Preliminary exploration of a WTaVTiCr high-entropy alloy as a plasma-facing material

Development of cost-effective high-entropy alloys with superior mechanical properties

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.

Achieving a balance between mechanical properties at room and elevated temperatures of lightweight NiAlFeCrMoV high-entropy alloy

Systematic study of (MoTa)xNbTiZr medium- and high-entropy alloys for biomedical implants- In vivo biocompatibility examination

Effects of AlCoCrFeNiTi high-entropy alloy on microstructure and mechanical properties of pure aluminum

This work presented an upgraded high-entropy alloy by the addition of chromium to the conventional Ti-10V-2Fe-3Al alloy to fabricate equiatomic Ti20V20Al20Fe20Cr20 high-entropy alloy via spark plasma sintering powder processing at different temperature of 700 °C, 800 °C, 900 °C, 1000 °C, and 1100 °C respectively under a constant heating rate of 100 °C /min, the pressure of 40 MPa, and holding time of 5 min. The microstructure and phase transformation of the sintered alloyed were studied with a scanning electron microscope equipped with energy dispersive spectroscopy. The constituent phases present in the sintered high-entropy alloy were analyzed by X-ray diffraction and were found to show increased development of body-centered cubic solid solution alloys across the temperature gradients. The mechanical properties over a temperature range of 700 ≤ T °C ≤ 1100 generally show an increase in hardness from 3363 to 8480 MPa, tensile strength from 1097.17 to 2766.58 MPa, and yield strength from793.61 to 2001.13 MPa respectively. The SEM-EDS of Ti20V20Al20Fe20Cr20 equiatomic high-entropy alloys show the existence of a nano-net-like spinodal structure at the optimum temperature of 1100 °C, which are rich in body centered cubic structure. At this elevated temperature, the presence of strong body-centered cubic–forming elements such as Cr, Fe, and Al was established from the corresponding EDS. Ti20V20Al20Fe20Cr20 high-entropy equiatomic alloy has been successfully fabricated by spark plasma sintering. The effects of temperature on microstructural and mechanical properties of the sintered Ti20V20Al20Fe20Cr20 alloy demonstrated a general improvement of the alloys at the elevated region.

Abstract

High entropy alloys (HEAs) have attracted great attention due to their impressive properties induced by the severe lattice distortion in comparison to the conventional alloys. However, the effect of severe lattice distortion on the mechanical properties in face-centered-cubic (FCC) and body-centered-cubic (BCC) structured HEAs is still not fully understood, which are critically important to the fundamental studies as well as the industrial applications. Herein, a theoretical model for predicting the lattice-friction resistance and the yield stress in the FCC and BCC HEAs accounting for the lattice distortion is presented. Both the calculated lattice-friction resistance and the yield strength are compared to the experimental results, to verify the rationality of the built theoretical model. Moreover, the effect of the grain-size distribution on the yield strength is theoretically considered, which reveal the origin of multistage grain structure strengthening. The numerical predictions considering the severe lattice-distortion effect agree well with the experimental results for both FCC and BCC HEAs, in terms of the yield strength and the lattice-friction resistance. The grain-boundary strengthening dominates the yield strength in the FCC Al0.3CrCoFeNi HEA. The yield strength is governed by the lattice-friction resistance in the BCC TaNbHfZrTi HEA, agreeing with the previous work. In AlxCrCoFeNi HEAs, the Al concentration dominates the lattice-friction resistance, and the atomic-radius mismatch and shear-modulus mismatch induced by other four-principal-elements govern the lattice-friction lattice. The atomic-size mismatch dominates the lattice distortion in HEAs, and this viewpoint differs from the traditional knowledge that the increasing incorporated principal element controls the lattice distortion. These results provide the insight into the effect of the severe lattice distortion on the yield strengths in HEAs from the theoretical perspective, for discovering advanced high-strength HEAs.

Molecular dynamics simulation and machine learning-based analysis for predicting tensile properties of high-entropy FeNiCrCoCu alloys

Evolution of high temperature yield strength of AlCoCrFeNiTi high entropy alloys

Lattice-distortion dependent yield strength in high entropy alloys

WC-based cemented carbide with NiFeCrWMo high-entropy alloy binder as an alternative to cobalt

High-entropy alloys (HEAs) have shown promising potential applications in advanced reactors due to the outstanding mechanical properties and irradiation tolerance at elevated temperatures. In this work, the novel low-activation Ti2ZrHfxV0.5Ta0.2 HEAs were designed and prepared to explore high-performance HEAs under irradiation. The microstructures and mechanical properties of the Ti2ZrHfxV0.5Ta0.2 HEAs before and after irradiation were investigated. The results showed that the unirradiated Ti2ZrHfxV0.5Ta0.2 HEAs displayed a single-phase BCC structure. The yield strength of the Ti2ZrHfxV0.5Ta0.2 HEAs increased gradually with the increase of Hf content without decreasing the plasticity at room and elevated temperatures. After irradiation, no obvious radiation-induced segregations or precipitations were found in the transmission electron microscope results of the representative Ti2ZrHfV0.5Ta0.2 HEA. The size and number density of the He bubbles in the Ti2ZrHfV0.5Ta0.2 HEA increased with the improvement of fluence at 1023 K. At the fluences of 1 × 1016 and 3 × 1016 ions/cm2, the irradiation hardening fractions of the Ti2ZrHfV0.5Ta0.2 HEA were 17.7% and 34.1%, respectively, which were lower than those of most reported conventional low-activation materials at similar He ion irradiation fluences. The Ti2ZrHfV0.5Ta0.2 HEA showed good comprehensive mechanical properties, structural stability, and irradiation hardening resistance at elevated temperatures, making it a promising structural material candidate for advanced nuclear energy systems.

Effects of Ta addition on the microstructures and mechanical properties of CoCrFeNi high entropy alloy

Outstanding tensile properties of a precipitation-strengthened FeCoNiCrTi0.2 high-entropy alloy at room and cryogenic temperatures