TRANSFORMATION SUPERPLASTICITY OF SUPER AT TITANIUM ALUMINIDE

Transformation superplasticity of an intermetallic Ti3Al-based alloy ( Super alpha(2)) is demonstrated by thermal cycling about the alpha(2)/ beta transformation temperature range under a uniaxial tensile biasing stress. Failure strains up to 610% were recorded at a stress of 3 MPa , compared with 110% for deformation by isothermal creep at the same s tress. The strain increment produced during each half-cycle is determi ned as a function of applied biasing stress, thermal cycle amplitude, and cycling frequency. Since internal stresses are generated both by t ransformation mismatch and by thermal expansion mismatch between the t wo coexisting phases, an effective volume mismatch is defined to accou nt for both contributions. Introduction of this parameter into existin g continuum-mechanics models results in good agreement with experiment al data in both the low-stress regime, where cycling strain rate and s tress are proportional, and the high-stress regime where the stress se nsitivity is increasing. Cycling strain increments at various temperat ure amplitudes and cycling frequencies are considered in terms of the equilibrium thermodynamics and transformation kinetics of Super alpha( 2). Finally, thermal cycling produces ratcheting, where texture and cr ystallographic orientation of the beta/alpha(2) transformation front a ffect the direction of ratcheting strains. (C) 1998 Acta Metallugica I nc. Published by Elsevier Science Ltd. AII rights reserved.