Low-temperature behaviour of Ni–Fe–Ga shape-memory alloys

Three polycrystalline alloys with compositions Ni53+xFe20−xGa27 (x = 0.5, 1, 2 at %) have been studied, paying particular attention to the low-temperature behaviour, between the martensite finish and liquid nitrogen temperature. In this respect, the dynamic mechanical analysis has proven to be a powerful tool in detecting structural and microstructural changes. The studied alloys display, on cooling in the above-mentioned temperature domain, a broad internal friction peak accompanied by a marked increase in the elastic modulus slope; on further cooling an additional modulus drop and concurrent internal friction jump take place. The internal friction and elastic modulus evolution is not strictly reversible on heating. Basing in the earlier results obtained for Ni–Mn–Ga alloys, the internal friction peak is attributed to the oscillating movement of twin boundaries in the layered martensites, while the lowest temperature anomaly suggests the occurrence of slight changes in the mixture of modulated martensite structures formed on cooling.