Approaches to modelling chemically induced transformation superplasticity

Abstract

Transformation superplasticity is a Newtonian deformation mechanism observed when polymorphic materials are subjected to small external stresses during their phase transformation. Although mostly studied during thermal excursions, recent investigations have observed this mechanism during excursions in chemical composition in the Ti-H system. In this work, methodologies for modelling this so-called ‘chemically induced transformation superplasticity’ are discussed, simultaneously considering the kinetic moving-boundary diffusion problem associated with the titanium α-β transformation, as well as the internal strains due to chemical swelling and the transformation. Two existing models, including, firstly, an analytical model for thermal-cycling-induced transformation superplasticity and, secondly, a numerical model for creep under conditions of chemical cycling, are adapted to the case of chemically induced transformation superplasticity. Both modelling approaches predict the main features of transformation superplasticity and are found to agree reasonably with the existing experimental data.