Closing the gaps in multiscale materials modeling of precipitation free zones in alloys ("Mind the gap")

In this project we aim to build a mathematical model of tiny but vital zones in aluminium vehicle bumper systems to visualize how stacked-up atoms behave in these components in the event of a collision. The model will give us new knowledge of how the composition of their component materials affects the properties of aluminium alloys. By linking this knowledge to the expertise of the designers of vehicle components, we can create alloys that will make vehicles safer. In Aluminium the metal has a granular structure. Each grain is made up of atoms stacked in a highly ordered 3D matrix, all of them with the same spatial orientation. The grains themselves are extremely strong, but between them are thinner zones that can act something like a zip fastener. It is these that we are going to examine in detail, because no-one knows how these zips behave when we crash. In a crash, atoms in vehicle components can be pushed out of their preferred positions. This is called dislocations. If many dislocations occur virtually simultaneously, they can slow each other down, which helps to hold the material together. If atoms are dislocated without meeting a lot of resistance, the result can be that the material will fracture. Because it is more likely that this happens in the zips, it is important to understand these zones. We have used several methods to investigate the interaction between dislocations and precipitates. These methods are coupled with atomistic methods and utilize information from TEM experiments. We have found that the effect of the precipitates on the strength of the material is mainly due to the density and size of the precipitates, and to a lesser extent the strain field around the precipitates. We have also investigated the mechanism for dislocations to pass precipitates by atomistic methods, and in combination with experiments and found that the precipitates are mainly cut by dislocations and these shear planes are mainly distributed through the precipitate. We have also studied different methods for investigating precipitation-free zones with crystal plasticity and found that it is possible to capture important physics. Furthermore, we have calculated the cohesive strength of the precipitates on the grain boundary and the corresponding critical stress and investigated the importance of these grain boundary precipitates during fracture of the material.

Modellene har gitt oss ny viten om hvordan materialsammensetningen påvirker de mekaniske egenskapene til aluminiumlegeringer. Ved å koble denne kunnskapen til kompetanse som designere av bildeler sitter med, kan vi lage legeringer som vil gjøre biler sikrere og samtidig lettere.

The main idea of this project is to combine models and important physics at different scales, from quantum mechanics to continuum theory in a robust and consistently integrated multiscale framework. The goal is to understand the effect of precipitate free zones on the strain localization and ductility in aluminum alloys beyond the understanding gained from experiments and continuum mechanical models. This is a most daring challenge that demands complex coupling between models formulated at different time and length scales. The project aims to both contribute to the fundamental understanding of precipitation free zones in alloys and to the development of general methods for bridging scales in materials modeling. The project will hire a postdoc at NTNU wo rking on the crystal plasticity part of the project, in close collaboration with CRI SIMLab and SINTEF. The post doc will focus on the developing the atomistically informed crystal plasticity model. For the atomistic studies, already existing competence o n Molecular Dynamics, Density Functional theory, the Quasicontinuum method and kinetic Monte Carlo method within the project group will be employed to investigate underlying physical phenomena influencing the mechanical properties of the precipitate free zones in Al alloys. Numerical results and underpinning theory for higher scale models will be generated. Discrete dislocation dynamics simulations informed from MD and DFT will be performed. A crystals plasticity model using numerical results and inform ation on underpinning physics from lower scale models in the constitutive equations will be created. The results will be compared with experimental results and competence from within NTNU and SINTEF for validation on all scales.