Reverse Transformation in Fe-Ni Bicrystals

Abstract

The effect of grain boundary character on the reverse phase transformation was investigated using Fe-Ni bicrystals containing a 90° ‹211› symmetric tilt or a 90° {211} twist boundary, focusing on the transformation temperature during heating, the morphology of reversed austenite and the variant selection. Martensitic transformation behaviour in bicrystals depends strongly on the type of grain boundary. The tilt bicrystals showed a characteristic self-accommodation of martensites across the boundary called cooperative nucleation (C-N) and higher martensite-start temperature (Ms). On the other hand, the reverse transformation-start temperature (As) of bicrystals with the tilt boundary was higher than that of single crystals or bicrystals with the twist boundary. By in-situ observation, a retardation of reverse transformation of cooperatively nucleated martensites was confirmed in the vicinity of the tilt boundary on heating. The C-N reduces a local stress field around martensites which helps the reverse transformation. Therefore, the bicrystals with the tilt boundary containing the C-N show higher than the others. Many martensites near the tilt and the twist grain boundary returned to the austenite phase inheriting the initial orientation. However, numerous sub-boundaries were retained near the tilt boundary. The C-N seems responsible for the lack of information about initial orientation of austenite. Thus, the reverse transformation behaviour in bicrystals was also found to depend strongly on the grain boundary character.