npj: 预转变材料中的缺陷—纳米胚胎增强材料性能

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位移型相变材料在相变温度以上经常可观察到异常的热学、力学和磁性行为。这些异常的材料性质通常是从电子/原子的本征响应的角度来解释。然而，由于这些材料通常是非均匀的，晶格缺陷对材料性质具有重要的影响，因此基于本征响应的解释至少是不完整的。

来自南京信息工程大学的国家“千人计划” 饶伟锋教授团队，应用相场模拟的方法研究了缺陷（如位错和共格沉淀物）附近的应力分布，发现缺陷周围的应力集中能够在相变温度以上诱发局部的相转变，生成稳定的纳米产物相胚胎。这些纳米胚胎处于一种新的热弹性平衡中，其尺寸由温度、缺陷性质、外加应力和磁场等综合因素决定。因此，当改变温度，外应力或磁场时，胚胎的平衡尺寸和体积占比将相应地改变，从而使材料的相变行为发生改变，并具有超弹性或超磁致伸缩等独特的性能。概括地说，预转变材料中晶格缺陷产生的应力集中可引起纳米尺度的相变，形成纳米胚胎。对纳米胚胎的研究将有助于更好地理解橡胶金属中的超弹性效应、铁基形状记忆合金中的超磁致伸缩效应，以及因瓦（热不胀）和艾林瓦（热不软）效应。

该文近期发表于npj Computational Materials 4: 58 (2018)，英文标题与摘要如下，点击左下角“阅读原文”可以自由获取论文PDF。

Nanoembryonic thermoelastic equilibrium and enhanced properties of defected pretransitional materials

Ye-Chuan Xu, Wei-Feng Rao, John W. Morris Jr. & Armen G. Khachaturyan 

Behaviors of displacive phase-transforming materials above the temperature of transformation, where abnormal thermal, elastic, magnetic properties are often observed, are mostly explained by intrinsic peculiarities in electronic/atomic structure. Here, we show these properties may also be attributed to extrinsic effects caused by a thermoelastic equilibrium in highly defected pretransitional materials. We demonstrate that the stress concentration near stress-generating defects such as dislocations and coherent precipitates could result in the stress-induced transformation within nanoscale regions, producing equilibrium embryos of the product phase. These nanoembryos in thermoelastic equilibrium could anhysteretically change their equilibrium size in response to changes in applied stress or magnetic fields leading to superelasticity or supermagnetostriction. Similar response to cooling may explain the observed diffuse phase transformation, changes in the coefficient of thermal expansion and effective elastic modulus, which, in turn, may explain the invar and elinvar behaviors.

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