ELSEVIER Dislocation Materials Science and Engineering A192/193 (1995) 217-225 structures and deformation behavior of Ti-50/52Al alloys between 77 and 1173 K S. Srirama, Vijay IS. Vasudevana, Dennis M. Dimidukb zyxwvutsrqponmlkjihgfedcbaZYX “ Department of M aterials Science and Engineering, University of Cincinnati, Cincinnati, OH 45221, USA b W right Laboratory, Materials Directorate, W right-Patterson AFB, Dayton, OH 45433, USA Abstract Coarse grained, binary alloys of Ti-SOat.%Al and Ti-52at.%Al, containing low (approximately 250 wt.ppm) levels of interstitials, have been deformed in compression over a range of temperatures (77-l 173 K). The resulting flow stress-temperature profiles in both the alloys consist of three distinct regimes: (I) 77-600 K (normal); (II) 600-1073 K (anomalous); (III) above 1073 K (normal). The fine structure of dislocations in the deformed samples was observed using weak-beam microscopy. It appears that the l/2( 1 lo] dislocations are the most difficult to move at 77 K and room temperature (RT ). At low temperatures, deformation is controlled by twinning and the (0 111 superdislocations. The mobility of the l/2( 1121 dislocations is intermediate between the (0 111 and l/2( 1 lo] dislocations. As the deformation temperature increases, all the dislocations become relatively mobile, but the (0111 dislocations become increasingly blocked as they adopt a thermally activated non-planar configuration and thereby give rise to a flow anomaly. Finally, at very high temperatures, the (01 l] superdislocations decompose to l/2(1 lo] unit dislocations and l/2(1 121 superdislocations which are sufficiently mobile such that the strength decreases following the flow stress peak. Keywords: Dislocation; Deformation; Titanium; Aluminium; Alloys 1. Introduction The deformation behavior of the intermetallic com- pound TiAl has been the focus of many studies [l- 181. Both single crystals [5,9,10] and polycrystalline alloys [ l-4,6-8,1 l-l 81 have been investigated. y-TiAl has an f.c.c.-derived ordered Ll, structure and, consequently, three major types of dislocation occur: zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA b= l/2(1 lo] unit, perfect dislocations, b=(lOl] and b= l/2(112] superdislocations [ 1,4]. Furthermore, twinning is also a dominant deformation mode in TiAl[2]. Ti-56at.%Al single crystals (all compositions are in at.%) under compression [5,9,10] shows a flow stress peak at about 600 “C, similar to that observed in the Ll, compound N&Al [ 191. Kawabata et al. [9] also reported an increase in strength at sub-ambient temperatures in Ti-56Al single crystals. There have been varying opinions on the existence of the flow stress anomaly in polycrystalline binary TiAl. For example, Vasudevan et al. [6] and Prasad Rao and Tangri [7] have argued that the observation of the flow anomaly is dependent on grain size, similar to that in boron-containing N&Al [20] and Zr,Al[21]. Huang and Hall [S] have reported that it is dependent on the heat-treatment temperature and Al content. Hug et al. [4] identified and described the nature of various dislocations and their dissociations in deformed polycrystalline Ti-54Al samples, and con- cluded that the flow stress anomaly was caused by locking of (1011 dislocations by the Kear-Wilsdorf cross slip mechanism (i.e. cross slip of dislocations with b= l/2(101] from (111) to {OlO} where they become locked), similar to that in N&Al [ 193. There has been no systematic work on the mechani- cal properties of TiAl and the associated deformation structures as a function of temperature and alloy com- position. Thus the objective of this investigation was to determine the mechanical properties and deformation structures of coarse grained TiAl alloys over a range of deformation temperatures and Al contents. 2. Experimental details Binary Ti-5OAl and Ti-52Al alloy buttons were prepared by vacuum arc melting of high-purity electro- 0921-5093/95/$9.50 0 1995 - Elsevier Science S.A. All rights reserved SSDZ 0921-5107(94)03250-5