Body-centered-cubic (BCC) high entropy alloys show exceptional strengths up to 1900K [1]. They are characterized by Mo, Nb, Ta, V, W, Cr, often combined with Ti, Zr, Hf [2], occupying BCC lattice sites at random. Few such alloys have been tested. Fundamental understanding of the mechanisms originating such exceptional behaviour is crucial to formulate theories enabling combinatorial search over the immense compositional space.

We introduce a new holistic, parameter-free strengthening theory of screw and edge dislocations in BCC alloys [3,4]. In contrast with screw-controlled pure BCC metals, in non-dilute BCC alloys both edge and screw dislocations are pinned [5] due to strong local energy fluctuations [3]. Both screw and edge dislocations assume wavy/kinked minimum-energy configurations where dislocation segments of characteristic length zetac are pinned at low-energy sites.

Three strengthening regimes are found in screws: (1) low-temperature, Peierls-barrier controlled strength; (2) intermediate-temperature strength, due to kink migration over barriers scaling with solute/dislocation interaction; (3) high-temperature strength, scaling with energy of vacancy and self-interstitials forming after unpinning of cross-kinks.

Edge dislocation strengthening (scaling with misfit volumes and elastic moduli) is controlled at all temperatures by glide of the zetac segments across the large energy barriers due to energy fluctuations.

Theory captures quantitatively experiments in a vast range of alloy compositions and temperatures (Fe-Si, Nb-Mo, Nb-W; high entropy Ti-Nb-Zr-based and Nb-Mo-Ta-W-V alloys). Screws control strength of non-dilute binaries and Ti-Nb-Zr-based alloys. The exceptional high-temperature strengthening in Nb-Mo-Ta-W-V alloys is controlled by edge dislocations, due to much larger ($\sim$3 eV) energy barriers created by solute/edge dislocation interaction.

The edge theory is cast in an analytical form that is parameter-free and depends on physical quantities (alloy concentrations, lattice parameter, elastic constants, misfit volumes) that can be determined ab-initio or experimentally. The reduced edge theory enables screening over 10 million compositions in the whole Al-Cr-Mo-Nb-Ta-W-V-Ti-Zr-Hf alloy family to find the strongest BCC HEAs [6].

[1] O. N. Senkov, G.B. Wilks, J.M. Scott, D.B. Miracle (2011) Mechanical properties of Nb25Mo25Ta25W25 and V20Nb20Mo20Ta20W20 refractory high entropy alloys. Intermetallics 19, 698-706.

[2] D.B. Miracle, O.N. Senkov (2017) A critical review of high entropy alloys and related concepts. Acta Materialia 122, 448-511.

[3] F. Maresca, W.A. Curtin (2020) Mechanistic origin of high strength in refractory BCC high entropy alloys up to 1900K. Acta Materialia 182, 235-249.

[4] F. Maresca, W.A. Curtin (2020) Theory of Screw Dislocation Strengthening in Random BCC Alloys from Dilute to “High-Entropy” Alloys. Acta Materialia 182, 144-162.

[5] F. Mompiou, D. Tingaud, Y. Chang, B. Gault, G. Dirras (2018) Conventional vs harmonic-structured a-Ti-25Nb-25Zr alloys: A comparative study of deformation mechanisms. Acta Materialia 161, 420-430.

[6] C. Lee, F. Maresca et al. (2021) Strength can be controlled by edge dislocations in refractory high-entropy alloys. Nature Communications 12, 1-8.

Dr. Francesco Maresca