KINETICS OF MARTENSITIC-TRANSFORMATION INDUCED BY A TENSILE-STRESS PULSE

The kinetics of martensitic transformation induced by a tensile stress pulse (generated by the reflection of a compressive shock wave at a free surface) in time durations in the microsecond range were studied in an Fe-32wt%Ni-0. 035wt%C alloy. The tensile hydrostatic component of stress interacts with the dilatational strain of the martensitic transformation and increases the M//s temperature. Impact experiments were conducted by varying either the temperature ( minus 10 to minus 50 degree C), or pulse duration (0. 22-1. 76 mu s) at a constant pressure. The martensitic transformation exhibits an isothermal nature in the microsecond regime. The activation energies for gamma yields alpha prime transformation range from 38,000 J/mol at minus 10 degree C to 25,000 J/mol at minus 60 degree C. The activation volume for the transformation is approximately 60 atoms (0. 7 nm**3). This indicates that the martensitic nucleation is interface-mobility controlled.