Microstructure And Properties Of CoNiAlMe Magnetic Shape Memory Alloys(MSMA)

Co₃₈Ni₃₄Al_28-xSi_x and Co₃₈Ni₃₄Al_28-xSn_x magnetic shape memory alloys(MSMA) were prepared by means of Rapid Solidification Technology (copper-mold vacuum suction casting). The effect of alloy composition and heat treatment process on Martensitic transformation, microstructure and magnetic properties of Co₃₈Ni₃₄Al_28-xSi_x and Co₃₈Ni₃₄Al_28-xSn_x alloys were investigated by differential scanning calorimeter(DSC), Vibrating Sample Magnetometer(VSM), X-ray diffraction(XRD), optical microscopy(OM), etc. The results are as follows.1. The microstructure and reverse martensite transformation temperature of Co₃₈Ni₃₄Al_28-xSi_x alloys is related to composition and heat treatment process. The alloys with 33 at.% Ni is characteristic of B2 structure at ambient temperature. When Ni content is increased to 34at.%, the alloy as cast is characteristic of M structure. Co₃₈Ni₃₄Al₂₅Si₃ alloy treated by 1573K aging for 2h and quenching in ice-water is characteristic of obvious preferred orientation. Si of 3 at% content is beneficial to increase of the reverse martensitic transformation enthalpy. Comparing with the cast, martensite transformation temperature of Co₃₈Ni₃₄Al₂₅Si₃ Alloys increases after heat treatment of 1373K with the largest enthalpy. By the means of Rapid Solidification Technology, martensite is obtained in the Co₃₈Ni₃₄Al₂₅Si₃ and Co₃₈Ni₃₄Al₂₅Si₅ alloys as cast. The samples as cast and heat treated by 1573K aging 2h and quenching in ice-water are in the best microstructure.2. The saturation magnetization (M_s) and microhardness of Co₃₈Ni₃₄Al_28-xSi_x alloys is influenced by microstructure. Co₃₈Ni₃₄Al_28-xSi_x alloys with the homogeneous single-phase contributed to the higher M_s. The more boundary precipitates occur after heat treatment will reduce the Hc and increase the M_s. The microhardness of austenite is higher than martensite.The increases heat treatment temperature will improve the microhardness of alloys. The boundary precipitates (γphase) contributed to increase the microhardness of alloys.3. The microstructure of Co₃₈Ni₃₄Al_28-xSn_x alloys is related to composition and heat treatment process. The grain grows with the increase of Sn content. The homogeneous microstructure is abtained by the means of Rapid Solidification Technology. With the Sn content increasing, the structure of the as-cast changes from two-phase (M+B2) structure to B2+M+γthree-phase structure. A few Sn contributed to increase the reverse martensite transformation temperature. The reverse martensite transformation temperature decreases with the content of Sn continue to increase. The grain grows and grain boundary precipitates gradually reduced with the increases of heat treatment temperature. Martensite is obtained by the means of 1573K heat treatment temperature and quenching to ice-water. With the increasing of heat treatment temperature, the structure of alloys changes to M phase.4. The microhardness of the Co₃₈Ni₃₄Al_28-xSn_x alloys is influenced by structure. The increases content of Sn will improve the microhardness of alloys.The boundary precipitates (γphase) contributed to increase the microhardness of alloys...