Compositional design of strong and ductile (tensile) Ti-Zr-Nb-Ta medium entropy alloys (MEAs) using the atomic mismatch approach

New non-equiatomic Ti(25+x)-Zr25-Nb25-Ta(25-x) (x = 0, 5, 10, 15, 20, in at%) medium entropy alloys (MEAs) have been designed using the atomic mismatch approach and fabricated through a conventional arc-melting process. These novel MEAs were derived from a recently developed equiatomic Ti-Zr-Nb-Ta MEA by gradually replacing its Ta content with Ti. Each non-equiatomic MEA solidified as a single solid-solution phase, which was characterised in detail and compared with Pandat™ simulation and empirical rules. Systematic tensile mechanical property data revealed the existence of a brittle-to-ductile transition for Ti-Zr-Nb-Ta MEAs, i.e., when 15 at% of Ta in the equiatomic Ti25-Zr25-Nb25-Ta25 MEA was replaced by Ti to become a Ti40-Zr25-Nb25-Ta10 MEA. The transition occurs corresponding to a small reduction in atomic mismatch from 4.72% to 4.65% but a signficant drop in nanoindentation hardness from 4.2 GPa to 3.5 GPa. In particular, both the as-cast Ti40-Zr25-Nb25-Ta10 and Ti45-Zr25-Nb25-Ta5 MEAs exhibited excellent tensile strain to fracture (>18%) and tensile strength (>900 MPa) with much reduced density compared to the brittle Ti25-Zr25-Nb25-Ta25 MEA. They are both among a very small number of strong and ductile (tensile strain >15%) HEAs reported to date. Their tensile mechanical properties can be further tuned by adjusting the atomic mismatch of the resulting single solid-solution phase in conjunction with the improved understanding of the microstructures of these MEAs.