Neutron study of the martensitic transformation in Ni-Fe-Ga alloys.

We present results of an extensive neutron diffraction study of the martensitic transformation in Shape Memory Alloys of nominal composition Ni_{55-x}Fe_{19+x}Ga_{26} (x=0, 1, 2) at. % transforming close to room temperature. The x=0 sample shows a martensitic transformation temperature T_{M}≈ 250 K and Curie temperature T_{C}≈ 289 K. Neutron measurements for this alloy were performed between 300 K (austenite phase) and 150 K (martensitic phase). At temperatures T>T_{C} we have found the coexistence of two phases: an L2_{1} phase with space group Fm3m and cell parameter a=5.730Å and a γ-fcc with cubic symmetry and with cell parameter a=3.586Å This second phase was also detected by magnetic measurements, giving rise to a second Curie temperature of about 363 K. [ABSTRACT FROM AUTHOR]