Characterization of the TRIP/TWIP effect in austenitic stainless steels using Stress-Temperature-Transformation (STT) and Deformation-Temperature-Transformation (DTT) Diagrams

Stress-Temperature-Transformation (STT) and Deformation-Temperature-Transformation (DTT) diagrams are suitable to characterize the TRIP (transformation-induced plasticity) and TWIP (twinning-induced plasticity) effect in steels. The triggering stresses for the deformation-induced microstructure transformation processes, the characteristic temperatures, the yield stress and the strength of the steel are plotted in the STT diagram as functions of temperature. The elongation values of the austenite, the strain-induced twins and martensite formations are shown on the DTT diagram. The systematics of STT and DTT diagrams and the method for their development are described. Especially, the correlations between the STT and DTT diagrams and the thermodynamics are explained in the present paper. The developed STT and DTT diagrams for a novel austenitic Cr-Mn-Ni (16%Cr, 6% Mn, 6% Ni) as-cast steel and austenitic steel AISI 304 are presented. The Cr-Mn-Ni steel shows a deformation-induced α′-martensite and twin formation. In contrast, the AISI 304 shows a deformation-induced e- and α′-martensite formation. The differences between both steel grades are based on thermodynamic pre-conditions. Therefore the thermodynamic stability conditions of the phases and the kinetics of stress and deformation-induced martensite and the twins formation are reflected in the developed STT and DTT diagrams.