Micron-sized Ni-Mn-Ga wires for micro-sensors and actuators

Recent advancement in technology in micro-sensors and actuators applications requires Ni-Mn-Ga ferromagnetic shape memory alloys with distinctively good response frequency and high magnetic-field-induced strain (MFIS). To this note, fabrication of single-crystalline Ni-Mn-Ga alloys with better martensite twin boundary mobility has become an area of interest amongst researchers despite the difficulties involved such as high cost, low growth rate, and severe segregation. Although fabrication of polycrystalline Ni-Mn-Ga alloys appears simpler as compared to fabrication of single crystals, it is associated with small MFIS due to incompatibilities of the neighbouring grains and the resulting internal geometrical constraints during twining. Previously published work has emphasized on the various methods of drawing alloys and metals into wires. For instance, melt-extraction is an efficient and cost-effective method for large-scale production of brittle alloys with refined microstructure and reduced elemental segregation. Unfortunately, the microstructural evolution of Ni-Mn-Ga microwires is yet to be fully explored for enhanced mechanical and physical properties.

Mingfang Qian, Xuexi Zhang and colleagues at Harbin Institute of Technology in China investigated the microstructural evolution of Ni-Mn-Ga microwires during the melt-extraction process. This included the product morphology, nucleation, grain growth orientation and texture. Their work is published in Journal of Alloys and Compounds.

The research team commenced their experimental work by fabricating polycrystalline Ni-Mn-Ga microwires of length 3-20cm and diameter 35-80µm using the melt-extraction method. They then presented a microstructural description of melt-extracted Ni-Mn-Ga microwires and utilized the electron backscatter diffraction to determine the process of the grain evolution.

The authors observed a circular cross-section with a flattened side in the resulting melt-extracted microwires. The arc-chord length ration in the microwires ranged between 1.8-3.6. Consequently, nucleation was observed on the flattened side in contact with the copper wheel while grain growth occurred towards the semi-circular surface. Furthermore, a negligible compositional change was noted during the melt-extraction process as compared to the composition before the process.

The study has elucidated much on the microstructure evolution of Ni-Mn-Ga microwires during the melt-extraction process. The nucleation of the molten alloy varied with the increase in the wheel tip temperature, which changed from singular to dual nucleation sites as the melt-extraction process continued. That is to say, low temperatures at the beginning of the melt-extraction process resulted in the nucleation of the crystallized molten alloy at the wheel tip while high temperature resulted in nucleation of the molten alloy at the two sides of the tip associated with a reduction in the arc-chord length ratio.

Also, desirable conditions, that is, wheel velocity, feed rate and power of 23m/s, 60-90µm/s and 20kW respectively were necessary to produce continuous microwires with D-shape cross-section and uniform diameters. On the other hand, molten alloy exhibited unique grain growth during solidification phase that resulted in columnar grains in the radial direction of the microwires with the elongated direction parallel to the <001> crystal direction. Especially, this texture evolved into <001> crystal direction perpendicular to the flattened part of the microwire during the last stage of crystallization. According to the authors, the findings presented in the study like the resulting grain texture and structure is a promising solution for fabricating Ni-Mn-Ga microwires with desirable MFIS that will therefore improve their mechanical and physical properties.

This research team has been working on the synthesis, microstructure and the multi-functional properties of the micron-sized Ni-Mn-X (X=Ga, Sn, In, Sb…) wires using the melt-extraction method for nearly one decade. In the recent five years, 25 journal papers and 1 book chapter regarding the mechanical property, magnetocaloric/elastocaloric effect, shape memory effect, superelasticity, damping capacity of the micron-sized Ni-Mn-X wires have been published.