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

在铁磁形状记忆合金中，马氏体相由于马氏体孪晶的重排而产生了几个百分点的磁致应变。为了提高这种机制的输出功率，必须提高饱和磁化强度和磁各向异性，降低孪晶应力。本文研究了稀土元素对相变、饱和磁化强度和力学性能的影响。采用氩弧熔炼法制备了以Ni-Mn-Ga为基，添加Sm、Nd、Tb、Gd等稀土元素的合金锭。将铸锭均质化并在水中淬火。通过DSC测量相变温度。通过VSM测量磁化强度。用扫描电镜和透射电镜观察了显微组织。在室温下通过压缩试验测量机械性能。用Lorentz方法观察了样品的磁畴结构，并对马氏体相变过程进行了分析。 ...更多信息 居里温度对价电子浓度的依赖性与Ni-Mn-Ga三元合金相似。然而，在添加Gd的情况下，合金表现出较低的马氏体相变温度和居里温度。稀土合金的饱和磁化强度高于三元合金。结果表明，稀土元素的加入能显著提高合金的压缩塑性。扫描电镜和透射电镜观察表明，在小角度和大角度晶界处均存在稀土含量约为15at%的第二相。Lorentz观测结果表明马氏体孪晶板中的磁畴壁主要为180度壁。此外，孪晶界引起的对比度类似于磁畴壁。这是由于在四方马氏体中的高磁各向异性能。观测结果支持了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