Hardness and Aging of Ni2MnGa Ferromagnetic Shape Memory Alloys

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Abstract
The effect of heat treatment and aging on hardness was investigated using near-stoichiometric and off-stoichiometric Ni2MnGa ferromagnetic shape memory alloys. The alloys fabricated by Ar arc-melting method were homogenized at 1273 K, and systematic heat treatments from 773 to 1173 K. Moreover, in order to clarify the aging behavior, some of the alloys were homogenized at 1373 K followed by aging at temperatures of room temperature (RT), 373 K and 473 K. The effects of heat treatment and aging were evaluated through micro-Vickers hardness measurements. It was revealed that hardness increases with increasing heat treatment temperature. It was also found that the hardness values are also higher with deviating from the stoichiometric composition, and that the hardness is higher at the Mn-rich side than at the Mn-poor side of stoichiometry. Moreover, aging behavior was observed for both stoichiometric and off-stoichiometric alloys even at RT. The low temperature aging must be caused by excess vacancies introduced during heat treatment. It is concluded that hardness of Ni2MnGa is sensitive to the chemical composition, heat treatment and aging conditions.