Modeling of Dislocation Interaction with Solutes, Nano-Precipitates and Interfaces: A Multiscale Challenge

We review here several multiscale methods that we have developed to determine dislocation properties and interactions in metals. The review includes: (1) dislocation core properties in fcc and bcc metals; (2) the effect of solutes or nanoprecipitates on the mobility of a screw dislocation in bcc metals; (3) the interaction between dislocations and precipitates in intermetallic compounds; and (4) the transmission of dislocations through coherent and incoherent interfaces. In the concurrent quantum mechanical (QM) and molecular mechanical (MM) coupling approach, the quantum mechanical treatment is spatially confined to a small region, surrounded by a larger classical atomistic region. This approach is particularly useful for systems where quantum mechanical interactions in a small region, such as lattice defects or chemical impurities, can affect the macroscopic properties of a material. We discuss how the coupling across the different scales can be accomplished efficiently and accurately. We have applied this method to study the core structure and mobility of an edge dislocation in Al and of a screw dislocation in Ta, which are prototypical fcc and bcc metals. We find that the local environment of W solutes in Ta has a dramatic effect both on the dislocation mobility and slip paths. Isolated W solutes enhance the dislocation mobility, W nanoclusters of triangular shape pin the dislocation, while those of hexagonal shape result in spontaneous dislocation glide. The first sequential multiscale approach is a hybrid ab initio-based approach of Suzuki's atomic-row (AR) model, which allows the study of the dislocation core of a screw dislocation in bcc metals. The second hybrid approach, based on an extension of the Peierls-Nabarro model to study the dislocation-interface and the dislocation-precipitate interactions, integrates the atomistic nature from ab initio calculations of the generalized stacking fault energy surface (GSFS) into the parametric dislocation dynamics method. The ab initio-based calculations reveal that Cu nano-clusters in alpha-Fe dramatically alter the core structure of a screw dislocation from non-polarized in pure Fe to polarized, in agreement with experiments. In contrast, Cr clusters do not change the core polarization and increase the Peierls stress, thus hardening Fe. The hybrid method with four different interaction models was applied to study the interaction of a superdislocation with a spherical gamma-precipitate embedded in the gamma'-matrix of a nickel-based superalloy. The dislocation core structure was found to play an important role in determining the critical resolved shear stress. Based on these simulations, analytical equations for the precipitate strengthening are derived. For the Cu/Ni interface, the dislocation is found to dissociate into partials in both Cu and Ni, and the dislocation core is squeezed near the interface facilitating the spreading process, and leaving an interfacial ledge. It is shown that the strength of the bimaterial can be greatly enhanced by the spreading of the glide dislocation, and also increased by the pre-existence of misfit dislocations.