Unconventional twin deformation of Ni-Mn-Ga 7M martensite under tension mediated by the collective lattice reorientation from a-c twin to b-c twin

Abstract

Deformation of martensite twins is a prerequisite to obtain the shape memory effect in shape memory alloys, which is conventionally conceived to be realized by twinning/detwinning based on the simple shear. Here, we report the unconventional twin deformation of seven-layered modulated (7M) martensite under tension in a directionally solidified Ni50Mn30Ga20 alloy. Based on the neutron diffraction and EBSD measurements, it is evidenced that the deformation of 7M martensite in the variant colonies with unfavorable Schmid factors is achieved by the collective lattice reorientation from four twin-related variants (a-c twin) to another type of four twin-related variants (b-c twin), rather than the detwinning of existing variants. Such deformation is not caused by the simple shear, but originated from the local atomic rearrangements that convert the initial lattice to the reoriented lattice. Moreover, the collective conversion from the initial self-accommodated variants to the reoriented variants follows a specific orientation relationship, where the deformation of individual variant yields the consistent dilatational strain along the loading direction and effectively accommodates the macroscopic strain. The present study provides clear evidence on the unconventional twin deformation mediated by the collective lattice reorientation, which is expected to deepen the understanding of martensite deformation behaviors in shape memory alloys.