Effect of ageing on the martensitic transformation of Ni–Fe–Ga alloys

Three polycrystalline Ni53.5+x–Fe19.5−x–Ga27.0 alloys, with x = 0, 0.5, 1.5, have been aged at 520, 670 and 770 K. All the alloys show good long-term thermal stability after treatments at 520 K, whereas ageing at 670 and 770 K causes degradation of the martensitic transformation after ∼105–106 s, which is more intense in the alloy with lower Fe content. Transmission electron microscopy reveals the appearance of small and mostly elongated precipitates after long ageing periods at 670 and 770 K, associated with a drop of temperatures and heat of transformation measured by differential scanning calorimetry.