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Equilibrium vacancy concentrations in alloys at different temperatures. (a) The change of vacancy concentration with configuration entropy for u n = 1.4 eV and (b) the change of vacancy concentration with vacancy formation energy.
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By means of thermodynamic analyses, we provide the intriguing evidence that the equilibrium vacancy concentrations and their clusters in high entropy alloys (HEAs) are substantially larger than those in pure metals and simple binary alloys. The increased defect concentrations strongly change the thermodynamic and kinetic properties of the HEAs. Wit...
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Context 1
... let us take NiCoFeCrMn for example to show the variation of equilibrium concentration in fcc HEAs. In this HEA alloy, the vacancy formation energy of different elements cannot be directly used because of their different crystal structures. For pure fcc Ni, the vacancy formation energy (VFE) is 1.4 eV by specific heat and electrical conductivity measurements, but 1.79 eV by positron annihilation spec- troscopy measurements [15]. The theoretical calculation of VFE for bcc Fe and Cr are 2.3 and 2.8 eV, respectively [15]. For HCP Co, the VFE is 1.34 eV by positron annihilation spectroscopy measurements, but 2.22 eV by theoretical cal- culation [15]. In the fcc HEAs, without a better choice, we just take the 1.4 eV for all the calculations. The equilibrium vacancy concentrations at different temperatures for different configuration entropy are shown in Fig. 3(a), which shows that the vacancy concentration increases rapidly as the temperature increases. The increased vacancy concentration from the con- figuration entropy contribution in quinary alloys is equivalent to the vacancy concentration of pure metal by increasing about 85 K at this temperature range. For NiCoCrFeMn at 800 K, the vacancy concentration will be 1.68 × 10 −8 , and vacancy density will be around 10 15 /cm 3 , by assuming a molar volume of 8 cm 3 /mol. If the vacancy formation energy can be reduced in HEAs, their vacancy concentration will further increase. As shown in Fig. 3(b), the vacancy concentration will increase by one order of magnitude when the vacancy formation energy decreases from 1.4 to 1.2 eV at the given temperature ...
Context 2
... let us take NiCoFeCrMn for example to show the variation of equilibrium concentration in fcc HEAs. In this HEA alloy, the vacancy formation energy of different elements cannot be directly used because of their different crystal structures. For pure fcc Ni, the vacancy formation energy (VFE) is 1.4 eV by specific heat and electrical conductivity measurements, but 1.79 eV by positron annihilation spec- troscopy measurements [15]. The theoretical calculation of VFE for bcc Fe and Cr are 2.3 and 2.8 eV, respectively [15]. For HCP Co, the VFE is 1.34 eV by positron annihilation spectroscopy measurements, but 2.22 eV by theoretical cal- culation [15]. In the fcc HEAs, without a better choice, we just take the 1.4 eV for all the calculations. The equilibrium vacancy concentrations at different temperatures for different configuration entropy are shown in Fig. 3(a), which shows that the vacancy concentration increases rapidly as the temperature increases. The increased vacancy concentration from the con- figuration entropy contribution in quinary alloys is equivalent to the vacancy concentration of pure metal by increasing about 85 K at this temperature range. For NiCoCrFeMn at 800 K, the vacancy concentration will be 1.68 × 10 −8 , and vacancy density will be around 10 15 /cm 3 , by assuming a molar volume of 8 cm 3 /mol. If the vacancy formation energy can be reduced in HEAs, their vacancy concentration will further increase. As shown in Fig. 3(b), the vacancy concentration will increase by one order of magnitude when the vacancy formation energy decreases from 1.4 to 1.2 eV at the given temperature ...
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... The multiple components in HEMs cause weak dependence on any single component, resulting in numerous opportunities for tailoring properties as well as electrochemical performance. Furthermore, the increase of ΔS conf results in increment of equilibrium vacancy concentration that impacts on overall electrochemical performance of LIBs, such as the reaction kinetics and cycling stability [42,43]. A five component HEA or HEO is excepted to present significantly high vacancy concentration relative to pure metals [23]. ...
Lithium-ion batteries (LIBs) has extensively utilized in electric vehicles and portable electronics due to their high energy density and prolonged lifespan. However, the current commercial LIBs are plagued by relatively low energy density. High-entropy materials with multiple components have emerged as an efficient strategic approach for developing novel materials that effectively improve the overall performance of LIBs. This article provides a comprehensive review the recent advancements in rational design of innovative high-entropy materials for LIBs, as well as the exceptional lithium ion storage mechanism for high-entropy electrodes and considerable ionic conductivity for high-entropy electrolytes. This review also analyses the prominent effects of individual components on the high-entropy materials’ exceptional capacity, considerable structural stability, rapid lithium ion diffusion, and excellent ionic conductivity. Furthermore, this review presents the synthesis methods and their influence on the morphology and properties of high-entropy materials. Ultimately, the remaining challenges and future research directions are outlined, aimed at developing more effective high-entropy materials and improving the overall electrochemical performance of LIBs.
... different elements in the same alloys [27] or the absence of diffusion retardation in many complex alloys [28][29][30][31]. Moreover, some theoretical works [32,33] predict that an enhancement of atomic diffusion in CCAs compared to dilute alloys is a more common phenomenon and this was experimentally documented later [31,34]. ...
... As was mentioned, the WMN alloy can be conveniently compared to pure Mo due to the similarity of E f v0 : the vacancy concentration in the WMN alloy is in 4-5 times higher than in the pure metal. Perhaps this large increase in C v for the CCA results from the configuration entropy contribution discussed in several papers [32,85]. ...
... An effect of the configurational entropy was thought to be responsible for such increase in the vacancy concentration in multi-component alloys. Indeed, Wang et al. [32] found that C v in a HEA consisting of n elements is greater than C v in a pure metal having the same G f v by a factor of exp(n−1) n . During the derivation, vacancies were considered as (n + 1)th element of the n-component alloy [32]. ...
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... Analytical model predictions of thermal conductivity in the duplex MPEA are plotted as solid lines assuming a 50/50 phase fraction of the BCC-like and FCC-like phases. The three lines, in order of decreasing thermal conductivity, represent vacancy concentrations corresponding to an equilibrium value for a fully-dense arc-melted sample [63], a concentration of 0.01 (the order-ofmagnitude typical for laser quenched elemental metals [64]), and a concentration of 0.1 (the orderof-magnitude expected for a laser quenched sample plus one for the stabilization of additional point defects in HEAs [65]). The strongest matching between analytical model and experiment when the largest population of vacancies are considered suggests that the point defects introduced by the rapid solidification rates during laser-engineered net-shaping are impeding phonon transport in the as-built condition. ...
Density-functional theory (DFT) is used to identify phase-equilibria in multi-principal-element and high-entropy alloys (MPEAs/HEAs), including duplex-phase and eutectic microstructures. A combination of composition-dependent formation energy and electronic-structure-based ordering parameters were used to identify a transition from FCC to BCC favoring mixtures, and these predictions experimentally validated in the Al-Co-Cr-Cu-Fe-Ni system. A sharp crossover in lattice structure and dual-phase stability as a function of composition were predicted via DFT and validated experimentally. The impact of solidification kinetics and thermodynamic stability was
explored experimentally using a range of techniques, from slow (castings) to rapid (laser remelting), which showed a decoupling of phase fraction from thermal history, i.e., phase fraction was found to be solidification rate-independent, enabling tuning of a multi-modal cell and grain size ranging from nanoscale through macroscale. Strength and ductility tradeoffs for select processing parameters were investigated via uniaxial tension and small-punch testing on
specimens manufactured via powder-based additive manufacturing (directed-energy deposition). This work establishes a pathway for design and optimization of next-generation multiphase superalloys via tailoring of structural and chemical ordering in concentrated solid solutions.
... Analytical model predictions of thermal conductivity in the duplex MPEA are plotted as solid lines assuming a 50/50 phase fraction of the BCC-like and FCC-like phases. The three lines, in order of decreasing thermal conductivity, represent vacancy concentrations corresponding to an equilibrium value for a fully dense arc melted sample [37], a concentration of 0.01 (the order-ofmagnitude typical for laser quenched elemental metals [55]), and a concentration of 0.1 (the orderof-magnitude expected for a laser quenched sample plus one for the stabilization of additional point defects in HEAs [56]). The strongest matching between analytical model and experiment when the largest population of vacancies are considered suggests that the point defects introduced by the rapid solidification rates present during laser engineered net shaping are impeding phonon transport in the as-built duplex MPEA. ...
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... Considering S f v~ 2.2k for the binary alloy, the estimated C equilibrium v is ~ 5.1 × 10 -7 . We mention here that we have not taken into account the effect of configuration entropy which for binary alloys may increase the vacancy concentration by 1.36 times as compared to pure metals [91]. ...
... properties. 17,164,177,178 Sluggish Diffusion -Largely resulting from lattice distortion which impedes atomic mobility, sluggish diffusion in HEAs is characterised by a decreased rate of atomic diffusion. ...
High-entropy alloys (HEAs), and in particular, refractory HEAs (RHEAs), can offer remarkable thermomechanical properties, corrosion tolerance, and irradiation tolerance, making them candidates for structural materials in extreme environments such as advanced nuclear applications. Despite their apparent potential, the current understanding of the irradiation tolerance of RHEAs and their suitability for such nuclear applications is limited. This dissertation provides an in-depth study into the irradiation response of these alloys, and the mechanisms behind their radiation resistance, and further explores the potential of RHEAs in nuclear applications through material design and optimisation. The first part of this research focusses on the TiZrNbHfTa RHEA – primarily in its nanocrystalline (NC) state – irradiated under conditions representative of advanced nuclear applications. The alloy demonstrated excellent microstructural stability and retained essential mechanical properties post-irradiation, with significantly less hardening compared to traditional alloys irradiated under like conditions. The research then explores the unique mechanisms intrinsic to HEAs, such as high configurational entropy, severe lattice distortion, and sluggish diffusion, clarifying how such mechanisms may improve the irradiation tolerance of RHEAs. However, the research also highlights the sensitivity of RHEAs to phase constitution, with the introduction of additional gregation affecting the alloys’ response to irradiation. Furthermore, the competitive viability of HEAs against existing nuclear structural materials is explored, focussing on potential applications where HEAs could provide superior performance. To foster the development of such alloys, nuclear-relevant property calculations and a framework for alloy design is proposed. A novel RHEA, Ti55Zr30Ta6V5Cr2Fe2 (Ti55), was synthesised and tested, designed with an aim to minimise neutron capture cross section, transmutation losses, and gamma activity. The novel alloy exhibited promising mechanical properties which provide motivation for further material development. This doctoral dissertation significantly contributes to the field of RHEAs for advanced nuclear applications. It offers insights into their behaviours under irradiation and provides guidelines for their design and optimisation. With continued exploration and refinement, the vast potential of RHEAs can be harnessed, potentially addressing materials challenges in the field of nuclear materials.
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Refractory multi-element alloys (RMEA) with body-centered cubic (bcc) structure have been the object of much research over the last decade due to their high potential as candidate materials for high-temperature applications. Most of these alloys display a remarkable strength at high temperatures, which cannot be explained by the standard model of bcc plasticity dominated by thermally-activated screw dislocation motion. Recent research on Nb-Mo-Ta-W alloys points to a heightened role of edge dislocations on deformation, which is generally attributed to atomic-level chemical fluctuations in the material and their interactions with dislocation cores during slip. However, while this model accounts for a strengthening effect due to the chemical complexity of the alloy, it is not sufficient to explain its strength across the entire thermal range. Here we propose a new mechanism that captures the existing theories about enhanced lattice strengthening and a thermally-activated component that emanates directly from the chemical complexity of the RMEA. This compositional complexity results in unique vacancy formation energy distributions with tails that extend into negative energies, leading to spontaneous, i.e., athermal, vacancy formation at edge dislocation cores. These vacancies relax into atomic-sized super-jogs on the dislocation line, acting as extra pinning points that increase the activation stress of the dislocation. At the same time, these super-jogs can displace diffusively along the glide direction, relieving with their motion some of the extra stress, thus countering the hardening effect due to jog-pinning. The interplay between these two processes as a function of temperature confers an extra strength to edge dislocation at intermediate-to-high temperatures, in remarkable agreement with experimental measurements in Nb-Mo-Ta-W and across a number of different RMEA.
