Modulated martensite: why it forms and why it deforms easily
Modulated martensite: why it forms and why it deforms easily
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DOI：
10.1088/1367-2630/13/5/053029
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发表时间：
2011-05-16
影响因子：
3.3
通讯作者：
Faehler, S.
中科院分区：
文献类型：
--
作者：
Kaufmann, S.;Niemann, R.;Faehler, S.
无扩散相变是（磁性）形状记忆合金、铁电体和多铁性材料的多功能性的核心。在低对称调制马氏体相中获得了外场下的巨应变效应。我们概述了调制相的起源，它们与四元马氏体和后果，由于它们的功能特性通过分析从原子到宏观尺度的外延Ni-Mn-Ga薄膜的马氏体显微结构。马氏体-马氏体相界的几何约束作用到原子尺度。因此，可以形成纳米孪晶四元马氏体的马氏体显微结构。孪晶变体的粗化可以降低孪晶边界能量，这是一个我们可以从原子到毫米尺度观察到的过程。粗化是一个分形过程，通过加倍孪晶周期以离散的步骤进行。集体缺陷能量导致大量的滞后，这允许在室温下保留调制马氏体作为亚稳相。在这种亚稳态下，弹性能量通过形成“双胞胎中的双胞胎”微观结构释放，这种微观结构可以从纳米到毫米尺度观察到。这种分级孪生导致介观孪晶边界。我们的分析表明，介观边界是广泛的和扩散的，与常见的原子尖锐的孪晶晶界的四相马氏体。我们认为，所观察到的非常高的流动性，这种介观孪晶界源于其扩散性质，使钉扎原子点缺陷无效。
Diffusionless phase transitions are at the core of the multifunctionality of (magnetic) shape memory alloys, ferroelectrics and multiferroics. Giant strain effects under external fields are obtained in low symmetric modulated martensitic phases. We outline the origin of modulated phases, their connection with tetragonal martensite and consequences owing to their functional properties by analysing the martensitic microstructure of epitaxial Ni-Mn-Ga films from the atomic to the macroscale. Geometrical constraints at an austenite-martensite phase boundary act down to the atomic scale. Hence, a martensitic microstructure of nanotwinned tetragonal martensite can form. Coarsening of twin variants can reduce twin boundary energy, a process we could observe from the atomic to the millimetre scale. Coarsening is a fractal process, proceeding in discrete steps by doubling twin periodicity. The collective defect energy results in a substantial hysteresis, which allows the retention of modulated martensite as a metastable phase at room temperature. In this metastable state, elastic energy is released by the formation of a 'twins within twins' microstructure that can be observed from the nanometre to the millimetre scale. This hierarchical twinning results in mesoscopic twin boundaries. Our analysis indicates that mesoscopic boundaries are broad and diffuse, in contrast to the common atomically sharp twin boundaries of tetragonal martensite. We suggest that the observed extraordinarily high mobility of such mesoscopic twin boundaries originates from their diffuse nature that renders pinning by atomistic point defects ineffective.