Quantitative analysis of grain subdivision in cold rolled aluminium

Torsion deformation was used to investigate dislocation substructure evolution at large strains in high purity nickel and NiCo solid solutions. Observations of small strain dislocation structures formed in stage III revealed that the laminar dislocation structure observed after large strains in stage IV develops from short paired dislocation sheets within the tangled dislocations of an equiaxed cell wall. The development of these short paired dislocation sheets into long microbonds occurs gradually by a multiple-slip process in accordance with the principles of low energy dislocation structures and without the occurrence of a shear instability. The plane of these sheets and /or microbands does not correspond to a {111} slip plane. As these microbands form, a misorientation between the interior of the paired sheets and the surrounding matrix develops and increases with increasing strain.

Strain hardening and substructural evolution in NiCo solid solutions at large strains

The evolution of the microstructure of cold-rolled pure aluminium (99.996%) was studied by transmission electron microscopy (TEM), extending a previous study that was limited to 30% strain ( e = 0.36). The present work concentrates on rolling strains from 50% ( e = 0.69) to 90% ( e = 2.3). The observations are explained through the governing principle that, in the course of polyslip, grains break up into volume elements within each of which fewer slip systems operate simultaneously than required by the Taylor model. The boundaries between the volume elements accommodate the lattice misorientations arising from the correspondingly different glide. They are therefore geometrically necessary boundaries and in TEM they appear variously as dense dislocation walls, different types of microbands and subgrain boundaries. The morphology and the spatial arrangement of the geometrically necessary boundaries were characterized and it is discussed how these boundaries form with increasing strain as an integral part of the microstructural evolution.

Microstructural evolution in rolled aluminium

A detailed description of possibilities given by the developed Cellular Automata—Finite Element (CAFE) multi scale model for prediction of the initiation and propagation of micro shear bands and shear bands in metallic materials subjected to plastic deformation is presented in the work. Particular emphasis in defining the criterion for initiation of micro shear and shear bands, as well as in defining the transition rules for the cellular automata, is put on accounting for the physical aspects of these phenomena occurring in two different scales in the material. The proposed approach led to the creation of the real multi scale model of strain localization phenomena. This model predicts material behavior in various thermo-mechanical processes. Selected examples of applications of the developed model to simulations of metal forming processes, which involve strain localization, are presented in the work. An approach based on the Smoothed Particle Hydrodynamic, which allows to overcome difficulties with remeshing in the traditional CAFE method, is a subject of this work as well. In the developed model remeshing becomes possible and difficulties limiting application of the CAFE method to simple deformation processes are solved. Obtained results of numerical simulations are compared with the experimental results of cold rolling process to show good predicative capabilities of the developed model.

Development of the Multi-scale Analysis Model to Simulate Strain Localization Occurring During Material Processing

The deformation behavior and failure mechanisms in 6061 Al alloy matrix particulate composites were examined via uniaxial tension and compression tests. The dependence of the deformation behavior on the matrix properties was studied by varying the heat treatment conditions (T4 and T6) of the matrix. The influence of the heat treatment condition on the deformation behavior of the composite was observed to be stronger than the influence of particle shape, size and volume fraction. In both tension and compression testing, composites in the T4 condition experience a larger degree of work-hardening as compared to the materials in the T6 condition. When subject to uniaxial tension, the deformation and failure behavior is strongly dependent on the relative strengths of the particles and matrix, and on the work-hardening behavior of the matrix, which is influenced by heat treatment condition. In compression, the particles merely act as obstacles to the flow of the matrix, whereas in tension they influence the failure mechanism of the composite.

Liu Yi-Lin

Papers

Substructure development and mechanical properties in cold-rolled aluminium alloy 3004 I: Substructure development

Substructure development and mechanical properties in cold-rolled aluminium alloy 3004 II: Mechanical properties