On the Critical Driving Force for Deformation-Induced α′-Martensite Formation in Austenitic Cr-Mn-Ni Steels

The influence of martensite formation on the mechanical properties of an Fe-16Cr-6Ni-6Mn (concentrations in wt%) austenitic stainless steel is studied by tensile tests between -70 and 300 degrees C. As-quenched martensite formation is observed at temperatures below -30 degrees C. Deformation-induced alpha ' martensite formation, on the other hand, is triggered below 100 degrees C. The temperature dependence of proof stress, tensile strength, and triggering stress for martensite formation are represented on a Stress-Temperature-Transformation (STT) diagram. In order to determine the critical Gibbs free energy for the formation of martensite at temperatures between Ms and Md, the chemical and mechanical contributions to deformation-induced martensite formation are determined. The chemical term is obtained from thermodynamic calculations. The mechanical term, on the other hand, is obtained by determining the mechanical energy supplied to tensile specimens to trigger martensite formation. This is done using the model proposed by Patel and Cohen. The magnitudes of shear strain (gamma 0=0.23) and dilatational strain (epsilon 0=0.023), required for the calculations, are obtained based on the martensite crystallography theory of Wechsler-Lieberman-Read. The sum of the chemical and mechanical contributions yields the critical driving force for the martensitic transformation. The results indicate a slight increase in the critical driving force for martensitic transformation at lower temperatures.