Electron holography studies on the pattem formation preceding to displacive phase transformations

本研究项目的目的是揭示在某些铁磁形状记忆合金的马氏体相变开始之前发生的宏观图案形成的机制。为了揭示Ni-Fe-Ca，Fe-Pd和Ni-Co-Mn-In合金母相中磁通分布的温度依赖性，我们用电子全息技术研究了这种宏观图案的形成过程。在Ni-Fe-Ca和Fe-Pd合金中，母相中的磁通模式随着接近Ms（马氏体相变开始温度）而急剧变化。有趣的是，在略高于Ms的温度下观察到的磁通量图案被继承到马氏体相中的磁畴结构。相反，在Ni-Co-Mn-In合金中没有观察到这种类型的图案形成。漫散射能量滤波分析 ...更多信息 红电子衍射结果表明，Ni-Fe-Ca和Ni-Co-Mn-In合金的母相在冷却过程中{110}面上出现明显的横向晶格位移，而Ni-Co-Mn-In合金的晶格位移信号很弱。这一结果意味着Ni-Fe-Ca和Ni-Co-Mn-In合金中磁通模式的变化。在Ni-Fe-Ga和Fe-Pd合金中，母相中的磁通模式随着接近Ms（马氏体相变开始温度）而急剧变化。有趣的是，在略高于Ms的温度下观察到的磁通量图案被继承到马氏体相中的磁畴结构。相反，在Ni-Co-Mn-In合金中没有观察到这种类型的图案形成。能量过滤电子衍射漫散射分析表明，Ni-Fe-Ga和Ni-Co-Mn-In合金的母相在冷却时{110}面上的横向晶格位移明显，而Ni-Co-Mn-In合金的晶格位移信号很弱。这一结果表明，在Ni-Fe-Ga和Ni-Co-Mn-In合金中的磁通模式的变化是由母相中的前兆晶格畸变触发的。在外加磁场（1.9T）和零场条件下对马氏体变体结构的TEM观察支持了上述解释，马氏体变体结构是影响形状记忆性能的重要因素。本文的研究结果对于理解铁磁形状记忆合金中马氏体变体的形成机理具有重要意义。少

The aim of this research project is to reveal the mechanism of the macroscopic pattern formation that occurs before the onset of martensitic phase transformations in some class of ferromagnetic shape memory alloys. This macroscopic pattern formation was discovered by the head investigator recently, while the mechanism remained unsolved.Detailed electron holography studied were carried out in order to reveal the temperature dependence of magnetic flux distribution in the parent phase of Ni-Fe-Ca, Fe-Pd, and Ni-Co-Mn-In alloys. In the Ni-Fe-Ca and Fe-Pd alloys, the magnetic flux pattern in the parent phase changes dramatically with an approach to Ms (martensitic transformation start temperature). Interestingly, the magnetic flux pattern observed at a temperature slightly higher than Ms was inherited to the magnetic domain structure in the martensitic phase. On the contrary, this type of pattern formation was not observed in a Ni-Co-Mn-In alloy. Diffuse scattering analysis by energy-filte … More red electron diffraction has revealed that the transverse lattice displacement on {110} planes is pronounced upon cooling in the parent phase of Ni-Fe-Ca and Ni-Co-Mn-In alloys, while the signal of the lattice displacement is quite weak in the Ni-Co-Mn-In alloy. This result implies that the change in the magnetic flux pattern in the Ni-Fe-Ca and Ni-Co-Mn-In alloys. In the Ni-Fe-Ga and Fe-Pd alloys, the magnetic flux pattern in the parent phase changes dramatically with an approach to Ms (martensitic transformation start temperature). Interestingly, the magnetic flux pattern observed at a temperature slightly higher than Ms was inherited to the magnetic domain structure in the martensitic phase. On the contrary, this type of pattern formation was not observed in a Ni-Co-Mn-In-alloy. Diffuse scattering analysis by energy-filtered electron diffraction has revealed that the transverse lattice displacement on {110}planes in pronounced upon cooling in the parent phase of Ni-Fe-Ga and Ni-Co-Mn-In alloys, while the signal of the lattice displacement is quite weak in the Ni-Co-Mn-In alloy. This result implies that the change in the magnetic flux pattern in the Ni-Fe-Ga and Ni-Co-Mn-In alloys is triggered by the premonitory lattice distortion in the parent phase. TEM observations of the martensite variant structures, which were obtained in an applied magnetic field (1.9T) and zero-field, respectively, supported the above interpretation.The structure of martensite variants is a very important factor that is responsible for the shape memory properties. The result of the present work sheds light on the understanding the formation mechanism of martensite variants in ferromagnetic shape memory alloys. Less