Twin microstructure, line defects and twnning stress of magentic shape-memory alloys

The magneto-mechanical properties of magnetic shape-memory alloy single crystals depend strongly on the twin microstructure which is established during the martensitic transformation, and through thermo-magneto-mechanical training. For selfaccommodated martensite, twin thickness and magnetic-fieldinduced strain are very small. For effectively trained crystals, a single twin may comprise the entire sample and magnetic-fieldinduced strain reaches the theoretical limit. The strong effect of the twin microstructure on magneto-mechanical properties is thought to be due to the mutual interaction of twinning disconnections and their interaction with twin boundaries and martensite variant boundaries. The present study explores twin boundary motion in self-accommodated martensite on a mesoscopic length scale. The defect structure of selfaccommodated martensite contains disclinations which are located where twin boundaries of different hierarchical levels meet. The disclinations are arranged in self-screened walls. To maintain selfscreening, twin boundaries must move collectively. The energy content of various hierarchical twin structures and the deformation produced by collective twin boundary motion are analyzed. The critical stress for deformation by collective twin boundary motion is derived from comparing the strain energy density with the work done by the applied stress. The critical stress is one order of magnitude smaller than the stress required for dislocation pull-out which initiates motion of an individual twin boundary.