γ interface in TiAl alloys

Scripta Materialia 45 (2001) 383±389

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A study of Ti5Si3/c interface in TiAl alloys

Fu-Sheng Suna*, Seung-Eon Kimb, Chun-Xiao Caoa, Yong-Tai Leeb, and Ming-Gao Yanb a

Titanium Alloys Laboratory, Beijing Institute of Aeronautical Materials, P.O. Box 81-15, Beijing 100095, China b Korea Institute of Machinery and Materials, 66 Sangnam, Changwon, Kyungnam, South Korea

Received 3 October 2000; accepted 6 March 2001

Keywords: Compounds; Intermetallic (TiAl); Phase transformation; Interface; Ti5 Si3

Introduction Gamma based TiAl alloys have recently received signi®cant attention for high temperature structural applications due to their low density, high speci®c strength, and sti€ness at elevated temperatures [1±3]. One of the principal limitations for their practical use is the poor ductility and toughness at room temperature. Alloying with third elements, such as V, Cr, Mn, Nb, Zr, Ta, Mo and W can improve the room temperature and high temperature tensile properties of TiAl alloy [2,4,5]. Recent investigations show that the addition of Si into TiAl alloy introduces round Ti5 Si3 particles by powder metallurgy [6,7], and Ti5 Si3 whiskers by ingot metallurgy [8]. The precipitation of Ti5 Si3 particles is heterogeneous. The objective of this study was to investigate the orientation relationship and character of Ti5 Si3 /c interfaces in TiAl alloys.

Experimental The alloys in this study have nominal compositions of Ti52 Al48 ±3Si, Ti52 Al48 ±3Si2V, Ti52 Al48 ±3Si2Cr and Ti52 Al48 ±3Si2Cr2V (at.%). Ingots were melted four times by using non-consumable electrode arc melting in order to ensure homogeneity. The ingots were subjected to hot isostatic pressing (HIP'ing) at 1200°C for 3 h and heat treatment at 1200°C for 12 h, followed by air cooling. Specimens were cut from the center of ingots

*

Corresponding author. E-mail address: [email protected] (F.-S. Sun).

1359-6462/01/$ - see front matter Ó 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 6 4 6 2 ( 0 1 ) 0 1 0 1 2 - 0

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and then prepared by grinding and twin jet polishing in a solution of 60% methanol, 35% butanol, and 5% perchloric acid. A JEM 2010 with a Link ISIS energy dispersive spectrometer (EDS) was used at 200 kV. Results Si-bearing TiAl alloys consist of the two phases c ‡ Ti5 Si3 . In addition to Ti5 Si3 whiskers, ®ne Ti5 Si3 particles are precipitated at c=c boundaries or in the c matrix. Fig. 1 shows the distribution of Ti5 Si3 particles in Ti52 Al48 ±3Si, Ti52 Al48 ±3Si2Cr and Ti52 Al48 ±3Si2V alloys. The addition of Cr or V increases the volume fraction of Ti5 Si3 particles and decreases the c lamellar spacing. Some Ti5 Si3 particles were found to be round or elongated in shape, and others were characterized by polygonal shape. The Ti5 Si3 particles were precipitated during heat treatment at 1200°C. Ti5 Si3 particles were also found to precipitate during creep deformation or heat treatment at 1000°C in other Si-bearing alloys [7,9]. Fig. 2 shows the morphologies of precipitated Ti5 Si3 particles in Ti52 Al48 ±3Si2Cr2V. In addition to round and elongated particles, there are some needlelike Ti5 Si3 particles. Most of the needle-like Ti5 Si3 particles were precipitated along c=c boundaries in Ti52 Al48 ±3Si2Cr2V. The amount of needle-like Ti5 Si3 particles in the V-bearing TiAl is much higher than that in the V-free TiAl. In addition, V and Cr are e€ective Ti5 Si3 phase stabilizers. The solution of V or Cr in Ti5 Si3 is higher than that in c (Table 1). The lattice parameters of c and Ti5 Si3 phase were measured by X-ray di€raction at a scanning rate of 0.2°/min (Table 2). Ti5 Si3 has a hexagonal D88 structure, and c has an orthogonal L10 structure. With the addition of V, the ratio c=a in Ti5 Si3 phases of Ti52 Al48 ±3Si2V and Ti52 Al48 ±3Si2Cr2V alloys was found to decrease. Examinations were carried out to study the orientation relationship between Ti5 Si3 particles and c phase. Fig. 3a shows a selected area di€raction pattern (SADP) and

Fig. 1. TEM micrographs showing the distribution of Ti5 Si3 particles in: (a) Ti52 Al48 ±3Si, (b) Ti52 Al48 ±3Si2Cr and (c) Ti52 Al48 ±3Si2V alloys.

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Fig. 2. TEM micrograph showing precipitated Ti5 Si3 particles in Ti52 Al48 ±3Si2Cr2V alloy. Table 1 Compositions of c and Ti5 Si3 phases (at.%) Alloys

Phases

Ti

Al

Si

Cr

V

Ti52 Al48 ±3Si

c phase Ti5 Si3 c phase Ti5 Si3 c phase Ti5 Si3 c phase Ti5 Si3

52.4 60.9 51.9 58.6 51.3 57.1 50.6 56.2

47.5 13.4 46.2 14.4 46.7 15.3 44.6 13.7

0 25.7 0 24.4 0 24.4 0 24.1

± ± 1.90 2.57 ± ± 2.21 2.77

± ± ± ± 1.96 3.29 1.95 3.30

Ti52 Al48 ±3Si2Cr Ti52 Al48 ±3Si2V Ti52 Al48 ±3SiCr2V

Table 2 Lattice parameter of TiAl alloys Composition (at.%) Ti52 Al48 ±3Si Ti52 Al48 ±3Si2Cr Ti52 Al48 ±3Si2V Ti52 Al48 ±3Si2Cr2V

c (nm) 0.4094 0.4088 0.4119 0.4091

c phase

Ti5 Si3 phase

a (nm)

c=a

c (nm)

a (nm)

c=a

0.4020 0.4018 0.4016 0.4024

1.0185 1.0174 1.0257 1.0167

0.5250 0.5224 0.5161 0.5136

0.7527 0.7517 0.7517 0.7517

0.6975 0.6950 0.6866 0.6833

corresponding schematic diagram of a Ti5 Si3 /c interface in Ti52 Al48 ±3Si2Cr. The electron beam was parallel to ‰0 0 0 2ŠTi5 Si3 , showing the hexagonal symmetry of Ti5 Si3 . In addition, it was found that …0  1 1†ck…0 1  1 0†Ti5 Si3 , and ‰2 3 3Šck‰0 0 0 2ŠTi5 Si3 . The spacing of  …0 1 1 0† in Ti5 Si3 phase is 0.6510 nm, almost three times the spacing of …0 1 1† in c (0.2866 nm). A di€raction spot of …0  1 1† in c coincides with that of …0 3 3 0† in Ti5 Si3 phase (Fig. 3a). This orientation relationship leads to an incoherent Ti5 Si3 /c interface due to big di€erence in plane spacing. Detailed investigation reveals that most Ti5 Si3 particles with a polygonal shape have this orientation relationship. Fig. 4a is a HRTEM micrograph showing a Ti5 Si3 particle with a polygonal shape in Ti52 Al48 ±3Si2Cr alloy. The Ti5 Si3 /c interface (marked with an arrow in Fig. 4a) has an orientation relationship

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Fig. 3. Selected area di€raction patterns (SADP) and corresponding schematic diagrams of Ti5 Si3 /c interfaces in (a) Ti52 Al48 ±3Si2Cr and (b) Ti52 Al48 ±3Si2V alloy.

of …0  1 1†ck…0 1  1 0†Ti5 Si3 , and ‰2 3 3Šc k‰0 0 0 2ŠTi5 Si3 . Moreover, needle-like Ti5 Si3 particles precipitated along the c=c boundaries also have an orientation relationship of …1 0  1†ck…1 0  1 0†Ti5 Si3 . This orientation relationship was di€erent from that in a previous study, where an orientation relationship of …4 1 5 0†Ti5 Si3 k…1 1 0†c was observed in Ti±Al± V±Si [10]. In addition, we use index …1 0  1† rather than index …1 1 0† in c. Because the former plane in c has alternate arrangement of Ti and Al atoms, the latter plane in c has only Ti or Al atoms. It was found that …1 0  1† plane rather than …1 1 0† plane in c was parallel to …0 1  1 0† plane in Ti5 Si3 . Orientation relationships of …1 0 1†ck…1 0 1 0†Ti5 Si3 , and 12 1 6ŠTi5 Si3 , or ‰2 3 3Šc k‰0 0 0 2ŠTi5 Si3 have been extensively observed in Ti52 Al48 ± ‰1 4 1Šc k‰ 3Si2V, Ti52 Al48 ±3Si2Cr and Ti52 Al48 ±3Si2Cr2V alloys. Another orientation relationship between c and Ti5 Si3 was observed in Ti52 Al48 ± 3Si2V, Ti52 Al48 ±3Si2Cr and Ti52 Al48 ±3Si2Cr2V alloys. Fig. 3b shows a selected area di€raction pattern (SADP) and corresponding schematic diagram of a Ti5 Si3 /c interface in Ti52 Al48 ±3Si2V alloy. A basal plane …0 0 0 2† of Ti5 Si3 phase is parallel to …1 1 1† of the c phase. The spacing of …0 0 0 2†Ti5 Si3 is 0.2581 nm, while that of …1 1 1†c is 0.2338 nm. It was found that most rectangular or elongated Ti5 Si3 particles (Fig. 2) at or near

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Fig. 4. HRTEM images showing (a) an incoherent Ti5 Si3 /c interface in Ti52 Al48 ±3Si2Cr alloy, (b) a semicoherent Ti5 Si3 /c interface in Ti52 Al48 ±3Si2V and (c) a semicoherent Ti5 Si3 /c interface in Ti52 Al48 ±3Si2Cr alloy.

c=c boundaries have an orientation relationship of the type …1 1 1†c k…0 0 0 2†Ti5 Si3 and 12 1 0ŠTi5 Si3 . This orientation relationship results in a semicoherent Ti5 Si3 /c in‰1 4 5Šc k‰ terface. Fig. 4b and c are HRTEM micrographs showing the Ti5 Si3 /c interfaces in Ti52 Al48 ±3Si2V and Ti52 Al48 ±3Si2Cr alloys. Those Ti5 Si3 /c interfaces are semicoherent. For a round Ti5 Si3 particle in a c matrix (Fig. 2), there was no orientation relationship at the Ti5 Si3 /c interface. It may nucleate at vacancies. Discussion There are two types of orientation relationship between c and Ti5 Si3 found in the present study. Type I is …1 0  1†ck…1 0  1 0†Ti5 Si3 , and‰1 4 1Šc k‰1 2 1 6ŠTi5 Si3 , or ‰2 3 3Šc k  ‰0 0 0 2ŠTi5 Si3 . Type II is …1 1 1†c k…0 0 0 2†Ti5 Si3 and ‰1 4 5Šc k‰1 2 1 0ŠTi5 Si3 . The former orientation relationship is associated with an incoherent Ti5 Si3 /c interface.

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Table 3 Mismatch of c and Ti5 Si3 phases Alloy

c phase

Spacing (nm)

Ti5 Si3 phase

Spacing (nm)

d

1=d

Ti52 Al48 ±3Si2Cr

h1 1 1† h1 0 1† or h0 1 1† h1 1 0†

0.2333 0.2866 0.2841

…0 0 0 2† …3 0 3 0† …3 0 3 0†

0.2612 0.2170 0.2170

0.1068 0.3207 0.3092

9.363 3.118 3.233

Ti52 Al48 ±3Si2V

h1 1 1† h1 0 1† or h0 1 1† h1 1 0†

0.2338 0.2876 0.2840

…0 0 0 2† …3 0 3 0† …3 0 3 0†

0.2581 0.2170 0.2170

0.0941 0.3253 0.3087

10.63 3.074 3.239

The … 1 1 1†c k…0 0 0 2†Ti5 Si3 and ‰1 4 5Šc k‰ 12 1 0ŠTi5 Si3 orientation relationship (type II) leads to a semicoherent Ti5 Si3 /c interface. Semicoherent interfaces consist of mis®t dislocations at a periodical distance (Fig. 4b and c). In Ti52 Al48 ±3Si2V, the mis®t dislocations at Ti5 Si3 /c interfaces occur periodically for every eleven …1 1 1†c planes (marked with arrows in Fig. 4b). In Ti52 Al48 ±3Si2Cr, the mis®t dislocations occur periodically for every nine … 1 1 1†c planes (marked with arrows in Fig. 4c). It is well known [11] that the mismatch d is de®ned as: d ˆ j‰aa

ab Š=ab j;

…1†

where aa and ab represent the plane spacing in c and Ti5 Si3 , respectively. Table 3 shows the mismatch between … 1 1 1†c and …0 0 0 2†Ti5 Si3 , and that between …1 0 1†c or …1 1 0†c and  …3 0 3 0†Ti5 Si3 in Ti52 Al48 ±3Si2Cr and Ti52 Al48 ±3Si2V alloys. 1=d represents a periodicity of mis®t dislocations. It is found that the orientation relationship of …1 1 1†c k …0 0 0 2†Ti5 Si3 , yields about 9.4 and 10.6 for a periodicity …1=d† of mis®t dislocations in Ti52 Al48 ±3Si2Cr and Ti52 Al48 ±3Si2V alloys, respectively. This is in good agreement with the HRTEM images in Fig. 4. Previous studies reported the orientation relationship of 1 1 1†c in Ti-45Al±2.7Si alloy [7]. For type I, if …1 0 1†ck…1 0 1 0†Ti5 Si3 , a …0 0 0 2†Ti5 Si3 k… semicoherent relationship would noticeably increase the Ti5 Si3 /c interface energy due to about 30±32% di€erence in plane spacing. Therefore, an incoherent Ti5 Si3 /c interface formed for the type I orientation relationship. Conclusions The addition of Cr or V to TiAl signi®cantly increases the volume fraction of Ti5 Si3 particles. Most needle-like Ti5 Si3 particles are precipitated along c=c boundaries in Ti52 Al48 ±3Si2V and Ti52 Al48 ±3Si2Cr2V alloys. Two types of orientation relationships between c and Ti5 Si3 phase were found in Si-bearing TiAl alloys. Type I is 12 1 6ŠTi5 Si3 , or ‰2 3 3Šc k‰ 0 0 2ŠTi5 Si3 , with the Ti5 Si3 /c in…1 0  1†ck…1 0  1 0†Ti5 Si3 and ‰1 4 1Šc k‰ terface incoherent. Most Ti5 Si3 particles with this orientation relationship featured with a polygonal shape or a needle-like shape and were located in the c matrix or at c=c boundaries. Type II is … 1 1 1†c k…0 0 0 2†Ti5 Si3 and ‰1 4 5Šc k‰1 2 1 0ŠTi5 Si3 , which gives rise to a

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