Transformation superplasticity of super α2 titanium aluminide

Abstract

Transformation superplasticity of an intermetallic Ti3Al-based alloy (Super α2) is demonstrated by thermal cycling about the α2/β transformation temperature range under a uniaxial tensile biasing stress. Failure strains up to 610% were recorded at a stress of 3MPa, compared with 110% for deformation by isothermal creep at the same stress. The strain increment produced during each half-cycle is determined as a function of applied biasing stress, thermal cycle amplitude, and cycling frequency. Since internal stresses are generated both by transformation mismatch and by thermal expansion mismatch between the two coexisting phases, an effective volume mismatch is defined to account for both contributions. Introduction of this parameter into existing continuum-mechanics models results in good agreement with experimental data in both the low-stress regime, where cycling strain rate and stress are proportional, and the high-stress regime where the stress sensitivity is increasing. Cycling strain increments at various temperature amplitudes and cycling frequencies are considered in terms of the equilibrium thermodynamics and transformation kinetics of Super α2. Finally, thermal cycling produces ratcheting, where texture and crystallographic orientation of the β/α2 transformation front affect the direction of ratcheting strains.