Improvmemnt of Properties of Ferromagnetic Shape Memory Alloys by Addition of Rare Earth Elements and Mechanism of Giant Magnetostriction

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Large magnetically induced strain of a few % has been reported for martensitic phase in ferromagnetic shape memory alloys, caused by rearrangement of martensitic twins. In order to improve the output power by this mechanism it is necessary to increase saturation magnetization and magnetic anisotropy and to decrease twinning stress.In the present investigation, effect of addition of rare earth elements on phase transformation, saturation magnetization and mechanical properties was studied. Ingots of alloys based on Ni-Mn-Ga with addition of rare earth elements such as Sm,Nd,Tb,Gd were prepared by argon arc melting furnace. Ingots were homogenized and quenched into water. Phase transformation temperatures were measured by DSC. Magnetization was measured by VSM. Microstructural observations were made by SEM and TEM. Mechanical properties were measured by compression test at room temperature. Magnetic domain structures were observed by TEM using Lorentz method.Martensitic transformation te … More mperatures, Curie temperatures exhibit systematic dependence on valence electron concentration similar to that in Ni-Mn-Ga ternary alloys. However in the case of Gd added alloys exhibit lower martensitic transformation temperatures and Curie temperatures. Saturation magnetization in rare earth containing alloys was higher than the ternary alloy. This tendency was significant in Nd added alloys.It was found that compressive ductility can be significantly improved by the addition of rare earth elements. SEM and TEM observations revealed that the second phase containing about 15at% of rare earth elements exists at low angle and high angle boundaries. These second phase precipitates maybe responsible for the improved mechanical property.Results of Lorentz observations revealed magnetic domains walls in martensite twin plates are mainly 180 degree walls. In addition, twin boundaries give rise to the contrast similar to magnetic domain walls. This is due to the high magnetic anisotropy energy in the tetragonal martensite. The observed results support the previously proposed models of the large magnetic strain. Less