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

Abstract: 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.

• The paper demonstrates that edge dislocations interacting with random solute fields underlie high strength retention in refractory BCC HEAs up to 1900K.
• It employs stochastic analysis and molecular static simulations to derive quantitative expressions for flow stress and activation barriers without adjustable parameters.
• The study facilitates high-throughput screening of over 600,000 compositions to guide the design of next-generation, high-temperature refractory alloys.

Mechanistic Theory of High Retained Strength in Refractory BCC High Entropy Alloys

Background and Motivation

Refractory body-centered cubic (BCC) high entropy alloys (HEAs) such as MoNbTaW and MoNbTaVW exhibit exceptional strength retention up to 1900 K, significantly surpassing conventional superalloys that fail around 1100 K. The underlying mechanism enabling this persistent high strength was not theoretically understood, particularly given the random, near-equal occupancy of BCC lattice sites by different refractory elements. Traditional models attributed the strength of BCC metals to screw dislocation motion mediated by thermally-activated double-kink nucleation, with strength dropping swiftly as temperature rises. In contrast, this work establishes the dominance of edge dislocations and their interaction with random solute fields as the mechanistic origin responsible for the superior strength retention of BCC HEAs at high temperature.

Edge Dislocation Strengthening Theory

The theory developed rigorously analyzes edge dislocation motion in random BCC alloys. Unlike screw dislocations, edge dislocations are flexible and interact strongly with local solute concentration fluctuations. The random solute distribution produces a complex energy landscape, which traps segments of the edge dislocation in statistically-favorable regions. The dislocation thus adopts a wavy configuration characterized by an amplitude $w_c$ and lateral length $\zeta_c$, dictated by a balance between energy minimization from solute interaction and line tension penalty.

A detailed stochastic analysis quantifies the energy barriers for dislocation glide, yielding parameter-free expressions for the zero-temperature flow stress $\tau_{y0}$ and for the characteristic thermal activation barrier $\Delta E_b$. The theory models the dislocation as advancing through a landscape in which motion of segments is governed by the local energy barrier, with applied resolved shear stress reducing the barrier and facilitating thermal activation. The resultant expressions for $\tau_{y0}$ and $\Delta E_b$ depend fundamentally on the alloy’s elastic moduli, Burgers vector, and solute misfit volumes.

Numerical Validation and Model Fidelity

The model is validated against extensive molecular static simulations using periodic arrays of edge dislocations embedded in truly random alloy configurations. EAM-type interatomic potentials and precise relaxation protocols confirm the predicted wavy core structure and derive simulated values for $w_c$ and $\zeta_c$, which closely match theoretical predictions. Simulated yield strengths at $T = 0$ K for various compositions in the Mo-Nb-Ta-V-W family exhibit strong quantitative agreement with theoretical results, all achieved without adjustable parameters.

Experimental results at ambient and elevated temperatures are systematically captured by theory. The analytic model, using elastic constants and misfit volumes derived from DFT and Vegard’s law, quantitatively predicts yield strength trends and absolute values across the studied alloys, including the distinction that V-containing alloys are notably stronger due to the largest misfit volume, despite V’s lower melting point. Activation volumes calculated from theory are consistent with measured values, corroborating underlying mechanistic length scales.

Analytic Model and High-Throughput Screening

A simplified analytic version of the mechanistic model based on elasticity enables rapid prediction of yield strengths across vast composition spaces. The essential strength and energy barrier scale with solute misfit volume squared, reflecting the edge-dislocation mechanism’s dependence on random solute trapping. Over 600,000 compositions in the Mo-Nb-Ta-V-W system are screened, revealing thousands of alloys with superior or comparable strength and strength/weight ratios to existing top-performing HEAs.

Selected compositions predicted to maximize high-temperature strength or strength/density ratio are identified for potential experimental fabrication. These results demonstrate that combinations of large and small misfit volumes and high-stiffness elements are crucial. The model provides actionable guidance for composition optimization balancing mechanical performance and other practical constraints such as mass density.

Implications for Materials Design and High-Temperature Performance

The intrinsic, solute-driven edge-dislocation strengthening mechanism renders these BCC HEAs robust against conventional high-temperature softening processes that defeat superalloys. The energy barriers are distributed throughout the alloy microstructure, eliminating easy paths for dislocation climb and diffusional escape, and ensuring consistent strength up to near $T_m$. Screw-dislocation strengthening, reliant on jog formation, is undermined at high temperature, highlighting the critical transition to edge-dominated strengthening in complex BCC alloys.

The analytic model’s high-throughput capability allows optimization across multiple properties, including oxidation resistance, creep, ductility, and avoidance of undesirable phase formation. This mechanistic framework, when coupled with models for screw dislocation motion and thermodynamic predictions, provides a comprehensive computational platform for designing next-generation high-performance materials for extreme environments.

Conclusion

The paper establishes and quantitatively validates a parameter-free mechanistic theory for edge-dislocation strengthening in refractory BCC HEAs, capturing their exceptional yield strength retention up to 1900 K. The theory reveals an unexpected strengthening mechanism, rooted in the atomic-scale complexity and random solute trapping, that is absent in traditional alloys. The analytic model enables unprecedented alloy composition screening and guides the targeted discovery of new HEAs with tailored multi-property performance. The insights and methodology provided open significant avenues for theory-guided, computational materials design for ultra-high-temperature applications.