Martensitic transformation and detwinning in directionally solidified two-phase Ni-Mn-Ga alloys under uniaxial compression

Abstract

A kind of Ni-Mn-Ga alloy consisting of orderly arrayed austenite and non-modulated (NM) martensite was designed and directionally solidified. The influences of step-wise uniaxial compression on martensitic transformation and detwinning processes of the above-mentioned sample were investigated by the electron backscatter diffraction (EBSD) tracing. Experimental results show that the stress has induced martensitic transformation at ambient temperature, which is completed after the three cycles (2%, 4% and 6% strain, respectively). This should be attributed to the increase of martensitic transformation temperature under compression. Moreover, the stress has caused detwinning of the twins with high Schmid factor, resulting in martensite only composed of favorable variants with 〈110〉M orientation parallel to the compression axis. The kernel average misorientation (KAM) maps reveal that the dislocation density decreases during stress-induced processes.