Structure-property control of polycrystalline Ni-Mn-Ga by moderate Co-doping

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It is well-known that the Co-doping of ferromagnetic shape memory alloys (FSMAs) is a crucial tool to control their multifunctional properties. The present work investigates the use of small quantities of Co to fine-tune the transformation, structure, microstructure, mechanical and magnetic properties of the polycrystalline Ni49.8Mn28.5Ga21.7 (at%) alloy, as the representative of the most elaborated magnetostrain-active Ni-Mn-Ga FSMAs. A series of seven alloys, undoped and doped with 0.5, 1, 1.5, 2, 3, and 4 at% Co, was fabricated using a standard induction melting technique with subsequent heat treatment. Alongside establishing the evolution of the basic characteristics as a function of the Co content in this series, an unusual effect of Co-doping on the grain microstructure was found. A Co-doping up to 1.5 at% instead of Mn significantly increased the average grain size while importantly maintaining the 10 M martensitic crystal structure. At Co concentrations of 1–1.5 at%, a microstructure with an average grain size of about 2.00 mm was formed with a twin structure, enabling the experimental observation of magnetic-field-induced twin variant rearrangement. At higher levels of Co-doping, the grain size was essentially reduced, and the crystal structure of the martensitic phase became 2 M martensite. The decreasing grain size and changing crystal structure are attributed to the progress of γ-phase precipitates. Alongside the academic aspect, the results of the present work point to the commercial advantage of fabricating 10 M Co-doped Ni-Mn-Ga actuating elements made from large grains of polycrystalline ingots obtained by a standard melting facility instead of grown single crystals.