Effect of grain size on macroscopic plastic behavior of nearly lamellar γ-TiAl-based alloys

An analytical micromechanical model in a framework of a micropolar continuum was proposed to investigate the size effect of macroscopic plastic behavior for a nearly lamellar γ-TiAl based alloy, which consisted of PST (polysynthetically twinned crystal) grains with random orientation. The γ-TiAl based alloy was considered as a two-phase composite composed of micropolar matrix and PST particles. The Mori-Tanaka's method was advanced to evaluate the effective elastic moduli, and a new secant moduli method based on the second moment of strain and torsion of the matrix was extended to analyze the nonlinear behaviour of the γ-TiAl based alloy. The modeling results show that the size dependence is more pronounced when the PST particle's size approaches to the matrix characteristic length. The plastic stress of a nearly lamellar γ-TiAl based alloy increases with the decrease of PST grain size or with the increasing volume fraction of the PST grains. The characteristic length of plasticity of the micropolar matrix is approximately as the same order of magnitude as the average diameter of the matrix particles.