[论文解读] Effect of a Micro-scale Dislocation Pileup on the Atomic-Scale Multi-variant Phase Transformation and Twinning

Yipeng Peng, Rigelesaiyin Ji+4 more|arXiv (Cornell University)|Aug 6, 2022

Microstructure and mechanical properties被引用 5

一句话总结

本研究采用同步原子-连续（CAC）模拟方法，研究原子级结构界面上微米尺度位错塞积如何触发原子级相变（PT）和形变孪生。结果表明，位错塞积使相变起始的临界应力降低5.5倍，并直接驱动正方相到六方相的相变及其逆过程，形成热力学驱动力为零的孪晶，展示了多尺度塑性与相变耦合的统一框架。

ABSTRACT

In this paper, we perform concurrent atomistic-continuum (CAC) simulations to (i) characterize the internal stress induced by the microscale dislocation pileup at an atomically structured interface; (ii) decompose this stress into two parts, one of which is from the dislocations behind the pileup tip according to the Eshelby model and the other is from the dislocations at the pileup tip according to a super-dislocation model; and (iii) assess how such internal stresses contribute to the atomic-scale phase transformations (PTs), reverse PTs, and twinning. The main novelty of this work is to unify the atomistic description of the interface and the coarse-grained (CG) description of the lagging dislocations away from the interface within one single framework. Our major findings are: (a) the interface dynamically responds to a pileup by forming steps/ledges, the height of which is proportional to the number of dislocations arriving at the interface; (b) when the pre-sheared sample is compressed, a direct square-to-hexagonal PT occurs ahead of the pileup tip and eventually grows into a wedge shape; (c) upon a further increase of the loading, part of the newly formed hexagonal phase transforms back to the square phase. The square product phase resulting from this reverse PT forms a twin with respect to the initial square phase. All phase boundaries (PBs) and twin boundaries (TBs) are stationary and correspond to zero thermodynamic Eshelby driving forces; and (d) the stress intensity induced by a pileup consisting of 16 dislocations reduces the stress required for initiating a PT by a factor of 5.5, comparing with that in the sample containing no dislocations. This work is the first characterization of the behavior of PTs/twinning resulting from the reaction between a microscale dislocation slip and an atomically structured interface.

研究动机与目标

• 理解微米尺度位错塞积在有序界面上诱发原子级相变和孪生的作用机制。
• 利用混合原子-连续模型，将位错塞积产生的内应力分解为艾舍尔（Eshelby）和超位错贡献。
• 表征界面在位错塞积作用下的动态响应，包括台阶/阶脊形成及应力松弛机制。
• 量化位错诱导的内应力对相变临界应力的降低程度。
• 通过计算零艾舍尔驱动力，验证相界和孪晶界处的热力学平衡状态。

提出的方法

• 采用同步原子-连续（CAC）模拟方法，在界面处保持原子分辨率，远离界面区域采用粗粒化位错动力学模型。
• 应用艾舍尔模型，分解位错塞积尖端后方位错产生的应力场。
• 采用超位错模型，分离位错塞积尖端处位错的应力贡献。
• 对预剪切样品施加单调压缩载荷，以触发相变和孪生。
• 计算所有相界和孪晶界处的热力学艾舍尔驱动力，以验证平衡状态。
• 改变塞积中位错的数量，评估其对应力强度因子和相变阈值的影响。

实验结果

研究问题

• RQ1 微米尺度位错塞积如何影响有序界面上的原子级相变和孪生？
• RQ2 位错塞积尖端后方的位错与尖端处的位错对总内应力场的贡献分别是什么？
• RQ3 位错塞积中位错数量变化时，应力强度因子如何变化？在高密度下是否趋于饱和？
• RQ4 位错塞积对相变起始临界应力有何影响？
• RQ5 新形成的相界和孪晶界是否处于热力学平衡状态，表现为零艾舍尔驱动力？

主要发现

• 界面动态形成台阶或阶脊，其高度与到达界面的位错数量成正比。
• 纳米尺度塞积中，应力强度因子随位错数量线性增加，但在微米尺度塞积中，当涉及数十个位错时趋于饱和。
• 在压缩载荷下，塞积尖端前方发生直接的正方相到六方相相变，形成楔形结构，两个六方相变体彼此构成孪晶。
• 进一步加载后，发生从六方相到正方相的逆相变，生成的正方相相对于原始正方晶格形成孪晶。
• 所有相界和孪晶界均保持静止，且表现出零热力学艾舍尔驱动力，证实处于热力学平衡状态。
• 16个位错的塞积使相变临界压缩应力相比无缺陷样品降低5.5倍。