Spontaneous Martensite Detwinning in Ni-Mn-Ga Single Crystal

Abstract

It is well known that the transformation between cubic Austenite phase and tetragonal Martensite phase of Ni-Mn-Ga single crystal Shape Memory Alloy can be controlled by thermo-magneto-mechanical fields. Particularly, the directional driving forces (magnetic fields and mechanical forces) can also trigger Martensite detwinning or Martensite variant reorientation due to the anisotropy of the tetragonal Martensite variant. By contrast, the temperature (a scalar quantity) was not expected to trigger Martensite detwinning. However, our experiments show that the cooling-induced Austenite → Martensite transformation is via two steps: Austenite → twin (consisting of two different Martensite variants), and twin → single-Martensite-variant via a detwinning process. The result implies that the material can switch between the Austenite phase and a single Martensite variant with a large cyclic macroscopic deformation in the simple heating-cooling cycle (a pure thermal loading without external stress or magnetic field), so-called stress-free “Two-Way Memory”.