The Studying Of The Magnetism And Martensitic Tansformation Of The New Type Of Forromagnetic Shape Memory Alloys

In this paper, the phase composition, martensitic transformation and magnetic properties of new magnetic shape memory alloys(MSMA) Ni-Mn-Sn, Ni-Mn-In and Co-Mn-Sn were investigated by using X-ray diffraction (XRD), Vibrating Sample Magne -tometer(VSM) and other equipments. The relationship between the martensite transfor -mation, magnetic properties and components of the MSMA was revealed.The results show that, Ni₅₂Mn_23+xSn_25-x(x=0,1,2), Ni₄₀Mn_50+xSn_10-x(x=-6, -4, -2, 0) and Co₅₂Mn_23+xSn_25-x(X=0, 1, 2, 3) alloys prepared by vacuum arc melting were in good crystallization condition and possess single-phase cubic L2₁ structure at room temperature. The lattice constants decreased with the increase of Mn content. The shrinkage of the lattice constants was attributed to Sn atom radii which is larger than Mn. Ni₄₀Mn_50+xIn_10-x(x=-6, -4, -2, 0) alloys prepared by the same method have also cubic L2₁ structures at room temperature, however, there was a small amount of the second phase. During to the larger radii of In atom than Mn, the lattice constants also decreased with the content of In.All of alloys Ni₅₂Mn_23+xSn_25-x(x=0, 1, 2), Ni₄₀Mn_50+xSn_10-x(x=-6, -4, -2, 0), Ni₄₀Mn_50+x -In_10-x(x=-6, -4, -2, 0) and Co₅₂Mn_23+xSn_25-x(x=0, 1, 2, 3)included obvious martensitic transformation which is from the ferromagnetic austenite to the paramagnetic (or anti -ferromagnetic) martensite. Except the Co₅₂Mn_23+xSn_25-x(x=0, 1, 2, 3) alloys whose Curie temperature was over 400K, the Curie temperatures of the others were slightly above room temperature located in the region of 300K～330K. This means that, cobalt is an element which can increase Curie temperature in the Heusler alloys.The temperature of martensitic transformation of Ni₅₂Mn_23+xSn_25-x(x=0, 1, 2) alloys was around 253K and the temperature range of martensitic transformation was less than 5K, such a narrow temperature range of the transformation was unusual in polycrystaIline MSMA. Compared with Ni₅₂Mn_23+xSn_25-x(x=0, 1, 2), the temperature of martensitic transformation of Co₅₂Mn_23+xSn_25-x(x=0, 1, 2, 3) were very low, i. e. below 140K. However, the transformation temperature region was very large, about 90K. This shows that, compared with Ni, Co is an elements which can decrease the martensitic transformation temperature, but it can increase the activation energy of the transformation, thus induced the increase of the temperature range of the phase transition. For above two series alloys, martensitic transformation temperature increased with the decrease (increase) of Sn(Mn) content, Curie temperature of Ni₅₂Mn_23+xSn_25-x(x=0, 1, 2) was also increased (Curie temperature of CoMnSn was not measured).The magnetism and the characteristics of martensitic transformation of Ni₄₀Mn_50+x -Sn_10-x(x=-6, -4, -2, 0) and Ni₄₀Mn_50+xIn_10-x(x=-6, -4, -2, 0) were very similar. Their martensitic transformation temperature were low, about 110K, while the tempera- ture range of the transition were about 50K. Their Curie temperatures were also very closed to each other, within 290～350K. On average, both the Curie temperature and the austen -ite magnetization in the magnetic field of 200Oe of Ni₄₀Mn_50+xIn_10-x(x=-6, -4, -2, 0) was higher than that of Ni₄₀Mn_50+xSn_10-x(x=-6, -4, -2, 0). The martensitic transformation temperature decreased with the increase of the In and Sn contents, which was accord with the conclusion that the temperature of martensitic transformation will increase with the increasing of electron concentration. Thus, although the effects of In and Sn atoms on the martensitic transformation are similar, there are differences influence on the magnetic parameters.In the thermal magnetism curves of Ni₄₀Mn_50+xSn_10-x(x=-2, 0) alloys, the abnormal change of the magnetism were observed at 200K, which may be related to the unstable crystal structure, such as phase separation or phase transition within the local induced by the internal stress.At appropriate temperature, the martensitic transformation process induced by the magnetic field can clearly observed in the magnetization curve, which are similar to the thermal-induced phase transformation. Once the transition took place, it could not restored with the decreasing of the magnetic field. The saturation magnetization increased with the increasing of Mn content, which was accordance with the conclusions that magnetism in the ferromagnetic Heusler alloys mainly come from the Mn atoms.