Papers by Keyword: Dislocation Cells

Abstract: A fully coupled micro-macro interaction model is proposed for the grain refinement caused by severe plastic deformation of cell-forming metallic materials. The model is a generalization of a previously proposed two-phase composite model suggested for the evolution of dislocation populations corresponding to the interior of the dislocation cells and dislocation cell walls. Just as within the original framework, the evolution of the material microstructure depends on the applied hydrostatic pressure, strain rate, and the loading path. Backstresses are used to define a measure of the strain path change. Thereby, the model can describe the experimentally observed dissolution of dislocation cells and the reduction of dislocation densities occurring shortly after load path changes. The large strain kinematics is accounted for in a geometrically exact manner using the nested split of the deformation gradient tensor, proposed by Lion. Within the extended model, the macroscopic strength of the material depends on the microstructural parameters. In that sense, the new model is fully coupled. It is thermodynamically consistent, objective, and w-invariant under isochoric changes of the reference configuration. A physical interpretation is provided for the nested multiplicative split in terms of the two-phase microstructure composite model.

Abstract: The thermo-simulation test and transmission electron microscopy (TEM) were applied to investigate the evolution of dislocation configuration and strain induced precipitation behavior during relaxation at 850°C in a deformed Fe-40Ni-Ti alloy. The stress relaxation curve can be divided into three stages, namely, the process of incubation, nucleation and growth, and the coarsening of strain-induced precipitates. The highly dense and twisted dislocations formed during the deformation develop into dislocation cells and finally, the sub-grains can be observed when relaxing to 1000s. The strain induced precipitates occur both onto the dispersed dislocations and dislocation cells. The precipitates pin the dislocations which results in retarding the progress of dislocation configuration evolution. As precipitates start to coarsen, the pinning effect weakens and the dislocations get rid of the pinning though bypassing mechanism. Adopting the same simulation test to bainitic steel, the optimum refinement could be obtained at 60-200s during relaxation processing, corresponding to the perfect dislocation cells formation of Fe-40Ni-Ti alloy.