Microstructure And Properties Of Ni-Fe-Ga-Co Ferromagnetic Shape Memory Alloys

The microstructure, thermally-induced martensitic transformation, magnetic properties and mechanical behavior of Ni-Fe-Ga-Co magnetic shape memory alloys have been studied by means of optical microscope, TEM, XRD and DSC as well as TG and tensile test at room temperature. The influence of composition and heat-treatment on the formation ofγphase was discussed, and the mechanism for increasing toughness byγphase was also revealed.The experimental results show that the microstructure of as-melted alloys is composed of martensite phase andγphase at room temperature. With increasing Co content, the volume fraction ofγphase increases. The saturation solubility of Fe and Co atoms in the matrix is about 16 at.%. When the total content of Fe and Co atoms exceeds 16 at.%, the residual Fe and Co atoms exist in form ofγphase.γphase is in the state of Fe-rich, Co-rich and Ga-deficient. Besides, the saturation solubility of Fe and Co atoms inγphase is around 23 at.%.Heat-treatment has notable influence on the microstructure of Ni₅₆Fe₁₇Ga_27-xCo_x alloys. After heat-treated at 1200℃for 3 hours, the microstructure of Ni₅₆Fe₁₇Ga₂₇ alloy changes to single martensite phase. When the temperature is between 1100 and 500℃, second-formedγphase is observed in form of tiny spiculum. With the temperature decreasing, the volume fraction of second-formedγphase increases and then decreases at 500℃. For Ni₅₆Fe₁₇Ga₂₅Co₂ alloy, the microstructure and the volume fraction ofγphase almost have no change until the temperature decreases to 850℃. However, when the temperature decreases to 500℃, the volume fraction of second-formedγphase comparatively reduces.The experimental alloys exhibit one-step martensitic transformation during the process of cooling and heating. The crystal structure of martensite phase is indexed as monoclinic 6M. With the increase of Co content, the transformation temperatures elevate. The alloys exhibit excellent thermal stability.Ni₅₆Fe₁₇Ga₂₇ alloy is hard to be magnetized at room temperature, while Ni₅₆Fe₁₇Ga₂₇-xCox(x=2,4,6) alloys appears to be easy magnetized. Furthermore, both the saturation magnetization and Tc increase with the increase of Co content.