Stability of lamellar structure in a Ti-47.8at%Al alloy

Scripta METALLURGICA et MATERIALIA

Vol. 30, pp. 165-168, 1994 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

STABILITY OF LAMELLAR STRUCTURE IN A Ti-47.8at%Al ALLOY

II.H. Tian, Z. Huang and C.Q. Chen Dept. of Materials Science and Engineering, Belling University of Aeronautics and Astronautics, Belling 100083, P.R. China (Received May 20, 1993) (Revised October 5, 1993)

Introduction The stability of the lamellar structure is known to be highly structure sensitive. Although some reports (1,2) have investigated the s t a b i l i t y of lamellac in lower aluminum content alloys, few reports have studied the s t a b i l i t y of lamellae in near gamma titanium aluminide alloys. The purpose of this paper is to investigate the s t a b i l i t y of lamellar structure with d i f f e r e n t features in Ti-47.8at%Al alloy.

Experimental Ti-47.8at%Al alloy was prepared by a nonconsumable arc-melting technique and followed by an extruding process. Samples were cut from the alloy and coated with a p r o t e c t i v e coating to reduce the oxidization rate. Protected samples were homogenized at 1400~ for 1 hour and then cooled at various rates, namely, furnace cooled (FC), air cooled (AC), oil quenched (OQ) and water quenched ~Q). The quenched samples were aged in the two phase fields. Microstructures were examined using OM, 81~1 and TEM, Results and Discussion

Table i l i s t s the volume f r a c t i o n of a coarse lamellar structure in FC, AC, OQ and WQ microstructures, after d i f f e r e n t aging treatments, it can be seen from the table that the faster the cooling rate, the greater the volume fraction of coarse lamellae. TABLE I the volume fraction of coarse lamellar structure in FC, AC, OQ and WQ microstructures after different aging treatments

heat treatment

FC

AC

OQ

1000~/24h 1080~/24h 1180~/24h

~ 0 ~ 0 ~ 0

~ 0 ~ 0 ~ 0

55 80 100

WQ I00 -

The lamellae obtained in the FC sample were very similar to these in the AC sample, Fig. l. The interlamellar spacing of these lamellae ranged from 0,5 to 1,5~m, No discontinuous coarsening occured in FC and AC samples af ter aging at 1000~, 1080~, I180~ for 24 hours. This indicates that the s t a b i l i t y of these lamellae is r e l a t i v e l y high. The OQ microstructure is shown in Fig. 2a. The interlamellar spacing was in the range of 0 . 1 5 - 0 . 5 m, which was smaller than that of the FC and AC microstructures. Discontinuous coarsening occurred when the OQ sample was aged in the two phase region (Fig. 2b). The 165 0956-716X/94 $6.00 + .00 Copyright (c) 1993 Pergamon Press Ltd.

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volume fraction of the coarse lamellar increased with increasing temperature, implying that the coarsening rate increased with temperature. The coarse structure became finer with decreasing temperature, Fig. 3. The original ~ grain boundaries were discsrnabie in the coarse microstructure, Fig, 2b. The alignment of coarse lamellse was complex, which was in sharp contrast with the perfect alignment of the primary lamellae. The reason is : when primary lamellae precipitate from an a phase, the orientation relationship between ~ and ~ plates is {111}//(0001) 1]<110> //<1120), the habit plane of the ~ phase is the close-packed plane of the ~ matrix, and there is only one close-packed plane in an ~ grain so that the alignment of lamellse in one original ~ grain is perfect (3) . Although the coarse lamellar structure has the same crystallographic orientation as the primary lamellao (1), coarse lamellse nucleate in both the ~ and ~a matrices so that more than one habit plane exists: for example, the y matrix has four close-packed planes, so coarse lamellae show different alignment within one original ~ grain. Parallel coarse lamellae, with same close-packed plane index, form a coarse packet. The alignment of coarse lamellae may also have something to do with the orientation variant. The primary lamellar structure has six orientation variants, and d i f f e r e n t variants can coexist within one Y plate (8,4) (Fig. 4). The same variants form a domain. When a coarse lamellar meets with the domain boundary, its orientation can change. The WQ microstructure is mostly an ~ s single phase ( Fig. 5). Fig. 6a shows the characteristic lamellar structure, obtained when the WQ sample was aged at 1000~ for 0 seconds. As aging time increased, discontinuous coarsening occurred, Fig. 6b. Notably discontinuous coarsening i n i t i a t e s at grain boundaries, and grows continuously to the other side of the grain. During subsequent coarsening, the crystallographic features play a leading role in the nucleation and growth of coarse lamellae, so that the alignment of coarse lnmellae change often. Compared to the lamellae formed during cooling, the volume fraction of coarse lamellse is the WQ sample, aged at 1000~ for 2 hours, was 85~, while that of the OQ sample after the same treatment was not more than 0~. This indicates that the coarsening rate of the WQ sample was greater than that of the OQ sample. The reason for this may be attributed to the smaller interlamellar spacing introduced by the lower temperature aging treatment in the WQ sample. The coarse microstructure in the WQ sample is finer than that of the OQ sample (Fig. 3,6). One possible reason is that the crystallographic changes in unit interlamellar spacing are greater in the Wq sample, Thus, more frequent adjustment is needed when coarse lamellae grow. Conclusions (1) A lamellar structure was obtained in FC, AC and OQ samples. As the cooling rate increased, the interlamellar spacing decreased. Thus, the s t a b i l i t y of lamellae decreased. The coarsening rate increased with increasing temperature. (2) An a+ single phase was obtained in the WQ sample. The iamellae transformed from the a s phase are less stable, and hence have a faster coarsening rate and finer coarse structure compared with the lamellnr formed during cooling. (3) Since coarse lamellae nucleate in the a s and Y matrices, more than one habit plane exists, so that the alignment of coarse lamellae is complex. Parallel coarse lamellee with same close-packed plane index form a packet. Acknowledgement This work was part of the research of TiAI Intermetallic joint group of NIN.NEUT.BUAA

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References 1 2 3 4

D. Simon Shong, Y-W. Kim. Scr. Metall.,1989, vol. 23, ppo257 Y. Yamabe, N. Honjo, M. Kikuchi, JIMIS-6, 1991, pp. 8~I M. KikuchL Y. yamabe, JIMIS-B, 1991, pp. 815 Y.S. Yang andS. K. Wu, Scr. Metall.,t9St, vol. 25, pp. 255

FIG. I microstructure

FIG. 2 mierostructure

FIG. 3

of

of 1400"C/lh

1400"C/lh/OQ

(a)FC (b)AC

(a) and subsequent aging (b)

1400"C/Ih/OQ + aging at (a) 1000"C/24h

(b) llSO'C/24h

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C

FIG. 4 orientation variants of primary lamellar (a) variants coexisting in the same "f lamellar (b) [101] pattern of T~ (c)[110] pattern of Y~

FIG. 5 microstructure of 1400"C/lh/WQ (a) featureless region (b)DF TF_~ micrograph showing ~2 single phase

FIG. 6 1400"C/Ih/WQ+IO00"C aging

for (a)Ss

(b) 24h

2