Effect of phase decomposition on the mechanical properties of Ti-Zr-Nb-Ta-Mo multi-principal element alloys

Body-centered cubic Ti-Zr-Nb-Ta-Mo multi-principal element alloys (MPEAs), boasting a yield strength exceeding one gigapascal, emerge as promising candidates for demanding structural applications. However, their limited tensile ductility at room temperature presents a significant challenge to their processability and large-scale implementation. This study identifies phase decomposition as a critical factor influencing the plasticity of these alloys. The microscale phase decomposition in these MPEAs during solidification, driven by miscibility gaps, manifests as dendritic structures within grains. Closer examination reveals that the MPEAs with a pronounced thermodynamic propensity for phase decomposition are also susceptible to analogous phenomena at the atomic level. The atomic phase decomposition is characterized by the localized aggregation of some elements across nanometric domains, culminating in the establishment of short-range orderings (SROs). It is observed that phase decomposition for these MPEAs, occurring at both microscale and atomic scale, adheres to thermodynamic principles and can be predicted using the CALPHAD approach. The impact of phase decomposition on the plasticity of MPEAs fundamentally stems from the induced heterogeneities at three distinct levels: (1) Fluctuations in mechanical properties at the micron scale; (2) Variations in the strain field at the atomic scale; (3) Bond polarization and bond index fluctuations at the electronic scale. Consequently, the key to designing high-strength and high-plasticity MPEAs lies in maximizing lattice distortion while simultaneously minimizing the adverse effects of phase decomposition on the alloy's plasticity (grain boundary cohesion). This research not only clarifies the mechanisms underpinning the ductile-to-brittle transition in high-strength Ti-Zr-Nb-Ta-Mo MPEAs but also offers crucial guidelines for developing advanced, high-performance alloys.