Velocity of the austenite-martensite interface in reverse martensitic transformation in FeNiC alloys

The austenite-martensite interface movement on heating above room temperature in FeNiC alloys with 0.1–0.8 wt.%C was observed in situ by light microscopy, and the velocities of reverse-moving interface motion were measured quantitatively at various heating rates. The interface motions during heating under various loads were also measured. The effect of a high density of pre-existing dislocations on the interface velocity was examined, using ausformed specimens. For an Fe-31Ni-0.3C alloy, it was found that the velocity, when normalized to a heating rate of 1C °/s, is scattered in the range of 10–230nm/s. Normalized velocities measured for other alloys containing various amounts of carbon were found to exhibit a similar range of scatter. Normalized velocities in these alloys are two orders of magnitude smaller than interface velocities measured for a Cu-Zn-Al shape memory alloy. Contrary to popular belief, carbide precipitates, formed in martensite during heating, do not behave as major obstacles to the interface movement. It is also a surprising finding that the interface velocity is not affected by applying a constant load, which should resist the reverse motion of the interface. The interface velocity is hardly decreased in an ausformed specimen.