基于原子体积场拉普拉斯算子对金属玻璃剪切转变区的预测

PREDICTION OF SHEAR TRANSFORMATION ZONES IN METALLIC GLASSES BASED ON LAPLACIAN OF ATOMIC VOLUME

剪切转变区(shear transformation zone,STZ)作为金属玻璃塑性事件的一个基本特征单元,已被研究者们逐渐接受,但STZ产生的机制和来源仍具争议.本文采用分子模拟方法对Cu64Zr36金属玻璃在受简单剪切加载时的变形行为展开了研究.结果表明,体系的初始构型与加载后STZ的产生是相关的.虽然原子体积场及其梯度可以用来有效表征金属玻璃中局部原子构型的非均匀性,但它们与STZ产生的区域没有直接明显的对应关系.基于此,提出一个新的局域结构参数ξ来用于金属玻璃中STZ产生区域的预测,它由两部分构成:原子体积场的拉普拉斯算子和体积场梯度分量的绝对差值.原子体积场的拉普拉斯算子为负且绝对值较大时,体积场梯度向量呈现向内指的分布特征,代表体系中的局域软区;而体积场梯度分量的绝对差值则用于遴选体积场梯度不同的分布模式.进一步地,建立了该结构参数与非仿射位移和剪切局部化三者关系,发现特定的体积场梯度向量分布模式,将导致局部剪切增强的非仿射位移场,从而更容易诱发STZ的形成.相关性分析表明,该参数与STZ区域平均相关性高于78%,因此,该参数能有效用于金属玻璃剪切转变区的预测,且运用拉普拉斯算子的思想有望应用于金属玻璃力学行为的理论分析.

Shear transformation zone(STZ), as a basic characteristic unit of plastic events in metallic glasses(MGs), has been widely accepted by researchers, but the source of its origin and activation mechanism are still controversial. Deformation behaviours of Cu64Zr36 MGs under simple shear loadings are investigated using molecular simulation method in this paper. The results indicate that the activation locations of STZ are related to the initial configuration of MGs. Though the field of atomic volume and its gradient are a direct representation of the local atomic structural heterogeneity of MGs,they lack an obvious correlation to the regions of STZ activation. A new local structural parameter ξ is proposed in this paper based on the initial configuration of MG to predict the potential regions of STZ. ξ is the product of two factors:the Laplacian of atomic volume field(AVF) and the absolute difference between components of the gradient of AVF.Vectors of the AVF gradient present a distribution pattern of pointing inside if the Laplacian of AVF is negatively large,representing the localized soft regions in MGs. The absolute difference of AVF gradient components is used to select different patterns of the AVF gradient distribution. Furthermore, the relationship among structural parameter ξ, nonaffine displacement and shear localization is established, revealing that only certain patterns of AVF gradient distribution would lead to nonaffine displacements field strengthening shear localization, which is more likely to result in activation of STZs.The correlation analysis shows that the averaged spatial correlation index of ξ and STZ is larger than 78%, so ξ can be used as an effective parameter for predicting the activation regions of STZs in MGs. Moreover, the ideology of using Laplacian of local AVF in predicting potential STZ regions in MGs would bridge the analysis between atomic simulations of MGs, the mechanism of STZ activations and the traditional mechanical theory.