Kinetics of Austenite to Martensite Transformations in Iron-Nickel-Carbone Alloys During their Cooling or Tensile Testing

Abstract
Two austenitic Fe-Ni-C alloys, one having 23.9 wt.% Ni, 0.39 wt.% C, and a martensit start point of 228 K, the other with 21.8 wt.% Ni, 0.48 wt.% C, and an M_s of 233 K, were tested to determine the kinetics of their austenite to martensite transformation both during cooling, at a rate of 5 K.min^-1, and during tensile testing at strain rates from 8.3x10^-5 to 8.3x10^-3 s^-1 and at temperatures from 233 to 323 K. Both alloys displayed variations in the critical plastic strain ε_c for the onset of serrated yielding. Metallography confirmed that serrated yielding can occur in austenite before any strain-induced martensite is formed. Although strain-induced martensite formed at all of the examined strain rates and temperatures, inflections on the true stress versus true strain, or σ- ε diagrams, were observed only at low test temperatures. The effect of transformation-induced plasticity, or TRIP, assessed by the maxima of uniform plastic strain ε_u, ( ε_u=0.8), was detected only in tests run at 253, 273 and 293 K. Kinetic diagrams for the formation of strain-induced martensite, in dependence on ε , were compiled for all the test temperatures and strain rates, and were compared with kinetic diagrams of heat release gained by differential scanning calorimetry for the austenite to martensite transformation. Diagrams of the amounts of heat released, ΔQ^A→M, in the martensite formation process, against the plastic strain magnitude in the presence of similar proportions of martensite, were found to yield linear relationship.