Solid solution softening in bcc transition metals has traditionally been attributed to either extrinsic effects (interstitial scavenging) or intrinsic effects (direct solute-dislocation interactions). We investigate intrinsic mechanisms using first principles methods. First, the interaction energy of a transition metal solute with a single straight screw dislocation in Mo and Fe is calculated using density functional theory. Next, the interaction energies and changes in resistance to dislocation motion are incorporated into a mesoscopic double-kink model of dislocation mobility to predict changes in yield stress with temperature and solute concentration. Quantitatively accurate predictions require a model that accounts for clusters of solutes interacting with dislocations. Previous methods employing the solute response in bulk as an approximation to the solute response in a dislocation lead to incorrect predictions. Using solute-dislocation interactions coupled with a realistic mesoscopic model, we reproduce the strength behavior of different alloy systems despite the range of intrinsic interactions.
Materials Science and Engineering, University of Illinois, Urbana-Champaign USA