Shape memory effect and magnetic properties of Co-Fe ferromagnetic shape memory alloys

In 1996, after Ullakko et al. first reported a measurement of 0.2% strain along the [001] direction of a Ni2MnGa single crystal when subjected to a magnetic field of 8KOe at 265K, the researches of ferromagnetic shape memory alloys became popular. The reversible strain induced by magnetic-field in Ni2MnGa has been proved to be nearly 10% in a magnetic field of less than 1T, which is much higher than that of the rare-earth giant magnetostrictive alloys. But the high brittleness of Ni2MnGa hinders its practical application. Therefore, the development of ferromagnetic shape memory alloys with good ductility is of primary importance. In this paper, the microstructure, martensitic transformation temperature, shape memory effect, as well as magnetic and mechanical properties of Co-Fe alloys were investigated by optical observation, X-ray diffraction, DSC, bending test and vibrating sample magnetometer. It was confirmed that the shape memory effect in Co-Fe alloys is associated with the fcc/hcp martensitic transformation. Moreover, Co-Fe alloys exhibit high saturation magnetization with the values of above 170 emu/g, which are much higher than that of Ni2MnGa (66 emu/g). So the driving force under magnetic field will be large for Co-Fe alloys. Additionally, Co-Fe alloys possess good ductility for practical application with tensile elongation higher than 17.5%. All these results indicate that Co-Fe alloys are promising candidates for developing as ferromagnetic shape memory alloys.