On phase stability of Mo-Nb-Ta-W refractory high entropy alloys

• Published in: International journal of refractory metals & hard materials 2022-02, Vol.103, p.105780, Article 105780
• Main Authors: Zhang, Enkuan, Tang, Ying, Wen, Mingwen, Obaied, Abdulmonem, Roslyakova, Irina, Zhang, Lijun
• Format: Article
• Language: English
• Subjects: 3rd-generation thermodynamic descriptions; A2/B2 phase transition; Casting alloys; High entropy alloys; High temperature; Intermetallic phases; Low temperature; Materials selection; Mo-Nb-Ta-W RHEAs; Molybdenum; Niobium; Non-equilibrium solidification; Phase diagrams; Phase equilibria; Phase stability; Quaternary systems; Regression models; Solidification; Tantalum; Temperature; Thermodynamic equilibrium; Thermodynamic properties; Thermodynamics
• ISSN: 0263-4368; 2213-3917
• DOI: 10.1016/j.ijrmhm.2022.105780

Refractory high entropy alloys (RHEAs) emerge as promising candidate materials for ultrahigh-temperature applications. Accurate phase stability is the prerequisite for design of high-performance RHEAs, but still missing in the literature due to the big challenge in experiments. In this paper, the reliable 3rd-generation Gibbs energy expressions for pure Mo, Nb, Ta and W were firstly evaluated by integrating the physical based Segmented Regression model with thermal vacancy description from 0 K to high temperatures. The thermodynamic database of the Mo-Nb-Ta-W quaternary system was then developed by combining the established 3rd-generation Gibbs energies with the phase equilibrium and thermodynamic data by using the CALPHAD (CALculation of PHAse Diagram) approach. The calculated phase equilibria, thermodynamic properties as well as the A2/B2 ordering transition behaviors show good agreement with those from experimental determination and theoretical calculation. The phase constitutes and elemental distributions in an equiatomic MoNbTaW as-cast alloy were experimentally investigated and compared with the non-equilibrium solidification simulations to further verify the database reliability. The high-throughput mapping of phase stability within the composition-temperature space in various Mo-Nb-Ta-W alloys was then constructed by applying thermodynamic calculations. The alloying effects on the formation of B2 phase were systematically analyzed. The Mo-Nb-Ta-W alloys have a wide temperature region with a single A2 phase, while the A2 + B2 phase regions exist at very low temperatures. It is highly anticipated that the present accurate 3rd-generation thermodynamic database provides an important basis for efficient design and development of novel Mo-Nb-Ta-W RHEAs. [Display omitted] •Reliable 3rd-generation thermodynamic descriptions for Mo-Nb-Ta-W system were evaluated.•Non-equilibrium solidification experiment was carried out to verify database reliability.•Phase stability within composition-temperature space in Mo-Nb-Ta-W RHEAs was constructed.•Alloying effects on the formation of single A2 and A2 + B2 phase regions were analyzed.