Morphology of Critically-Sized Crystalline Nuclei at Shear-Induced Crystal Nucleation in Amorphous Solid

TL;DR

This study investigates the morphology and critical size of crystalline nuclei during shear-induced crystallization in an amorphous system using molecular dynamics simulations, revealing how shear rate and temperature influence nucleus shape, orientation, and size.

Contribution

It provides a detailed quantitative analysis of the morphology and critical parameters of nuclei under shear, introducing new insights into their orientation, shape evolution, and size scaling laws.

Findings

Critical nuclei are oriented within the shear-gradient plane at moderate/high shear rates.

Nucleus shape becomes more elongated and aspherical with increasing shear rate.

Critical size scales with shear rate as n_c ∝ (γ̇ τ_c)^{1/3}.

Abstract

In this work we study morphological characteristics of the critically-sized crystalline nuclei at initial stage of the shear-induced crystallization of a model single-component amorphous (glassy) system. These characteristics are estimated quantitatively through statistical treatment of the non-equilibrium molecular dynamics simulation results for the system under steady shear at various (fixed) values of the shear rate $\dot\gamma$ and at different temperatures. It is found that the sheared glassy system is crystallized through nucleation mechanism. From analysis of a time-dependent trajectories of the largest crystalline nuclei, the critical size $n_{c}$ and the nucleation time $\tau_{c}$ were defined. It is shown that the critically-sized nuclei in the system are oriented within the shear-gradient $xy$-plane at moderate and high shear rates; and a tilt angle of the oriented nuclei depends on the shear rate. At extremely high shear rates and at shear deformation of the system more than $60$\%, the tilt angle of the nuclei tends to take the value $\simeq45^\circ$ respective to the shear direction. We found that this feature depends weakly on the temperature. Asphericity of the nucleus shape increases with increasing shear rate, that is verified by increasing value of the asphericity parameter and by the contour of the pair distribution function calculated for the particles of the critically-sized nuclei. The critical size increases with increasing shear rate according to the power-law, $n_{c}\propto(\dot\gamma\tau_{c})^{1/3}$, whereas the shape of the critically-sized nucleus changes from spherical to the elongated ellipsoidal. We found that the $n_{c}$-dependencies of the nuclei deformation parameter evaluated for the system at different temperatures and shear rates are collapsed into unified master-curve.