Modeling solution hardening in BCC refractory complex concentrated alloys: NbTiZr, Nb1.5TiZr0.5 and Nb0.5TiZr1.5

Abstract

Large scale, atomistic simulations of the core structure and mobility of ½[111] screw, edge and mixed dislocations in ternary multicomponent alloys (e.g. High Entropy alloys), NbTiZr, Nb1.5TiZr0.5 and Nb0.5TiZr1.5, are presented. The core structure of ½[111] screw dislocations continuously varies from compact to 3-fold with decreasing Nb content. The screw dislocation core structures in NbTiZr and Nb1.5TiZr0.5 are calculated using Embedded Atom Potentials (Johnson-Zhou) and compared with first-principles calculations of the screw dislocation in a quasi-random structure. In both simulations the dislocation core spreads on different (110) glide planes as the composition varies along the dislocation line in stoichiometric NbTiZr. The Nb-rich composition Nb1.5TiZr0.5 shows a compact core with very little core structure variation along the dislocation line in both First Principles and atomistic simulations. The screw dislocation deposits interstitial and vacancy dipole debris as it moves under stress. Average solute-dislocation core interaction energies in NbTiZr, Nb1.5TiZr0.5 and Nb0.5TiZr1.5 are derived from the average interatomic potential derived for each of the three systems. The interaction energies are used to determine the critical stress for the motion of ½[111] screw dislocations in the three systems as a function of temperature using the Suzuki model of kink migration controlled mobility developed for concentrated BCC random alloys. This analysis shows that the relatively high barrier for kink migration caused by fluctuations in solute concentration along the screw dislocation line and the dipole dragging stress associated with the screw dislocation motion results in a shallow fall-off of critical stress with temperature in these alloys as compared to simple BCC metals. Finally, the screw dislocation to edge and mixed dislocation critical stress ratio in NbTiZr are shown to be low, ∼2 at 5 K, in contrast to simple BCC metals, where it could be as high as 100–1000.