Prediction of yield stress for polysynthetically twinned TiAl crystals Jun Yang a , Gengkai Hu a * , and Yonggang Zhang b a Department of Applied Mechanics, Beijing Institute of Technology, Beijing 100081, China b Department of Material Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100083, China Received 2 November 2000; accepted 2 April 2001 Keywords: Yield stress; Lamellar structure; TiAl; Micromechanics; Modelling Introduction TiAl based alloys have good resistance to oxidation, creep and fatigue, and are con- sidered to be one of the most promising structural materials for elevated temperature applications [1±3]. Polysynthetically twinned PST) TiAl crystals, which consist of a series of well-oriented lamellae of TiAl c) and Ti 3 Al a 2 ), have been extensively used as a model material to study mechanical behavior of the lamellar microstructure. The mechanical properties of the PST crystal depend signi®cantly on the lamellar orienta- tion de®ned by the angle between the lamellae and the loading axis h [4]. When h  0° or 90°, usually referred to as hard deformation modes, the PST crystals have the highest yield stress. In this case, dislocations have to overcome the interface barriers. When h  45°, on the other hand, the motion of dislocations is relatively easy and the crystal has a low yield stress, corresponding to a soft deformation mode. The variation of the yield stress as a function of h was determined experimentally by Fujiwara et al. [5]. Lebensohn et al. [6] adopted a pair of the matrix and the twin as a basic element, and proposed a method to evaluate the yield stress of PST crystals. Grujicic and Zhang [7] proposed a ®nite element method incorporating dislocation theory to analyze the yield and deformation of TiAl polycrystals. Kad et al. [8,9] studied the plastic deformation and fracture of PST crystals by numerical simulation and a micro-mechanical method. Sun [10] advanced a dislocation pile-up model to study the in¯uence of the layer Scripta Materialia 45 2001) 293±299 www.elsevier.com/locate/scriptamat * Corresponding author. E-mail address: hugeng@public.bta.net.cn G. Hu). 1359-6462/01/$ - see front matter Ó 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII:S1359-646201)01030-2