Effect of ageing in Ni–Fe–Ga ferromagnetic shape memory alloys

Three polycrystalline Ni53.5+x–Fe19.5−x–Ga27.0 alloys, with x = 0, 0.5, 1.5 (at.%), have been aged at 520 and 670 K. The initial increase of martensitic transformation temperatures upon ageing has been attributed to a decrease of L21 order. The alloys with lower Fe content show remarkable thermal stability for ageing times larger than 107 s at 520 K. In the case of ageing at 670 K, degradation of the alloys occurs after ∼106 s. Optical and transmission electron microscopy reveal the initial presence of γ′ particles mostly located at the grain boundaries, as well as the appearance of small (<1 μm) elongated precipitates after long ageing periods at 670 K, associated to a drop of transformation temperatures.