npj: 形状记忆陶瓷的自愈—纳米尺寸下的机制
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形状记忆陶瓷，如钇稳定的四方氧化锆（YSTZ），具有独特的性能，包括工作温度高、抗氧化能力强、热稳定性好和抗腐蚀性能优异等诸多优点，它们在制造和/或机械变形过程中容易形成缺陷，其缺陷的纳米结构演变机制至今仍不清楚。
来自美国科罗拉多州矿业学院的Ning Zhang和Mohsen Asle Zaeem教授等，使用分子动力学计算揭示，YSTZ纳米柱中裂缝和空隙的闭合是材料体积的扩大和相变作用的结果。裂纹存在一个临界裂纹宽度，在此宽度以下的裂纹在压缩h后可以完全闭合。当裂缝宽度增大到超过临界值时，相变引起一定的体积膨胀（~4％），此时仅有部分裂缝可以愈合。原子应力分析表明，负压应力集中在新的单斜带附近。值得注意的是，空隙尺寸对力学响应和空隙愈合具有显著影响，而裂缝尺寸的影响则可忽略不计。位错成核和位错迁移导致堆垛层错和非晶态相形成，成为裂纹和孔洞闭合的中间态。该研究结果为人们提供了深入研究裂缝和空隙尺寸调控的可能性，以研制所需力学性能的形状记忆陶瓷，并指导设计适当的结构部件与自愈合能力，做到使用寿命更为长久。
该文近期发表于npj Computational Materials 5: 54 (2019)，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。
Nanoscale self-healing mechanisms in shape memory ceramics
Ning Zhang & Mohsen Asle Zaeem
Shape memory ceramics, such as yttria-stabilized tetragonal zirconia (YSTZ), offer unique properties including ultra-high operating temperatures and high resistance to oxidation. However, they are susceptible to formation of defects during manufacturing and/or by mechanical deformation. To completely take advantage of their shape memory properties, it is necessary to fully understand the nano-structural evolution of defects under external stimuli. In this study, defect evolution behaviors in YSTZ nanopillars are investigated by atomistic simulations. Two characteristic orientations of \(\left[ {01\bar 1} \right]\) and [001] are selected to represent the dominant deformation mechanisms of phase transformation and dislocation migration, respectively. Volume expansion associated with the tetragonal to monoclinic phase transformation is observed to promote healing of crack and void. Atom stress analyses reveal stress concentrations along the newly formed monoclinic phase bands. A critical crack width is identified, less than which the crack can be fully closed in compression. For [001]-oriented YSTZ nanopillars, dislocation migration leads to formations of an amorphous phase, which also assist the crack and void closure process. The revealed crack/void healing mechanisms may provide a path for mitigating internal defects that influences the mechanical properties and deformation mechanisms of shape memory ceramics.
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