Thermal stability and ordering effects in Ni-Fe-Ga ferromagnetic shape memory alloys

The martensitic transformation (MT) from L21 (Heusler) structure to layered martensites in three polycrystalline Ni53.5+x-Fe19.5−x-Ga27.0 alloys, with x = 0, 0.5, and 1.5 at.%, has been monitored by differential scanning calorimetry and transmission electron microscopy in order to follow the evolution under ageing in parent phase as well as the effect of atomic order on the MT temperatures. Ageing at 670 and 770 K produces degradation of the MT after 105-106 s, concomitant with the appearance of small (<1 μm), elongated γ particles. Water quenching from different temperatures (470-1070 K) causes an increase in MT temperatures compared to slow cooled samples. This effect is related to a decrease of the L21 order of the parent phase upon quenching, promoting an increase in the MT temperatures. The effect is maximum after quenching from 970 K; a temperature just above the B2-L21 order transition. As for the magnetic properties, order leads to larger saturation magnetization and higher Curie temperatures.