Theoretical and experimental study on phase stability of TiVZrMoW refractory high entropy alloy

ABSTRACT

An equiatomic quinary TiVZrMoW refractory high entropy alloy (RHEA) is selected for studying phase evolution and stability. The prediction of phases that may form on synthesis is made by (i) Semi-empirical /empirical methods (based on extended Hume-Rothery rules), (ii) CALPHAD and (iii) ab-initio methods. Mixing enthalpy (calculated by Miedema method) and valence electron concentration predicted the formation of the BCC phase (; and VEC = 5). However, the atomic mismatch factor, δ (7.17%) value being higher than the desired value, can de-stabilize the lattice to form a multi-phase structure. CALPHAD technique predicted that at room temperature three BCC and two Laves phases would form. The enthalpy of mixing (4.25 kJ/mol) calculated using Density Functional Theory (DFT) also suggests phase separation at room temperature. The phases and microstructure that evolved in the as-cast and annealed samples are studied by X-ray diffraction (XRD), scanning electron and transmission electron microscopy (TEM) techniques. Two BCC phases and a minor amount of ordered B2 and C15 type Laves phase are found to co-exist in the as-cast sample. The primary BCC phase (BCC#1) is rich in Mo and W (a = 3.17 ± 0.02 Å), while the minor BCC phase (BCC#2) is rich in Zr and Ti (a = 3.65 ± 0.02 Å). The annealed sample shows the transformation of both the disordered BCC phase to the ordered B2 phases and the increase in the amount of C15 Laves phase. The phase evolution and their relative stability in this RHEA has been discussed after analysing the experimental and theoretical results.

KEYWORDS:

• Refractory high entropy alloy
• density functional theory
• CALPHAD
• Laves phase

Acknowledgement

The authors would like to thank Profs S. Lele, R. K. Mandal, Dr J. Basu and Dr Vikas Jindal for many stimulating discussions. The authors acknowldge to help received from Dr R. Manna for extending the facilities of the Advance research center for Iron Steel (ARCIS) as its coordinator and from Dr Vikas Jindal for providing the arc-melting facilities.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The authors would also like to acknowledge the support of the Department of Science and Technology (DST), Government of India under the scheme of ‘Fund for the Improvement of S&T Infrastructure’ (DST-FIST) (Grant No. TPN-32537) for providing Electron Microscopy facility.