High-Temperature Micromechanical Properties of (MoTa)₂₅Ti₂₅V₂₅Zr₂₅ High Entropy Alloy

Refractory high entropy alloys have promising complexity and structural stability for next-generation nuclear reactors [1] and high-temperature applications [2]. A large number of compounds as single-phase solid solutions or multiphase options with additional precipitates from directly as-cast or triggered by post-treatment have been investigated in many studies [3], [4]. Thanks to the employment of low-activation refractory elements [5], these high entropy alloys can demonstrate better characteristics to meet the requirements of design and material performance. These alloys have favorable combinations of high strength and chemical stability at elevated temperatures. Recently, Ayyagari and co-workers [6], [7] reported a BCC+BCC microstructure with V-rich decoration around W-Ta dominated phase. This alloy has met hardness stability at elevated temperatures. However, this equiatomic alloy could not provide sufficient ductility at room temperature because of more than 60 wt.% W-Ta content. As a conceptual alternative to WTaTiVZr, MoTaTiVZr alloy was produced via arc-melting. The amount of Mo and Ta was selected as 25 at.% to decrease the brittle phase ratio embedded in TiVZr plus less than 10 at.% Mo-Ta binder phase. V-rich decorations were also observed in both as-cast and homogenized forms. However, Ti distribution in as-cast form was much more homogenous than in Ayyagari’s studies [6], [7]. The grain size stability and phase transition characteristic of (MoTa)₂₅Ti₂₅V₂₅Zr₂₅ was investigated after different homogenization treatments. The effects of selected homogenization treatments on microstructure and formation of inherent brittle precipitates were determined through X-ray diffractometer, scanning electron microscopy with energy dispersive X-ray, and electron backscatter diffraction analysis. After 1400 °C for 24 hours, V segregated to Mo-Ta rich phases whereas the binder phase decomposed to the Zr-rich and Zr-Ti dominated phases. Microstructural stability and micromechanical properties of (MoTa)₂₅Ti₂₅V₂₅Zr₂₅ alloy up to 600 °C will be discussed by nanoindentation analysis.<br/>References<br/>[1] E. J. Pickering, A. W. Carruthers, P. J. Barron, S. C. Middleburgh, D. E. J. Armstrong, and A. S. Gandy, “High-entropy alloys for advanced nuclear applications,” <i>Entropy</i>, vol. 23, no. 1. MDPI AG, pp. 1–28, Jan. 01, 2021. doi: 10.3390/e23010098.<br/>[2] O. N. Senkov, G. B. Wilks, D. B. Miracle, C. P. Chuang, and P. K. Liaw, “Refractory high-entropy alloys,” <i>Intermetallics (Barking)</i>, vol. 18, no. 9, pp. 1758–1765, Sep. 2010, doi: 10.1016/j.intermet.2010.05.014.<br/>[3] M. C. Troparevsky, J. R. Morris, P. R. C. Kent, A. R. Lupini, and G. M. Stocks, “Criteria for predicting the formation of single-phase high-entropy alloys,” <i>Phys Rev X</i>, vol. 5, no. 1, 2015, doi: 10.1103/PhysRevX.5.011041.<br/>[4] Y. Zhang, Y. J. Zhou, J. P. Lin, G. L. Chen, and P. K. Liaw, “Solid-solution phase formation rules for multi-component alloys,” <i>Adv Eng Mater</i>, vol. 10, no. 6, pp. 534–538, Jun. 2008, doi: 10.1002/adem.200700240.<br/>[5] A. Kareer, J. C. Waite, B. Li, A. Couet, D. E. J. Armstrong, and A. J. Wilkinson, “Short communication: ‘Low activation, refractory, high entropy alloys for nuclear applications,’” <i>Journal of Nuclear Materials</i>, vol. 526, Dec. 2019, doi: 10.1016/j.jnucmat.2019.151744.<br/>[6] A. Ayyagari, R. Salloom, S. Muskeri, and S. Mukherjee, “Low activation high entropy alloys for next generation nuclear applications,” <i>Materialia (Oxf)</i>, vol. 4, pp. 99–103, Dec. 2018, doi: 10.1016/j.mtla.2018.09.014.<br/>[7] M. Pole, M. Sadeghilaridjani, J. Shittu, A. Ayyagari, and S. Mukherjee, “High temperature wear behavior of refractory high entropy alloys based on 4-5-6 elemental palette,” <i>J Alloys Compd</i>, vol. 843, Nov. 2020, doi: 10.1016/j.jallcom.2020.156004.