Stability and elastic anisotropy of the β′ and γ'' phases in Mg-Gd-Ag alloys at finite temperatures by first principles calculations

• Published in: Journal of materials research and technology 2024-11, Vol.33, p.9195-9207
• Main Authors: Xiao, Zhenyu, Xu, Shiwei, Huang, Weiying, Jin, Chen, Lin, Zhanhong
• Format: Article
• Language: English
• Subjects: Elastic anisotropy; First-principles calculations; Mg alloys; Thermal properties; γ'' phase
• ISSN: 2238-7854
• DOI: 10.1016/j.jmrt.2024.11.225

The β′ and γ'' phases are both critical strengthening phases in Mg-Gd-Ag alloys, yet research on the intrinsic properties of γ'' phase is limited. This study employs first-principles calculations to assess the stability and elastic properties of γ'' phase and compare them with the β′ phase. For the first time, the reason why the γ'' phase cannot periodically stack along its axis is revealed: the improper position of Gd atoms leads to imaginary frequencies and prevents its period stacking independently. However, introducing three layers of α-Mg between the γ'' phases eliminates these imaginary frequencies and forms the typical α/γ''/α phase of sandwich structure with good thermal, mechanical and dynamical stability. The results also show that the α/γ''/α phase exhibits superior thermal stability and better thermoelastic properties than both the α-Mg and the β′ phase. Moreover, the α/γ''/α phase shows lower elastic anisotropy than β′ phase, maintaining superior elastic modulus across all directions and temperatures. Specifically, the shear modulus mismatch between the β′ phase and the α-Mg matrix is lower and even becomes negative in some directions, making it less efficient in shear strengthening. By contrast, the higher modulus mismatch and larger lattice mismatch of the α/γ''/α phase, enhance its shear strengthening effects on the α-Mg matrix. These findings enhance the understanding of the γ'' phase's role in strengthening, informing the design of high-performance heat-resistant Mg alloys for future applications.