Deuterium Surface Segregation in Titanium Alloys

PHILIP N. ADLER, ROBERT L. SCHULTE, and HAROLD MARGOLIN

Deuterium surface segregation has been investigated in a, a + /3, and/3-phase titanium that were deuterium charged over the range of 2 to approximately 300 wppm. Surface segregation was observed in samples that were essentially a-phase materials, i.e., high-purity commercial a-Ti, Ti-6A1, and Ti-3A1-2.5V, whereas Ti-6A1-4V had slight enrichment and/3-Ti-13Mn had no detectable segregation. Nuclear reaction analysis (NRA) techniques were used to measure the near-surface deuterium concentration, and the segregation has been localized to within 50 nm of the surface. The time-dependent increase of deuterium at the surface is consistent with deu- terium diffusion from the bulk to the surface and a room-temperature diffusivity of approxi- mately 3 • 10 -9 cm2/s. Surface enrichment in excess of 30 times the bulk concentration was observed in charged samples and in excess of 60 times for samples that had been charged and then vacuum annealed. Polishing was found to be of importance in causing segregation. The presence of deuterides or a surface defect state is suggested to explain the deuterium surface enrichment.

I. INTRODUCTION

IN an earlier ,-[l] wor~:, we reported on deuterium enrich- ment within the near-surface region, i.e., < 0 . 3 / z m from the surface, of high-purity commercial a-Ti (hcp) that had been deuterium charged and measured using nuclear reaction analysis (NRA). Surface enrichment was not observed in fl-phase (bcc) titanium regions of the same diffusion-bonded samples. Because of this phase depen- dence, the existence of the surface segregation peak was used as a means of identifying the presence of a phase within localized regions being probed.

Hydrogen and deuterium surface segregation have been reported in other metal systems. Laursen and Koelbl ~2] found a deuterium-rich surface layer less than 20-nm thick and 10 times the bulk concentration in a Zr-2.5 wt pct Nb alloy (hcp) that had been deuterium charged and ex- amined using NRA. More recent works on this Zr alloy and single-crystal Zr (hcp) indicate that surface polishing and oxygen in solid solution contribute to the observed surface segregation, t3,41 Deuterium surface segregation has also been reported for Nb-V alloys (bcc), tS] where measurements were made using secondary ion mass spectroscopy (SIMS). For Ni (fcc), Fukushima and Birnbaum t6] observed surface and grain boundary seg- regation of deuterium using SIMS and found a correla- tion with surface sulfur segregation. Hydrogen surface segregation has also been observed in polycrystalline Mg (hcp); t71 in this case, time-of-flight analysis of direct atomic recoils of pulsed Ar ions was used, and a mechanism involving oxygen-induced segregation to oxide-rich sur- face layers was adduced. In contrast to these observa- tions of surface segregation, NRA work on Nb (bcc) foils t8~ showed that hydrogen depletion occurs near the surface when oxides and dissolved oxygen exist. These inves-

PHILIP N. ADLER, Director of Materials and Structures, and ROBERT L. SCHULTE, Laboratory Head of Nuclear Detection and Analysis, are with the Grumman Corporate Research Center, Bethpage, NY 11714. HAROLD MARGOLIN, Professor, is with the Department of Metallurgy and Materials Science, Polytechnic University, Brooklyn, NY 11201.

Manuscript submitted June 5, 1987.

tigations suggest that different factors can affect the hy- drogen and/or deuterium concentration in the near-surface of metals.

The present work attempts to characterize deuterium surface segregation in titanium alloys. Deuterium en- richment in the near-surface region, kinetics of segre- gation, generality of the effect, and factors contributing to segregation were investigated and are discussed.

II. EXPERIMENTAL

A. Materials and Sample Preparation

Most of the samples examined in this work were high- purity commercial materials supplied as 1.3-cm-diameter rod. Single-phase a (hcp) and/3 (bcc) materials as well as a + /3 alloys were investigated. The materials and their compositions are shown in Table I. In addition to the rod materials, 25-/zm-thick commercially pure (CP) Ti foil was examined. This foil had a yield strength des- ignation of 70 ksi (482 MPa) and, therefore, contained a higher content of interstitials than the rod materials.

Sample discs of approximately 1-cm length were pre- pared from the rod materials. One of the circular faces was hand ground using standard metallographic proce- dures and final polished with 1.0-~m alumina grit. The samples were cleaned, weighed, and deuterium charged at 800 ~ in a Sieverts-type apparatus. Prior to charging, samples were vacuum annealed at 800 ~ for 2 hours at approximately 1 x 10 -5 torr, and then a predetermined amount of deuterium was introduced into the sample chamber tube from a 1 L standard volume. Samples were maintained at 800 ~ for an additional 2 hours to achieve equilibrium and were cooled by removing the furnace from the container tube and blowing air across the tube. Cooling to 50 ~ took place over an approximate 3/4 hour period; the maximum cooling rate was 160 ~ and the cooling rate decreased with decreasing temperature. Pressure and temperature were monitored throughout. The residual deuterium gas that was in equilibrium at 800 ~ was absorbed during cooling. To provide a surface rep- resentative of the bulk, samples were reground using SiC

METALLURGICAL TRANSACTIONS A VOLUME 21 A, JULY 1990-- 2003

grit to remove at least 50 /zm from the "as-charged" sur- face and were then final polished using 1.0-,~Lm alumina. The foil samples, however, were examined without sur- face preparation, i.e., in the "as-charged" or vacuum- annealed condition, unless otherwise specified.

Samples with different deuterium concentrations were obtained by charging fixed quantities of deuterium from the standard volume. Earlier, calibration experiments with hydrogen using an inert gas fusion method to measure bulk concentrations had established a relationship be- tween the charging pressure and sample concentration. The deuterium concentrations reported herein were mea- sured in the subsurface region of samples using NRA techniques (described subsequently). The measured values were generally consistent with the expected con- centrations from the deuterium chargings to within 10 pet. Therefore, the concentrations measured in the sub- surface region have been termed nominal bulk concentrations.

B. Deuterium Measurements

Deuterium concentration measurements were made using a NRA technique based on the D(3He,p)4He re- action, t91 A 1.2 MeV beam of 3He ions from a Van de Graaff accelerator is incident on the sample and interacts with the deuterium within the first 1.7/zm. High-energy protons that are emitted in the reaction are detected using a silicon surface barrier detector, and the number of emitted protons is proportional to the deuterium content. The proton count rate is converted to deuterium concen- tration by comparison to a deuterium standard, tl~ The energy spectrum of emitted protons is related to the depth of occurrence of the reaction within the sample; thus, we are able to distinguish between the average near-surface (0 to 0.3/xm) and subsurface (0.3 to 1.7/xm) deuterium content. The ability to localize a surface enrichment is limited to the depth resolution at the surface; in this case, it is approximately 0.18/.~m. For the purpose of extract- ing the deuterium content at the surface, the extent of the surface region has been chosen to be approximately 0.3/xm. A 3He beam of 1 na and a beam diameter of 0.8 mm were used. The duration of a typical measure- ment was less than 4 minutes, and the accumulated charge was 0.25/zC.

To achieve better localization of the surface enrich- ment, the depth resolution can be improved by measur- ing the energy spectrum of the associated alpha particle. A completely different detection configuration is used in this case. The 3He beam energy is 0.9 MeV, and the alpha particles are detected at an angle of 72 deg from

the incident beam. The effective depth resolution for this reaction at the surface of titanium samples is about 0.05/xm.

III. RESULTS AND DISCUSSION

A. Description of Surface Segregation

Initial observations of surface segregation were made on a-Ti rod samples that were deuterium charged and subsequently polished. Typical spectra of the proton en- ergy distribution for o~-Ti and fl-Ti-13Mn samples that were charged to approximately 200 wppm are shown in Figure 1. The proton spectrum for a uniform deuterium content within the 0 to 1.7/xm depth is typified by the spectrum of the Ti-13Mn alloy. On the other hand, there is a large peak within the <0 .3 /~m region from the sur- face for the o~-Ti that indicates a significant concentra- tion enrichment within this region. The 0 .3 /zm value is not a measure of the actual depth of the segregation, but it is determined by the depth resolution of the technique. The extent of deuterium segregation within the <0 .3 /~m region was found to be approximately 10 to 15 times the bulk concentration for samples charged to concentrations from 50 to 200 wppm. Interestingly, an a-Ti sample that was charged with deuterium and then vacuum annealed exhibited a surface segregation of almost 30 times the bulk 5 wppm value. These observations strongly suggest that segregation is the result of diffusion from the bulk, since the ambient could not supply the copious quantities of deuterium that were observed.

The kinetics associated with surface segregation were investigated by periodically measuring the deuterium profiles in a-Ti samples over a 31-day period subsequent to surface polishing. Samples containing nominal bulk concentrations of 5, 45, and 85 wppm were examined starting within 1 to 3 hours after polishing. The changes in deuterium concentration as a function of time in the 0 to 0 .3 /zm near-surface and 0.3 to 1.7/xm subsurface regions are shown in Figure 2. Each data point in Figure 2 is an average of four concentration measurements taken at 1-mm intervals along the polished surface. The de- viation of the near-surface concentrations is typically less than 8 pet, indicating that the enrichment occurs uni- formly over the surface. The error bars reflect the un- certainty due to counting statistics. After the initial measurement, the concentration in the subsurface region remains constant and is in reasonable agreement with the nominal concentration. In the near-surface region, the concentration systematically increases with time. To de- scribe the time dependence, a diffusion model based on

Table I. Sample Composition* (Weight Percent)

Material AI V Mn Fe O N

a-Ti - - - - - - 0.08 0.08 0.001 Ti-13Mn - - - - 12.9 0.02 0.09 0.008 Ti-3AI-2.5V 3.2 2.5 - - 0.08 0.09 0.003 Ti-6AI 6.2 - - - - 0.06 0.07 0.002 Ti-6AI-4V 6.2 3.5 - - 0.08 0.07 0.007 CP Ti foil . . . . 0.40** 0.07**

*Spectrographic analysis for A1, V, Mn, Fe, and vacuum fusion for O and N. **Specified maximum for this material.

2004--VOLUME 21A, JULY 1990 METALLURGICAL TRANSACTIONS A

500

400

u~ 300 I - z

O o 2OO

A ~ u-T i �9 fl-Ti-13Mn

z~

z~ A

100 ~, �9 ~OOo

zx ~ z~'~ QIJ zkA~,

o ,~ , , , , i ,~ ,o* ~ , . ~ _ . . 160 180 200 220

C H A N N E L NUMBER

Fig. 1 - - P r o t o n emission spectra fo r t i tan ium al loys showing a dis- tribution indicative of a uniform deuterium content in/~-Ti-13Mn and a surface enrichment (channel 180) in a-Ti.

the formalism of Lea and Seah In] was used to fit the data from the near-surface region. Room-temperature dif- fusivities varied from 2 • 10 -8 to 2 x 10 -9 cm2/s, and a weighted mean of 3 x 10 -9 cm2/s was obtained from these measurements. These results agree with the extrap-

1500

E 1000

z o

8 500

, . , K a) NEAR-SURFACE (0 - 0.3~rn) ~

D ~ 1.8 X 10 "9 cm21s

cm2/s

I I I I I I 1 2 3 4 5 6

t 112 (days 112)

NOMINAL BULK COMPOSITIONS

�9 5 wppm A45 wppm �9 85 wppm

200

r ~: 150

o

100 u

S c~ 50

b) SUBSURFACE (0.3 - 1.7 ,,=m)

.L k & I I I I~ ~" I T I i I I I I I I I I '

4 8 12 16 20 24 28 32

t (days)

Fig. 2 - - T i m e dependence of deuterium concentration in the near- surface and subsurface regions of a-Ti samples after polishing.

olated room-temperature value for hydrogen of approx- imately 2 x 10 -9 cm2/s obtained from measurements by Brauer et al. [12] ofD,~.Ti = 2 • 10 -6 cm2/s at 176 ~ and an activation energy of 12.4 kcal/mol, as reported by Wasilewski and Kehl. tl3] Considerably lower values for the room-temperature diffusivity of hydrogen, i.e., 10 -n to 10 -12 cmE/s, have also been extrapolated from high- temperature data (600 ~ to 1000 ~ Despite the rather wide range of diffusivities, the comparison sug- gests that the overall mechanism of surface segregation in a-Ti is describable by diffusion of deuterium from the bulk to the polished surface of the sample.

B. Alloying and Polishing Effects

To evaluate the generality of surface segregation in titanium alloys, five materials were investigated so that comparison could be made among single-phase t~ and fl, a + /3 with differing amounts of/3, and a-phase mate- rials with differing propensity for deuteride formation. It has been reported that it is more difficult to nucleate hydrides in Ti-6AI than in a-Ti.! 161 It is also generally recognized that Ti-6A1-4V contains more/3 phase than Ti-3A1-2.5V. The /3-Ti-13Mn alloy was utilized as a single-phase/3-Ti control.

Samples of these materials were deuterium charged together to four bulk concentration levels in four sepa- rate chargings and examined. Deuterium concentrations were measured in the near-surface and subsurface re- gions approximately 40 days after charging and polish- ing, and results are given in Table II. The equilibrium pressure for each of these chargings is also indicated in Table II. (The samples annealed at < 10 -5 torr were first charged at an equilibrium pressure of 0.190 torr.) Most of the measured subsurface concentrations were consis- tent with the expected bulk concentrations based on the predetermined quantity of deuterium that was introduced during charging and the measured equilibrium pressure. However, the measured subsurface concentrations for three of the samples were higher than expected. Examination of the proton energy spectrum for these samples indi- cated that the results were affected by the overlap of a predominant near-surface peak. Therefore, for these cases (which are footnoted in Table II), the reported subsur- face concentrations were determined from the measured equilibrium pressure-concentration relationships assum- ing a constant activity coefficient for each material, i.e., linear relationship between the square root of the equi- librium pressure and concentration.

The overall behavior for the three materials that are primarily a phase, i.e., a-Ti, Ti-6A1, and Ti-3AI-2.5V, is quite similar, and significant surface segregation oc- curs in each. The relative extent of surface segregation, as indicated by the ratio, R, of near-surface to subsurface concentrations, is highest for the lowest bulk concentra- tion materials, where a factor of 20 to 30 is observed. For bulk concentrations over the range of 20 to 280 wppm, the extent of surface segregation is roughly a factor of 10 to 16. The extent of segregation is far lower in the t~ + /3 Ti-6AI-4V alloy, where a factor of 2 to 5 en- hancement is indicated. Surface segregation is not ob- served in the fl-phase Ti-13Mn alloy.

The reduced extent of segregation in Ti-6AI-4V and

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Table II. Deuterium Content* (wppm) in Near-Surface and Subsurface Showing Effect of Alloying on Surface Segregation

/3-Ti- 13 Mn

a-Ti Ti-6A1 Ti-3AI-2.5V

Sub- Sub- Sub- Surface surface R** Surface surface R** Surface surface R**

Ti-6A1-4V

Sub- Surface surface R**

Equilibrium Charging Pressure *

(tort)

7 145 5 29 40 2 20 120 5 24 35 7 5 <10 -5 70 450 45 10 320 20 16 745 55 14 75 40 2 0.014

145 1180 85 14 685 55 12 1385 115' 12 140 110 1 0.051 320 2820 185' 15 1525 150 10 3940 245* 16 435 225 2 0.237

*Measured approximately 40 days after charging and polishing. **R = ratio of near-surface (0 to 0.3/xm) to subsurface (0.3 to 1.7/~m) concentration.

*Determined from concentration-equilibrium pressure relationships. *At 800 ~

its absence in the B-phase Ti-i3Mn alloy suggest that the presence of fl inhibits this phenomenon. On the other hand,/3 exists in Ti-3A1-2.5V, as detected using X-ray diffraction, but surface segregation still occurs. The vol- ume percentage of/3, however, was far less in Ti-3A1- 2.5V than in Ti-6A1-4V. For both alloys, the/3 resides in the grain boundary region of essentially equiaxed a, as shown in the optical micrographs of Figure 3. How- ever, the extent of retained/3 within the region of/3 that existed at 800 ~ and is observable in the micrographs cannot be discerned optically. The small quantity of/3 in the Ti-3A1-2.5V is apparently insufficient to inhibit segregation.

In addition to the rod materials discussed above, 25-/xm-thick foil samples of CP Ti were examined. Sam- ples were charged to 250 wppm, and "as-charged" sur- faces were probed. Interestingly, a uniform deuterium concentration was found within the near-surface region. Samples were then hand polished to reproduce the pro- cedure utilized for preparing the rod materials, and sur- face segregation was observed. Other approaches such

(a) (b)

:ig. 3--Microstructures of (a) Ti-3AI-2.5V and (b) Ti-6A1-4V after 'acuum annealing at 800 ~ showing equiaxed a with prior/3 at the :/a boundaries. Krolls etch, Magnification 360 times.

as rubbing to produce surface deformation, plastic de- formation by three-point bending or by rolling, and ex- posure to a saturated aqueous solution containing different SiC and alumina polishing grits were investigated, but only polishing initiated the surface segregation. The im- portance of polishing in introducing deuterium and hy- drogen surface segregation has also been reported in Zr-2.5 wt pct Nb. 13J

The reproducibility of polishing in affecting surface segregation was evaluated by using the same metallo- graphic procedures in repolishing a-Ti rod samples. Three polishing experiences were evaluated for the four sam- ples of different nominal compositions shown in Table II. In all cases, surface segregation reoccurred; however, the extent of surface enrichment associated with com- parable polishing conditions and holding times was highly variable. Subtle differences in surface preparation ap- pear to have a profound effect on this phenomenon.

C. Further Characterization of Surface Segregation

To resolve the depth of the surface segregation with greater precision, the associated NRA reaction, in which the energy distribution of alpha particles is measured, was utilized. This approach has a 0.05 k~m depth reso- lution, and the deuterium surface segregation in a-Ti, Ti-6AI, and Ti-3A1-2.5V polished rod samples was found to be within this resolution capability. For these mea- surements, the average deuterium content within a 0.1/xm depth was found to be at least twice the surface concen- trations given in Table II. The extent of surface segre- gation, i.e., ratio of surface to bulk concentration, therefore, is also at least twice the values indicated in Table II.

The high concentrations of deuterium that are mea- sured in the surface suggest that deuterides may form in this region. Titanium deuteride is in equilibrium in bulk a-Ti above concentrations of approximately 40 wppm at room temperature, and the preferential formation of deu- terides in the surface region has been proposed TM because of reduced constraint for the expansion accompanying formation. For bulk samples, the presence of deuterides were detected in a-Ti containing concentrations of 45, 85, and 185 wppm deuterium as well as in the Ti-3A1- 2.5V alloy containing 245 wppm using a recently de- veloped method of differential scanning calorimetry

06 - - VOLUME 21 A, JULY 1990 METALLURGICAL TRANSACTIONS A

( D S C ) . [17] This method utilizes a disc sample of approx- imately 8-mm diameter • 1-mm thickness, and the pres- ence and morphology of hydrides and/or deuterides within the bulk are identified by their dissolution behavior. The identification of deuterides in these samples is consistent with their bulk concentrations. Deuterides were not de- tected by DSC in any of the Ti-6A1 and Ti-6A1-4V sam- pies or in the Ti-3A1-2.5V samples containing 55 and 155 wppm deuterium.

Unfortunately, the sensitivity of the DSC technique is inadequate to identify whether deuterides exist in the lo- calized near-surface region for the concentrations of the samples in Table II. Thus, we have no direct evidence for the formation of deuterides accompanying surface segregation. However, the fact that surface segregation occurs in Ti-6AI, a material of low propensity for deu- teride formation and one in which no deuterides were detected for bulk concentrations as high as 150 wppm, may indicate that formation of deuterides is not essential to surface segregation. Surface defects or interstitial ox- ygen could also be acting to reduce the chemical poten- tial at the surface and act as trapping sites for deuterium. For other metal systems, defects and interstitial impu- rities such as oxygen have been found to be associated with surface segregation, t4-71 In view of these results, we plan to utilize NRA techniques to investigate the role of oxygen in the surface segregation of titanium alloys.

IV. CONCLUSIONS

Deuterium segregation occurs within the near-surface of polished t~-phase titanium, but not in/3-phase mate- rial, and it has been localized to within 50 nm of the surface. A model based on diffusion of deuterium from the bulk adequately describes the observed increase in the extent of segregation with time. The high concen- trations associated with segregation suggest that deuter- ides may form in the near-surface region, but the introduction of surface defects and/or interstitial oxygen during polishing may create a surface state that acts to trap large quantities of deuterium without deuteride formation.

ACKNOWLEDGMENTS

This work was partially supported by ONR Con- tract No. N00014-80-C-0742, and the interest and encouragement of Dr. Bruce MacDonald are appreci- ated. We also wish to acknowledge the participation by W. Poit, Jr. in sample preparation, T. Kantorcik in op- eration of the Van de Graaff accelerator and assistance with data collection, and W. Rooney, Jr. in X-ray dif- fraction analysis. We are also grateful to Dr. John M. Papazian and James R. Kennedy for technical dis- cussions and their contribution in evaluating the DSC behavior.

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