Ni-Fe-Ga-Co

In Ni-Fe-Ga-Co quaternary system, the martensitic transformation temperature T_m and the Curie temperature T_c can be tailored in a wide range by changing composition and heat treatment, T_m ~ 170 to 370 K and T_c ~ 280 to 400 K. We focus on an alloy composition Ni₅₂Fe₁₇Ga₂₇Co₄, which is promising for practical application due to its relatively high martensitic and magnetic transformation temperatures (T_m = 363 K, T_c = 360 K). The bulk sample has a single martensite phase at room temperature, exhibiting a self-accommodation morphology (figure A, left).

The different transformation characteristics caused by different treatments are attributed to the change of atomic order (figure A, right). A lower cooling rate gives rise to a higher degree of order, leading to a reduced transformation temperature and vice versa. In other words, the transformation temperature can be controlled easily by varying the cooling rate.

Aging at 500 ºC leads to the presence of large γ particles distributed along the grain boundaries, where also some small needle-like γ precipitates exist (figure B, left). Small needle-like γ particles are dispersed inside the austenite grains. The very fine martensitic plates (M) with a width of about 1 µm can be seen. The thin area surrounding the large γ phase particles remains in the austenite phase, i.e. the martensitic plates do not reach the large γ phase particles. There exists a large difference in the Fe content for each area. The γ phase is a Fe-rich and Ga-poor phase, which depletes the surrounding matrix.

The martensitic transformation present in the bulk sample totally disappears after powderisation. From room-temperature X-ray diffraction (figure B, right), it is found that the tetragonal non-modulated martensite in the bulk sample is completely transformed to a disordered fcc phase by powderisation, but gradually changes into a highly ordered L2₁ austenite phase during the subsequent annealing.