Analysis of NiAlTa precipitates in β-Nial + 2 at.% Ta alloy

Scripta

METALLURGICA

V o l . 21, pp. Printed in

283-288, 1987 the U.S.A.

ANALYSIS OF NiAITa PRECIPITATES B-NiAI + 2 at.?o Ta ALLOY V. Pathare, G. M. Michal Department of Metallurgy & Case Western Reserve Cleveland, Ohio

Pergamon Journals, Ltd. All rights reserved

IN

and K. Vedula Materials Science University 44106

and M. V. Nathal NASA Lewis Research Center, Cleveland, (Received (Revised

September December

Ohio 44135

18, 1986) Ii, 1986)

Introduction A material of great interest for structural use at elevated temperature is the intermetallic compound B-NiAI(1). One of B-NiAl's major assets is its lower density (5.9 g/cc) compared to Ni or Co based superalloys (7 to 9 g/cc). However, its creep strength needs to be increased in order to compete with existing alloys on a strength to weight basis. Previous work has shown that the creep strength of NiAI can be substantially increased through alloying additions of i to 2 at.?' of Ta, Nh or Hf (2). Transmission electron microscopy (TEM) of a B-NiAI alloy containing 2 at.?' Ta, has been shown to contain a dispersion of submicron size second phase precipitates in its matrix (3). These precipitates are believed to play a major role in improving the high temperature creep properties of B-NiAI. A similar finding has been reported by Sherman and Vedula (4) in a B-NiAI alloy containing 2 at.% Nb. Only limited information exists about the ternary Ni-AI-Ta phase diagram (5). The 1273°K isotherm predicts that greater than 2 at.?. of Ta can exist in solid solution with B-NiAI. However, when pure Ta is added to B-NiAI via powder processing to produce a nominally 2 at.% Ta alloy, enrichment of the microstructure locally to much higher Ta levels will occur as the pure Ta diffuses into the NiAI matrix during homogenization annealing. At higher Ta levels at least three possible ternary compounds between Ni, AI and Ta are reported in the literature. NiAITa has a hexagonal MgZn 2 (C14) type structure (space group P63/mmc) with a unit cell containing 4 molecular units (6). Ni2AITa has the cubic Heusler alloy structure (Cu2AIMn type) with a o = 0.5949nm (7). (AI0.5Tao.5)Ni3 has the hexagonal Ni3Ti type (DO24) structure with a o = 0.5112nm and c o = 0.8340nm. (8). This paper describes the results of a systematic study to identify the precipitates and their orientation relationship with respect to their matrix in a ~-NiAI alloy containing a nominal adition of 2 at.?' Ta, that was subjected to creep testing at 1300°K. In addition, the size, morphology and habit plane were determined using TEM. Such basic information concerning the precipitates and matrix can lead to a better insight into dislocation-precipitate interaction. Experimental

Procedure

The alloy examined was chemically analyzed and contained 50.4 ± 2.5 at.?' Ni, 47.5 2.4 at.% AI and 2.1 • 0.I at.?' Ta. The specimens were made by hot extrusion of metal powders. Steel cans filled with blended B-NiAI and Ta powders were extruded at 1350°K with an area reduction ratio of 16:1. A homogenization treatment at 1623°K for i00 hours in argon followed the extrusion. The alloy was tested in compression at 1300°K in air at a nominal strain rate of 6x10-6s -I. Test durations were typically 50 hours. Specimens for TEM analysis were machined from the compression tested samples using an electrodischarge technique. Final thinning was carried out in a twin-jet electropolishing unit using an electrolyte containing 2 parts methanol per 1 part nitric acid by volume at 253°K and a potential of 15 V. TEM bright and dark field imaging combined with X-ray energy dispersive spectroscopy (EDS) and selected area electron diffraction (SAD) were used to identify the structure, size, shape, habit plane and orientation

0036-9748/87 Copyright (c) 1 9 8 7

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relationship with respect to the matrix of the precipitate phase. Analysis of SAD patterns were aided by use of a computer software package that generated composite SAD patterns for various orientations of the precipitate and matrix phases. Identification of the precipitate phase was obtained by X-ray diffractometry run on a pulverized specimen taken from a section of the alloy that was compression tested. Monochromated CuaK radiation was used. Results The l a t t i c e s p a c i n g s and p e a k i n t e n s i t i e s for the precipitate p h a s e , o b t a i n e d f r o m an X - r a y d i f f r a c t o m e t e r s c a n on a p u l v e r i z e d s e c t i o n o f a c o m p r e s s i o n t e s t e d s p e c i m e n o f t h e a l l o y , a r e l i s t e d i n Table I . The o b s e r v e d d - s p a c i n g s m a t c h e d w e l l t h o s e f o r t h e compound NiA1Ta a s r e p o r t e d by Kusma and Nowotny ( 6 ) and M a r k i v e t a l . ( 9 ) . The d - s p a c i n g s o b t a i n e d f r o m t h e d i f f r a c t o m e t e r s c a n were u s e d i n a c o s e c o t e extrapolation t o compute t h e l a t t i c e p a r a m e t e r o f t h e NiA1Ta p r e c i p i t a t e p h a s e . The c a l c u l a t e d l a t t i c e p a r a m e t e r s were a o = 0.4903±0.0013nm and c o s 0.7929±0.0029nm. Peak i n t e n s i t i e s f o r t h e NiA1Ta p h a s e w e r e a l s o c a l c u l a t e d t a k i n g into account the structure factor, Lorentz-polarization f a c t o r and m u l t i p l i c i t y factor. The s t r u c t u r e f a c t o r and t h e m u l t i p l i c i t i e s of reflections w e r e c o m p u t e d a s s u m i n g an i d e a l HgZn 2 t y p e (C14) s t r u c t u r e ( 6 ) . The d - s p a c i n g s o b t a i n e d f r o m t h e c a l c u l a t e d l a t t i c e p a r a m e t e r s and t h e c a l c u l a t e d peak i n t e n s i t i e s a r e compared w i t h the e x p e r i m e n t a l l y d e t e r m i n e d v a l u e s in Table I. The good a g r e e m e n t b e t w e e n t h e o b s e r v e d and c o r r e s p o n d i n g c a l c u l a t e d v a l u e s c o n f i r m s t h a t the precipitate p h a s e i n t h e a l l o y was NiA1Ta. F i g u r e 1 c o n t a i n s a TEM b r i g h t f i e l d image o f t h e p r e c i p i t a t e p h a s e v i e w e d a l o n g a [100] matrix direction. E v i d e n c e t h a t c o m p l e t e h o m o g e n i z a t i o n o f t h e b l e n d e d B-NiA1 and Ta p o w d e r d i d n o t o c c u r was o b t a i n e d f r o m t h e o b s e r v a t i o n o f a v e r y n o n - u n i f o r m d i s t r i b u t i o n of precipitates. The p r e c i p i t a t e s e x h i b i t e d an i r r e g u l a r p l a t e m o r p h o l o g y . The p l a t e e d g e s v a r i e d from s h a r p and f a c e t t e d t o b l u n t and s m o o t h l y c u r v e d . F i g u r e 1 shows t h a t t h e h a b i t p l a n e o f t h e precipitates was c l o s e t o {012 } o f t h e m a t r i x . C h e m i c a l m i c r o a n a l y s i s o f t h e a l l o y u s i n g EDS d e t e r m i n e d t h a t t h e m a t r i x was B-NiA1 w i t h a s m a l l amount o f Ta i n s o l i d s o l u t i o n . The p r e c i p i t a t e s w e r e f o u n d t o be c o n s i d e r a b l y r i c h e r i n Ta. T y p i c a l EDS s p e c t r a f r o m t h e m a t r i x and a precipitate a r e shown i n F i g u r e 2. A single matrix Eegion containing a large precipitate was t i l t e d i n t h e TEM t o p o s i t i o n i t s [ 1 1 1 ] , [011] and [100] z o n e a x e s p a r a l l e l t o t h e e l e c t r o n beam. Superimposed single crystal SAD p a t t e r n s o f t h e m a t r i x and t h e p r e c i p i t a t e s were o b t a i n e d f o r each o f the t h r e e o r i e n t a t i o n s . They a r e shown i n F i g u r e 3. The i n d e x i n g o f t h e p r e c i p i t a t e SAD p a t t e r n s m a t c h e d w e l l w i t h t h e d - s p a c i n g s and i n t e r p l a n a r a n g l e s p r e d i c t e d f o r t h e NiA1Ta p h a s e . A consistent precipitate matrix orientation relationship amon ~ t h e t h r e e SAD p a t t e r n s was o b s e r v e d . The o r i e n t a t i o n relationship can be e x p r e s s e d a s [ l l l ] H / / { 1 2 1 6 ] p p t and ( O l l ) H / / ( 1 0 1 0 ) p p t Thus, the p r i s m p l a n e s o f t h e h e x a g o n a l NiA1Ta p h a s e w e r e f o u n d t o ' b e p a r a l l e l to the c l o s e s t packed p l a n e s i n t h e E2 s t r u c t u r e o f t h e m a t r i x . Discussion Several different values for the lattice p a r a m e t e r s o f NiA1Ta h a v e b e e n r e p o r t e d . Kusma and Nowotny ( 6 ) d e t e r m i n e d a o m 0 . 4 9 6 9 n m and c o - 0.7985nm f o r p u r e NiA1Ta. Markiv e t a l . ( 9 ) h a v e r e p o r t e d a o - 0 . 4 9 3 2 n m and ¢ o = 0.7980um. R e c e n t l y , Nash and West ( 5 ) h a v e r e p o r t e d a o - 0.501Sum and c o - 0 . 8 1 7 1 n m f o r NiA1Ta p r e c i p i t a t e d i n an a l l o y c o n t a i n i n g 5 9 . 3 a t . ~ N i , 3 5 . 8 a t . % A1 and 4 . 9 a t . ~ Ta. The l a t t i c e p a r a m e t e r s o f a o - 0 . 4 9 0 3 n m and c o - 0.7929nm determined In this study most closely match those of Markiv et al. (9). The s c a t t e r i n t h e reported lattice p a r a m e t e r s c o u l d be f r o m s e v e r a l s o u r c e s . The NiA1Ta p h a s e may h a v e a r a n g e o f a t o i c h t o m e t r y , and t h u s l a t t i c e parameters. Also the difficulty of analyzing high angle p e a k s i n t h e NiA1Ta p h a s e c a u s e s r e l a t i v e l y large uncertainties in the determination of its lattice parameters. A well-defined orientation relationship b e t w e e n t h e NiA1Ta p r e c i p i t a t e s and t h e B-NiA1 m a t r i x was o b s e r v e d . The c r y s t a l s t r u c t u r e s o f t h e two p h a s e s a r e q u i t e d i f f e r e n t . It is ~ery i n t e r e s t i n g t o s e e how w e l l t h e a t o m i c d e n s i t i e s and l o c a t i o n s o f a t o m s m a t c h on t h e p r e c i p i t a r e and m a t r i x p l a n e s w h i c h a r e p a r a l l e l to each other. F i g u r e 4 ( a ) shows the s t a c k i n g of 1010 } p l a n e s o f t h e NiA1Ta p h a s e s t o be ABCDEABCDE. The d e n s i t y o f a t o m s on t h e s e p l a n e s i s q u i t e low. The A, B, and E p l a n e s h a v e 1 9 . 4 3 8 x 10 -20 m2 p e r a t o m and t h e C and D p l a n e s h a v e L2.959 x 10 - 2 0 m2 p e r a t o m . The {011} p l a n e s i n B-NiA1 a r e s t a c k e d ABAB. T h e s e p l a n e s a r e o o r e d e n s e l y p a c k e d w i t h 5 . 8 9 4 x 10 - 2 0 m2 p e r a t o m . By c o m b i n i n g t h e E÷A÷B o r ~ D 2 P l a n e s o f ~he NiA1Ta p h a s e , e a c h c o m b i n a t i o n w i l l p r o d u c e a d e n s e r p a c k i n g o f 6 . 4 7 9 x 10- m p e r

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atom which is comparable to the {011} planes in NiAI. The atomic positions on the grouped planes in the NiAITa phase still do not match very well those on the (011} planes of 8-NiAI. This point is illustrated in Figure 4(b) which shows an overlay of a (011) plane of NiAI on the combined E+A+B (i010) planes of NiAITa. Not the {011} plane of the B-NiAI, but the lower packing density {012} planes were observed to be the habit planes for the NiAITa precipitates. The {012} planes of the 8-NiAI phase have 18.638 x 10 -20 m 2 per atom which is comparable to the low atomic densities of planes in NiAITa. Conclusions The second phase precipitates in the £-NiAI+2 at.~ Ta alloy were identified by X-ray diffraction to be the intermetallic compound NiAITa with a hexagonal C14 structure. The precipitates had a plate-like shape and a habit plane close to {012}. Selected area electron

d i f f r a c t i o n p a t t e r n s e x h i b i t e d an o r i e n t a t i o n r e l a t i o n s h i p hexagonal NiAITa p r e c i p i t a t e s were p a r a l l e l t o t h e c l o s e s t matrix.

i n which t h e p r i s m p l a n e s o f t h e packed p l a n e s i n t h e c u b i c B-NiA1

Acknowledgement The s u p p o r t of t h i s work by NASA Lewis R e s e a r c h C e n t e r , C l e v e l a n d , Ohiop appreciated.

is sincerely

References 1. 2. 3. 4. 5. 6. 7. 8. 9.

J . R . S t e p h e n s , " H i g h - T e m p e r a t u r e Ordered I n t e r m e t a l l i c A l l o y s " , 39~ p. 381, ed. C.C.Koch, C.T.Liu and N . S . S t o l o f f , MRS. P i t t s b u r g h (1985). V . H . P a t h a r e , K.M.Vedula and R . H . T i t r a n , "Modern Developments i n Powder M e t a l l u r g y " , 16~ p. 695, ed. E.N.Aqua and C . l . ~ r h i t m a n , MPIF, P r i n c e t o n (1985). K.M.Vedula, V . M . P a t h a r e , l . A s l a n i d i s and R . H . T i t r a n , " H i g h - T e m p e r a t u r e Ordered I n t e r m e t a l l i c A l l o y s " , 39~ p. 411, e d . C.C.Koch, C.T.Llu and N . S . S t o l o f f , MRS, P i t t s b u r g h (1985). M.Shermau and K.M.Vedula, J . o f M a r l s . S c . , 21~ 1974 (1986). P.Nash and D.R.F.West, M e t a l S c i e n c e , 13~ 670 (1979). J.B.Kusma and H.Nowotny, Montash, Chem., 95~ 428 (1964). B . C . G i e s s e n and N . J . G r a n t , A c t a . C r y s t . , 18~ 1080 (1965). W.Heine and U.Zwicker, N a u t r e w i s s , 49~ 391 (1962). V . J a . Markiv, J u . V . V o r o s i l o v , P . I . K r i p j a k e v i c and E . E . C e r k a s i n , K r i s t a l l o g r a f i j a , SSR, 9_~ 737 (1964). TABLE I: NiA1Ta Peaks Observed i n X-Ray D i f f r a c t i o n

Reflection

d nm (obsrved)

d nm (calculated)

I(relative) (observed)

I(relative) (calculated)

I0.0

0.4245

0.4246

16

28

00.2 10.1 11.0 10.3 20.0 10.4

0.3967 0.3744 0.2454 0.2244 0.2119 0.1796

0.3965 0.3743 0.2452 0.2244 0.2123 0.1796

9 7 63 100 23 8

17 21 78 100 16 2

21.0 10.5 21.3 30.2 20.5 22.0

0.1604 0.1487 0.1372 0.1334 0.1273 0.1226

0.1605 0.1486 0.1372 0.1333 0.1271 0.1226

6 33 52 43 24 33

4 17 43 22 26 20

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p

~:~i~

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FIG. 1. A bright field TEM micrograph of a compression tested specimen of 8-NiAI + at.% Ta alloy, viewed along a [i00] direction in the matrix.

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FIG. 2. EDS spectra from (a) the matrix and (b) the precipitates.

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planes i n NiA1Ta and ( b ) a n overlay of (011) planes of B - N i A I a n d c o m b i n e d E+A+B ( i 0 1 0 ) planes of NiA1Ta.