Acta mater. 49 (2001) 3803–3809 www.elsevier.com/locate/actamat FORMATION OF DEFORMATION TWINS IN TiAl E. CERRETA 1 † and S. MAHAJAN 2 1 Los Alamos National Laboratory, MST-8, Mail Stop G755, Los Alamos, NM 87545, USA and 2 Department of Chemical and Materials Engineering, Arizona State University, Tempe, AZ 85287-6006, USA ( Received 28 March 2001; received in revised form 2 July 2001; accepted 2 July 2001 ) Abstract—Mechanisms have been proposed for the formation of fault-pairs in the L1 0 structure that involve both ordinary and super glide dislocations having co-planar Burgers vectors. These fault-pairs could serve as two-layer embryonic twins and twin thickening may occur by the coalescence of fault-pairs located at different levels within a slip band. The crystallographies of deformation twins are examined in as-processed TiAl by transmission electron microscopy and are correlated with that of slip required for the formation of fault-pairs. The correlations are consistent with the suggestion that fault-pairs maybe involved in the formation of deformation twins. Slip is observed ahead of a twin terminating within a crystal. Arguments are developed to rationalize this observation.  2001 Published by Elsevier Science Ltd on behalf of Acta Materialia Inc. Keywords: Intermetallic; Twinning 1. INTRODUCTION The gamma-based titanium aluminides are of interest to the aerospace industry for high temperature struc- tural applications, such as gas turbine engine compo- nents. For this reason, much of the research on these alloys has focused on their creep behavior. While some of the grains of deformed, polycrystalline samples contain high densities of ordinary glide dislo- cations, twinning and slip may co-exist in other grains. The single-phase TiAl deforms at room tempera- ture by the glide of 101] superdislocations. How- ever, twinning becomes active at temperatures around 750–800°C. Furthermore, slip at high temperatures is dominated by ordinary 1 2 110] dislocations [1]. This is an unusual behavior because twinning in body cent- ered cubic (BCC) and face centered cubic (FCC) met- als and alloys is usually a low temperature defor- mation mode [2]. TiAl crystallizes in the L1 0 structure and its projec- tion on a (111) plane is schematically shown in Fig. 1. Deformation twins on a (111) can be produced by imposing 1 6 1 ¯ 1 ¯ 2] or 1 3 2 ¯ 11] or 1 3 12 ¯ 1] displacements on consecutive (111) planes. The 1 6 1 ¯ 1 ¯ 2] displace- ment is in the same sense as that for twinning in FCC crystals. However, the 1 3 2 ¯ 11] and 1 3 12 ¯ 1] displace- † To whom all correspondence should be addressed. Tel.: 505-665-2576. 1359-6454/01/$20.00  2001 Published by Elsevier Science Ltd on behalf of Acta Materialia Inc. PII:S1359-6454(01)00264-6 Fig. 1. Crystallography of the L1 0 structure. (a) L1 0 ordered structure and (b) the stacking of the {111} planes. ments are double of that required for twinning in the FCC structure and are also opposite in sense. Many explanations have been proposed to rational- ize the formation of deformation twins in TiAl [1, 3–5]. According to Girshick and Vitek [1], the core structure of the 1 2 1 ¯ 1 ¯ 2] screw dislocation can evolve into a two-layer twin. Hug and Veyssie ´re [3] have shown that embryonic twins may nucleate from 1 2 