Analysis of interface properties and associated void of nanoscale Al precipitates in Al-Si alloys: First-principles calculations and experiment

Abstract

In this paper, the properties of the interface between nanoscale Al precipitate and Si matrix were studied by the first-principles calculations combined with high-resolution transmission electron microscopy (HRTEM) observation. The purpose of this work is to reveal its role on the mechanical properties of the eutectic Si particle in Al-Si alloys. Meanwhile, the formation mechanisms of the void associated with every Al precipitate were also discussed in depth. The most thermodynamically stable interface structure was obtained by the calculation of the work of adhesion (Wad) and interfacial energy (γint) of the 12 models of Al(11¯2)/Si(220) interface. By analyzing the electronic structure of the interface, such as charge density difference, and density of states (DOS), it is found that strong AlSi covalent bonds are formed at the interface, making the bonding strength higher than the inside of the Al precipitate. Therefore, the microcracks tend to form and propagate firstly inside the nanoscale Al precipitates instead of Al/Si interface. In order to explain the interesting phenomenon that there is always a void associated with every Al particle, the diffusion barrier of Al atom in the Si matrix and the binding energy of solute atom and a vacancy were calculated. The results show that the vacancies can be introduced by the change of the volume and the migration of the solute-vacancy pair during the diffusion and precipitation progress. Then these vacancies will be discharged along the close-packed direction (Al<111>) and gather at the Al{111}/Si{001} interface, forming the visible void. This agrees very well with the experimental observation.