Structural, Magnetic and Mechanical Properties of Oligocrystalline Ni-Mn-Ga Shape Memory Microwires

In single-crystalline Ni-Mn-Ga alloys, high reversible strain (~10%) induced by an external magnetic field due to twin variants reorientation can be achieved. However, twin boundary motion is hindered by grain boundaries in polycrystalline Ni-Mn-Ga alloys, resulting in a low strain output. In this work, oligocrystalline Ni-Mn-Ga microwires with near bamboo-like grain structure (1-3 grains across the wire diameter of 30-80. μm) were prepared by melt-extraction technique and subsequent annealing treatment. Single twin variant, with twin lamella thickness up to 6. μm, spans across the entire diameter of the microwire. The oligocrystalline microwires are of ferromagnetic nature at ambient temperature. Twining boundary motion was demonstrated by tensile test performed on a Q800 DMA where different twining stresses corresponding to various grain orientations in a single microwire were detected. A maximum strain of ~1.57% was obtained and related to the orientation of a subset grains. The martensite twin variant orientations after tensile test were observed using a SEM.