Fabrication of Ferrite-Coated Magnetic Fe–Mn–Si–Cr–Ni Alloy Utilizing Selective Oxidation of Mn Element

The development of magnetic properties extends application fields of Fe-Mn-Si-based shape memory alloys (SMAs) into the field of electromagnetic sensors and actuators. In the previous studies, magnetic Fe-Mn-Si-based shape memory ribbons have been prepared by the melt-spinning technique. However, the melt-spinning technique could only obtain 2-D thin materials. In this paper, a ferrite-coated Fe-18.81Mn-5.61Si-9.31Cr-5.36Ni bulk shape memory material, exhibiting both magnetic and shape memory functions, was fabricated by means of the oxidation at 1173 K for different times. The ferritic layer was depleted in the Mn element due to the selective oxidation of the Mn element, and its growth behavior followed the parabolic law. Furthermore, we prepared not only ferrite/austenite/ferrite sandwich sheets but also matrix at ferrite core-shell wires in the case of ferrite-coated Fe-18.81Mn-5.61Si-9.31Cr-5.36Ni alloy. The shape recovery ratio of sandwich sheets decreased to 74.1% from 78.3% of the ferrite-free state while their saturation magnetization increased to 7.8 emu/g. The shape recovery ratio of core-shell wires decreased to 54.8% from 78.3% of the ferrite-free state with increasing the saturation magnetization to 36.2 emu/g. For both sandwich sheets and core-shell wires, there is a tradeoff relationship between shape memory effect (SME) and saturation magnetization. Accordingly, it is necessary to strike a balance between their shape memory and magnetic properties according to engineering applications.