Characterization of High-temperature Polishing Techniques for Magnetic Shape-memory Alloy Ni2MnGa

Magnetic shape-memory alloys (MSMA) such as Ni2MnGa exhibit a magnetic field-induced, reversible strain through the motion of twin boundaries. Twin boundaries arise from the diffusionless transformation from the cubic austenite to the tetragonal martensite phase. Twin boundary formation changes X-ray diffraction patterns, defines the stress-strain behavior, and leads to surface reliefs which can be characterized with optical microscopy, electron microscopy, and atomic force microscopy. The accurate characterization of the surface relief requires a high quality surface finish with minimal surface roughness. Samples that are in the martensite phase at room temperature show surface topography that is indicative of the twin boundary size, angle, and position. Polishing at room temperature can remove this topography, yielding a smooth surface which is not indicative of the highly twinned microstructure of the bulk. After room temperature polishing, thermal or mechanical cycling can result in historic twins whose angles and position do not necessarily coincide with the bulk microstructure. We polished Ni2MnGa at elevated temperature in the cubic austenite phase. Upon cooling to martensite, the resulting surface relief more accurately represented the twinning structure in the bulk sample. We present the micrographic evidence supporting our theory that high-temperature polishing is necessary for surface characterization of twinned MSM alloys