Martensitic Transformation And Magnetocaloric Property Of Fine-grained Ni-Mn-Ga Alloys

In this paper, various powders with different particle size distribution were prepared by controlling the ball mill parameters, followed by a heat treatment. Through X-ray diffraction, differential scanning calorimeter, scanning electron microscopy and superconducting quantum interferometer, we characterized and uncovered the microstructure and martenstic transformation and magnetocaloric effect of these powders with different particle size distribution.It has been discovered that, during ball mill, the particle size of Ni-Mn-Ga powders did not declines continually with the increase of ball mill time. Under the uniform condition of 500 round per minute and the ratio of ball between powder 5:1 and the same ball mill medium acetone, for the bulk sample with 3mm particle size prepared for ball mill, the particle size falls into a magnitude of 100μm after a 60 min ball mill process. The powder size decreses gradually with the prolong of ball mill time. With a further prolong of ball mill time to 24 h, the size of the fine-gained powders decreased to a 20μm magnititude. After that, when the ball mill time was extended to 72 h, particle size becomes 200 nm. Any further prolong of ball mill time incurred no manifest fluctuation to the powder size distribution, for which just showed a stable size.DSC and XRD tests showed, at room temperature, bulk sample occupied one-step transformation due to the Ni2 Mn Ga was Austenitic L21 structure, while Ni47.4Mn32.1Ga20.5 the non-modulated body-centered tetragonal structure. Due to strong internal stress induced by ball mill process, the crystal structure distorts strongly, which resulted in no manifest peak occurred in thermal analysis of the ball milled powders, only several samples embodied faint phase transformation latent heat peaks in DSC curves. After a 500℃/5h heat treatment was exerted on the five powder samples ball milled from the bulk Ni47.4Mn32.1Ga20.5, we discovered that the Ms drifts to lower temperature scope with a reduced particle size, which can be attributed to suppression of nucleation and grown-up with a large percentage of grain boundary induced by grain refinement, consequently, driving the Ms to lower temperature range. XRD analysis of before and after annealing samples manifested that the internal stress influenced the transformation path and t he annealed Ni47.4Mn32.1Ga20.5 powder showed 7M structure in room temperature, which denotes that the 7M variant is thermodynamically more stable than the tetragonal variant at room temperature.The magnetic characterization revealed that the curie point of Ni47.4Mn32.1Ga20.5 powders stayed stable with the fluctuation of particle size, and did not sensitive to the composition of Ni-Mn-Ga alloys. Curie piont is higher than Ms and martensite stayed at a ferromagenetic state in Ni47.4Mn32.1Ga20.5. Maximum magnetic entropy change was got in bulk Ni47.4Mn32.1Ga20.5 in a 5T magnetic field. With the decrese in Ni47.4Mn32.1Ga20.5 powder particle size, magnetic entropy change undergoes a declination, the maxium magnetic entropy change of all powder samples occurred at the martenstic point.