高Mn鉄合金多結晶におけるγ\ ightleftarrowsεマルテンサイト変態に及ぼす外力とひずみの影響

抄録

Austenite (γ: fcc)→epsilon (ε: hcp) martensitic transformation behaviour in Fe-16 wt%Mn, Fe-24 wt%Mn, and Fe-24 wt%Mn-6 wt%Si alloys has been studied to clarify the influences of plastic strain and applied stress.
Microstructures of as-quenched specimens in these alloys consist of γ and ε. The specimens were heated up to 673 K above the A_f temperature and then cooled to a test temperature at which the tension test was carried out. Positive temperature dependence was found under the 0.2% proof stress due to the stress-induced γ→ε transformation in each of the Fe-Mn alloys and the Fe-24Mn-6Si alloy. Extremely low yield strength and high work hardening were observed in the beginning of deformation due to the γ→ε martensitic transformation, when deformation was given at room temperature in the Fe-24Mn alloy in which the γ phase was stabilized by ten times that of cyclic transformations.
The change in the length of the specimens was measured during cooling from 673 K under a constant tensile stress. When the applied stress was larger than the yield strength at 673 K, a rapid elongation in the tensile direction was observed from a temperature between A_s and M_s. This is ascribed to a preferential transformation shear, i.e. the movement of Schockley partial dislocation caused by a linked effect of the plastic strain at 673 K and the applied stress. The change in the specimen length vs. temperature curve showed a slightly different trend between the Fe-Mn alloys and the Fe-24Mn-6Si alloy. Furthermore, a remarkable shape recovery due to the ε→γ reverse transformation was observed in the Fe-24Mn-6Si alloy but scarcely in the Fe-16Mn and Fe-24Mn alloys, when the elongated specimens were again heated above A_f under an applied stress of 2 MPa. The solid-solution hardening by Si appears to be responsible for the observed refinement of the shape memory effect.