Effects of grain size and mechanical pretreatment on strain localization in FCC polycrystals (original) (raw)
Related papers
International Journal of Plasticity, 2005
Grain size is a critically important aspect of polycrystalline materials and experimental observations on Cu and Al polycrystals have shown that a Hall-Petchtype phenomenon does exist at the onset of plastic deformation. In this work, a parametric study is conducted to investigate the effect of microstructural and deformation-related length scales on the behaviour of such FCC polycrystals. It relies on a recently proposed non-local dislocation-mechanics based crystallographic theory to describe the evolution of dislocation mean spacings within each grain, and on finite element techniques to incorporate explicitly grain interaction effects. Polycrystals are modeled as representative volume elements (RVEs) containing up to 64 randomly oriented grains. Predictions obtained from RVEs of Cu polycrystals with different grain sizes are shown to be consistent with experimental data. Furthermore, mesh sensitivity studies revealed that, when there is a predominance of geometrically necessary dislocations (GNDs) relative to statistically-stored dislocations (SSDs), the polycrystal response becomes increasingly mesh sensitive. This was found to occur specially during the early stages of deformation in polycrystals with small grains.
International Journal of Fatigue, 1999
Fatigue tests were carried out on copper polycrystals that had a texture with strong components along directions inside the standard stereographic triangle (single slip orientations in monocrystals). The experiments showed that this "single slip" texture, ramp loading as a mechanical pretreatment and extended cycling at a given stress to insure saturation provide the conditions to obtain a plateau in the Cyclic Stress-Strain Curve (CSSC) of copper polycrystals in a reproducible fashion. Transmission Electron Microscopy (TEM) studies revealed the presence of numerous Persistent Slip Bands (PSB's) in samples deformed within the plateau regime.
Complex strain paths in polycrystalline copper: microstructural aspects
Materials Research, 1999
Microstructural aspects of polycrystalline copper sheets subjected to complex strain paths were analysed in this work. Dislocation structures developed during the strain paths (rolling and tension) and the evolution of this microstructure during reloading have been studied. The active slip systems developed in each strain path were used to explain the microstructural evolution. The heterogeneous surface deformation observed on polished tensile specimens prestrained in rolling was also analysed. The structural aspects are related with the mechanical behaviour of the material, namely with the increase in yield stress in reloading, the work hardening evolution and the premature occurrence of plastic instability for some prestrain values.
Key Engineering Materials, 2009
This article reports on a study of the microstructure and mechanical response of copper polycrystals with grain sizes in the micrometer range. Three-dimensional dislocation dynamics simulations are used for the first time to investigate grain boundary strengthening and the Hall-Petch law. The methodology, which involves constructing a microcrystalline representative volume element with periodic boundary conditions, is briefly presented. Simulation results show that the initial density of dislocation sources and the cross-slip mechanism are two key factors controlling the heterogeneity of plastic deformation within the grains. At yield, the smaller the grains size, the more plastic deformation is heterogeneously distributed between grains and homogeneously distributed inside the grains. A size effect is reproduced and it is shown that the Hall-Petch exponent decreases from the very beginning of plastic flow and may reach a stable value at strains larger than the conventional proof st...
THE ROLE OF STRAIN COMPATIBILITY IN THE CYCLIC DEFORMATION OF COPPER BICRYSTALS
Acta Materialia, 1997
In order to improve our understanding of the link between the cyclic deformation of single crystals and of polycrystallinematerials, a study was performedon copper bicrystals.Testingunder strain cpntrol within the high cycleregimewas performed in isoaxial [149](90°and 180°)twist boundaries and [149]/[001] bicrystals, all with boundaries perpendicular to the tensile axis. A Grain Boundary Affected Zone (GBAZ), where multiple slip dominated, appeared when the boundaries developedcompatibility stresses.Experimentsshowedthat the behaviorof microscopicallycompatiblebicrystals, 180°boundaries, is approximately similar to that of a monocrystal, whereas the fatigue responses of the other misorientations show a grain boundary effect, which increases the cyclic stress. The strain in each grain of the [~49]/[001] sample was measured separately. The plastic deformation in the adjacent crystals oscillatedduring initial hardening;finally,the soft grain carried approximatelyfivetimes the plastic strain of the other grain.~1997Acta Meta[lurgicaInc.
Acta Materialia, 2011
The room temperature macroscopic and microscopic plastic behavior of four face-centered cubic metals (Al, Au, Cu and Ni) is investigated experimentally over a wide strain range, and theoretical modeling is used to simulate the established major micromechanisms describing the evolution of mobile and forest dislocations during plastic flow. It is shown that forest dislocations develop primarily due to interaction between mobile dislocations, while the contribution from forest-mobile interactions is only minor. The trapping of mobile dislocations and the annihilation of forest dislocations are both controlled by the same thermally activated dislocation motion. These observations permit a simplification of the theoretical model that leads to an analytical relationship for the evolution of the total dislocation density as a function of strain. From this analysis, correlations are drawn between the macroscopic parameters describing the stress-strain relationship and the fundamental characteristics of the microscopic processes.
Strain path change effect on dislocation microstructure of multicrystalline copper sheets
Materials Chemistry and Physics, 2006
In this study, coarse-grained copper sheets were subjected to tension-rolling and rolling-tension strain path sequences. In both cases, two different types of strain path change were studied: the tensile and rolling directions were parallel and normal to each other. TEM observations of deformed samples showed the typical dislocation structures for the prestraining paths in tension and rolling. Special microband features, not observed during prestrain, were found during the second strain path, whatever the sequence and type of strain path change. The microstructure observed during reloading is discussed in terms of the sequence and type of strain path change, parallel or normal. The frequency of appearance of microbands is discussed in terms of the activity of new slip systems, i.e. not active during the prestrain path and connected with the number of the active slip systems after reloading. The results from this study, obtained for coarse-grained multicrystalline copper sheets, are compared with previous ones for fine and medium-grained copper.
2013
A study of microstructural characterization of copper polycrystals during strain control fatigue has been studied for three decades. However, the reported results are quite controversial in regard to the plateau behavior, which has been well established for single copper crystals. The absence of complete and reliable results on the effects of the copper grain size when polycrystals are subjected to strain control fatigue testing was the primary motivation for the present study, with the goal being to address the cause of a clear answer in the existing literature. Saturation stresses measured in strain control fatigue tests were plotted as a function of corresponding plastic strain amplitudes to obtain a cyclic stress strain curve (CSSC) for three different grain sizes. After testing, cycled specimens were sectioned and cut longitudinally in the gauge length section of the specimens to produce samples for etching and microscopic investigation. Scanning electron microscopy (SEM) was conducted on the etched samples to determine the presence of persistent slip bands (PSBs) in each grain size.