Controlling factor for nucleation of martensite at grain boundary in Fe-Ni bicrystals

A favourable nucleation of martensites at a grain boundary was investigated using Fe-Ni bicrystals containing a <211> symmetric tilt boundary with various tilt angles, focusing on the martensite-start temperature (Ms), the morphology of martensites and the variant selection. Near the grain boundaries, some variants with the habit plane almost parallel to the boundaries were preferentially selected and their variants changed depending on the tilt angle. In addition, the equivalent variants were symmetrically adjoined at the boundary to maintain the compatibility of shape strain of martensites across the boundary. Such characteristic nucleation can be regarded as a self-accommodation across the boundary, which is called cooperative nucleation (CN). The CN effectively reduces the strain energy due to the formation of martensites compared with the independent nucleation within a grain resulting in an increase in Ms. The characteristic variant selection of martensites can also be explained from the minimisation of strain energy.