WEB Theory and prediction of strength in refractory BCC High Entropy Alloys up to 1900KWednesday (23.09.2020) 15:00 - 15:15 S: Structural Materials 2 Part of:
Body-centered-cubic (BCC) high entropy alloys show exceptional strengths up to 1900K [Senkov, Wilks, Scott, Miracle (2011) Intermetallics 19, 698]. They are characterized by Mo, Nb, Ta, V, W, Cr, often combined with Ti, Zr, Hf, 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 [Maresca & Curtin (2020) Acta Mater 182, 144; Maresca & Curtin (2020) Acta Mater 182, 235]. In contrast with screw-controlled pure BCC metals, in non-dilute BCC alloys both edge and screw dislocations are pinned due to strong local energy fluctuations. 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 and qualitatively 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 (~3 eV) energy barriers created by solute/edge dislocation interaction.
We have therefore formulated a holistic theory of strengthening in BCC alloys covering a vast range of compositions and temperatures. Theory rationalizes and captures experiments on BCC alloys. A reduced form of the theory enables combinatorial search for stronger alloys, opening avenues for materials discovery.