Fe-based Magnetic Shape Memory Alloys Design And Studies Based On A New Magnetic-field-driven Mechanism

As the development of modern society and new technology, there is an increasing demand for functional behaviors in materials. Magnetic-field-driven shape memory alloys (MSMAs), which have a large magnetic-field-induced strain/stress (MFIS) and fast response frequency, has stimulated much attention due to their potential candidate for magnetic sensors and actuators. Base on the different actuation mechanisms, magnetic-field-driven shape memory alloys can be classified as two kinds. The first kind is the Ni-Mn-Ga alloy system, in which the martensite is ferromagnetic, and the MFIS is caused by the reorientation of ferromagnetic martensitic variants under an applied magnetic field. The second kind is the Ni-Mn-Co-In alloy system. In this alloy system, the high-temperature parent phase (austenite) is ferromagnetic, whereas the low-temperature martensite phase is antiferromagnetic or paramagnetic, and the MFIS is caused by the magnetic-field-induced phase transformation from the martensite to parent phase. The two kinds of MSMAs have their own unique characterizations, which have made them become one of the attractive research focuses in materials science and physics.So far, most of studies on MSMAs mainly have been focused on these two typical systems. The magnetic-field-driven reorientation is able to provide a large strain with the limited output stress. On the contrary, the alloy can obtain a higher output stress under the mechanism of magnetic-field-induced phase transformation, but the output strain will decrease. To overcome these problems, this thesis is dedicated mainly to two aspects. On the one hand, the paper put forward a new idea that it is possible to combine these two mechanisms in one MSMA system based on their common physical nature. On the other hand, several Fe-based MSMA systems are selected to investigate the possibility of this new magnetic-field-driven mechanism, the results and conclusions can be summarized as follows:(1) The magnetic shape memory alloy with both of two magnetic-field-driven mechanisms (the magnetic-field-driven reorientation and the magnetic-field-induced transformation) must have the following characteristics:the martensite transformation, a significant difference of magnetization between parent phase and martensite, the ferromagnetic martensite.(2) In the specific composition range of Fe-Mn-Ga system,(a) Fe-Mn-Ga alloy is an ideal candidate for the application of the new magnetic-field-driven mechanism, the magnetic-field-induce martensite transformation from paramagnetic austenite to ferromagnetic martensite was obtained in Fe43Mn2sGa29alloys under7T applied magnetic field,△Ms=4.1K/T;(b) the martensite transformation temperature (Ms) increases with increasing Mn content and constant Ga, whereas the Ms decreases with increasing Ga and constant Mn;(c) Addition of Co element can suppress the martensite transformation and increase Tc of austenite.(3) Ge element has a strong effect on microstructures of Fe-Co-Ge alloys. With the decrease of Ge content, the microstructure varies from the dual-phase structure (>20at.%) to single austenite phase (<20at.%). The austenite is the disorder A2structure. It means the martensite transformation which is necessary for the magnetic-field-induced strain/stress can hardly occur in this alloy.(4) Ge addition depresses the martensite transformation in Fe-Mn-Ge alloys. The antiferromagnetic martensite was observed at room temperature (Ge<6at.%). The tendency from antiferromagnetic martensite to ferromagnetic martensite was found with the addition of Co element in Fe-Mn-Ge alloys, which is essential for the magnetic-field-driven reorientation of martensite.