Tensile properties of high- and 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.

Mechanical and wear characteristics of Aluminium-7068-composites structurally modified with medium-entropy-alloy.

Medium-entropy alloys (MEAs) have attracted considerable attention in recent decades due to their exceptional material properties and design flexibility. In this study, lightweight and non-equiatomic MEAs with low density (~5 g/cm3), high strength (yield strength: 1200 MPa), and high ductility (plastic deformation: ≧10%) were explored. We fine-tuned a previously developed Ti-rich MEA by microalloying it with small amounts of Ni (reducing the atomic radius and increasing the elastic modulus) through solid solution strengthening to achieve a series of MEAs with enhanced mechanical properties. Among the prepared MEAs, Ti65Ni1 and Ti65Ni3 exhibited optimal properties in terms of the balance between strength and ductility. Furthermore, the Ti65Ni3 MEA was subjected to thermo-mechanical treatment (TMT) followed by cold rolling 70% (CR70) and cold rolling 85% (CR85). Subsequently, the processed samples were rapidly annealed at 743 °C, 770 °C, 817 °C, and 889 °C at a heating rate of 15 °C/s. X-ray diffraction analysis revealed that the MEA could retain its single-body-centered cubic solid solution structure after TMT. Additionally, the tensile testing results revealed that increasing the annealing temperature led to a decrease in yield strength and an increase in ductility. Notably, the Ti65Ni3 MEA sample that was subjected to CR70 and CR85 processing and annealed for 30 s exhibited high yield strength (>1250 MPa) and ductility (>13%). In particular, the Ti65Ni3 MEA subjected to CR85 exhibited a specific yield strength of 264 MPa·cm3/g, specific tensile strength of 300 MPa·cm3/g, and ductility of >13%.

Functional Medium Entropy Alloys for Joint Replacement: An Atomistic Perspective of Material Deformation and a Correlation to Wear, Corrosion, and Biocompatibility

Interstitial carbon content effect on the microstructure and mechanical properties of additively manufactured NiCoCr medium-entropy alloy

Enhancement in mechanical properties through an FCC-to-HCP phase transformation in an Fe-17.5Mn-10Co-12.5Cr-5Ni-5Si (in at%) medium-entropy alloy

Effect of room temperature rolling and annealing on microstructure and mechanical properties of (CrCoNi)96V4 medium entropy alloy

Phase evolution, microstructure, and mechanical behaviors of the CrFeNiAlxTiy medium-entropy alloys

The temperature dependence of the mechanical properties of the CoCrFeNi medium-entropy alloy (MEA) manufactured by laser-directed energy deposition (L-DED) and additionally annealed at 1200 °C for 24 h was studied. The microstructure of the as-deposited alloy was represented by a single-phase face-centered cubic structure with coarse columnar grains and a high density of dislocation. Annealing resulted in the development of recrystallization and a reduction in dislocation density. The CoCrFeNi alloy produced by L-DED demonstrated mechanical properties comparable with those of the fine-grained equiatomic CoCrFeMnNi alloy, produced by casting followed by thermomechanical processing. Namely, as-deposited CoCrFeNi had a yield strength (YS) and ultimate tensile strength (UTS) of YS = 370 MPa and UTS = 610 MPa at room temperature, and YS = 565 MPa and UTS = 965 MPa at cryogenic temperature, along with a ductility of ~60%. Annealing resulted in a decrease in strength to YS = 180/350 MPa at 293/77 K. A quantitative analysis of various strengthening mechanisms showed that some strength increment of the as-deposited alloy was ensured by the high dislocation density formed during L-DED.

Microstructures and deformation mechanisms of the medium-entropy alloy (NiCoCr)76(Ni6AlTi)3

Ultrastrong medium entropy alloy with simultaneous strength-ductility improvement via heterogeneous nanocrystalline structures

Plastic deformation of single crystals of the equiatomic Cr-Fe-Co-Ni medium entropy alloy – A comparison with Cr-Mn-Fe-Co-Ni and Cr-Co-Ni alloys

Superior strength-ductility synergy and strain hardenability of Al/Ta co-doped NiCoCr twinned medium entropy alloy for cryogenic applications

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

Microstructure, tensile properties and deformation behaviour of a promising bio-applicable new Ti35Zr15Nb25Ta25 medium entropy alloy (MEA)