[Paper Review] Solute Induced Defect Phase Transformations in Mg Grain Boundaries

P. Mathews, S. Zhang+4 more|arXiv (Cornell University)|Mar 16, 2023

Microstructure and mechanical properties4 citations

TL;DR

This study investigates solute-induced defect phase transformations in Mg grain boundaries using ab initio simulations and defect phase diagrams, revealing that Ga segregation stabilizes the A-type structure in a Σ7 (12̄30) [0001] GB under compressive stress or elevated temperature. Key findings include two distinct defect phase transitions: one driven by stress/temperature and another by chemical potential, with Ga preferentially occupying site a1 at low concentrations and shifting to site a3 at higher concentrations, accompanied by Mg5Ga2 precipitate formation upon nucleation barrier overcoming.

ABSTRACT

The study of defect phases is important for designing nanostructured metals and alloys. Grain boundaries (GBs) form one class of defects that directly influence materials properties, such as deformability and strength. At the same time, alloying can introduce GB phase transformations and can therewith alter mechanical performance. In this work, the defect phases of a $Σ$7 $(12\bar{3}0)$ [0001] 21.78$^\circ$ symmetric tilt GB in hcp Mg are investigated. Ab-initio simulations as a function of stress and temperature (using quasi-harmonic approximation) are performed, and different types of phase transformations are revealed. To this end, the influence of the chemical degree of freedom on the defect phases is studied for the example of Ga addition, using an efficient screening approach that combines empirical potentials and accurate ab-initio calculations. By exploiting the concept of defect phase diagrams, a phase transformation from the T to the A structural type and as well as a systematic transition of the segregation site preference is revealed. The results qualitatively agree well with experimental observations from scanning transition electron microscopy. The underlying physical mechanisms have an impact on grain-boundary engineering in metallic alloys.

Motivation & Objective

• To understand how solute addition (Ga) induces defect phase transformations in Mg grain boundaries.
• To construct a defect phase diagram (DPD) for Ga in a Σ7 Mg grain boundary, capturing structural and segregation transitions.
• To identify the thermodynamic and kinetic drivers of phase stability and solute site preference under varying stress, temperature, and chemical potential.
• To bridge computational predictions with experimental observations from scanning transmission electron microscopy (STEM).

Proposed method

• Ab initio density functional theory (DFT) calculations were used to compute defect phase stability as a function of stress and temperature, employing the quasi-harmonic approximation.
• An efficient screening protocol combined empirical Mg-Al MEAM potentials with DFT to pre-select low-energy configurations for 2–6 Ga atoms at the GB.
• Defect phase diagrams (DPDs) were constructed by varying the Ga chemical potential, enabling mapping of structural and segregation transitions.
• Segregation energies were calculated for multiple configurations to identify stable solute sites (a1, a3) and track site preference evolution with solute concentration.
• Nucleation barriers for Mg5Ga2 precipitate formation were evaluated to assess kinetic accessibility of equilibrium states.
• Experimental validation was performed by comparing predicted defect structures with STEM observations.

Experimental results

Research questions

• RQ1 How does compressive stress or elevated temperature induce a structural transition from the T-type to A-type defect phase in the Σ7 Mg grain boundary?
• RQ2 How does Ga segregation alter the stability of A and T-type grain boundary structures, and what is the resulting site preference evolution with increasing Ga concentration?
• RQ3 What is the role of chemical potential in driving a second-order defect phase transformation involving a shift in Ga segregation from site a1 to site a3?
• RQ4 How do nucleation barriers influence the formation of Mg5Ga2 precipitates in Ga-rich conditions, and when does this phase become thermodynamically accessible?
• RQ5 To what extent do the predicted defect phase transitions and solute site preferences agree with experimental STEM observations?

Key findings

• The T-type structure is the ground state at 0 K, but compressive stress or temperatures above 450 K stabilize the A-type structure, indicating two first-order defect phase transitions.
• Ga segregation stabilizes the A-type structure, with the lowest-energy configuration involving one Ga atom at the a1 site at low solute concentrations.
• A systematic shift in Ga segregation preference occurs from site a1 to site a3 as the Ga chemical potential increases, indicating a chemistry-induced defect phase transition.
• At high Ga concentrations, the system transitions to a state where four Ga atoms occupy the a3, b, e, e’ sites in a stable configuration, with the a3 site becoming energetically favorable only under solute-rich conditions.
• The formation of Mg5Ga2 precipitates is thermodynamically favored but kinetically hindered by a high nucleation barrier of 0.606 eV, which must be overcome for precipitation to occur.
• The predicted defect phase behavior and solute site preferences show strong qualitative agreement with experimental STEM observations of Ga-decorated grain boundaries.