is not recommended where edgeretention is critical. The sequence used for handpolishing is typically:

1. A 6 lm diamond slurry on nap cloth is used.2. A 3 lm diamond slurry on nap cloth is used.

3. Apply as much pressure as possible withoutrocking the specimen.

A recommended intermediate hand-polishingmethod using a beeswax wheel is an effective,inexpensive method that routinely producesquality samples. About 2.5 mm (0.10 in. of bees-wax is cast on the platen into which relief grooves (Fig. 6) are cut at a spacing of 10 to 14grooves per inch. Polishing is done with a pastemade with 5lm alumina and hydrogen peroxide.While polishing with the paste, drops of 3% hy-drogen peroxide (or higher percentage with pro-tective gloves) may be applied to the wheel.Higher percentages of hydrogen peroxide can bemore effective. Another way is to polish with the

alumina paste, followed by a Kroll’s etch, re-peating this cycle until a clean microstructurecan be observed. A combination of these stepscan prove effective as well. This method hasproven very effective for labs without automatedpolishers. The beeswax wheel would be an ex-cellent choice for the small lab or classroom.

Final Polishing. There are three methods forfinal polishing: hand polishing, semiautomated,and vibratory polishing. For hand polishing a 50/ 50 mix of 3% hydrogen peroxide and a 10% so-lution of a 0.05 lm premixed alumina suspen-sion is used. These suspensions are availablefrom several suppliers. A nap cloth or a closed-cell chemical-resistant foam pad is recom-

mended. For increased edge retention a Dacronor polyester cloth is preferred.A semiautomatic method is used for most ti-

tanium alloys as well as commercially pure (CP)titanium when not inspecting for hydrides. Thesame 50/50 mix of 3% hydrogen peroxide and adiluted 0.05 lm premixed alumina suspension isused on a closed-cell chemical-resistant foam

pad (neoprene rubber cloth). The wheel speedshould be 120 to 150 rpm and the force shouldbe 15 N (3.4 lbf) per 38 mm (1.5 in.) diammount. Head rotation should be in the comple-mentary direction. For even better edge reten-tion, a Dacron or polyester cloth may be usedwith the same parametersexcept the force shouldbe 25 to 40 N (5.6 to 9.0 lbf) per 38 mm (1.5in.) diam mount. The foam pad usually produces

a clearer overall microstructure, but with slightedge rounding. Polishing times will vary, withmuch longer times required for CP titanium thanthe more highly alloyed materials.

The vibratory polisher set up with a short-napwoven synthetic cloth is the preferred method toproduce a finish with the least amount of defor-mation to the microstructure. However, this pro-cess can take a considerable amount of time. A10% solution of premixed 0.05 lm alumina ona short-nap synthetic cloth may be used. Thiswill optimize removal of deformation from pre-vious steps, but will yield slight edge roundingand require polishing for 8 to 16 h with a weightof only about 0.3 kg (11 oz). More weight will

round the edges more.The vibratory polisher set up with a non-nap

polyester cloth is the preferred method to pro-duce a microstructure that provides optimal edgeretention and little or no deformation to the mi-crostructure. The same suspension is used on aDacron or polyester non-nap cloth. A weight of 1.0 to 1.5 kg (2.2 to 3.3 lb) is attached to a 38to 50 mm (1.5 to 2 in.) mount with double-back tape. The polishing time is 1 h. This methodworks well on hybrid materials containing tita-nium as well as other materials. It optimizes edgeretention.

Example: Comparison of Polishing Meth-ods. Micrographs of a beta-annealed Ti-6Al-4V

structure are shown in Fig. 7 for three differentpolishing preparations, as described below. Eachprocedure is designed for a different purpose.

Polishing to Optimize Removal of Deformed Material (Fig. 7a). In this example, polishingwas accomplished with a four-step method foroptimizing removal of deformed material. Thismethod is best for overall polish quality:

1. 200 grit diamond-embedded platen2. 9 lm proprietary grinding platen3. 3 lm polyester cloth4. 16 h on vibratory polisher with short-nap s

thetic cloth and 10% solution of premi0.05 lm alumina suspension

Polishing to Optimize Edge Retention (F

7b). In this example, the same material and e

is used, but polishing was accomplished by following four-step method for optimizing eretention:

1. 220 grit diamond-embedded platen2. 9 lm proprietary grinding platen3. 3 lm polyester cloth4. 1 h on vibratory polisher with non-nap po

ester cloth and 10% solution of premixed 0lm alumina suspension

Another example is Fig. 8 from Ti-6Al-4V mterial solution treated at 925 C (1700 F) aaged. It was polished with the four-step methfor edge retention. It was etched with the oxatint etch to reveal the deformed grain struct

from drilling. Figure 8(a) shows the depth of cold work as evidenced by the disturbed micstructure to a depth of 310 lm. Figure 8shows the normal microstructure for compson.

Polishing to Optimize Preparation Time (F7c). In this example, the same material and eis used. Polishing was accomplished withthree-step, semiautomated method for optiming preparation time:

1. 220 grit diamond-embedded platen2. 9 lm proprietary grinding platen3. Closed-cell chemical-resistant foam pad w

a 50/50 mix of 3% hydrogen peroxide an0.05 lm premixed alumina suspension

Total time to prepare sample is less than 20 mNote that the addition of an intermediate 3 polyester cloth step will further improve this pcess. Also note the lack of detail in the darkgions in Fig. 7(c).

Etchants. There are numerous choicesetchants for revealing titanium alloy microstr

Fig. 10 Ti-6Al-4V plate heated at 885 C (1625 F) for 15 min, air cooled. (a) Ammonium bifluoride (ABF) tint etch, 60 s; slightly uncrossed polarized light. (b) Kroll’s reages. (c) Oxalic tint etch, 15 s




ture. Table 1 lists various etchants, and a moreextensive listing is also contained in Ref 1. How-ever, this article focuses on three types of etch-ants that can easily handle nearly all the needsin any laboratory. The three etchants are Kroll’sreagent and two tint etches: an oxalic-acid tintetch and an ammonium bifluoride (ABF) tintetch (Table 2).

The etching time will vary depending on the

alloy and heat treat condition of the sample. It isa good practice to document successful etchingpractices in the laboratory. When using the tintetchants, it is critical not to swab the specimenafter etching. Just wash with warm tap water.

Any swabbing or contact will disturb the surface.The etchant products highlight the grain orien-tation. The use of tint etchants on specimenswhere cold work (plastic and elastic deforma-tion) is left from the sample preparation is notrecommended. Kroll’s reagent is more forgivingin that case.

Comparison of contrast developed by thethree etchants is shown for three types of struc-

tures or conditions:

● Coarse lamellar structure after slow furnacecool of T-6Al-4V from beta anneal at 1035C (1900 F) revealed by Kroll’s reagent andtint etches (Fig. 9)

● Worked structure in Ti-6Al-4V plate aircooled after heat treatment at 885 C (1625F) revealed by Kroll’s reagent and tint etches(Fig. 10)

● Bimodal structure representative of an alpha/ beta forging revealed by Kroll’s reagent andtint etches (Fig. 11). The light phase in Fig.11(b)—primary alpha in a matrix of trans-formed beta, a lamellar alpha/beta structureas clearly illustrated in the inset of Fig. 11(c)

Figures 12 to 15 are all of a Ti-6Al-4V solu-tion treated and aged fastener with rolled threads.Examples are shown etched with both oxalicacid and Kroll’s reagent. Both etchants show the

fine detail without overetching. However, it difficult to show etched microstructure in titanium alloy fasteners due to the cold work fromrolling the threads. Figures 12 and 13 show thcrest area of the thread. Note the lighter structurat the crest where the cold work is greatest.

Kroll’s reagent, the most common etchant oreagent used on titanium alloys (Table 2), is usefor bringing out the general microstructure in a

pha-beta alloys. It is a relatively low-contraetchant. Figure 16 shows examples of etched mcrostructures from the same Ti-6Al-4V overageplate with varying etch severity with (Kroll’reagent. Etchant time is a compromise betweedetail and contrast. The shorter times revemore detail, while longer etching times result imore contrast. As etching time increases, all detail is lost in the contrast (see Fig. 16d). It ibetter to underetch than overetch. Figure 16(ais underetched; however, it has all the detail necessary to analyze the specimen correctly. Figu16(b) is a good compromise at 15 s.

Oxalic acid is a tint etch that stains the microstructure and provides more contrast in the m

Fig. 11 Ti-6Al-4V die forging, mill-annealed. (a) Am-monium bifluoride (ABF) tint etch,60 seconds;

slightly uncrossed polarized light. (b) Kroll’s reagent, 15 s;slightly uncrossed polarized light. (c) Oxalic tint etch, 15s; slightly uncrossed polarized light. See also Fig. 60 in thearticle “Selected Color Images” in this Volume for colorversion.

Fig. 12 Oxalic tint etch for 15 s. Ti-6Al-4V fastener so-lution treated and aged. 1 h vibratory polisher,

non-nap polyester cloth and alumina. Note: the mountedparts were vacuum impregnated withhydratedrhodamine-dyed epoxy. Note the crest lap, which is typical for a rolledthread.

Fig. 13 Etched with Kroll’s reagent for 15 s. Ti-6Al-fastener solution treated and aged. 1 h vibr

tory polisher, non-nap polyester cloth and alumina. Nothe crest lap, which is typical for a rolled thread.

Fig. 14 Etched with Kroll’s reagent for 15 s. Ti-6Al-4Vfastener solution treated and aged. 1 h vibra-

tory polisher, non-nap polyester cloth and alumina.

Fig. 15 Oxalic tint etch for 15 s. Ti-6Al-4V fastener slution treated and aged. 1 h vibratory polish

non-nap polyester cloth and alumina. Note: This mouwas vacuum impregnated with hydrated, rhodamine-dyepoxy.




crostructure in alpha-beta alloys. Etching withoxalic acid requires a better quality polish. It isgood for revealing grain orientation, heat effects,and alpha-rich regions formed by exposure toalpha stabilizers such as oxygen or nitrogen.

Note: do not remove the etching products orswab during the procedure.

Ammonium bifluoride (ABF) is another tintetch. It is similar to oxalic acid and also provides

more contrast in the microstructure of alpha-betaalloys. Both reagents will reveal grain orienta-

tion. It is most often used for investigating alphacase or alpha-enriched areas in the microstruc-ture. Note: do not remove the etching productsor swab during the procedure.

Macroexamination

Macrostructural examination of titanium al-

loys provides useful information about materialprocessing, both melting and metalworking. It is

used for detection of melting defects or anomlies, qualitative assessment of grain refinemand uniformity, as well as determination of grflow in forged products. Macroetching of tnium alloys is discussed in the article “Mcroetching” in this Volume.

Four principal defects are to be found in mcrosections of ingot, forged billet, or other seifinished product forms. These include high-a

minum defects (HADs or type II defects), hinterstitial defects (HIDs, also referred to as tyI defects or low-density interstitial defects), bflecks and high-density inclusions (HDI), Hialuminum defects are areas containing an abnmally high amount of aluminum. These are sareas in the material (Fig. 17, 18) and are areferred to as “alpha segregation.” Defects ferred to as “beta segregation” are sometimessociated with alpha segregation. These are arin which aluminum is depleted. The high intstitial defects (Fig. 19, 20) are normally highoxygen and/or nitrogen, which stabilize thepha phase. These defects are hard and britthey are normally associated with porosity,

shown in Fig. 21.

Fig. 16 Micrographs from solution treated and overaged Ti-6Al-4V plate after etching with Kroll’s reagent for (a) 5 s,(b) 15 s, (c) 30 s, and (d) 60 s. All specimens polished for 1 h with vibratory polisher, non-nap polyester cloth

and alumina. In the severe etch (d), note that fine detail is etched away and the relief is becoming excessive. Fig. 17 Ti-6Al-4V alpha-beta processed billet illuing the macroscopic appearance of a high

minum defect. See also Fig. 18. 1.25. Courtesy oScholl

Fig. 18 Same as Fig. 17. There is a higher volume frac-tion of more elongatedalpha in theareaof high

aluminum content. 50. Courtesy of C. Scholl

Fig. 19 Ti-6Al-4V alpha-beta processed billet illustrat-ing macroscopic appearance of a high intersti-

tial defect. See also Fig. 20. Actual size

Fig. 20 Same as Fig. 19. The high oxygen contensults in a region of coarser and more brittl

ygen-stabilized alpha than observed in the bulk mate100




Beta flecks are regions enriched in a beta-sta-bilizing element due to segregation during ingotsolidification. Their occurrence in alpha-beta al-loys is uncommon. Flecking becomes more of aproblem with beta alloys, which have muchhigher amounts of beta-stabilizing additions.The problem is most prevalent in iron- and chro-mium-bearing alloys. This enrichment of a lo-calized region with beta stabilizers lowers thebeta transus, locally changing the microstructureand thereby enabling their detection.

This microstructural modification can taketwo forms. In alpha-beta alloys, such as Ti-6Al-6V-2Sn, vanadium enrichment lowers the beta

transus, but is not sufficient to stabilize the betato room temperature. When working or heattreating the material high in the b phasefield, the microstructure observed (after coolingback to room temperature) will consist of pri-

mary alpha and transformed beta. The beta fleck

is a result of the local composition with a higherbeta-stabilizer content that results in a local beta

transus lower than that of the bulk material. Bet

flecks occur if the temperature is above the transus in the “flecked” region. This condition is ap

Fig. 21 Ti-8Al-1Mo-1V, as forged. Ingot void (black),surrounded by a layer of oxygen-stabilized al-

pha (light). The remaining structure consists of elongatedalpha grains in a dark matrix of transformed beta. Etchant:Kroll’s reagent (ASTM 192). 25

Fig. 22 Ti-6Al-6V-2Sn alpha-beta alloy forging, solution treated, quenched, and aged. Hand forging at 925 C (170F), solution treated for 2 h at 870 C (1600 F), water quenched, aged 4 h at 595 C (1100 F), and air coole

(a) “Primary” alpha grains (light) in a matrix of transformed beta containing acicular alpha. Kroll’s reagent (ASTM 19150. (b) Same structure is the same as in (a), except that alloy segregation has resulted in a dark “beta fleck” (centemicrograph) that shows no light “primary” alpha. See also Fig. 23 and 24. Etchant: Kroll’s reagent (ASTM 192). 75

Fig. 23 Ti-6Al-6V-2Sn forging, solution treated for 11 ⁄ 4h at 870 C (1600 F), water quenched, and

aged 4 h at 575 C (1070 F). Structure: same as in Fig.22(b), but higher magnification shows a small amount of light, acicular alpha in the dark “beta fleck.” See also Fig.24. Etchant: 2 mL HF, 8 mL HNO3, 90 mL H2O. 200

Fig. 24 Ti-6Al-6V-2Sn b forged billetillustratingmacroscopic appearance of beta flecksthat appear as darkspoSee also Fig. 22 and 23. Etchant: 8 mL HF, 10 mL HNO3, 82 mL H2O, then 18 g/L (2.4 oz/gal) of NH4HF2

H2O. Less than 1. Courtesy of C. Scholl

Fig. 25 Ti-10V-2Fe-3Al pancake forging. (a) Beta forged about 50% alpha-beta finish forged about 5%, with hetreatment at 750 C (1385 F), 1 h, water quench, 540 C (1000 F), 8 h. (a) Lamellar alpha with a sm

amount of equiaxed alpha in an aged beta matrix. Etched 10 s with Kroll’s reagent, then 50 mL of 10% oxalic acid, 5mL of 0.5% HF. 400. Courtesy of R. Boyer. (b) Same as (a), but amount of b finish forging is 2%. Micrograpillustrates darkened aged beta surrounding a lighter etched beta fleck. See also Fig. 26. Same etch as (a). 50. Courteof T. Long




parent in Fig. 22 and 23. A beta fleck could go

undetected if the final processing and heat treat-ment are conducted at a temperature low enoughthat the beta transus suppression is not sufficientto cause a microstructural perturbation. The ef-fects of beta flecks on properties in such alloysas Ti-6Al-4V and Ti-6Al-6V-2Sn are still inquestion, but the effect is not a major one.

Beta flecks are more of a problem with near-beta alloys; they are observed macroscopicallyas shiny spots or flecks. Their appearance issimilar in b alloys (Fig. 24). The beta-sta-bilizer enrichment in the flecked regions of betaalloys, however, is sufficient to stabilize the betadown to room temperature. To guarantee mate-rial that will be fleck-free, producers must solu-

tion treat samples at a certain temperature belowthe beta transus, assuring the user that the ma-terial will not form beta flecks if heat treated toa temperature up to or below the test tempera-ture. The material will then form alpha, but betafleck regions will be above or much nearer thetransus. Therefore, they will be void of alpha orcontain a significantly lower volume fraction of alpha upon cooling to room temperature, asshown in Fig. 25 and 26. These regions in betaalloys will be harder, will have higher strengthand lower ductility, and will have lower low-cy-cle fatigue strength than the bulk material.

Tree rings (Fig. 27) are another macrostruc-tural anomaly observed in titanium alloy macro-sections. This phenomenon represents very mi-

nor composition variations that occur duringmelting. The appearance of tree rings is normallyonly a cosmetic nuance, not a cause for concern.

Grain flow of forgings is useful for evaluatingthe forging process. For high-quality forgings, ingeneral, the grain flow should conform to thegeneral shape of the part. There should be noforging laps, seams, or areas of grain flow thatappear as though they could produce forging lapsin subsequent operations. In addition, the partshould be uniformly recrystallized and suffi-ciently worked in all areas.

Microexamination

Bright-field illumination reveals microstruc-ture of properly prepared specimens in mostcases, but image enhancement can be achieved

in nearly all cases by using just the polari(plane-polarized light) or using the analyzerslightly uncrossed polarized light. This also aas a neutral density filter for capturing the digimage. High-quality, strain-free objectives desirable for any polarized light microsco

Fig. 26 Same as Fig. 25(b), but at higher magnificationto demonstrate the reduced amount of alphain

the beta fleck. The alpha observed (light) is primary alpha;the alpha that forms upon aging is too fine to resolve. Sameetch as Fig. 25(a). 200. Courtesy of T. Long

Fig. 27 Ti-6Al-2Sn-4Zr-2Mo alpha-beta forged billet macroslice illustrating “tree rings,” which represent minorpositional fluctuations. The slices are from two ingot locations. Etchant unknown. 0.63. Courtesy o

Reinsch

Fig. 28 Variation in appearance with changes in illumination of a Ti-6Al-4V specimen with oxalic tint etch (Material was beta annealed at 1050 C (1925 F) and furnace cooled. (a) Illuminated and examined

slightly uncrossed (45–129) polarized light. (b) Illuminatedand examinedwith slightly uncrossed(45–139)polarized(c) Plane-polarized light illumination. (d) Bright-field illumination. See also Fig. 61 in the article “Selected Color Imin this Volume for color version.




while non-strain-free objectives or objectiveswith a long working distance will compromisethe polarized light image. Figure 28 provides

comparative micrographs from the examinationof a Ti-6Al-4V specimen with different illumi-nation modes.

Alpha Structures

Generally, two types of alpha are present: prmary alpha and secondary alpha or transformebeta (Fig. 29, 30). The primary alpha is that preent during prior hot working, remnants of whicpersist through heat treatment. The secondary apha is produced by transformation from betThis may occur upon cooling from above th

beta transus (Fig. 29d) or high within the alphabeta phase field (Fig. 29b and c) by aging or baging of the beta (Fig. 30d). The aged alpha usually too fine to resolve using light (opticamicroscopy. The alpha in these areas has diffeent appearances and may be acicular or lamellaplatelike, serrated, or Widmanstatten.

Equiaxed alpha grains, such as are shown iFig. 31 and 32 are usually developed by anneaing cold-worked alloys above the recrystalliztion temperature. Elongated alpha grains (Fi33, 34) result from unidirectional working of thmetal and are commonly found in longitudinsections of rolled or extruded alloys.

The microstructure of titanium alloys

strongly influenced by the processing history anheat treatment. The effect of cooling rate on T5Al-2.5Sn annealed above the beta transus cabe seen in Fig. 35. This is also illustrated for T6Al-4V in Fig. 36. As the cooling rate increasethe lamellar alpha (or martensite, depending othe alloy and cooling rate) becomes finer. Coarsand finer lamellar structures in alloy Ti-6Al-4Vare also shown, respectively, in Fig. 9 and 2after furnace cooling from different temperatureabove the beta transus. The extent of lamellaalpha in the Ti-10V-2Fe-3Al lean beta alloy shown in Fig. 37. The structure is completellamellar alpha (Fig. 37a) when heat treatment below the beta transus. When heat treated jubelow the beta transus, a beta structure develop

with some residual alpha (Fig. 37b). Wheheated above the transus and cooled, the structure of Ti-10V-2Fe-3Al is completely beta.

The effect of forging temperature is illustratefor Ti-8Al-1Mo-1V in Fig. 38. As the forgin

Fig. 29 Ti-6Al-2Sn-4Zr-6Mo, forged at 870 C (1600 F). (a) Solution treated 2 h at 870 C (1600 F), waterquenched,and aged 8 h at 595 C (1100 F), and air cooled. Elongated “primary” alpha grains (light)in agedtransformed

beta matrix containing acicular alpha. (b) Solution treated at 915 C (1675 F) instead of at 870 C (1600 F), whichreduced the amount of “primary” alpha grains in the b matrix. (c) Solution treated at 930C (1710 F), which reducedthe amount of alpha grains and coarsened the acicular alpha in the matrix. (d) Solution treated at 955 C (1750 F), whichis above the beta transus. The resulting structure is coarse, acicular alpha (light) and aged transformed beta (dark). All

etched with Kroll’s reagent (ASTM 192). 500

Fig. 30 Ti-15V-3Cr-3Al-3Sn cold-rolled strip that has been annealed at 790 C (1450 F) for 10 min and aged at various times to illustrate the progression of aging and whattermed “decorative aging,” a technique used to determine the extent of recrystallization. (a) Not aged. (b) Aged 2 h at 540 C (1000 F). (c) Aged 4 h. (d) Aged 8 h. Grai

in center are completely aged (uniform alpha precipitation throughout the grains). An 8 h age results in a fully aged structure. All etched with Kroll’s reagent. All 200. Courtesy ofBania




temperature increases, the amount of trans-formed beta increases until the forging tempera-ture is above the beta transus, at which point thestructure is 100% transformed beta. The effectof the amount of forging deformation is illus-trated for Ti-6Al-2Sn-4Zr-2Mo and Ti-5Al-6Sn-2Zr-1Mo-2.5Si, respectively in Fig. 39 and 40.Sufficient working of the cast Widmanstattenstructure at a temperature below the beta transuscauses recrystallization of the lamellar structureto a more equiaxed structure. Sufficient working

and proper heat treatment can produce a com-pletely equiaxed crystal structure (Fig. 32). Themicrostructural behavior trends will be similarfor all-alpha and b alloys.

Acicularor lamellar alpha is the most commontransformation product formed from beta duringcooling. It is a result of nucleation and growth oncrystallographic planes of the prior beta matrix.Precipitation normally occurs on multiple vari-ants or orientations of this family of habit planes,as illustrated in Fig. 29(d) and 41. A packet orcluster of acicular alpha grains aligned in thesame orientation is referred to as a “colony.”Whencorrelating thistype of microstructure withproperties such as fatigue or fracture toughness,colony size is often regarded as an important mi-

crostructural feature.The multiple orientationsof alpha have a basketweave appearance character-istic of alpha Widmanstatten structure. Lamellaralpha forming from small beta grains also mayhave a singular orientation (Fig. 42).

Under some conditions, the long grains of al-pha produced along preferred planes in the betamatrix take on a wide, platelike appearance, asshown in Fig. 35(a). Under other conditions,grains of irregular size and with jagged bound-aries, called “serrated alpha,” are produced(Fig. 43).

Alpha Case. Unless heat treatments are p

formed in an inert atmosphere, oxygen andtrogen will be absorbed at the surface, stabilthe alpha, and form a hard, brittle layer refer

Fig. 31 High-purity (iodide-process) unalloyed tita-nium sheet, cold rolled, and annealed 1 h at

700 C (1290 F). Equiaxed, recrystallized grains of alpha.Etchant: Kroll’s reagent (ASTM 192). 250

Fig. 32 Ti-6Al-4V plate, recrystallize annealed at 925C (1700 F) 1 h, cooled to 760 C (1400 F) at

50 to 55 C/h (90 to 100 F/h), then air cooled. Equiaxedalpha with intergranular beta. The alpha-alphaboundariesare not defined. Etchant: 50 mL oxalic acid in H2O, 50 mL1% HF in H2O. 500. Courtesy of J.C. Chesnutt

Fig. 33 Commercial-purity (99.0%) unalloyed titanium sheet. (a) As-rolled to 1.0 mm (0.040 in.) thickat 760C (1F). Grains of alpha, which have been elongated by cold working. (b) Same as in (a), but annealed 2 h at

C (1290 F) andair cooled. Recrystallized alpha grains, particlesof TiH (black), andparticles of beta (alsoblack) stabby impurities. (c) Same as in (a), but annealed 1 h at 900 C (1650 F)—just below the beta transus—and air cooRecrystallized grains of “primary” alpha and transformed beta containing acicular alpha. (d) Same as in (a), but ann2 h at 1000 C (1830 F) and air cooled. Colonies of serrated alpha plates; particles of TiH and retained beta (both bbetween the plates of alpha. All etched with Kroll’s reagent (ASTM 192). 250

Fig. 34 Ti-6Al-4V, as-forged at 955C (1750 F), bethe beta transus. Elongated alpha (l

caused by low reduction (20%) of a billet that had coplatelike alpha, in a matrix of transformed beta contaacicular alpha. Etchant: Kroll’s reagent (ASTM 192). 2




Fig. 35 Ti-5Al-2.5Sn, hot worked below the alpha transus, annealed 30 min at 1175 C (2150 F), which is above the beta transus. (a) Furnace cooled to 790 C (1450 F) in 6 and furnace cooled to room temperature in 2 h. Coarse, platelike alpha. Etchant: Kroll’s reagent (ASTM 192). 100. (b) Air cooled from the annealing temperature inste

of furnace cooled. The faster cooling rate produced acicular alpha that is finer than the platelike alpha in (a). Prior beta grains are outlined by the alpha that was first to transfoEtchant: Kroll’s reagent (ASTM 192). 100. (c) Water quenched from the annealing temperature instead of furnace cooled and shown at a higher magnification. The rapid cooliproduced fine acicular alpha. A prior beta grain boundary can be seen near the center of the micrograph. Etchant: Kroll’s reagent (ASTM 192). 250

Fig. 36 Ti-6Al-4V bar, held for 1 h at 1065 C (1950 F), above the beta transus. (a) Furnace cooled. Platelike alpha(light) and intergranular beta (dark). (b) Air cooled. The structure consists of acicular alpha (transformed beta);

prior beta grain boundaries. Etchant for both (a) and (b): 10 mL HF, 5 mL HNO3, 85 mL H2O. 250

Fig. 37 Effect of heat treatment temperature below, near, and above the transus temperature on etched appearance of lean beta alloy Ti-10V-2Fe-3Al. All specimens were poliswith four-step procedure ending up with 16 h on vibratory polisher (10% alumina slurry), etched with Kroll’s reagent for duration noted, and examined under slig

uncrossed polarized light (a) lamellar alpha after air cool (AC) from temperature about 70 C (130 F) below beta transus (730 C, or 1350 F, for 2 h). (b) Heat treated just below tbeta transus (788 C, or 1450 F, for 2 h, AC), where almost all of the alpha has gone back i nto solution. One grain in this view contains residual alpha. (c) All-beta structure from bheat treatment. Duration of etching with Kroll’s reagent: (a) 15 s, (b) and (c) 60 s




to as an “alpha case” (Fig. 44, 45). This case isnormally removed by chemical milling or ma-chining. A part should not be put into serviceunless this alpha case has been removed.

Figures 46 to 50 show alpha case layerscaused by interstitial oxygen migration throughthe surface. This illustrates the increase in alphacase thickness as the thermal exposure is in-creased with longer times or higher tempera-

tures. The oxygen migration causes an increasein hardness beyond the visible depth of the alphacase layer.

Ti3Al (Alpha-2) Ordered Phase. The alphaphase can decompose to Ti3Al, an ordered

phase, at compositions greater than about 6 wt%Al. This ordered phase is submicron in size andcan be observed only by electron microscopy(Fig. 51).

Martensite

Martensite is a nonequilibrium supersaturaalpha-type structure produced by diffusion

Fig. 38 Ti-8Al-1Mo-1V forging. (a) Forged with a start-ing temperature of 900 C (1650 F), which is

below the normal temperature range for forging this alloy.Structure: equiaxed alpha grains (light) in a matrix of trans-formed beta (dark). (b) Forged with starting temperature of 1005 C (1840 F), which is within the normal range, andair cooled. Equiaxed grains of “primary” alpha (light) in amatrix of transformed beta (dark) containing fine acicularalpha. (c) Starting temperature for forging was 1095 C(2000 F), whichis above thebeta transus temperature,andthe finished forging was rapidly air cooled. The structureconsists of transformed beta containing coarse and fineacicular alpha(light). All etched withKroll’sreagent(ASTM192). 250

Fig. 39 Ti-6Al-2Sn-4Zr-2Mo forged ingot. (a) Forged and held 1 h at 1010 C (1850 F), air cooled, heated to 97(1775 F), and immediately air cooled. Acicular alpha (transformed beta); prior beta grain boundarie

Same as (a), but reduced 15% by upset forging while at 970 C (1775 F). The structure consists of slightly deforacicular alpha (transformed beta), boundaries of elongated prior beta grains. Both etched withKroll’s reagent (ASTM100

Fig. 40 Ti-5Al-6Sn-2Zr-1Mo-2.5Si forging. (a) Reduced 75% by upset forging starting at 980 C (1800 F), annealh at 980 C (1800 F), air cooled, and stabilized 2 h at 595 C (1100 F). Fine alpha grains (light); intergra

beta. (b) Same as (a), except upset forged starting at 1150 C (2100 F), which is above the beta transus temperaDistorted acicular alpha (light constituent); intergranular beta; and boundaries of elongated prior beta grains. Both ewith HF, HNO3, HCl, glycerol (ASTM 193). 100

Fig. 41 Ti-6Al-5Zr-4Mo-1Cu-O.2Si casting. (a) As-cast. Microstructure: transformed beta containing acicular (light platelets). A thin film of alpha phase (light) is evident at the prior beta grain boundaries. (b) Same

but solution treated 1 h in argon at 845 C (1550 F), air cooled, and aged 24 h at 500 C (930 F). Acicular alpha (land aged beta; alpha platelets at prior beta grain boundaries. Both etched with 10 mL HF, 30 mL HNO3, 50 mL H(ASTM 187). 500




(martensitic) transformation of the beta. Thereare two types of martensite: , which has a hex-agonal crystal structure, and , which has anorthorhombic crystal structure. Martensite canbe produced in titanium alloys by quenching(athermal martensite) or by applying externalstress (stress-induced martensite). The can beformed athermally or by a stress-assisted trans-formation (see Fig. 52, 53). However, can be

formed only by quenching. Examples of structures are exhibited in Fig. 54 and 55. Agingof the martensite results in its decomposition to b.

Beta Structures

In alpha-beta and beta alloys, some equilib-rium beta is present at room temperature. A non-

equilibrium, or metastable, beta phase can beproduced in alpha-beta alloys that containenough beta-stabilizing elements to retain thebeta phase at room temperature on rapid coolingfrom high in the b phase field. The com-position of the alloy must be such that the tem-perature for the start of martensite formation isdepressed to below room temperature. One hun-dred percent beta can be retained by air cooling

beta alloys. The decomposition of this retainedbeta (or martensite, if it forms) is the basis forheat treating titanium alloys to higher strengths.

Aged Structures

The alpha that forms upon aging of retainedbeta is often too fine to be resolved by opticalmicroscopy, particularly with beta and near-beta

alloys. Aging of martensite results in the fomation of equilibrium b, but most agemartensite structures cannot be distinguishefrom unaged martensite by optical microscopy

Unresolved alpha precipitation is shown iFig. 56 for alloy Ti-10V-2Fe-3Al with Krolletch. Figures 57 and 58 are light micrographs ocold-rolled and aged Ti-15V-3Cr-3Al-3Sn foiThe white regions indicate there was less or n

heat treatment response. Precipitation of alphduring aging of beta results in some darkeninof the aged beta structure. The progression oaging response in alloy Ti-15V-3Cr-3Al-3Sn shown in Fig. 30.

Other precipitation products include:

● Eutectoid products● x phase (Fig. 59), which is a transition pha

(potentially resulting in severe embrittlemen● Phase splitting

Phase splitting, or phase separation only, occurin the solute-rich beta alloys; b r br b1 wher

Fig. 42 Ti-6Al-4V forging. (a) Solution treated 1 h at 955 C (1750 F), air cooled, and annealed 2 h at 705 C (1300F). Equiaxed alpha grains (light) in transformed beta matrix (dark) containing coarse, acicular alpha. (b) Same

as in (a), except water quenched from the solution treatment (before the anneal) instead of air cooled. Structure is similarto that in (a), but the faster cooling resulted in finer acicular alpha inthe transformed beta.Both etched withKroll’sreagent(ASTM 192). 500

Fig. 43 Unalloyed titanium sheet. Same as Fig. 33, bannealed 2 h at 1000 C (1830 F) and a

cooled. Colonies of serrated alpha plates; particles of Tand retained beta (both black) between the plates of alphEtchant: Kroll’s reagent (ASTM 192). 250

Fig. 44 Ti-7Al-2Mo-1V plate, heated to 1010 C (1850F), which is above the beta transus. Surface

layer of white, oxygen-stabilized alpha (alpha case); theremainder of the structure is acicular alpha (transformedbeta). Etchant: 2 mL HF, 8 mL HNO3, 90 mL H2O. 450

Fig. 45 Ti-6Al-4V platediffusion-bonded joint(bondedat 925 C, or 1700 F) illustrating bond-line

contamination. The white horizontal band is an area of O2

and/or N2 enrichment. An alpha case is also observable onthe exterior surface. Etchant: 50 mL H2O, 50 mL 10% ox-alic acid, 1 mL HF. 58. Courtesy of J.C. Chesnutt

Fig. 46 Alpha case in Ti-6Al-4V after exposure to 7C (1400 F) for 90 min. Preparation: oxalic t

etch for 60 s, and four-step edge-retention process endiwith 1 h on vibratory polisher with a non-nap polyestcloth and alumina. See also Fig. 62 in the article “SelectColor Images” in this Volume for color version.




br is solute-rich beta and b1 is solute lean beta.The solute lean beta is designated b (Fig. 60).This is not an important decomposition productfrom a practical standpoint, because it does notoccur in commercial alloys with heat treatmentsthat are used. The x phase and phase splittingcan only be observed using electron microscopy.

Other Structures

Hydrides. Figure 61 shows hydrides in com-mercially pure (CP) titanium sheet located at theweld heat-affected zone (HAZ). The black nee-dles in this micrograph are a result of hydrogenmigrating to high residual stress areas and form-ing a titanium hydride. In most cases, because of

their brittle nature, hydrides will result in micro-cracks. Material with this extent of hydrides willbe very brittle. Commercially pure titanium ismore difficult to polish than more highly alloyedtitanium alloys because it is softer and retainsmore cold work. The same polishing methodused for titanium alloys will also work for CPtitanium, but longer final polishing times may beneeded. Do not use hydrogen peroxide or etch-polish to remove residual cold work induced byprevious polishing steps. Etching will mask thehydrides. It is best just to add more time to thelast two steps. If hand polishing with a 5 lmalumina and beeswax wheel, it can take 20 min.

Revealing hydrides in CP titanium with an

ABF etch (Fig. 61) requires a perfectly polishedsurface. The lactic hydride reagent (Table 2) maybe more easily applied to reveal the presence of hydrides with etching times ranging from 11 ⁄ 2 to3 min. A 3 min etch can provide good contrastto reveal hydrides with a light microscope. Anetch of 11 ⁄ 2 min may provide subtle contrast un-der a light microscope, but examination in ascanning electron microscope can easily revealmicrocracks associated with hydride cracking.

Heat-Affected Zones. Heat damage frommachining, sample excision, or preparation is of-ten confused with alpha case. The difference can

be determined by making 10 g Knoop indentssince the heat-damaged layer will be softer thanthe base material but would be harder in an alpha

case area. Unlike heat damage from straight ther-mal exposure (which results in a harder surface),heat damage from mechanical sources duringpreparation results in a softer surface. In the ex-ample of Fig. 62, the layer is softer from heatdamage, yielding a larger indent. Another ex-

ample is shown in Fig. 63 of a heat-affected zfrom a molten fine, which had a 50% knockdofactor on fatigue life.

Figures 64 and 65 show heat damage fromlab-induced lightening strike. Note the compmise in etching time in Fig. 64, where the heaffected zone is underetched, while the base mterial is overetched. Figure 65 shows the effeof intraply arcing and heat-affected zone fromlab-induced lightning strike.

Other Techniques

Several metallographic techniques have bdeveloped for specific purposes, including crystallization studies and microstructure/frture topography correlations. Decoration ag

Fig. 47 Alpha case in Ti-6Al-4V after exposure to 885C (1625 F) for 90 min. Preparation: ammo-

nium bifluoride tint etch for 60 s, and four-step edge-reten-tion process ending with 1 h on vibratory polisher with anon-nap polyester cloth and alumina. See also Fig. 63 inthe article “Selected Color Images” in this Volume for colorversion.

Fig. 48 Alpha case in Ti-SP 700(Ti-4.5Al-3V-2Mo-2Fe)after exposure to 900 C (1650 F) for 90 min.

Preparation: ammonium bifluoride tint etch for 60 s, andfour-step edge-retention process ending with 1 h on vibra-tory polisher with a non-nap polyester cloth and alumina.See also Fig. 64 in the article “Selected Color Images” inthis Volume for color version.

Fig. 49 Alpha case in Ti-SP 700 after exposure toC (1400 F) for 90 min. Preparation: am

nium bifluoride tint etch for 60 s, and four-step edge-retion process ending with 1 h on vibratory polisher wnon-nap polyester cloth and alumina

Fig. 50 Alpha case in Ti-10V-2Fe-3Al after exposureto790 C (1450 F) for 2 h. Preparation: Oxalic

tint etch for 60 s, and four-step edge-retention process end-ing with 1 h on vibratory polisher with a non-nap polyestercloth and alumina

Fig. 51 Ti-8Al (with 1800 ppm O2) sheet aged to cipitate the ordered alpha-2 (Ti3Al) phase.

dark-field transmission electron micrograph illustrateprecipitates (light) in an alpha matrix. 105,600. Couof J.C. Williams




was developed to study the extent of recrystal-lization in beta alloys. After recrystallization an-nealing, the material is given a partial age at atime and temperature appropriate for the alloy of interest. The incompletely recrystallized grains

retain some dislocation substructure (stored en-ergy) that accelerates the aging process, resultingin a more rapidly aged grain. These grains thenetch darker than the recrystallized ones, makingit easy to identify the extent of recrystallization.This effect is illustrated in Fig. 30.

Another technique utilizes deep macroetchingand thermal etching. The deformed specimen ispolished, then subjected to overetching to pro-duce deep grooves at the deformed grain bound-aries. Next, the specimen is subjected to the re-crystallization cycle of interest in a hard vacuum(106 torr), followed by oil quenching. The ma-terial recrystallizes and thermal etching occurs,which differentiates between different grains,be-cause surface atoms evaporate or sublimate at

different rates on different crystallographicplanes. Different grains will have different crys-tallographic planes at the exposed surface.

The original grain boundaries are observableas ghost boundaries, due to the deep macroetch-ing used previously. Therefore, the recrystallized

and original microstructures can be observed si-multaneously. This permits studying not only therecrystallized structure, but also the recrystalli-zation nucleation sites. The ghost boundaries canbe removed by repolishing and chemically etch-

ing. This technique is illustrated in Fig. 66. Fig-ure 66(a) demonstrates the as-deformed structurethat has been heavily etched. The specimen wasrecrystallized at 925 C (1700 F) for 1 h in avacuum of 106 torr. Recrystallization invacuum caused thermal etching of the recrystal-lized grains (Fig. 66b shows recrystallized struc-ture). The prior unrecrystallized structure canstill be observed as ghost boundaries remainingfrom the initial overetching.

Subgrain boundaries can be revealed using arelatively simple technique. The specimen iselectropolished and viewed in the scanning elec-tron microscope in the backscattered electronmode. The contrast and delineation of subgrainsare due to differences in crystallographic orien-

tation. Electropolishing occurs at different rateson different crystallographic planes, similar tothe thermal etching phenomenon.

Several techniques have been developed tobserve fracture topography and microstructusimultaneously in the scanning electron microscope using its large depth of field. A very simple method involves selective polishing anetching of the fracture face. The fracture face anmachined surfaces are first masked with a sui

able maskant, such as a stop-off lacquer, whiccan be applied with a small paint brush. Selecteareas of the fracture face are left unmasked. Th

Fig. 52 Ti-8.5Mo-0.5Si water quenched from1000C (1830 F). Thin-foil transmissionelectronmicrographillustratingheavily twinned athermal martensite. 5000. Courtesy of J.C. Williams

Fig. 53 Ti-10V-2Fe-3Al, beta solution treated, waquenched, and strained 5% at room temper

ture. This Nomarski interference micrograph illustrates dformation-induced martensite in a beta matrix. No etc500. Courtesy of J.E. Costa

Fig. 54 Ti-6Al-2Sn-4Zr-2Mo forgings, finish forgedstarting at 970 C (1775 F), air cooled, ma-

chined to 13 mm (0.5 in.) diam test bars, reheated to 995C (1825 F), the beta transus, held for 1 h, and air cooled.The microstructure is entirely . Etched with Kroll’s re-agent (ASTM 192). 100

Fig. 55 martensite in Ti-6Al-4V. (a) Light micrograph of bar, held for 1 h at 955 C (1750 F), which is below tbeta transus, andwater quenched.Equiaxed“primary” alpha grains(light) ina matrixof (martensite).Etche

with 10 mL HF, 5 mL HNO3, 85 mL H2O.250. (b) Thin foil transmission electron micrographof the samemicrostructas in (a), but at higher magnification. The large light grains are primary alpha; the darker region is acicular martensin a beta matrix. 5880. Courtesy of J.C. Williams




specimen is then electropolished, which will af-fect only the unmasked areas, and etched. Study-ing the interface between the polished and etched

and the masked areas permits a correlation of microstructural features and fractographic de-tails, as shown in Fig. 67. This technique is use-ful for correlating general microstructural de-

tails, but it may be difficult to pinpoint a specificarea to study.Precision sectioning techniques have also

been developed. The area of interest on the frac-ture face, such as crack origin, is first located.The specimen is then cut on a plane perpendic-ular to the fracture face close to the area of in-terest. The distance from the cut face to the areaof interest is measured. Next, the specimen isplaced in a metallurgical mount, then ground andpolished the measured distance for metallurgicalanalysis of the precise area of interest and cor-relation of microstructure to fractographic fea-tures. An example of this technique is shown inFig. 68. The microstructure and fracture face canbe observed simultaneously using the scanningelectron microscope by carefully dissolving themount material. This fatigue specimen had aninternal origin at point A, which initiated at an

iron inclusion, as determined in Fig. 68(b)precision sectioning. The cleavage zone at poC in Fig. 68(a) is due to the TiFe2 zone seenpoint C in Fig. 68(b). Below the TiFe2, the strture consists of transformed Widmanstatten pha. The section (Fig. 68b) was taken at line in Fig. 68(a).

REFERENCES

1. G.F. Vander Voort, Metallography: Princip

and Practice, McGraw-Hill, 1984, reprinby ASM International, 1999

2. Struers Metallographic Application Gu

Titanium Alloys, Struers3. M.J. Donachie, Jr., Titanium: A Techn

Guide, 2nd ed., ASM International, 2000

Fig. 56 Fine, unresolved alpha precipitation in light micrograph of aged Ti-10V-2Fe-3Al alloy. The white phase isprimary alpha in an aged beta matrix, dark background. Slightly uncrossed polarized light, and a four-step

polishing ending up with 16 h on vibratory polisher 10% alumina slurry. Etched with Kroll’s reagent for 15 s (a) and 7 s(b)

Fig. 57 Structure from cold-rolledand aged foil ofalloy Ti-15V-3Cr-3Al-3Sn. Oxalic tint etc

3 s, 1 h vibratory polisher, non-nap polyester clothalumina

Fig. 58 Structure from cold-rolledand aged foil of beta

alloy Ti-15V-3Cr-3Al-3Sn. Oxalic tint etch for15 s, 1 h vibratory polisher, non-nap polyester cloth andalumina. The white regions indicate there was no or alesser heat treat response (alpha precipitation).

Fig. 59 A titanium-iron binary alloy, beta solutiontreated, water quenched, and aged to form x.

The x is the light precipitate in this thin-foil transmissionelectron micrograph. In alloys where the x has a high lat-tice misfit, the x is cuboidal to minimize elastic strain inthe matrix. 320,000. Courtesy of J.C. Williams

Fig. 60 Ti-40Nb (at.%), beta solution heat treated at900 C (1650 F), water quenched, then aged

at 400 C (750 F) for 24 h. Thedark precipitate isb (solutelean beta phase) in a solute-enriched beta matrix. Thin-foiltransmission electron micrograph. 31,000. Courtesy of J.C. Williams

Fig. 61 Ammonium bifluoride tint etch for 10 s. Cmercially pure titanium sheet. Four-steppo

ingending upwith 16h onvibratory polisher, 10%alumslurry




Fig. 62 Ti-6Al-4V heat damage. Oxalic tint etch for 15 s. Four-step edge retention process ending up with 1 h on avibratory polisher and alumina

Fig. 64 Oxalic tint etch for 15 s. Ti-15V-3Cr-3Al-3foil heat-affected zone. This is from a lab-

duced lightning strike. Note the compromise in etchi

time. The heat-affected zone is underetched and the bamaterial is overetched. 1 h vibratory polisher, non-npolyester cloth and alumina

Fig. 63 Ti-6Al-4V plate fatigue specimen with molten fine and heat-affected zone 1 h vibratory polisher with non-nap polyester cloth and alumina

Fig. 65 Hybrid Ti-6Al-4V carbon-reinforced polymcomposite with arcing and heat damage fro

a lab-induced lightning strike. Notethe heat-affected zoThe vertical line shows the original surface of the titaniufastener and the extent of intraply arcing. Because of complex shape, the specimen was vacuum impregnatewith hydrated rhodamine-dyed two-part epoxy after setioning on a wafering saw. This sample was prepared wa five-step edge retention process; 220 grit, 9 lm, 3 lsilk, 3 lm non-nap polyester and ending up with 1 h onvibratory polisher with a non-nap polyester cloth and 10alumina solution.




Fig. 66 Ti-10V-2Fe-3Al deformed at 1150C (2100F).(a) Etched with 60 mL H2O, 40 mL HNO3, 10

mL HF for 30 min. (b) Etched with 60 mL H2O, 40 mLHNO3, 10 mL HF for 30 min thermally etched at 925C (1700 F) for 1 h in vacuum (106 torr). Magnificationnot given. Courtesy of D. Eylon

Fig. 67 Scanning electron micrograph (SEM) imagefrom Ti-6Al-4V beta-annealed fatigued plate

specimen. (a) SEM at the polished and etched/unetchedfracture topography interface showing microstructure/frac-ture topography correlation. Secondary cracks are a resultof intense slip bands. (b) SEM that illustrates “furrows” or“troughs” that are defined by the lamellar alpha plates.These furrows link up as the crack progresses. Kroll’s re-agent. 2000. Courtesy of R. Boyer

Fig. 68 Ti-6Al-4V powder metallurgy compact,isostatically pressed at 925 C (1700 F),

MPa (15 ksi), for 2 h. (a) Scanning electron micrographetch. 80. (b) Optical micrograph. Etchant: Kroll’s rea16. Courtesy of D. Eylon



Color Micrographs of Titanium Alloys / 5

Fig. 59 Coarse lamellar alpha revealed by differentetches in Ti-6Al-4V structure after beta anneal

at 1040 C (1900 F) and furnace cooling. Preparation wasfour-step polishing, with final polish of 16 h on vibratorypolisher and 10% alumina slurry. Slightly uncrossed polar-ized light illumination for all three etches. (a) Ammoniumbifluoride tint etch, 60 s. (b) Kroll’s reagent, 15 s. (c) Oxalictint etch, 60 s. Color version of Fig. 9 in the article “Met-allography and Microstructures of Titanium and Its Alloys”

Fig. 60 Ti-6Al-4V die forging, mill-annealed. (a)monium bifluoride tint etch, 60 s. Slightly

crossed polarized light illumination. (b) Kroll’s reagens. Slightly uncrossedpolarizedlight illumination.(c) Otint etch, 15 s. Slightly uncrossed polarized light illumtion. Color version of Fig. 11 in the article “Metallogrand Microstructures of Titanium and Its Alloys”

Fig. 58 Ti-6Al-4V alloy with Widmanstatten alpha in abeta matrix after furnace cooling from above

the transus. Beta-anneal temperature was 1040 C (1900F). Samples were etched with the oxalic tinting reagent for15 s after polishing by (a) the four-step method for optimiz-

ing removal of deformed material, (b) the four-step methodfor optimizing edge retention, or (c) the three-step semi-automated method for optimizing preparation time (notethe lack of detail in the dark regions). See text for descrip-tion of polishing procedures. Color version of Fig. 7 in thearticle “Metallography andMicrostructuresof TitaniumandIts Alloys”



544 / Color Micrographs of Titanium Alloys

Fig. 61 Variation in appearance with changes in illumination of a Ti-6Al-4V specimen with oxalic tint etch (15 s).Material was beta annealed at 1050 C (1925 F) and furnace cooled. (a) Illuminated and examined with

slightly uncrossed (45–129) polarized light. (b) Illuminatedand examinedwith slightly uncrossed(45–139)polarizedlight.(c) Plane-polarized light illumination. (d) Bright-field illumination. Color version of Fig. 28 in the article “Metallographyand Microstructures of Titanium and Its Alloys”

Fig. 62 Alpha case in Ti-6Al-4V after exposure to 760C (1400 F) for 90 min. Preparation: oxalic tint

etch for 60 s, and four-step edge-retention process, endingwith 1 h on vibratory polisher with a non-nap polyestercloth and alumina. Color version of Fig. 46 in the article“Metallography and Microstructures of Titaniumand Its Al-loys”

Fig. 63 Alpha case in Ti-6Al-4V after exposure to 885C (1625 F) for 90 min. Preparation: ammo-

nium bifluoride tint etch for 60 s and four-step edge-reten-tion process, ending with 1 h on vibratory polisher with anon-nap polyester cloth and alumina. Color version of Fig.47 in the article “Metallography and Microstructures of Ti-tanium and Its Alloys”

Fig. 64 Alpha case in Ti-SP 700 (Ti-4.5Al-3V-2Mo-2after exposure to 900 C (1650 F) for 90 mi

Preparation: ammonium bifluoride tint etch for 60 s afour-step edge-retention process, ending with 1 h on vibtory polisher with a nonnap polyester cloth and aluminColor version of Fig. 48 in the article “Metallography aMicrostructures of Titanium and Its Alloys”

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Metallography and Microstructures of Titanium