Ordered ϖ-derivatives in a Ti37.5Al20Nb at% alloy

Scripta METALLURGICA et M A T E R I A L I A

V o l . 24, pp. 1 5 4 1 - 1 5 4 6 , Printed in t h e U . S . A .

ORDERED w-DERIVATIVES

L.A. Bendersky,

1990

IN A Ti-37.5AI-20Nb

Pergamon

Press

plc

at% ALLOY

B.P. Burton, W.J. Boettlnger, F.S. Biancanlello Metallurgy Division NIST, Galchersburg MD 20899 (Received H a y 14, 1 9 9 0 ) (Revised J u n e 4, 1 9 9 0 ) Introduction

In recent studies of phase equilibria in the TI-AI-Nb system, chemical ordering of an phase was reported for some alloys [i-3]. The Ti~AIbN-b alloy, cooled from a g2 phase field above 1100°C, has the following transformation path - B2 ~ w" ~ 58 z - which involves strongly coupled chemical and displaclve order-dlsorder transitions. The trlgonal (PJml) w" phase exhibits partial collapse of 111 planes of the 52 phase and reordering relative to its 52 parent [2]. Evidently the stable transformation path would be 52 - 58 z , but the observed path includes the metastabie intermediate phase (w"). Furthermore, ~" (P3ml) is of lower symmetry than either 52 (PmJm) or 582 (P63/~mmc), and it is of lower conflguratlonal entropy than 582. The 582 structure (InNi 2 prototype, [4]) was formed after prolonged annealing at 700°C. Both w" and 58 z structures were verified by means of transmission electron microscopy (TEH) and single crystal X-ray diffraction, and their structures are showln in the schematic drawing of Figure I. In the present work we report a new Ti-AI-Nb phase which is apparently a more ordered derivative of the 582 phase. The phase was found as fine precipitates in a 58 z matrix. possible structure of the new phase will be discussed based on the results of electron diffraction and high-resolution microscopy. ExDerlmental An alloy with the TI-37.5A1-20Nb (at%) composition was prepared by arc melting according to the procedure described in [2]. Three sets of specimens (HTI, HT2 and HT3) were studied in this work. HTI was annealed at 1400"C for 3 hours and cooled at about 400"C/min. Specimen HT2 was prepared from one of the HTI specimens by annealing at 1100"C for 25 hours followed by quenching in water. The HT2 specimen was prepared by encapsulating Ta foilwrapped slices in evacuated He-backfilled quartz tubes. A third set of samples (HTJ) was obtained by a heat treatment of some of the HTI in similar tubes at 700"C for 18 days followed by a water quench. All three specimens (HTI-HT3) were studied by transmission electron microscopy (TF.M). TEM thin foils were prepared by standard twln-Jet electropollshlng using a 300 ml methanol, 175 ml n-butanol, 30 ml HC10~ electrolyte at 0"C.

M i c r o s t r u c t u r e s a t 1400"C. ~100°C an4 700°C All specimens cooled or quenched from 1&00 and 1100"C (HT1, HT2 and HTJ) contain two different microstructural scales - one resolved by optical metallography, and another only by TEM. The coarse scale represents the mlcrostructure present at high temperature while the fine scale represents a phase transformation of the matrix during continuous cooling [2]. First we describe the coarse structure in order to indicate the equilibrium phases at temperature and second we identify the mlcrostructure of the transformed matrix.

O p t i c a l m e t a l l o g r a p h y shows chat the 1A00"C samples (HT1) c o n s i s t o f l a r g e s i n g l e phase grains. According to TEM a n a l y s i s o f i t s transformed s t r u c t u r e (as w i l l he e v i d e n t l a t e r ) the phase has the B2 s t r u c t u r e , The 1100"C samples (HT2) c o n s i s t o f l a r g e g r a i n s o f the same phase w i t h b l o c k y and n e e d l e - l i k e p r e c i p i t a t e s (few tens o f ~m long) as sho~-n i n Fig.2a. The p r e c i p i t a t e s occurred i n an i r r e g u l a r morphology, and TEM a n a l y s i s i n d i c a t e s t h a t they are an i n t e r g r o w t h o f r e s p e c t i v e l y o and L10 phases. The 700"C (HT3) samples have n e e d l e - l i k e p r e c i p i t a t e s f i n e r t h a t chose i n the HT2 specimens ( F i g . 2 b and 3a), p r o b a b l y due

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Co the l o w e r p r e c i p i t a t i o n temperature. As TEM a n a l y s i s shows, t h e p r e c i p i t a t e s have a p e c u l i a r s t r u c t u r e o f c h i n l a y e r s o f b o t h DO18 and LI 0 phases ( F i g . 3 b , c ) . The o r i g i n o f t h i s s t r u c t u r e was noC i n v e s t i g a t e d i n the p r e s e n t wor k. The i d e n t i t y o f the m a t r i x phase a t 1400 and l l 0 0 ° C f o r the HT1 and HT2 specimens can o n l y be i n f e r r e d , whereas f o r the HT3 the a c t u a l 700"C phases a r e b e l i e v e d t o be o b s e r v e d . S i m i l a r Co o u r p r e v i o u s w o r k [2] on a d i f f e r e n t c o m p o s i t i o n we f i n d t h e d e c o m p o s i t i o n o f a B2 phase d u r i n g c o o l i n g f r o m 1400°C and l l O 0 ° C t e m p e r a t u r e s t o t h e w" phase. No B2 APB's have been observed, indicating stabllicy of the B2 order as high as 1400*C. The microstructure of the HT2 specimen Indicates that the decomposition of the B2 phase occurs only below IIO0°C during continuous cooling.

Crvstallo~raDhv of the new phase Fig. 4a shows mlcrostructure of the matrix of the HT3 specimen which consists of rotational (four) and CranslaClonal (three for each roCatlonal) domains of the m-type phase. The structure is similar to thac of the 582 phase found for the TI4AI3Nb alloy [2] except for the presence of spherical (elipsoidal) precipitates uniformly dlscributed in the matrix. The precipitates can be imaged in dark-field separately from the maCrlx using reflections addlcional to those of the B82 phase (Fig. 4b). These reflections are in fact superlaccice spots of the hexagonal reciprocal lattice o f t h e B8 z p h a s e , as can be seen on a s e r i e s o f selected area diffraction (SAD) pa~terns t a k e n from an area including all four rotatlonal domains, Fig. 5a-c, (compared with those of only ~he B8 z phase of the TI, AI3Nb alloy. Fig. 5d-f). From the SAD patterns and mlcrodiffractlon from individual domains It was found cha~ ~he superlat~ice g vectors are I/3<1120> ° of the B82 reciprocal lattice. This corresponds to a tripled hexagonal latclce of the precipitate phase: a~ = 2a(sln60°); cp = c (a=0.458 nm, c=0.552 nm for the B8 z phase [2]). Axes of the new cell wlth respect co the B82 are: 2 a p L = a I + 2a2, ap2 = -2a I - e2, Cp = c. The results on resolution ( ~ E M ) B82 matrix of one On chat image the

the crystallography of the new structure are directly confirmed by high imaging. Fig. 6 shows such an HREM i m a g e where two preclpltates and the rotational domain, both in [0001] orientation, are imaged simultaneously. precipitates are translated wi~h respect to each other by a.

Several structural models for the new phase can be suggested assuming further substituClonal ordering of the B8 z phase and a group/subgroup relation between the phases. The maximal non-lsomorphlc subgroup of the B82 P6a/mmc space group corresponding to our experimental results is centered hexagonal H63/mmc which is equivalent to p~Imltive" hexagonal P63/mcm with 90 ° rotated axes (and the c-gllde plane) [5]. The subgroup has index [3] which gives the three translational variants (in fact observed in the HREM image, Flg 6). The index [3] also implies, based on the Landau-Lifschicz theory of phase transformation [6], that the ordering transformation is first order, which is conslstenC with the preclplcacion nature of the transformation. Because the cell was trlpled by the ordering, the number of atoms per unit cell is 18 (6 atoms per the B82 phase unit cell). For the cell wlch space group P 6 3 / m c m and Pearson symbol hP18 we flnd a possible prototype phase, Ga, Ti 5 [4]. This structure has four Vyckoff posiclons: (2)b 0,0,0: (4)d I/3,2/3,0; (6)g I x~,0,1/~; (6)g z xz,O,I/4. For the B8 z phase referred to these poslClons the El slre is occupied by TI and g2 by Al atoms in a double layer in Fig.1 [2]. We suggest chat the observed ordering does noc change the double layer, and therefore o c c u r s entirely on a single layer. Based on the estlmaCed compositions of the average matrix, B82 phase and precipitates, we suggest an Ideal ordering on the single layer wich Nb on (4)d and AI on (2)b positions. This gives a (TI3AI3)(AINb 2) stolchlometry for the new phase, with occupancies Al (2)b, Nb (4)d, Ti (6)gx, AI (6)g 2 and x I end x 2 slightly dlfferenC from 1/3 and 2/3 of the ~-type. Another possibility could be ~he Strukturberichc D8 s structure, Mn~Si 3 prototype, also wlth P6~/mcm space group buc wlch 16 a t o m s per unlt cell. For this structure the (2)b

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position is vacant. The (TI~AI~)Nb 2 stoichiomecry has a good correspondence co mass balance requirements_ Further HREM and X-ray diffraction work is in progress to find the correct structure. AcknowiedRement l'he authors thank M.E.Willlams for the TEM specimen preparation and L. Smith for the optical metaliography. The work was supported by DARPA under order No. 6065. References I. 2_ 3. 4. 5. 6.

R. Scrychor, J.C. Williams and W.A. Soffa, Met. Trans. 19A, 225 (1988) L.A_ Bendersky, W.J. BoerCinger, B.P. Burton, F.S. Biancaniello, C.B. Schumaker, to appear in Acta Met. (1990) L.A. Bendersky and W.J. Boettlnger, Proc. of 47th EMSA Meeting, San Francisco Press, Inc (1989) p.324 P. Villars and L.D. Calvert, "Pearson's Handbook of Crystallographic Data for Intermetailic Phases", Vol_l, ASM, Metals Park Oh (1985) "International Tables of Crystallography", Vol. A, T. Hahn, ed., Reldel Publishing Co., Dordrecht (1978). L_D. Landau and E_M. Lifschicz, "Statistical Physics", Pergamon, London (1976).

a

b

c

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82

W'"

B82

Pm3m

P3ml

P63/mrnc

I. Schemaclc drawing of (a) B2, (b) co" and (c) B8 z crystal structures. B2 structure is shown as a scacklng of (111) planes l Different symbols represent different occupancies. (From Ref. 2).

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Optical metallography

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of the (a) HT2 and (b) ST3 samples.

Fig. 3. TEM images of the needle-like precipitates in the HT3 (700"C) specimen matrix. (a) Low magnification image. (b) Internal structure of the precipitate and (c) the corresponding SAD pattern (smaller celt - ~he DO1g phase, [1120]; larger cell - the L10 phase, [ii0]) showing alternating layers of the DO1g and L10 phase and their orientation relationship.

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Fig. 4_ M i c r o s t r u c t u r e of t h e HT3 s p e c i m e n m a t r i x and spherical p r e c i p i t a t e s . (s) Brightfield image of s e v e r a l r o t a t i o n a l and t r a n s l a t i o n a l domains (b) D a r k - f i e l d image of the p r e c i p i t a t e s u s i n g s u p e r l a t t l c e (to B82) refleet£on.

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Fig_ 5_ (a-c) SAD p a t t e r n s taken from the HT3 s p e c i m e n and c o m p a r e d to those (d-f) from the B8 z phase of the Ti4Ai3N'5 alloy. Zone axes u s i n g cubic indices are: (a,d) - ; (b,e) ~ (c,f) - .

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Fig. 6. HREH image of the B82 matrix and che prec£piCates [0001] parallel to the electron beam direction.

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