Construction and analysis of surface phase diagrams to describe segregation and dissolution behavior of Al and Ca in Mg alloys

TL;DR

This study uses density functional theory and surface phase diagrams to analyze how Al and Ca behave on Mg alloy surfaces, revealing water's significant impact and contrasting segregation behaviors.

Contribution

It introduces a combined ab initio and cluster expansion approach to construct surface phase diagrams for Mg alloys with Al and Ca, elucidating segregation and dissolution trends.

Findings

Ca strongly segregates and dissolves in water.

Al remains in the Mg bulk, showing anti-segregation.

Water dramatically affects surface stability and composition.

Abstract

Segregation and dissolution behavior of Mg alloyed with Ca and Al are studied by performing density functional theory calculations considering an extensive set of surface structures and compositions. Combining ab initio surface science approaches with cluster expansion for ordered surface structures we construct surface phase diagrams for these alloys. We utilize these diagrams to study segregation phenomena and chemical trends for surfaces in contact with a dry environment or with an aqueous electrolyte. We show that the presence of water dramatically impacts the stability and chemical composition of the considered metallic surfaces. We furthermore find that the two alloying elements behave qualitatively different: whereas Ca strongly segregates to the surface and becomes dissolved upon exposure of the surface to water, Al shows an anti-segregation behavior, i.e., it remains in Mg bulk. These findings provide an explanation for the experimentally observed increase/decrease in corrosion rates when alloying Mg with Al/Ca.