Effects of high magnetic field on martensitic transformation and its reverse transformation in Fe-based alloys
Effects of high magnetic field on martensitic transformation and its reverse transformation from lath martensite to austenite have been investigated in Fe-based alloys. An Fe-0.4C alloy was solution treated at 900°C for 15min and water-quenched without magnetic field, which produces the lath martensite single phase structure. Then this specimen was reheated to 750°C (ferrite and austenite two phase region) with or without magnetic field and held for 20min and then rapidly cooled to room temperature by He gas. In this heat treatment, austenite is formed by the reverse transformation and then transformed to lath martensite again during the cooling to room temperature, making the final structure of ferrite and lath martensite. Without magnetic field, equiaxed ferrite grains are formed and both ferrite and lath martensite are distributed homogeneously. With magnetic field, lath martensite, that is, austenite grains in the two phase region, and ferrite grains are aligned along the direction of magnetic field. On the contrary, neither martensite nor ferrite phases are aligned along the direction of magnetic field when the reheating temperature is rather high such as 770°C or the prior austenite grain size is rather large. In both specimens, austenite is not necessarily aligned along the direction of magnetic field, and rather formed along the packet or block boundaries. Effects of high magnetic field on austenite to lath martensite transformation behavior was also studied, but no structural alignment along the magnetic field was observed.