Grain growth and thermal stability in multi-phase nanocomposite materials

: a Monte Carlo study

Abstract

The interplay between phase composition and grain structure (size and distribution) is of paramount importance in understanding of a new material. This applies in particular to multicomponent systems in multiphase nanocomposite materials where interfaces, structure boundaries, amorphous volumes, interfacial boundaries and other features lead to complex behavior. Understanding and eventually controlling these effects is a prerequisite for designing and improving materials. In principle, modelling and simulation are ideally suited to complement and guide experimental efforts, especially as dimensions shrink and phase complexity increases. Monte Carlo (MC) method is an effective simulation technique to simulate the microstructural evolution process and provide valid structural significance. Also the microstructural evolution depends only on the topology of the grains while not on any kind of geometric simplification. These all make MC methods widely used for the grain growth simulation. In this PhD study, a modified MC method based on Q-state Potts model is applied to systematically study the grain growth behavior and microstructures in the multi-phase nanocomposite materials. The first important work was achieved by studying the grain refining effect of energy competition and amorphous phase in nanocomposite materials by means of two-dimensional (2D) MC method. For a two-phase nanocomposite comprising of nanocrystalline (nc-) phase surrounded by amorphous (a-) matrix, the simulations showed that the ratio of nc-nc grain boundary energy (Jgb) and nc-a interfacial energy (Jint) under a small fraction (f) of amorphous phase determines the two-phase microstructure and controls its stability. The optimal stable microstructure was found with Jgb/Jint = 10 and f = 0.15, where almost all the grains are surrounded by the thinner amorphous phase and the grain size distribution obeys the log-normal form very well. The second significant work in this study was to study the grain growth, the structural and thermal stability in the three-phase nanocrystalline-amorphous-nanocrystalline (nc-a-nc) system consisting of two immiscible nanocrystalline phases embedded in an amorphous matrix by modified MC Potts method. Compared to the grain growth behavior in the two-phase nc-nc system, the addition of amorphous phase in the nc-a-nc system has significant influence on the grain growth behavior of both the nanocrystalline phases and thus the microstructure of the system. The amorphous matrix along the grain boundaries can change the boundary compositions and grain growth behaviors of both nanocrystalline phases and thus lead to a remarkable reduction in grain size and kinetic grain growth exponent, and also it changes the grain size distribution to approach log-normality and enhances the grain size stability under high temperature. The coverage of nanocrystalline grains by the amorphous phase, in other words, the nc-a interfacial boundary area around the nanocrystalline grains was found to be a decisive factor in enhancing structural and thermal stability, where the system gains thermodynamic stability only when maximizing the nc-a interfacial boundary area and simultaneously minimizing both the nanocrystalline grain size and amorphous thickness. The optimal stable nanostructure can maintain grain size stability up to a relatively high temperature of 1350 K with the nanocrystalline grains surrounded by one monolayer amorphous phase and log-normal grain size distribution. Finally, the three-dimensional (3D) modified MC model (the code was written and the method was developed) was also applied to study the two-phase nc-a nanocomposite materials. Since the microstructural evolutions of nanocomposite materials mostly take place in 3D spaces, thus the 3D grain growth simulation has more persuasive and valid physical significance. The dimensional extension from 2D to 3D simulations and the choice of simulation variables affect the grain growth characteristics in some ways. The results of the grain growth simulation in grain growth kinetics, grain size and topology were found to be similar to the experimental results. The topological evolution during the grain growth process can also be explicitly shown through the 3D simulations.

Date of Award	2 Oct 2015
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yaogen SHEN (Supervisor) & Kwok Yan LI (Co-supervisor)