J. Phys. IV France 05 (1995) C8-323-C8-328
Kinetics of Austenite to Martensite Transformations in Iron-Nickel-Carbone Alloys During their Cooling or Tensile TestingI. Kozelkova1, C. Dagbert1, P. Gregoire1, J. Galland1 and L. Hyspecka2
1 Ecole Centrale Paris, 92295 Châtenay-Malabry, France
2 Technical University of Minning, 70800 Ostrava, Czech Republic
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 Ms 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 εc 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 εu, ( εu=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, ΔQA→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.
© EDP Sciences 1995