Formation of ε Phase in Fe-Mn Alloys Under High Pressure and Its Stability Under Tensile Stress

Abstract

The A_S and M_S temperature in the martensitic transformation of Fe-Mn alloys under hydrostatic pressures up to 35.5 kbar were measured by the differential thermal analysis. The progresses of γ→ε and ε→α transformation as influenced by the pressure and temperature changes and the effect of the tensile stress on them were discussed from a thermodynamical point of view. The main results obtained are as follows:
1) The A_s and M_s temperatures in the γ→←α transformation of alloys containing 4.8% or 6.76% of Mn, by increasing the pressure, were lowered at an approximate rate of 40°C/10kbar, while those in the γ→←ε transformation of alloys containing 6.76 to 28.24% of Mn were raised at an approximate rate of 40°C/10 kbar
2) The A_S and M_S temperatures calculated on the basis of the free energy changes of the phases were in a good agreement with those observed.
3) The γ→ε transformation progressed rapidly in the vicinity of the M_S^γ→ε temperature and then gradually with further decreasing the temperature. temperatures calculated on the basis of the free energy changes of the phases were in a good agreement with those observed.
4) In order to stabilize at the ambient pressure and temperature, the β phase which had been formed by pressurizing, it was required that the M_S^ε→α and the A_S^ε→γ temperatures of the alloy were lower and higher, respectively than the room temperature.
5) The increase in the tensile strength was obtained only for the specimens in which the ε phase had been formed by pressurizing. The reason for the increase in the strength was explained on the basis of the thermodynamics for the γ→ε transformation under tensile stress.