Detwinning of a non-modulated Ni-Mn-Ga martensite: From self-accommodated microstructure to single crystal

The microstructural changes observed during the early stages of mechanical training of a self-accommodated martensite of Ni-Mn-Ga single crystal with a non-modulated structure were studied in detail by the electron backscatter diffraction technique. Training was done by quasi-static compression tests performed parallel to the 〈0 0 1〉 directions. It has been shown that primary deformation takes place inside individual martensite variants being internally twinned at the nanoscale. As a consequence of this nanodetwinning process, a necessary rigid crystal lattice rotation within neighboring martensite variants occurs, bringing them into a twin relation. Thus, further deformation can take place by detwinning of martensite variants at the microscale. During the first compression of training, both processes (i.e. nanodetwinning and detwinning) from the macroscopic point of view are observed in a localized region which gradually propagates further throughout the sample. The second compression is more homogeneous and is totally dominated by detwinning between thick martensite plates, leading eventually to a single crystalline state of a non-modulated martensite.