Critical Assessment : Martensite – Start Temperature for the γ → ε Transformation

It is increasingly important in the context of high–manganese steels of the kind that lead to twinning–induced plasticity to be able to estimate the temperature at which ε–martensite forms when austenite is cooled. We find that the thermodynamic method used in similar calculations for α martensite cannot in many cases be implemented because of apparently imprecise thermodynamic data, a conclusion partly validated using limited first–principles calculations. Alternative, empirical methods are also evaluated. The austenite grain size dependence of the martensite–start temperature has also been rationalised in terms of existing theory for α martensite. Experiments have also been conducted to show that the problem in dealing with the ε–martensite does not lie in the precision with which the transformation can be measured using dilatometry.