Metastable defect phase diagrams as a tool to describe chemically driven defect formation: Application to planar defects

TL;DR

This paper introduces metastable defect phase diagrams as a new tool to understand chemically driven defect formation, extending traditional thermodynamic methods by incorporating kinetic effects, and demonstrates their application to specific alloy systems.

Contribution

It develops and applies metastable defect phase diagrams to better predict defect formation in alloys, accounting for kinetic limitations that traditional thermodynamics overlook.

Findings

Successfully explained planar defect formation in Fe-Nb alloys.

Identified conditions for defect occurrence in Mg-Al-Ca alloys.

Extended defect phase diagram concept to include kinetic effects.

Abstract

Thermodynamic bulk phase diagrams have become the roadmap used by researchers to identify alloy compositions and process conditions that result in novel materials with tailored microstructures. Recent experimental studies show that changes in the alloy composition can drive not only transitions in the bulk phases present in a material, but also in the concentration and type of defects they contain. Defect phase diagrams in combination with density functional theory provide a natural route to study these chemically driven defects. Our results show, however, that direct application of thermodynamic approaches can fail to reproduce the experimentally observed defect formation. Therefore, we extend the concept to metastable defect phase diagrams to account for kinetic limitations that prevent the system from reaching equilibrium. We successfully applied this concept to explain the formation of large concentrations of planar defects in supersaturated Fe-Nb solid solutions and to identify in a joint study with experiments conditions in Mg-Al-Ca alloys for defect phase occurrence. The concept offers new avenues for designing materials with tailored defect structures.