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Multi-principal-element metallic alloys 1,2 have created a growing interest that is unprecedented in metallurgical history, in exploring the property limits of metals [3][4][5][6][7] and the governing physical mechanisms [8][9][10][11] . Refractory high-entropy alloys (RHEAs) have drawn particular attention due to their (i) high melting points and excellent softening-resistance, which are the two key requirements for high-temperature applications 12 ; and (ii) compositional space, which is immense even after considering cost and recyclability restrictions 13 . However, RHEAs also exhibit intrinsic brittleness and oxidationsusceptibility, which remain as significant challenges for their processing and application. Here, utilizing naturalmixing characteristics amongst refractory elements, we designed a Ti38V15Nb23Hf24 RHEA that exhibits >20% tensile ductility already at the as-cast state, and physicochemical stability at high-temperatures. Exploring the underlying deformation mechanisms across multiple length-scales, we observe that a rare 𝛃′ precipitation strengthening mechanism governs its intriguing mechanical response. These results also reveal the effectiveness of natural-mixing tendencies in expediting HEA discovery.The attention that RHEAs received increased exponentially since the reporting of the first examples with single-phase bodycentered cubic (BCC) structures 14 , owing to the yield strength preservation tendency they exhibit at elevated temperatures.Since then, numerous alloys have been assessed both theoretically and experimentally, yet, the efficiency of this compositional search has been low. Apart from the immense compositional space that drastically hinders accurate phase diagram computations, there are several other serious obstacles 14,15 : (i) governed by the milder discrepancy between ideal shear strength and ideal tensile strength for cleavage formation 16 , the majority of the reported RHEAs demonstrate negligible tensile ductility and are intrinsically brittle at ambient temperature; (ii) extensive homogenization treatments are often inevitable due to the sluggish diffusion kinetics of refractory elements 17 ; and (iii) the presence of catastrophic oxidation at intermediate temperatures (800-1000 o C) retards the application of classical hot-processing treatments 18 . Due to these challenges and the absence of sufficiently-sophisticated thermodynamic-kinetic fundamentals, the RHEA rush has led to few alloys that exhibit applicationworthy mechanical performances 19,20 . We reveal in this work that by following an approach that exploits the natural mixing characteristics amongst refractory elements to minimize casting segregation, it is possible to expediently guide the RHEA composition search, and thereby to achieve excellent strength-ductility-high temperature stability combinations.

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By investigating a metastable high-entropy alloy, we report a latent strengthening mechanism that is associated with the thermally-induced epsilon-martensite-to-austenite back-transformation. We show this reversion-assisted hardening effect can be achieved in the same time-scale and temperature range as conventional bake-hardening treatment, but leads to both improved strength and cumulative ductility. Key mechanisms are discussed considering transformation kinetics, kinematics, strengthening and ductilization modules.