Rodney James Mccabe

Transmission of {10 (1) over bar2}{(1) over bar 011} mechanical twins across grain boundaries in Mg is a mechanism that can facilitate shear accomodation but also provide a path for failure via intergranular crack propagation. Until recently the twin research has focused on a 2D characterization of intragranular propagation and intergranular transmission along the forward propagation direction. Recent 3D studies of the twin domain interface reveal a faceted structure, anisotropic mobility, and a relative easiness of lateral twin propagation (as opposed to forward or normal propagation). Here we describe a study of the forward and the lateral twin transmission into neighbors applying a variety of experimental and computational characterization techniques, namely: (1) statistical EBSD analysis of twin sections; (2) 3D Phase Field and Molecular Dynamic simulations of twins propgating and reacting with grain boundaries. This study improves our understanding of the transmission mechanisms in a 3D aggregate, and helps us to develop criteria for treating twin modeling in CP simulations.

Recently it has been demonstrated that nanolayered hcp/bcc Zr/Nb composites can be fabricated with a severe plastic deformation technique called accumulative roll bonding (ARB) [1]. The final layer thickness averaged to approximately 90 nm for both phases. Interestingly, the texture measurements show that the textures in each phase correspond to those of rolled single-phase rolled Zr and Nb for a wide range of layer thickness from the micron to the nanoscales. This is in remarkable contrast to fcc/bcc Cu/Nb layered composites made by the same ARB technique, which developed textures that strongly deviated from theoretical rolling textures of Cu or Nb alone when the layers were refined to submicron and nanoscale dimensions. To model texture evolution and reveal the underlying deformation mechanisms, we developed a 3D multiscale model that combines crystal plasticity finite element with a thermally activated dislocation density based hardening law [2]. For systematic study, the model is applied to a two-phase Zr/Nb polycrystalline laminate and to the same polycrystalline Zr and polycrystalline Nb as single-phase metals. Consistent with the measurement, the model predicts that texture evolution in the phases in the composite and the relative activities of the hcp slip modes are very similar to those in the phases in monolithic form. In addition, the two-phase model also finds that no through-thickness texture gradient develops. This result suggests that neither the nanoscale grain sizes nor the bimetal Zr/Nb interfaces induce deformation mechanisms different from those at the coarse-grain scale.