Equiatomic and non-equiatomic Ti-Zr-Nb-Ta refractory medium entropy alloys

High entropy alloys have been developed over the last two decades and are most commonly defined as synthesizing from five or more principle elements in which the concentration of each element ranges from 5 at.% to 35 at.%. Yeh [1] classified the metallic alloys world into three groups:  high entropy alloys (HEAs), medium entropy alloys (MEAs), and low entropy alloys (common metallic alloys). This novel class of MEAs and HEA metallic materials offer a wide range of new alloy composition as well as promising properties for advanced applications such as jet-engine in aerospace and fusion power reactor components [2, 3]. In this thesis, using the high entropy concepts, both new equiatomic and non-equiatomic Ti-Zr-Nb-Ta MEAs have been designed and fabricated as novel candidate alloys for potential bio-applications as well as for structural applications at room temperature and elevated temperatures. A single phase equiatomic Ti-Zr-Nb-Ta MEA was first designed using the empirical rule, assisted with the CAPHAD approach. The equiatomic Ti-Zr-Nb-Ta MEA was fabricated using the arc-melting method and then the effect of homogenization annealing treatments on its microstructure and mechanical properties at room temperature and elevated temperatures was studied. After that, the alloy composition was tailored to obtain lighter and more affordable non-equiatomic Ti-Zr-Nb-Ta MEAs using the atomic mismatch (δ, %) approach. The microstructure and mechanical properties of these non-equiatomic MEAs were studied from room temperature to 1200 °C. The major developments made in this thesis are summarized below.A novel quaternary equiatomic Ti-Zr-Nb-Ta (δ = 4.8 %) MEA, was redesigned from a quinary equiatomic Ti-Zr-Nb-Ta-Mo (δ = 5.5 %) HEA, for much improved strength-ductility (tensile) combinations by reducing d through excluding Mo and for potentially improved biocompatibility.The effect of heat treatment (at 1200oC for 8 h and 24 h) on the phase stability, microstructure, and mechanical properties of the equiatomic Ti-Zr-Nb-Ta MEA has been investigated in detail. A cuboid-like nanostructure in the matrix and a lamellar structure at the grain boundary region formed after annealing of 8 or 24 hours at 1200oC. This nanostructure is responsible for a significant increase in compression yield strength from 1100 ± 90 to 1760 MPa ± 25 compared with its as-cast counterpart.The microstructure and compression behaviours of the homogenised equiatomic Ti-Zr-Nb-Ta ME were investigated at temperatures from 600oC to 1200oC. The yield strength, σ0.2, of the homogenised MEA halved from ~1760 MPa to ~800 MPa, with increasing deformation temperature from room temperature to 600oC. However, the alloy still exhibited excellent softening resistance at 1000oC and 1200oC; its yield strength still remained ~410 MPa at 1000oC and ~210 MPa at 1200oC.Four non-equiatomic Ti25+xZr25Nb25Ta25-x (x = 5, 10, 15, 20, in at. %) MEAs were designed using the atomic mismatch approach. These novel MEAs were derived from the equiatomic Ti-Zr-Nb-Ta MEA by replacing part of the Ta content with Ti. Each non-equiatomic MEA solidified as a single solid-solution phase, and their microstructures were characterized in detail and compared with PandatTM simulation and the empirical rules. In particular, a brittle-to-ductile transition was observed with decreasing Ta content. As a result, both the as-cast Ti40Zr25Nb25Ta10 and Ti45Zr25Nb25Ta5 MEAs exhibited excellent tensile strain to fracture (>18%) and tensile strength (>900 MPa) with much reduced density compared with the low-ductility Ti25Zr25Nb25Ta25 MEA. Both MEAs are among a very small number of strong and ductile (tensile strain >15%) alloys which have been reported as MEAs or HEAs so far. High-temperature annealing (at 1200oC for 8 h) affects the phase stability, microstructure, and mechanical properties of the non-equiatomic Ta25-xZr25Nb25Ti25+x (x= 5, 10, 15, 20, at. %) MEAs. After homogenisation at 1200oC, a nano-cuboidal structure formed in the matrix and a secondary phase precipitated at the grain boundary in Ta20Zr25Nb25Ti30 (Ta20-HT) and Ta15Zr25Nb25Ti35 (Ta15-HT). In contrast, the homogenized Ta10Zr25Nb25Ti40 (Ta10-HT) and Ta5Zr25Nb25Ti45 (Ta5-HT) showed a stable single BCC solid-solution phase up to 1200oC. Both Ta20-HT and Ta15-HT exhibited high yield strength, but limited ductility at room temperature under compression.  In contrast, Ta10-HT and Ta5-HT showed excellent ductility at room temperature under both tension and compression.The collective results of this thesis provide a detailed understanding of the microstructure and mechanical properties of a new group of equiatomic and non-equiatomic TaZrNbTi refractory MEAs. These new alloys, which consist of biocompatible Ti, Nb, Zr, and Ta as principal constituent elements, exhibit promising mechanical properties for both biomedical applications and structural materials.