Effects of element specific Co substitution in Ni-Mn-Ga-Fe and accelerated grain growth during heat treatment

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The substitution of small amounts of Co and Fe into Ni-Mn-Ga-based ferromagnetic shape memory alloys (FSMAs) is widely recognized for its ability to finely tune their multifunctional properties. This study investigated the effects of the substitution of each of the parent elements, in amounts of Co = 1, 3, and 5 (at.%), on the crystalline structure transformation temperature, microstructure, and magnetic properties of the nominal alloy Ni50Mn25Ga20Fe5. An induction melting technique was used to fabricate a series of 10 alloys with and without Co-substitution. In addition to determining the evolution of its magnetic and structural characteristics, Co-doping has been found to have an unexpected and beneficial effect on the grain microstructure. The findings of the study revealed that the Ni49Co1Mn25Ga20Fe5 alloy exhibited significant grain growth while demonstrating the mixed 10 M + 14 M martensitic crystal structure at ambient temperature. Separating an oriented grain of 1 × 2 mm enabled the experimental demonstration of magnetic field-induced structural rearrangement through twin-domain motion. Varying the levels of Co-substitution and the specific element being substituted allows tuning between the stabilisation of cubic austenite, modulated martensites (10 M or 14 M), or a non-modulated martensite at ambient temperature. Using standard melting facilities to fabricate large grains of polycrystalline ingots with 10 M + 14 M Co-doped Ni-Mn-Ga-Fe actuating elements offers a practical advantage over growing single crystals.