Modelling and Simulation of Dislocation and Crack Behavior in FCC Iridium and Rhodium

Dislocation and crack-tip behavior are thought to be well-understood in FCC structures. However, FCC Ir and Rh exhibit plenty of peculiarities during plastic deformation and fracture. They are the only two plastic cleavable elements in the FCC metal family. The anomalous behavior was attributed to the presence of a compact screw core which drives rapid cross-slip, exponential dislocation accumulation and consequent brittle cleavage. However, this proposition is not commensurate with the dislocation microstructure, plastic flow and fracture behavior in experiments. The “Ir problem” has thus remained unresolved despite long-time research efforts.    We recently acquired atomic-resolution lattice defects in Ir via state-of-the-art HAAD-FSTEM, which revealed extraordinarily high density defects never seen in other elemental metals. Burgers circuit analyses suggest their primary screw and 600 dislocation characters, but with highly-distorted cores. DFT/MD simulations with a preliminary XMEAM-Ir potential show unexpected dislocation cores and crack-tip behavior related to anomalous plasticity. These combined insights not only reveal unique, polymorphic nature of dislocation cores in Ir, but also shed lights on the path to truly understand deformation and fracture in Ir and Rh.   Leveraging these new advances, we propose a multiscale research to firmly establish the fundamental physics and mechanics of dislocation, crack, and their interaction and evolution in Ir/Rh. The research will focus on four aspects at the root of plastic deformation and fracture. First, the polymorphic dislocation cores and energetics will be unequivocally determined in DFT, followed by development of new XMEAM potentials to reproduce these properties. Second, XMEAM-based MD simulations will be performed to quantitatively determine (i) temperature/stress-dependent dislocation mobility tensors M( σ, T), and (ii) the junction/hardening matrix aij in the modified Taylor model. Third,M( σ, T) and σij will be incorporated into DDD simulations to reveal dislocation microstructure/density evolution, and the true plastic flow behavior. Finally, a new stress-based crack-tip model will be developed to account for surface step formation and tension-shear coupling, enabling more accurate, quantitative prediction of crack-tip plasticity and critical loadings. Success will lead to firm advances in several frontiers, including (i) direct atomistic simulations of plastic deformation and fracture without a priori assumptions of material constitutive laws at atomistic scales; (ii) new mobility laws and junction strength parameters; and (iii) a new crack-tip model enabling physically-accurate prediction of crack tip plasticity applicable to a broad family of metals and alloys.