Effect of grain boundary character on the martensitic transformation in Fe-32at.%Ni bicrystals

The effect of grain boundary character on the martensitic transformation wa s examined in two types of Fe-32at.%Ni bicrystals containing a 90 degrees < 211 > tilt or a 90 degrees {211} twist boundary focusing on the martensite -start temperature (Ms) during cooling, the morphology of the martensite an d the variant selection. The Ms of bicrystals with a tilt boundary was high er than that of single crystals, while bicrystals with a twist boundary sho wed no significant difference from that of single crystals. Coarse lenticul ar martensites formed symmetrically in neighbouring grains around the tilt boundary. In contrast, tiny martensites were uniformly distributed in bicry stals with a twist boundary, and in single crystals. In the vicinity of the tilt and twist boundaries, some variants with the habit plane almost paral lel to the boundaries were preferentially selected among 24 variants. Moreo ver, since the equivalent variants in neighbouring grains at the tilt bound ary are selected, strain compatibility of the shape strain of martensite ac ross the tilt boundary is satisfied, resulting in an increased Ms. The effe ct of pre-strain on the heterogeneous nucleation of martensite at the bound aries was also investigated. (C) 2001 Acta Materialia Inc. Published by Els evier Science Ltd. All rights reserved.