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

据报道，在铁磁形状记忆合金中，由于马氏体孪晶的重排，马氏体相产生了几%的大磁致应变。为了提高该机制的输出功率，必须提高饱和磁化强度和磁各向异性，降低孪晶应力。在本研究中，研究了稀土元素的加入对相变、饱和磁化和力学性能的影响。采用氩弧熔炼炉制备了添加Sm、Nd、Tb、Gd等稀土元素的Ni-Mn-Ga合金铸锭。钢锭均质后入水淬火。用DSC法测定相变温度。磁化强度用VSM测定。采用扫描电镜和透射电镜对样品进行了显微组织观察。通过室温压缩试验测定了材料的力学性能。用洛伦兹法透射电镜观察了磁畴结构。在更高的温度下，居里温度对价电子浓度的依赖性与Ni-Mn-Ga三元合金相似。而Gd合金的马氏体相变温度和居里温度较低。含稀土合金的饱和磁化强度高于三元合金。这种趋势在掺钕合金中表现明显。结果表明，稀土元素的加入可以显著提高合金的压缩塑性。SEM和TEM观察表明，低角和高角晶界处存在稀土元素含量约15%的第二相。这些第二相析出物可能是提高力学性能的原因。洛伦兹观测结果表明，马氏体孪晶板的磁畴壁以180度壁为主。此外，孪晶界会产生类似磁畴壁的对比。这是由于四方马氏体具有较高的磁各向异性能。观测结果支持先前提出的大磁应变模型。少

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