Mechanical behavior and thermal stability of Ti-Zr-Hf-Nb-Ta alloys

Refractory high entropy alloys (RHEAs) are interesting from both scientific and engineering viewpoints. Using group IV, V, and VI elements, the compositional space available to RHEAs grants greater freedom to obtain material properties that are often in competition with each other such as high melting temperature and room-temperature ductility. As a model system, Ti-Zr-Hf-Nb-Ta alloys show promise with their high melting temperature and ductility that allows for cold rolling. Despite over a decade since the discovery of the equiatomic alloy TiZrHfNbTa, there is still a dearth of basic composition-property data for its derivatives. This work aims to address three issues with respect to TiZrHfNbTa alloys: relationship between composition and (i) uniaxial tensile behavior, (ii) fracture toughness behavior, and (iii) thermal stability. The equiatomic alloy TiZrHfNbTa and four other alloys were investigated that represented relatively wide compositional changes within the Ti-Zr-Hf-Nb-Ta space that could be processed. With respect to tensile properties in the single-phase state, the alloys behave similarly and their yield strengths can be predicted well by an analytical theory based on edge dislocations. During fracture toughness testing, sudden crack extension is seen in TiZrHfNbTa but not the other alloys. Finally, the thermally stable phases in the four derivative alloys showed three unique microstructures: single-phase with incidental minor phases, HCP homogenously precipitated in BCC, and lamellar BCC-HCP microstructure akin to eutectoid microstructures.