Effect of Zr addition on the local structure and mechanical properties of Ti–Ta–Nb–Zr refractory high-entropy alloys

Solid solution strengthening is the major strengthening mechanism that accounts for the high strength of single-phase body-centered cubic (BCC) refractory high-entropy alloys (RHEAs). Local lattice distortion (LLD), often regarded as one of the core effects of HEAs, is generally believed to be deterministic in solid solution strengthening for RHEAs since the loosely packed BCC crystal structure can accommodate significant LLD. To systematically investigate the effect of LLD on solid solution strengthening, the present study deliberately introduced different degrees of LLD in the experimentally fabricated BCC Ti65-xTa25Nb10Zrx (x = 0, 5, 10, 15, and 20) RHEAs by varying the Zr content. Subsequently, by combining experimental analysis, first-principles calculations, and theoretical modeling, it is found that yield strength, hardness, atomic radii, and LLD increase with the increase of Zr content. Moreover, through quantitative solid-solution strengthening analysis, it is demonstrated that the modulus mismatch dominates solid solution strengthening over LLD even for severely distorted Zr-containing RHEAs, contrary to the generally accepted assumption that solid solution strengthening is mainly from LLD effect. What's even more surprising is that the increase of Zr content accelerates grain growth, opposite to the sluggish diffusion effect proposed for HEAs. Our results shall guide the elemental selection for the design of high-strength RHEAs eradicating the random sampling in the endless compositional pool.