Deformation texture of aluminium – A grain interaction simulation approach

We present plane strain simulations about the dependence of orientational in-grain subdivision and crystallographic deformation textures in aluminium polycrystals on grain interaction. The predictions are compared to experiments. For the simulations we use a crystal plasticity finite element and different polycrystal homogenization models. One set of finite element simulations is conducted by statistically varying the arrangement of the grains in a polycrystal. Each grain contains 8 integration points and has different neighbor grains in each simulation. The reorientation paths of the 8 integration points in each grain are sampled for the different polycrystal arrangements. For quantifying the influence of the grain neighborhood on subdivision and texture we use a mean orientation concept for the calculation of the orientation spread among the 8 originally identical in-grain orientation points after plastic straining. The results are compared to Taylor-Bishop-Hill-type and Sachs-type models which consider grain interaction on a statistical basis. The progress report reveals five important points about grain interaction. First, the consideration of local grain neighborhood has a significant influence on the reorientation of a grain (up to 20% in terms of its end orientation and its orientation density), but its own initial orientation is more important for its reorientation behavior than its grain neighborhood. Second, the sharpness of the deformation texture is affected by grain interaction leading to an overall weaker texture when compared to results obtained without interaction. Third, the in-grain subdivision of formerly homogeneous grains occurring during straining is strongly dependent on their initial orientation. [...

Effects of the grain size and shape on the flow stress: A dislocation dynamics study

Dislocation dynamics simulation is used to investigate the effect of grain size and grain shape on the flow stress in model copper grains. We consider grains of 1.25-10 $\mu$m size, three orientations (, and ) and three shapes (cube, plate and needles). Two types of periodic aggregates with one or four grains are simulated to investigate different dislocation flux at grain boundaries. It is shown that in all cases the flow stress varies linearly with the inverse of the square root of the grain size, with a proportionality factor varying strongly with the grain orientation and shape. Simulation results are discussed in the light of other simulation results and experimental observations. Finally, a simple model is proposed to account for the grain shape influence on the grain size effect.Comment: International Journal of Plasticity, Elsevier, 201

Crystal plasticity finite element simulations of cast α-uranium

α-uranium, the stable phase of uranium up to 670◦C, has a base-centred orthorombic crystal structure. This crystal structure gives rise to elastic and thermal anisotropy, meaning α-uranium exhibits complex deformation and fracture behaviour. Understanding the relationship between the microstructure and mechanical properties is important to prevent fracture during manufacture and usage of components. The lattice of α-uranium corresponds to a distorted close-packed-hexagonal crystal structure and it exhibits twins of both the 1st and 2nd kind. Therefore, detailed examination of the behaviour of α-uranium can also contribute to the general understanding of the interaction between plasticity, twinning and fracture in hcp crystals. Plastic deformation in α-uranium can be accommodated by 4 slip systems and 3 twin systems, previously identified by McCabe et al. These deformation modes are implemented into a crystal plasticity finite element (CPFE) material model. A temperature dependent, dislocation density based law is implemented to describe the critical resolved shear stress on the different slip/twin systems. The strong anisotropic thermal expansion behaviour is taken into account to simulate the development of internal residual stresses following casting of the material. During cooling, the internal stresses in α-uranium are sufficient to induce plasticity. This effect is quantified using polycrystal simulations, in which first the temperature is decreased, then plastic relaxation takes place, followed by application of a mechanical load. The asymmetry between mechanical properties in tension and compression, due to the presence of twins, is investigated. The model is calibrated using stress strain curves and the lattice strain found from published neutron diffraction experiments carried out on textured samples at ISIS. The strength of the slip systems is found to be lower than in fine grained material, while the strength of the twin system is similar to single crystals. The CPFE method allows the heterogeneity of the strain between neighbouring grains and its influence on the evolution of the internal stress state to be investigated