Mechanistic origin of high retained strength in refractory BCC high entropy alloys up to 1900K

The body centered cubic (BCC) high entropy alloys MoNbTaW and MoNbTaVW show exceptional strength retention up to 1900K. The mechanistic origin of the retained strength is unknown yet is crucial for finding the best alloys across the immense space of BCC HEA compositions. Experiments on Nb-Mo, Fe-Si and Ti-Zr-Nb alloys report decreased mobility of edge dislocations, motivating a theory of strengthening of edge dislocations in BCC alloys. Unlike pure BCC metals and dilute alloys that are controlled by screw dislocation motion at low temperatures, the strength of BCC HEAs can be controlled by edge dislocations, and especially at high temperatures, due to the barriers created for edge glide through the random field of solutes. A parameter-free theory for edge motion in BCC alloys qualitatively and quantitatively captures the strength versus temperature for the MoNbTaW and MoNbTaVW alloys. A reduced analytic version of the theory then enables screening over >600,000 compositions in the Mo-Nb-Ta-V-W family, identifying promising new compositions with high retained strength and/or reduced mass density. Overall, the theory reveals an unexpected mechanism responsible for high temperature strength in BCC alloys and paves the way for theory-guided design of stronger high entropy alloys.Comment: This version corrects the theory and provides more extensive explanation

Thermodynamics of concentrated solid solution alloys

This paper reviews the three main approaches for predicting the formation of concentrated solid solution alloys (CSSA) and for modeling their thermodynamic properties, in particular, utilizing the methodologies of empirical thermo-physical parameters, CALPHAD method, and first-principles calculations combined with hybrid Monte Carlo/Molecular Dynamics (MC/MD) simulations. In order to speed up CSSA development, a variety of empirical parameters based on Hume-Rothery rules have been developed. Herein, these parameters have been systematically and critically evaluated for their efficiency in predicting solid solution formation. The phase stability of representative CSSA systems is then illustrated from the perspectives of phase diagrams and nucleation driving force plots of the σ phase using CALPHAD method. The temperature-dependent total entropies of the FCC, BCC, HCP, and σ phases in equimolar compositions of various systems are presented next, followed by the thermodynamic properties of mixing of the BCC phase in Al-containing and Ti-containing refractory metal systems. First-principles calculations on model FCC, BCC and HCP CSSA reveal the presence of both positive and negative vibrational entropies of mixing, while the calculated electronic entropies of mixing are negligible. Temperature dependent configurational entropy is determined from the atomic structures obtained from MC/MD simulations. Current status and challenges in using these methodologies as they pertain to thermodynamic property analysis and CSSA design are discussed

Розрахунок твердорозчинного зміцнення багатокомпонентних жароміцних сплавів

Розглянуто моделі розрахунку твердорозчинного зміцнення багатокомпонентних жароміцних сплавів. Проаналізовано можливість застосування цих моделей до оцінки границь плинності сплавів з ОЦК гратками. Визначено відповідність розрахованих границь плинності і їх експериментальних значень.Рассмотрены модели расчета твердорастворного упрочнения многокомпонентных жаропрочных сплавов. Проанализирована возможность использования этих моделей для расчета предела текучести сплавов с ОЦК решеткой. Выполнена оценка соответствия расчетных значений предела текучести экспериментальным значениям.The models of solid solution strengthening of multicomponent high temperature alloys are considered. The possibilities of using the models for calculation of yield stress of BCC alloys are analyzed. Correspondence between calculated yield stresses and the corresponding experimental values are evaluated

A ductility metric for refractory-based multi-principal-element alloys

We propose a quantum-mechanical dimensionless metric, the local$-$lattice distortion (LLD), as a reliable predictor of ductility in refractory multi-principal-element alloys (RMPEAs). The LLD metric is based on electronegativity differences in localized chemical environments and combines atomic$-$scale displacements due to local lattice distortions with a weighted average of valence$-$electron count. To evaluate the effectiveness of this metric, we examined body$-$centered cubic (bcc) refractory alloys that exhibit ductile$-$to$-$brittle behavior. Our findings demonstrate that local$-$charge behavior can be tuned via composition to enhance ductility in RMPEAs. With finite$-$sized cell effects eliminated, the LLD metric accurately predicted the ductility of arbitrary alloys based on tensile$-$elongation experiments. To validate further, we qualitatively evaluated the ductility of two refractory RMPEAs, i.e., NbTaMoW and Mo$_{72}$W$_{13}Ta$_{10}Ti$_{2.5}Zr$_{2.5}, through the observation of crack formation under indentation, again showing excellent agreement with LLD predictions. A comparative study of three refractory alloys provides further insights into the electronic-structure origin of ductility in refractory RMPEAs. This proposed metric enables rapid and accurate assessment of ductility behavior in the vast RMPEA composition space.Comment: 36 pages, 12 figures, 5 Tabl

Effect of Y2O3 addition on the microstructure and mechanical properties of an Al1.8CoCrCu0.5FeNi BCC HEA

The present study investigated the influence of Y2O3 addition by mechanical alloying (MA) on the microstructure evolution of a BCC High Entropy Alloy (HEA). The characterisation and mechanical properties of the alloy were explored using X-ray diffraction, SEM, EBSD, and nano-indentation. The sintered Al1.8CoCrCu0.5FeNi HEA shows a microstructure formed by an ordered BCC phase (Al-rich) and a second disordered BCC (Cr-rich), while a minor FCC (Cu-rich) appears. These BCC phases show a wide morphology evolution from cuboidal and wave-like structures to irregular shapes. The minor FCC phase also adopts several morphologies as the MA is performed. The introduction of oxide reinforcements and microstructure refinement through mechanical alloying yields a change in phase quantification and grain structure. In accordance with the hardness and elastic modulus values from ordered/disordered BCC phases, the disordered BCC shows higher values than the ordered one. The grain size reduction as well as the solid solution strengthening from the microstructure evolution consequence of the MA are shown to be the main contributors to the increase in hardness and elastic modulus in the consolidated samples.This research was supported by the Regional Government of Madrid under the programme S2018/NMT-4381-MAT4.0-CM project. Funding from PID2019-109334RB-C32 awarded by the Spanish Ministry of Science, Innovation and Universities is also acknowledged. J. Cornide also acknowledges funding from the Spanish Ministry of Science and Innovation (IJCI-2017-31348) and TED2021-130831B-I00 funded by MCIN/AEI/10.13039/501100011033 and NextGenerationEU/PRTR. Funding for APC: Universidad Carlos III de Madrid (Read & Publish Agreement CRUE-CSIC 2023)

Alloy Design for High-Entropy Alloys: Predicting Solid Solubility, and Balancing Mechanical Properties and Oxidation Resistance

High-entropy alloys (HEAs) comprise of multi-principal elements in equi-atomic or near equi-atomic percentage. HEAs are considered as potential structural materials for high temperature applications, which require alloy design for optimum mechanical properties. In this regard, achieving both high strength and tensile ductility is still a great challenge. Compared to conventional alloys, HEAs have high configurational entropy, which tends to stabilize the solid solution formation, mainly face-centered-cubic (fcc) and/or body-centered-cubic (bcc) solid solutions. Generally, fcc-type HEAs are ductile but soft, while bcc-type HEAs are hard but brittle. \ua0\ua0This project has three working directions. The first part of this work is related to alloy design and aims to gain improved understanding of the solid solubility in HEAs. The difficulties that are encountered by HEAs are mostly related to the alloy design strategy. Previous approaches to describe the solid solubilities in HEAs could not accurately locate the solubility limit. Therefore, the need for single-phase solid solution and controlling the formation of secondary phases is addressed through the molecular orbital approach. The output of this approach is the introduction of the Md parameter, the d-orbital energy level to HEAs, which can well describe the solubility limit in HEAs. To further develop this approach, Md is also complemented with theoretical methods specifically, CALPHAD and experimental work.\ua0The second part of this work is to ductilize HEAs containing group IV (Ti, Zr, Hf), V (V, Nb, Ta) and VI (Cr, Mo, W) refractory elements, known as refractory HEAs (RHEAs), where inadequate ductility puts a limit on their mechanical performance for structural applications. A strategy is proposed to design RHEAs with sufficient yield strength combined with ductility at room temperature. Ductility is introduced by maintaining the bcc single-phase solid solution and keeping the number of total valence electrons low, which can be achieved through controlled alloying. More importantly, a mechanism and route for ductilizing RHEAs is proposed. \ua0The third part, which is the ultimate goal of this work, is to address the balance of mechanical properties and oxidation resistance for RHEAs, for the optimal development of RHEAs aiming at high-temperature applications. Based on the known facts for refractory alloys, the oxidation resistance is also problematic for RHEAs and there exists only limited work towards the study of high temperature oxidation of ductile RHEAs. Therefore, the oxidation mechanism is studied and it is found out that the insufficient oxidation resistance in existing ductile RHEAs is attributed to the failure in forming protective oxide scales accompanied by the accelerated internal oxidation leading to pest-disintegration or pesting. Efforts are also carried out to improve oxidation resistance via alloying and pack-cementation aluminizing. These studies provide important input to the further development of RHEAs as novel high-temperature materials and shed light on the design of refractory HEAs with optimal mechanical and oxidation resistance properties

Atomic Representations of Local and Global Chemistry in Complex Alloys

The exceptional properties observed in complex concentrated alloys (CCAs) arise from the interplay between crystalline order and chemical disorder at the atomic scale, complicating a unique determination of properties. In contrast to conventional alloys, CCA properties emerge as distributions due to varying local chemical environments and the specific scale of measurement. Currently there are few ways to quantitatively define, track, and compare local alloy compositions (versus a global label, i.e. equiatomic) contained in a CCA. Molecular dynamics is used here to build descriptive metrics that connect a global alloy composition to the diverse local alloy compositions that define it. A machine-learned interatomic potential for MoNbTaTi is developed and we use these metrics to investigate how property distributions change with excursions in global-local composition space. Short-range order is examined through the lens of local chemistry for the equiatomic composition, demonstrating stark changes in vacancy formation energy with local chemistry evolution.Comment: Version 2: editing and figure improvements, overall content unchanged. 15 pages, 6 main figures, 1 supplemental figur