具有高迁移率孪晶界的Ni-Mn-Ga马氏体中的非简谐非公度结构调制
Anharmonic Incommensurate Structure Modulation in Ni-Mn-Ga Martensite Exhibiting Highly Mobile Twin Boundaries
AI总结：
本研究通过中子衍射和从头算，揭示了Ni-Mn-Ga 10M马氏体中的非简谐非公度调制与a/b纳米孪晶的关联，解释了其结构转变并明确了低温正交构型的稳定性与极限尺寸。
摘要：
理解磁性形状记忆合金的晶体结构对于深入认识其独特性能（如孪晶界的高迁移率和磁场功能特性）至关重要。10–300 K范围内的复杂中子衍射图样表明，Ni50.0Mn27.7Ga22.3 10M马氏体中存在非公度且同时具有非简谐性的调制函数（AMF）。我们通过识别AMF中的主导傅里叶分量，将计算得到的衍射图样与实验结果进行了对比。当300 K下近公度的AMF随冷却转变为非公度AMF时，AMF能够解释特殊的小强度衍射峰的出现。进一步分析显示，非公度调制与晶格之间的周期性差异导致了额外纳米畴的形成。从基底面(110)的(2-3)2堆垛序列的角度解读纳米畴内的调制位移，可以理解这些纳米畴是正在形成的a/b纳米孪晶。与非公度调制相互关联的纳米孪晶化也解释了冷却过程中从四方对称到正交对称的转变。我们识别出了特定的低温正交结构，如34O、24O、14O，这些结构具有重要意义，因为它们的晶胞同时代表了一个a/b纳米孪晶。从头算计算证实了所有发现的纳米孪晶结构的稳定性，且它们的能量较低且相当。基于纳米孪晶态的推定稳定性以及与文献的对比，我们提出，名义上标记为五层调制、5M或10M的马氏体低温态，是34O、24O、14O等特定a/b纳米孪晶构型中的一种。具体选择取决于成分，但由q = 3/7产生的14O是极限情况，对应的最小孪晶畴尺寸为3.5 nm。
英文摘要：
Understanding the crystal structure of magnetic shape memory alloys is crucial for insights into their unique properties, such as the high mobility of twin boundaries and magnetic field functionality. The complex neutron diffraction patterns betweeen 10-300 K indicate an incommensurate and simultaneously anharmonic modulation function (AMF) in Ni50.0Mn27.7Ga22.3 10M martensite. Our identification of the dominant Fourier components in the AMF allowed for a comparison between calculated diffraction patterns and experiments. The AMF explains the appearance of peculiar small-intensity diffraction peaks when nearly commensurate AMF at 300 K turns to incommensurate AMF upon cooling. Further analysis reveals that divergent periodicity between the incommensurate modulation and the lattice leads to the formation of additional nanodomains. Interpreting the modulation displacements within the nanodomains in the terms of the (2-3)2 stacking sequence of basal (110) planes enables to understand that these nanodomains are emerging a/b nanotwins. The nanotwinning interlinked with incommensurate modulation also explains the transition from the tetragonal to the orthorhombic symmetry upon cooling. We identify specific low-temperature orthorhombic structures, like 34O, 24O, 14O, which are significant as their unit cell simultaneously represents an a/b-nanotwin. The ab initio calculations confirm the stability and low and comparable energy of all found nanotwinned structures. Based on the presumed stability of the nanotwinned state and literature comparisons, we propose that the low temperature state of the martensite nominally marked as five-layered modulated, 5M or 10M, is one of the specific a/b-nanotwinned configurations such as 34O, 24O, 14O. The exact choice depends on the composition, but 14O resulting from q = 3/7 is the ultimate limit with a corresponding smallest twin domain size of 3.5 nm.
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