First Principles Study Of Crystal Structures And Stabilities Of Strengthening Precipitates In Mg Alloys

Magnesium alloys are among the lightest commonly used structural materials and have been extensively studied to understand their structural behavior and in order to expand the range of their applications.However,the wide-spread adoption of magnesium alloys is limited by their inadequate strength.To tackle this problem,age hardening processes are widely utilized to form high-strength precipitates that impede the movement of dislocations in the alloy matrix.First-principles calculations based on density functional theory(DFT)are a useful tool to assess the structure of precipitates and their formation sequences.Several thermodynamic quantities that may be difficult to measure experimentally,such as the formation energies of the precipitates,can be readily computed.Based on a large amount of calculations,high-throughput computing contributes to the foundation of large databases containing both experimental structures and prototypes.Consequently,a new trend in materials design,is to screen data and search for new phases.In this work,we present a powerful approach that combines ab initio calculations and structure prediction techniques to analyze the stability of precipitates in Mg age-hardened alloy systems.Hypothetically,the formation sequences during age hardening process is based on thermodynamics of phases.Specifically,precipitates are classified by MgRE(rare earth elements),Mg-RE-RE,Mg-RE-Zn,Mg-Zn,Mg-Al,Mg-Sn,Mg-Ca-Zn,Mg-Al-Ca systems.For precipitates in each system,we calculate energetics related to their formation,and use it to predict the sequences in which they form from their precursors,thereby validating our hypothesis.In cases where the crystal structure for a precipitate has not yet been experimentally conclusively determined,we comprehensively search over decorations of many prototype structures,including hexagonal superstructures,and in addition perform global structural optimization using the minima hopping method to predict suitable crystal structures.In addition,we construct a monolayer model for the Guinier-Preston zones observed in the Mg-Nd-Zn system during early stages of age hardening,and thereby explain the formation of the γ′′(Mg5(Nd,Zn))phase from the G.P.zones,as observed in experiments.Our study aims to significantly improve our current understanding of age hardening precipitates in Mg,and serves as a guide for future experimental investigations of their behavior.