Solute Segregation and Grain Boundary Cohesion of Magnesium Binary Alloys: A First-Principles Study

Abstract

Solute segregation at grain boundaries (GBs) can significantly influence GB cohesion. In this work, the segregation energies of solutes (Zn, Al, Ag, Ca, and Gd) were first investigated at six symmetrical tilt GBs rotating around [0001] axis of Mg, to uncover the impact of GB characteristics on solute segregation behavior. The results reveal that solute segregation propensity is closely related to the local geometric environment of GB sites, but has little correlation with intrinsic GB properties (such as GB misorientation and GB energy). Furthermore, relationships between GB site characteristics and solute segregation tendencies were established. Ca-like solutes tend to occupy GB sites with larger Voronoi volumes (V), while Zn-like solutes prefer GB sites with smaller V as well as smaller shortest bond lengths (SBL). Based on this finding, we further evaluated the segregation capacities of 26 solutes at their most energetically stable segregation sites and their impact on GB cohesion. A descriptor that can effectively capture the strengthening/embrittling potency of segregated solutes on GBs was proposed by performing the crystal orbital Hamilton population (COHP) analyses. It was found that the discrepancies in bond strength between GBs and free surface dominate the solute-strengthening behavior. Finally, a first-principles “design map” regarding the segregation energies and strengthening energies was provided, which offers a database for designing Mg alloys with high fracture toughness.