Preparation And Properties Of CoNiAl Magnetic Shape Memory Alloys (MSMA)

Co₃₈Ni₃₄Al_28-xMn_x(x=0,0.5,1,1.5,2,3) and Co₃₈Ni₃₄Al_28-xCr_x(x=0,0.5,1,1.5, 2) magnetic shape memory alloys (MSMA) were prepared by copper-mold vacuum suction casting. The effect of alloy compositions and heat treatment on microstructure, martensitic transformation, magnetic transformation, magnetic properties and microhardness of alloys were investigated by optical microscopy (OM), X-ray diffraction (XRD), AC susceptibility, vibrating sample magnetometer (VSM) and microhardness tester. The major results as follows:1. The microscopic structures of Co₃₈Ni₃₄Al_28-xMn_x and Co₃₈Ni₃₄Al_28-xCr_x alloys are related to the content of Mn and Cr. With the increase of Mn and Cr content, the volume fraction of y phase of the alloys increases, and the morphology change from granulate to strip gradually; theβphase grains of alloys become smaller. Moreover, the content of martensite decreases as the increase of Mn and Cr.2. The content of y phase of Co₃₈Ni₃₄Al_28-xMn_x and Co₃₈Ni₃₄Al_28-xCr_x alloys deceases and the content of martensite increases as the increase of quenching temperature, respectively. When quenched at 1473K, the alloy is ofβ+γphase structure; when quenched at 1523K,β,γand martensite phase coexist in the alloy; when quenched at 1573K, The alloy Co38Ni34Al28 consists of single phase martensite, alloys with other compositions are of y+martensite phase. The content of y phase for Co38Ni34Al28, Co38Ni34Al27Mn1 and Co38Ni34Al27Cr1 decreases slightly as the increase of holding time. The two-stage heat treatment process can control the content of y phase and make it distribute along the grain boundry completely. Under the condition of 1573K/0.5h+1543K/5h (a typical two-stage heat treatment process), the y phase of Co38Ni34Al28 distributes along the grain boundry, and the y phase area ratio of the grain boundry is about 50%, and it indicating a promising magnetically controlled shape memory with great comprehensive performance.3. The martensitic transformation temperature of Co₃₈Ni₃₄Al_28-xMn_x is related to the content of Mn and heat treatment conditions. As the increase of Mn content, the martensitic transformation temperature Tms decreases from 302K to 216K, and the thermo-hysteresis AT increases from 4K to 22K. The Tms of Co38M34Al28 increses from 302K to 360K, while AT decreases from 4K to OK with increasing quenching temperature. And the Tms of Co38Ni34Al27Mni increses from 254K to 359K, while AT decreases from 15K to 10K with increasing quenching temperature.4. The Curie temperature of Co₃₈Ni₃₄Al_28-xMn_x is related to the content of Mn and heat treatment conditions. It increases from 397K to 422K as the increase of Mn content. In the range of 1473K-1573K, as the increase of quenching temperature, the Curie temperature of Co38Ni34Al28 increases from 327K to 430K, while from 335K to 417K of Co38Ni34Al27Mn1. And the Curie temperature changes linearly with quenching temperature.5. The saturation magnetization of Co₃₈Ni₃₄Al_28-xMn_x is related to the content of Mn and heat treatment conditions. Under the same heat treatment conditions, it increases as the increase of Mn content. In the case of as-cast, the saturation magnetization of Co38Ni34Al26.5Mn1.5 is the biggest, as high as 60.47emu/g, too much bigger than that of Co38Ni34Al28 which is only 35.08emu/g. In the range of 1473K-1573K, as the increase of quenching temperature, the saturation magnetization increases. When quenched at 1573K for different time, the saturation magnetization of Co38Ni34Al28 and Co38Ni34Al27Mn1 increases.6. The magnetocrystalline anisotropy constant of Co₃₈Ni₃₄Al_28-xMn_x increases as the increase of Mn content, with the order of magnitude of 106erg/cm3. In the range of 1473K-1573K, as the increase of quenching temperature, martensitic transformation occurs, the magnetocrystalline anisotropy constant of Co38Ni34Al28 and Co38Ni34Al27Mn1 increases.7. The microhardness of Co₃₈Ni₃₄Al_28-xMn_x and Co₃₈Ni₃₄Al_28-xCr_x increases as the increase of Mn and Cr content. In the range of 1473K-1573K, as the increase of quenching temperature, the microhardness of Co38Ni34Al26.5Mn1.5 and Cro.5 increases.