From first to second order nonequilibrium phase transition in crystal-amorphous interface: Effects of spatial and kinetic constraints

• Published in: Journal of alloys and compounds Vol. 850; p. 156841
• Main Authors: Zhu, Yiying, Wang, Hao, Wu, Lingkang, Li, Mo
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 05.01.2021; Elsevier BV
• Subjects: Amorphization; Amorphous alloys; Amorphous materials; Concentration gradient; Crystal defects; Crystal structure; Crystal-amorphous interface; Crystallinity; Hydrogen permeation; Melting points; Metallic glass; Metallic glasses; Molecular dynamics; Molecular dynamics simulation; Non-equilibrium transition; Phase transitions; Thermodynamic properties; Molecular dynamics simulation; Amorphization; Crystal-amorphous interface; Metallic glass; Non-equilibrium transition
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2020.156841

Crystalline materials undergo phase transitions to disordered state at the melting point when being heated; but the transition can also occur from crystal to amorphous solids when subject to various other stimuli such as chemical mixing, irradiation, or hydrogen permeation, etc. The amorphization transition observed in experiments is often first order, but theories predict a possibility of the second order. Here, we demonstrate that the first order transition can indeed become continuous in a diffusion couple using molecular dynamics simulation. The amorphization transition happens anisotropically at the interface between a crystal and an amorphous alloy with a steady concentration gradient. Under the non-equilibrium condition and the spatial constraint, the amorphization of the crystalline phase can proceed without the abrupt changes in thermodynamic properties often seen in the first order transition. A homogeneous model and theoretical analysis also confirm that an isentropic transition can appear when sufficient defects or impurity atoms are introduced under certain kinetic constraints. Our study provides a reasonable explanation for the inconsistency existing in the experiments and simulations or theories on the thermodynamic understanding of the disordering process in the crystal-amorphous transitions. (a-g) Atomic structural evolution of a crystal Al layer from 0 to 6000 ps. The upper panels are the atomic configurational views with Al atoms colored yellow, Cu blue and Zr red; the middle panels are the atomic configurational views colored by the values of the S(k) for each atom; and the bottom ones are the distribution of S(k) of the atoms with a bin of 0.05 Å. When Al layer is crystal, the values of S(k) is at the right side close to 1 and when more disorder occurs, the values of S(k) shifts to the left side, and eventually when amorphization occurs, the values approach zero. [Display omitted] •An unusual continuous transition in a diffusion couple made of an equilibrium crystal metal and a metastable amorphous phase is reported.•The transition from crystal to amorphous phase is driven by concentration gradient at the interface region where all finite changes of thermodynamic quantities disappear.•The cause for the transition is the kinetic constraint, i.e. the low temperature and limited time and space.