Formation of L12-structured Ni3Si

Formation of L12-Structured Ni3Si

IAN BAKER, JUN YUAN, and ERLAND M . SCHULSON

Nickel-23 at. pct Si produced by hot extrusion o f canned powders is two phase above 1035 °C,consisting o f particles o f the nickel (silicon) solid solution in a complex silicide matrix, labeled/32 on the phase diagram. Transformation to the L12-structured/3~-Ni3Si was examined at tem-peratures from 525 °C to 925 °C by both optical microscopy and transmission electron micros-copy. Transformation to/3~-Ni3Si was observed to o c c u r by three different processes, namely,by precipitation within the Ni(Si) particles, by the peritectoid transformation Ni(Si) + /32131, and by the eutectoid decomposition of/32, that is, through/32 ~/31 + yNi3~Si~2. All o f thetransformations are consistent with the phase diagram. The /31 which formed both by precipi-tation and by the peritectoid transformation is coherent with the Ni(Si) solid solution. The peri-tectoid transformation appears to be interface controlled (at the coherent /3~//32 interface), asevidenced by the anisotropic fingerlike growth o f the/31 into the/32 matrix.

I. INTRODUCTION

THE L12-structured [ordered face-centered cubic (fcc)]compound Ni3Si has been the subject o f an increasingamount o f research in recent years because o f its high-temperature strength, good low-temperature ductility whenalloyed with titanium and boron, excellent oxidationresistance, and resistance to attack by hot sulfuricacid.t M2~

According to the phase diagram, tt31 Ni3Si (labeled 131in Figure 1) can form in three ways:(1) /3 l-Ni3Si can be precipitated from the primary nickelsolid solution, Ni(Si). The nucleation o f Ni3Si precipi-tates has been studied previously, as has the subsequentgrowth o f the precipitates which was found to follow(time)~/3 kinetics.t ~4,t51(2) Nickel containing 23.7Si* transforms to/3~-Ni3Si by

*Compositions are given in a tomic percent throughout.

the peritectoid reaction Ni(Si) + f12 --~ /31 at 1035 °C,where/32 is a monoclinic high-temperature silicide (alsoreferred to as Ni3Si) which is slightly richer in silicon(25.1 pct) than /31.~j31 It is worth noting that there havebeen few studies o f peritectoid transformations, such asthat o f the formation o f Zr3A1 from Z r and Zr2A1; [161 thistype o f reaction may become o f increasing importancewith the growing interest in intermetallic compounds asa whole.

(3) Silicon-rich /31-Ni3Si also forms through the eu-tectoid decomposition /32 ~ /31 + y , where y has theformula Ni31Si12 and a complex hexagonal crystalstructure.

This article presents a study o f the transformation o fa two-phase microstructure in Ni-23Si, which consistedo f Ni(Si) particles in a/32 matrix, to the equilibrium L12-

IAN BAKER, Associate Professor, JUN YUAN, Ph.D. Candidate,and ERLAND M . SCHULSON, Professor of Engineering, are withthe Thayer School of Engineering, Dartmouth College, Hanover, NH03755-8000.

Manuscript submitted January 2 l , 1992.

structured /31-Ni3Si.* We will show that the /31-Ni3Si

*This particular material was used in two previous publicationsconcerning the effect of boron on the mechanical properties of Ni3Si. t5"81

typically forms by all three processes outlined above.

II. E X P E R I M E N T A L

Powders o f Ni-23Si were canned in 304 stainless steeland extruded to rod through an area reduction ratio o f6:1 after soaking for 2 hours at 1000 °C. The micro-structure in the as-extruded state was too fine to studyoptically. Thus, small samples o f each rod were an-nealed for 24 hours (unless otherwise noted) at 1100 °Cin flowing argon gas and water-quenched to room tem-perature. This treatment produced a microstructure whichconsisted of Ni(Si) particles dispersed within a 132 matrix(see below). Samples so heat-treated (with sizes rangingfrom 0.5 x 0.5 x 2 mm for anneals less than 2 minutesto 2 x 10 x 10 mm for anneals o f more than 2 hours)were then annealed at either 525 °C, 725 °C, 775 °C,825 °C, 875 °C, o r 925 °C to effect the transformationto the/31-Ni3Si; samples annealed for less than 2 hourswere placed on a heated iron plate (50 x 50 x 10 mm)to aid rapid heating to temperature.

The samples were subsequently mounted and polishedin the standard manner and etched with Marble's re-agent. The fraction o f the /31 phase present a f te r eachanneal was determined by a point-counting method inwhich six different areas were examined on the viewingscreen o f the optical microscope. Fo r each area, a gridconsisting o f 19 × 15 points, based on 6-ram squaremesh , was placed on the screen, and each point locatedin the/31 phase was counted as 1, while a point locatedon a/31 phase boundary was counted as 0.5. A total o f1578 points was used (6 x 19 x 15 - 6 x 22; 22 pointson the grid were obscured by markings on the viewingscreen).

Since the optical microstructures observed at differentannealing temperatures were similar, it was decided that

METALLURGICAL TRANSACTIONS A VOLUME 24A, FEBRUARY 1993--283

1500[-- with a TRACOR NORTHERN* TN5500 energy-

140C

1455*TRACOR NORTHERN is a trademark of Tracor Northern, Inc.,

Middleton, WI.

dispersive X-ray microanalysis system (EDS).

r,.)o

E-<U.l

1300

1200

II00

1000

900

1115

(Ni)

93~2

1035

-t---4 990

III. R E S U L T S A N D DISCUSSION

A . Microstructures after 1100 °C Anneal

Figure 2 shows an optical micrograph o f the alloy afterannealing for 24 hours at 1100 °C and water-quenching.It is evident from the shapes o f some Ni(Si) particles thatsome particle coalescence had occurred during anneal-ing. Transmission electron microscope examination o fthese samples showed that the particles observed opti-cally were Ni(Si) and that they were surrounded by acomplex crystal-structured silicide, presumably /32, al-though the crystal structure was not identified. As evi-dent in Figure 3(a), there was no sign o f f l l - N i a S i ,produced by the peritectoid transformation, between thesetwo phases. However, 20 to 25 nm diameter precipitateswere present in the Ni(Si) particles (Figure 3(b)). Thesmall size o f the particles and problems o f particle over-lap made them difficult to analyze immediately afterquenching; that these are Ni3Si precipitates will be dem-onstrated below. Thus, it is evident that the water quench

800

700 L J0 10 20 30

A t % Si

Fig. 1 - Nickel-rich portion of the binary phase diagram for Ni-Si.The L12-structured NisSi phase is designated/3~.

transmission electron microscope (TEM) examination o fthe microstructure annealed at 725 °C would be repre-sentative o f the structures at all temperatures after dueaccount was taken o f the slightly differing transforma-tion kinetics. Thus, to examine the details o f the micro-structure, thin foils o f samples which had been annealedat 725 °C (after annealing at 1100 °C for 24 hours) wereprepared by electropolishing 250-/xm-thick, 3-mm-diameter discs in a Struers Tenupol. A medium flow rateo f 13 vol pct hydrochloric acid in methanol at - 4 0 °Cwas used at a voltage o f 40 V, which produced a currento f 200 mA. Af t e r perforation, the thin foils were rinsedin ethanol and methanol before washing in hydrofluoricacid for 30 to 40 seconds. All o f the phases presentelectropolished well but, as evident from the thicknessdifferences in some micrographs, at different rates. Thefoils were examined in a JEOL 2000FX TEM, furnished

Fig. 2--Optical micrograph of Ni-23Si after annealing for 24 h at1100 °C and a water quench.

284--VOLUME 24A, FEBRUARY 1993 METALLURGICAL TRANSACTIONS A

Fig. 3 - - T E M images of Ni-23Si annealed at 1100 °C for 24 h and water quenched: (a) Ni(Si)/silicide interface (note the absence of Ni3Si atthe interface); (b) Ni3Si precipitates within a Ni(Si) particle; and (c) paired dislocations at the Ni(Si)/silicide interface.

is rapid enough to stop both the peritectoid and the eu-tectoid transformation from occurring, but that it cannotprevent the precipitation o f fll-Ni3Si. It appears that thereis little barr ier to nucleation and growth for the precip-itation transformation.

Some dislocations were present within the Ni(Si)(Figure 3(c)). These were generally paired, presumablydue to the necessity o f traveling as a couple through theordered Ni3Si precipitates.

The microstructures arising from each o f the transfor-mation routes to fl~-Ni3Si (precipitation, peritectoid, andeutectoid), which occur on annealing the Ni(Si) + /32microstructure at different temperatures, are describedbelow.

B . The Precipitation Transformation

As noted above, Ni3Si precipitates had formed withinthe Ni(Si) particles even after quenching, but due to their

small sizes (20 to 25 nm) and to the overlap o f images ,they were not easily analyzed. More detailed analysis o fthe Ni(Si) particles was possible a f te r annealing for15 minutes at 725 °C. This showed that the rounded par-ticles were generally single crystals containing precipi-tates 50 to 100 nm in diameter (Figure 4(a)). Comparisonwith Figure 3(b) shows that considerable coarsening o fthe Ni3Si precipitates had occurred at 725 °C, the ki-netics o f which presumably followed the (time)1/3 ob-served by others, t14'151 Convergent beam electrondiffraction (CBED) patterns from precipitates on the foiledge, such as that shown in Figure 4(b), demonstratedthat these were ordered fcc (Figure 4(c)), while CBEDpatterns from the adjacent matrix showed that they weresimply fcc (Figure 4(d)). The diffraction patterns alsoindicate that the precipitates and the matrix have a lmos tidentical lattice parameters and are in the same orien-tation. There is no evidence o f strain contrast at the pre-cipitate interface. Energy dispersive X-ray microanalysis


Fig. 4--Ni-23Si annealed for 15 min at 725 °C: (a) TEM image showing --50 to 100 nm Ni3Si precipitates ins ide a Ni (S i ) particle; (b) T E Mimage of a Ni3Si precipitate in a Ni(Si) par t ic le on the edge of a thin foil; and corresponding CBED patterns from (c) the precipitate and (d) thenearby nickel matrix. Note that the Ni3Si precipitate and Ni(Si) matr ix have the same orientation and closely similar lat t ice parameters.

showed that the precipitates had a much h ighe r Si con-tent than the matrix. Thus, the precipitates are Ni3Si co-herently bonded to the Ni(Si) solid solution.

The Ni3Si precipitated due to the decreased solubilityo f silicon in nickel at lower temperatures. Precipitationoccurred homogeneously owing to the low interfacial en-ergy between the Ni(Si) and the Ni3Si, as indicated bythe coherent interface between the two and presumablybecause o f the large driving force for the transformation.A low misfit between the two phases is also the reasonthat the Ni3Si precipitated as spheresJ~Tj

C. The Peritectoid Transformation

Af te r short annealing times (<1 minute) at any tem-perature, optical metallography showed that a rimlike /31-Ni3Si phase had appeared around the Ni(Si) particles.The rim arose from the peritectoid transformation

Ni(Si) + f12 --~ ill- At short annealing t imes , the Ni3Siwas usually observed to have grown nonuniformly aroundthe Ni(Si) particles; the Ni(Si)/Ni3Si interface was smooth,but the NiaSi had grown into the surrounding/32 phasein a clearly directional fingerlike fashion (Figure 5). Thatthe orientation o f these fingers depends on the relativeorientation o f the/32 grains is also evident in Figure 5,where fingers from three different particles (arrowed) aregrowing in the same direction into a/32 grain. That theyall grow in a single direction, rather than a number o fequivalent directions, presumably reflects the low sym-metry o f the/32 phase. At longer times at the higher an-nealing temperatures, when the rimlike /3~-Ni3Si phasehad grown thicker, less evidence o f the fingerlike growthwas evident, although a nonuniform thickness and a di-rectional character around the particles could still be seen(Figure 5).

Transmission electron microscope examination o f thin


Fig. 5--Optical micrographs of Ni-23Si after annealing: (a) 1 min at 725 °C, (b) 1 min at 825 °C, and (c) 30 rain at 925 °C. The letters in (a)indicate (A) the Ni(Si) particle, (B) a relatively uniform Ni3Si rim, (C) directional fingers of Ni3Si around a particle, (D) possible eutectoiddecomposition products, and (E) twinned f12 matrix.

foils o f samples annealed at 725 °C for 15 minutes re-vealed rounded particles o f the o rde r o f 10 /xm in di-ameter, surrounded by a slightly less-thinned, rimlike,second phase residing in an even thicker matrix.Figure 6 shows a typical example; some preferentialthinning has clearly occurred at the interface between theparticles and the surrounding phase. Energy dispersiveX-ray microanalysis o f the rounded particles indicatedthat they had an overall composition o f - 1 3 at. pct Si,which roughly corresponds to the solubility limit o f Siin the Ni solid solution at elevated temperature.

Figures 7(a) through (f) are TEM photos and diffrac-tion patterns o f Ni-23Si annealed at 725 °C for15 minutes. Figure 7(a) shows a Ni(Si) particle which

Fig. 6 - - T E M image of Ni(Si) particle surrounded by rimlike Ni3Siphase and the f12 matrix in Ni-23Si annealed at 725 °C for 15 min.Note the straightness of the Ni3Si/fl2 interface.

consists o f two grains. Figures 7(b) through (d) are se-lected area diffraction patterns from the upper grain, therimlike phase, and the matrix, respectively. The patternsfrom the Ni(Si) particle (Figure 7(b)) and the rimlike phase(Figure 7(c)) are a lmos t identical, indicating that therimlike phase has a very close lattice parameter and thesame orientation as the Ni(Si) particle. The EDS datafrom the rimlike phase indicated a composition o f- 2 3 at. pct Si. Thus, the rimlike phase has the chemistryand crystal structure corresponding to the L12-structuredNi3Si. That the rimlike Ni3Si, which forms peritec-toidally, was a single crystal and had the same orienta-tion as the Ni(Si) particle was also evident during tiltingexperiments when bend contours would move across theNi(Si) and Ni3Si in unison. Dark-field imaging with asuperlattice reflection was used to "light up" both thesurrounding rimlike phase and the Ni3Si particles in theNi(Si), Confirming that they had the same crystal struc-ture. However, because o f bending in the foils examined(see, for example, Figures 6 and 7(a)), it was not foundpossible to "light up" both the rimlike Ni3Si and the Ni3Siparticles simultaneously.

Figure 7(f) shows a higher magnification image o f atypical interface between the Ni(Si) particle and the rim-like Ni3Si. Note that dislocations are present in the sur-rounding Ni3Si rim, which extend from the interface toapproximately halfway across the Ni3Si phase. Occa-sionally, dislocations were found which extended com-pletely across the Ni3Si, but these were less common. Itis possible that these dislocations arose so that the elasticenergy from the transformation could be relaxedplastically. ~7~

Two features o f the interface between the rimlike Ni3Siand the f12 phase are worth noting. First, the interface isnot equidistant fom the Ni(Si) particle, as is clearly ev-ident in Figure 6. This feature makes an analysis o f thewidth o f the rimlike Ni3Si vs t ime, as performed by


Fig. 7--Ni-23Si annealed at 725 °C for 15 min: (a) T E M image of particle, consisting of two grains, surrounded by rimlike Ni3Si phase a n dthe 132matrix; corresponding SAD patterns from (b) upper Ni(Si) grain, (c) rimlike Ni3Si, and (d) 132 matrix. Note that the particle and surroundingNi3Si have similar lattice parameter and orientation. (e) T E M image of/32 matrix showing lathlike structure and dislocations; and ( f ) T E M i m a g eof Ni(Si)/Ni3Si interface. Note the dislocations on the Ni3Si side of the interface.


Fig. 8 - - ( a ) T E M image of 132 matr ix in Ni-23Si annealed for 30 min at 725 °C which has undergone the eutectoid decomposition to form Ni3Siand a complex silicide. SAD patterns from regions (b) and (c) indicated.

Schulson and Graham, 116~ difficult. Second, the interfaceis often very straight and, thus, appears to have a crys-tallographic character and a fingerlike character in places.Both o f these features, which are consistent with the op-tical microstructure, suggest that the peritectoid reactiono f the Ni(Si) with the /32 silicide to form /3~-Ni3Si isinterface-controlled and depends on the orientation o f theNi3Si/silicide interface.

D. The Eutectoid Transformation

While the crystal structure o f the matrix phase was notidentified [but it is clearly complex; Figure 7(d)], EDSshowed that it contained - 2 6 at. pct Si, consistent withthe /32 phase. Imaging o f the /32 silicide under strongdiffraction conditions showed that it had a lathlike"twinned" structure and contained some dislocations(Figure 7(e)). Interestingly, optical micrographs showedetching lines in this phase, in the same orientation as the

fll-Ni3Si fingers; these lines may have corresponded tothe/32 twins.

That the eutectoid transformation f12 ~/3~ + yNi3~Sil2had occurred af te r annealing at temperatures o f 825 °Cor be low was clearly evident in optical micrographs. Theeutectoidally formed fli-Ni3Si phase shows the same (op-tical) contrast and the same orientation relationships withthe/32 matrix as the rimlike fl~-Ni3Si around the Ni(Si)particles (Figures 5(a) and (b)). That the eutectoidallyformed y plates are all aligned in one direction in a grainagain reflects the low symmetry o f the f12 phase fromwhich they form. It is possible that some o f the eutec-toidally formed /3wNi3Si phase may have nucleated onthe peritectoidally formed/3~-Ni3Si, since the latter ap-pears to form almost immediately upon annealing whilethe former requires an incubation period before nucle-ation occurs.

The TEM specimens from samples which had beenannealed at 725 °C for 30 minutes showed areas o f the


e~@

.mm

e~

t...

1.0

0.8"

0.6"

0.4

0.2

0.0

O '

!

01 1 02 1 03 1 04 1 0 sT i m e ( S e c )

!

1 06 1 0 7p I ! I

l O = l o s l o 4 l o s

T i m e ( S e c )

@e a t

e ~

t...

L O "

0.8"

0.6

0 . 4

0.2"

0.01 0

@.mm

t...k,,

187s'c

!

1 0 $ 1 0 7

T i m e ( S e c )

1.0

0.8"

0.6"

0.4"

0.2

0.01 0 1

!

1 0 2 1 0 3 1 0 4 1 0 s 1 0 s 1

[ 8 2 5 " C

l O 2 l O 3 l O 4 l o s

T i m e ( S e c )

@Qml

¢ J

1.0

0.8"

0.6"

0.4-

0.2"

0.01 0 1

i 9 2 5 " C

I1 0 s 1 0 7

@emm

¢ J

t .

.0 ii7 2 5 " C

0.8"

0.6"

0.4- ~

0.2"

0.0 , , ,1 0 1 1 0 2 1 0 s 1 0 4 1'05 1 0 s 1 0 7

T i m e ( S e c )

1.0

0.8"

0.6"

0.4

0.2

0.001

!

1 0 2

I szs.c

I

1 0 3 1 0 4 1 0 5 1 0 6 1 0 7

T i m e ( S e c )

Fig. 9 - - G r a p h s of vo lume fraction of/3~-Ni3Si vs annealing time at different temperatures for Ni-23Si, The specimens were pre-heat-treated byannealing at 1100 °C for 24 h and then water quenched. The curves for temperatures up to and including 825 °C are a combination of eutectoidal lyformed and peritectoidally formed Ni3Si: the poin t at which the eutectoid transformation was first identified is an-owed on each curve. At 875 °Cand 925 °C, only the peritectoid transformation occurred. The data do not include the Ni3Si formed by precipitation, since this could not b eobserved optically.

2 9 0 - - V O L U M E 2 4 A , F E B R U A R Y 1993 M E T A L L U R G I C A L T R A N S A C T I O N S A

82 matrix which had undergone the eutectoid decom-position and contained alternate lamellae o f 13t-Ni3Si andthe more complex 3' silicide (Figure 8(a)). Figures 8(b)and (c) show selected area diffraction (SAD) patterns fromthe Ni3Si and 3, silicide lamellae indicated in Figure 8(a).The crystal structure o f this complex silicide was not de-termined, but EDS showed that it contained around28 at. pet Si, as expected fo r the 3' phase. The Ni3Silamellae produced by the eutectoid transformation oftencontained dislocation networks and low-angle boundaries.

After the eutectoid reaction had occurred, the matrixwas still not the equilibrium fl~-Ni3Si phase. For the lat-ter to form, Ni atoms must diffuse from the Ni(Si) par-ticles into the 3/ lamellae. Since these two phases arewell separated, this is not an easy process. Thus, thefinal stages o f the formation o f fl~-Ni3Si are sluggish.

E. Transformation Kinetics

Figure 9 shows the volume fraction of/31-Ni3Si as afunction o f time at different annealing temperatures. Un-fortunately, it was not always possible to distinguish the/3]-Ni3Si formed by the peritectoid transformation fromthat formed from the eutectoid transformation; what lookedoptically like eutectoidally produced/3~-Ni3Si could, insome cases, have been peritectoidally produced with theNi(Si) particle located be low the specimen surface. Inaddition, it must be emphasized that, since the volumefraction of/31-Ni3Si was determined from optical micro-graphs, the/31-Ni3Si formed by precipitation was not in-cluded in the kinetics: the amount o f /3~-Ni3Si producedby precipitation probably does not vary much with an-nealing time (annealing simply leads to coarsening o f themicrostructure), but it does vary a little with tempera-ture, due to the curvature o f the phase boundary on thephase diagram. Thus, the graphs in Figure 9 depict theoverall transformation kinetics to /3~-Ni3Si by both theeutectoid and the peritectoid transformations but neglectthe fixed contribution from the precipitation process. Theclearly nonsigmoidal nature o f the curves at tempera-tures up to 825 0(2 is indicative that more than one pro-cess is occurring, and at some temperatures, such as825 °C, the overall transformation curves appear to be acombination o f two sigmoidal curves. The time at whichthe eutectoid transformation was f i r s t clearly observedoptically is indicated on the curves. Note that, above825 °C, only the peritectoid transformation was occur-r ing , even though the phase diagram suggests that thereaction should occur up to 990 °C, and so the curvesreflect only this process.

An Arrhenius analysis was not performed because anapparent activation energy for the overall transformationso obtained would be difficult to ascribe to any particularprocess.

The data from the graphs o f Figure 9 are replotted asa time-temperature-transformation (TTT) curve inFigure 10. It is evident that the overall transformationrate is at a maximum around 850 °C, a feature charac-teristic o f diffusion-controlled nucleation and growthprocesses. The kinetics depicted in Figure 10 are, o fcourse, limited to materials whose microstructure is o f

1000 ~ I ra 10%a A I o

r~ 900- %

~ 800"

2~ 71111-

[,., 61111-

51111 . . . . .101 102 10 3 104 lO s 106 107

Time (See)Fig. 1 0 - - T T T curves (from data in Fig. 9) for Ni-23Si.

the same scale as that studied here. More finely struc-tured alloys will transform more rapidly.

It is worth noting that a parallel study was performedon Ni-23Si containing 0.19B. The results were essen-tially the same as those reported here. The peritectoidtransformation was found to be slower, but this may sim-ply have reflected the coarser starting microstructure thatwas present after the 24-hour anneal at 1100 °C.

Finally, it is useful to compare the results obtainedhere with those in the only previous detailed study o f aperitectoid transformation, that by Schulson andGrahamll61 on Zr3A1. In that case, Zr3AI forms throughthe reaction Zr2A1 + Zr(A1) ~ Zr3A1. The Zr3A1formeduniformly around Z r particles, allowing the thickness o fthe Zr3A1 to be measured as a function o f time; it wasfound to fol low a (time)1/2 dependence. Using this fact,it was shown that the isothermal transformation kineticscould be described quantitatively in terms o f nucleationand parabolic growth. Clearly, in the present case, thehighly anisotropic crystallographic nature o f the growtho f the NiaSi rim precludes a simple measurement o f rimthickness vs t ime. Thus, a more detailed kinetic analysis,as that performed by Schulson and Graham, tl6j is not ap-propriate. Interestingly, because o f the rapidly decreas-ing solubility o f A1 in the primary Z r phasetjSJ withdecreasing temperature, Zr3A1 formation must have oc-curred not only by a peritectoid reaction but by precip-itation from the primary solid solution, as occurred inNi3Si. Thus, the volume fraction o f Zr3AI reported bySchulson and Grahamt~61 also did not include that formedby precipitation, as was the case in Figures 9 and 10shown here.

IV. CONCLUSIONS

Optical microscopy and transmission electron micros-copy have been used both to examine the microstructureo f Ni-23Si annealed at 1100 °C and to fol low the trans-formation to L12-sla-uctured/31-Ni3Si at temperatures from525 °C to 925 °C. It is concluded that:

1. /31-Ni3Si forms by three different processes, namely,by precipitation from the Ni(Si) solid solution, by the


peritectoid transformation Ni(Si) + [32 --~ [3~, and bythe eutectoid decomposition of [32, that is , through[32 - > [3t + yNi31Si12• All of these transformationsare consistent with the p h a s e diagram, although ofcourse, their microstructures could not have beenpredicted.

2. T h e peritectoid transformation appears to b e interfacecontrolled (at the coherent [3J/ [32 interface), as evi-denced by the highly anisotropic, crystallographic na-ture of the growth of the [3~ into the [3~ phase.

3. [3,, formed by precipitation and by the peritectoidtransformation, is coherent with the Ni(Si) solidsolution.

A C K N O W L E D G M E N T S

This work was supported by the United StatesDepartment o f Energy, Office o f Basic Energy Sciences,through Grant Nos. DE-FG02-86ER45260 and DE-FG02-87ER4531 I.

REFERENCES

1 . T . Takasugi, E .P . George, D.P. Pope, and O. lzumi: ScriptaMetall., 1985, vol. 19 , pp . 551-56.

2 . A.I. Taub, C.L. Briant, S.C. Huang, K.-M. Chang, and M.R.Jackson: Scripta Metall., 1986, vol. 20 , pp . 129-34.

3 . W.C. Oliver and C.L. White: High Temperature Orderedlntermetallic Alloys H, Proc. MRS, MRS, Pittsburgh, PA, 1987,vol. 81 , pp . 241-46.

4 . A.I. Taub and C.L. Briant: Metall. Trans. A , 1989, vol. 20A,pp . 2025-32.

5 . I. Baker, R.A. Padgett, and E.M. Schulson: Scripta Metall., 1989,vol. 23 , pp . 1969-74.

6 . W.C. Oliver: High Temperature Ordered lntermetallic Alloys l l l ,Proc. MRS, MRS, Pittsburgh, PA, 1989, vol. 133 , pp . 397-402.

7 . B. Tounsi, P . Beauchamp, Y. Mishima, T . Suzuki, andP . Veyssi~re: High Temperature Ordered lntermetallic Alloys 111,Proc. MRS, MRS, Pittsburgh, PA, 1989, vol. 133, pp . 731-36.

8 . E.M. Schulson, L.J. Briggs, and I. Baker: Acta Metall., 1990,vol. 38 , pp . 207-13.

9 . T . Takasugi, D. Shindo, O. Izumi, and M . Hirabayashi: ActaMetall., 1990, vol. 25 , pp . 739-45.

10 . T . Takasugi, M . Nagashima, and O. lzumi: Acta Metall., 1990,vol. 25 , pp . 747-55.

11. T . Takasugi and M . Yoshida: J. Mater. Sci., 1991, vol. 26 ,pp . 3032-40.

12. T . Takasugi and M . Yoshida: J. Mater. Sci., 1991, vol. 26 ,pp . 3517-25.

13. Binary AUoy Phase Diagrams, T.B. Massalski, ed., ASM, MetalsPark , OH, 1986, p . 1754.

14. P.K. Rastogi and A.J. Ardell: Acta Metall., 1971, vol. 19 ,pp . 321-30.

15. M.A. Dvorack, J.E. Epperson, and H. Chen: Acta Metall., 1986,vol. 34 , pp . 117-24.

16. E.M. Schulson and D.B. Graham: Acta Metall., 1976, vol. 24 ,pp . 615-25.

17. Physical Metallurgy, R.W. Cahn and P . Haasen, e d s . , 3rd ed.,Elsevier Science Publishers, New York, NY, 1983, pp . 934-1030.

18. Binary Alloy Phase Diagrams, T.B. Massalski, ed., ASM, MetalsPark , OH, 1986, p . 187.


Documents

Formation of L12-structured Ni3Si