Research On Fabrication And Stress-Induced Strain Properties Of Ni-Mn-Ga Ferromagnatic Shape-Memory Microwires

In this paper continuous Ni-Mn-Ga magnetic-shape-memory wires with minimal elemental loss were synthesized by melt-spinning, and the preparation parameters were optimized. The effect of heat treatment on the microstructure, composition, phase transformation and magnetic properties are investigated for as-fabricated and heat-treated wires. The stress-induced strains of the wires in the austenitic and martensitic states for as-fabricated and heat-treated wires were studied. The effect of heat-treatment and training on the stress-induced strain were discussed.Continuous and composition intact Ni-Mn-Ga wires are made by melt-spinning with various parameters such as heating watt, copper wheel rotation speed and material feed speed. The morphology and microstructure of the wires were observed by Optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM), and the composition, phase, transformation temperature and magnetic properties by vibrating sample measurement (VSM), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and energy dispersive spectrum (EDS). Based on the abovementioned factors, an optimized synthesis parameter of heating watt 14-23kW, copper wheel rotation speed 1400rpm and material feed speed 40~120μm/s was obtained.The as-fabricated wires were subjected to various heat treatment and the resulting microstructure, composition, phase transformation and magnetic properties of the resulting wires were investigated by various measures such as OM,SEM,TEM,EDS,VSM,DSC and XRD. After a chemical ordering heat-treatment of 725℃2h, 700℃10h, 500℃20h, the wires exhibited increasing martensitic phase transformation temperature and magnetic properties. While for grain growth goal, a grain growth heat-treatment of 800℃1h is better compared to the chemical ordering heat-treatment.The stress-strain curves of the as-fabricated and heat-treated wires were carried out in a dynamic mechanical analyzer (DMA), found that the shape memory effect and superelasticity were both existed. The results show no twin boundary motion for the as-fabricated wire due to their fine grains and inhomogeneous diameter and the resulting difficulties in twin boundary motion. In contrast, twin boundary motion appeared in the chemical ordering heat treated wires due to increasing chemical ordering, coarse grains and the resulting increasing mobility of twin boundaries. After superelastic training, a higher threshold strain for shape memory strain recovery was obtained. For the Superelasticity of the wires, the heat-treated wires show better superelasticity than as-fabricated wires. The critical stress for stress-induced martensitic transformation decreases after two-way shape memory training. With increasing external stress, the martensitic phase transformation shifted to higher temperatures, and two-way shape memory strains increased at first, then kept intact after a critical stress was reached. The two-way shape memory strains significantly increased after superelastic training because of the reducing martensitic variants and thus better mobility of twin boundaries after training. SEM observation of the wire fracture surfaces show that the wires broke at smaller diameter locations. The cracks initiated at the boundaries of the granular grains then propagated along the boundary and finally broke at smaller diameter sites.