Kinetics of strain-induced fcc→hcp martensitic transformation

Abstract

The kinetics of the strain-induced γ (fcc)→ε (hcp) transformationi.e. the amount of phase transformationvs applied strain were determined by density measurements at various temperatures. The transformation curve has a sigmoidal shape and approaches saturation below 100 pct transformation. Assuming that ε-platelets form from stacking faults, the volume fraction can be expressed as an implicit function of strain. The saturation value is constant and can be evaluated from quantitative metallography. The approach to saturation is determined by only one temperature-dependent parameter related to the stacking fault energy. Good agreement with experimental results was obtained. The model was also applied to transformation kinetics after a prestrain inducing both slip and twinning. The prestrain stabilizes austenite with respect to the strain-induced transformation through a block-refining of austenite by the substructure. In addition the nucleation is enhanced through the introduction of stacking faults. This effect vanishes at high applied strains but causes the shape of the transformation curve to become parabolic. It is concluded that decreasing the size of the ε platelets provides a simple means for reducing the temperature dependence of the transformation kinetics.