Anomalous behaviour of cyclic deformation and fatigue properties of TiAl PST crystals under constant applied stress

Abstract

Cyclic deformation and fatigue properties of TiAl PST crystals were investigated at room temperature under constant applied stress by focusing on the effect of γ domains. Strong anisotropy in plastic behaviour was observed depending on the angle (Φ) between the loading axis and the lamellar planes. At Φ = 0° the γ domains were divided into two groups of T-type and V-type depending on deformation substructure. In T-type and V-type domains, deformation twins and vein-like structure were mainly formed, respectively. Plastic strain energy for each cycle increased with increasing applied stress amplitude (Δσ) but an anomalous drop in the energy was found around Δσ = 440 MPa at Φ = 0°. The anomalous dependence of the plastic strain energy on Δσ was closely related to the deformation mode in the V-type domains. Deformation twins which were induced in the V-type domains in an early stage interrupted the to-and-fro motion of ordinary dislocations resulting in a rapid cyclic hardening. At Φ = 45°, shear deformation occurred along the lamellar planes, and ordinary dislocations and twins easily passed through the domain boundaries. The plastic strain energy monotonically increased as Δσ rose.