Uncoupling the Effects of Strain Rate and Adiabatic Heating on Strain Induced Martensitic Phase Transformations in Steels

In this work, the effects of strain rate and adiabatic heating on the strain induced martensitic phase transformation were uncoupled and individually evaluated. Tension tests were performed at different strain rates ranging from 2x10-4 s-1 to 1400 s-1, covering both isothermal and adiabatic conditions. The adiabatic temperature rise of a sample tested at a high strain rate was replicated with heating resistors in a normally isothermal low strain rate test. This test allows studying the mechanical behavior and microstructural evolution of the material at a very low strain rate at thermal conditions similar to that of a high strain rate test. The phase transformation rates from austenite to α'-martensite were measured with the magnetic balance method. The phase transformation rate drops significantly with increasing strain rate, and the effect of adiabatic heating seems to be much smaller than the effect of strain rate. At a higher strain rate, the α'-martensite nucleates primarily on a single habit plane parallel to the primary slip plane of the parent austenite, while at a lower strain rate the α'-martensite nucleation occurs on several habit planes. At the studied plastic strains, the strain rate seems to have a stronger effect on the α'-martensite formation than the adiabatic heating. This is supported by thermodynamic stacking fault calculations, which indicate that the increase in the stacking fault energy due to adiabatic heating at low strains is small and therefore unlikely the only reason for the reduced phase transformation rate. Therefore, the strain rate itself seems to have an important role in the strain induced martensitic phase transformation rate.