Deformation modes in gamma-TiAl as derived from the single crystal yield surface

Based on a detailed analysis of the topography of the single crystal yield surface, a deformation mode map for slip and twinning of Ll(o) ordered gamma-TiAl is derived. Various different deformation systems have been considered, namely, glide of ordinary and superdislocations as well as shear contributions by ordered twins. In the framework of the Taylor model for all combinations of critical resolved shear stress, parameters for polycrystal deformation are determined such as Taylor factors, slip distribution among the various deformation modes and internal stresses set up between grains. The comparison of theoretical with experimental textures, and of predicted with observed anisotropy of plastic how, is utilized to determine the slip geometry and the relative strength levels of the different deformation systems. The results are used for a general discussion of deformation processes observed in TiAl and how these are affected by work hardening characteristics. It is concluded that superdislocations of one kind or another are necessary deformation modes, and that all modes have similar CRSS values at large strains.