Shear creep deformation in polysynthetically twinned (PST) Ti–47Al–2Nb–2Cr with 〈110〉 and 〈112〉 orientations

Abstract

Stress and temperature change creep experiments under a compression shear stress state were conducted on polysynthetically twinned (PST) crystals prepared from a cast and HIPed Ti–47Al–2Cr–2Nb alloy. Specimens with 〈110〉 and 〈112〉 directions parallel to the principle resolved shear stress were deformed and the deformation characteristics are compared. Transient creep of the PST crystal after stress and temperature changes (at 9.5–34 MPa and 760–806°C) exhibited `normal' behavior and indicates Class M type creep deformation in TiAl. The stress exponent n changes from 3.1 to 5.3 with increasing strain. The activation energy Q =137 kJ mol −1 at 28 MPa for the 〈110〉 PST indicates that dislocation glide aided by pipe diffusion is a likely mechanism of creep deformation in this deformation regime. The activation energy (185 kJ mol −1 at 13 MPa) of the 〈112〉 PST is higher than the 〈110〉 PST crystal, but primary creep in the 〈112〉 direction occurs much faster than for the 〈110〉 direction at a higher stress. Primary creep at 9.5 MPa in the 〈112〉 direction caused a 25% refinement of lamellar spacing that hardened the microstructure, indicating that mechanical twinning parallel to the lamellar planes provided an easy initial deformation path during primary creep. At strains near 1%, the creep strength of the two orientations becomes similar.