Effects of surface damage on twinning stress and the stability of twin microstructures of magnetic shape memory alloys

Twinning is the primary deformation mechanism in magnetic shape memory alloys (MSMAs). Obstacles such as inclusions, precipitates and defects hinder or even prevent twin boundary motion in the bulk of Ni–Mn–Ga MSMA single crystals. Here, we study the effect of surface damage on the mechanical properties and twin structure of Ni–Mn–Ga single crystals. Any methods that produce defects may be considered for modifying the near-surface microstructure. In this study deformations were produced by grinding and mechanical polishing using abrasive particles. The amount of damage was characterized with X-ray diffraction: damage causes peak broadening. Deformation and damage localized near the surface increases the twinning stress. Surface damage stabilizes a densely twinned microstructure. The twins are thin but extend over the entire sample and allow a large strain to be accommodated at moderate stress. This effect is critical for preventing damage accumulation in high-cycle magnetomechanical actuation and for achieving high dynamic performance.

Research highlights
► Electropolishing reduces residual stresses in surfaces of magnetic shape-memory alloys and reduces the twinning stress.
► Mechanical polishing and grinding produces a layer of defects on surfaces and increases the twinning stress of magnetic shape-memory alloys.
► Defects localized near the surface (surface damage) increase the twinning stress and smoothen the stress–strain curve.
► Defects localized near the surface create a stable and dense twin microstructure with mobile twins which extend throughout the entire crystal.
► A stable dense twin microstructure provides the basis for fatigue-resistant magnetic shape-memory alloys.