Plastic deformation of B19’ monoclinic martensite in NiTi shape memory alloys: HRTEM analysis of interfaces in martensite variant microstructures

Martensite variant microstructures in grains of nanocrystalline NiTi shape memory wire deformed in tensile test at room temperature up to 15% strain were analyzed in transmission electron microscope (TEM) with the aim to reveal the mechanism of plastic deformation of B19’ monoclinic martensite which created these microstructures. The analysed microstructures consisted of parallel and/or wedge arranged deformation bands within the martensite matrix oriented in a common <010> low index zone. High resolution TEM (HRTEM) analysis of 35 interfaces within a single grain was performed. The observed interfaces were reasonably planar on mesoscale of individual grains but highly irregular on atomic scale. Majority of analysed interfaces displayed interface planes and lattice misorientations similar to (100), (20-1) and (10-1) martensite twins, but not exactly, many interfaces deviated from the exact twin planes and misorientations. Some interfaces displayed crystal lattices only rotated within the <010> low index zone, i.e. lacking the mirror symmetry of adjoining lattices characteristic for twins. On the other hand, traces of (001) crystal planes were mirror symmetric across all observed interfaces. These experimental observations were rationalized by assuming that the analysed martensite variant microstructures were created by the plastic deformation of B19’ martensite by kwinking deformation proceeding via coordinated [100](001) dislocation slip based kinking combined with (100) twinning.