Grain orientation, deformation microstructure and flow stress (original) (raw)
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Mechanics of Materials, 2011
In this work a dislocation based constitutive description for modeling the thermo visco plastic behavior of FCC metals has been developed. The constitutive description, which is founded on the concepts of thermal activation analysis and dislocation dynamics, assumes the plastic flow additively decomposed into internal stress and effective stress. The internal stress represents the applied stress required for the transmission of plastic flow between the polycrystal grains and it is defined by the Hall Petch relationship. The effective stress formulation, which is the main innovative feature of this work, represents the thermally activated deformation behavior. This is defined taking into account the interrelationship between strain rate and temperature, and gathers structural evolution dependence. This structural evolution is described as a function of dislocations density, which acts as internal state variable in the material deformation behavior. A systematic procedure for identifica tion of the material parameters is developed and the model is applied to define the behav ior of annealed OFHC copper. The analytical predictions of the constitutive description are compared with the experimental data reported by Nemat Nasser and Li (Nemat Nasser, S., Li, Y., (1998). Flow stress of FCC polycrystals with application to OFHC Copper. Acta Mater. 46, 565 577). Good correlation between experiments and analytical predictions is found within wide ranges of strain rate and temperature.
Effects of grain size and mechanical pretreatment on strain localization in FCC polycrystals
International Journal of Fatigue, 2001
Polycrystalline copper samples where the grains in the cross-section had crystallographic axes parallel to the load related to 'single slip' directions and three grain sizes, were ramp loaded and then step-tested to obtain their Cyclic Stress-Strain Curves (CSSCs). Plateaux were found for medium grain size at about 89 MPa, whereas large grained samples showed plateaux at 72 MPa, which correlated with the measured Taylor and Sachs factors, respectively. No plateaux were found when grains were smaller than 200 µm or ramp-loading stresses were below 87 MPa. Comparisons are made with nickel polycrystals and it is found that the plateaux in copper are narrower than those reported in nickel. The differences are attributed to a more homogeneous dislocation structure in nickel, due to lower elastic interactions across grain boundaries and easier cross-slip behavior as compared to copper.
Dislocation impediment by the grain boundaries in polycrystals
Acta Mechanica, 2021
Thermodynamic dislocation theory incorporating dislocation impediment by the grain boundaries is developed to analyze the shear test of polycrystals. With a small set of physics-based material parameters, we are able to simulate the stress-strain curves for the load and its reversal, which are consistent with the experimental curves of Thuillier and Manach (Int J Plast 25:733-751, 2009). Representative distributions of plastic slip under load and its reversal are presented, and their evolution explains the extended length of the transition stage during load reversal.
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.
Materials Science Forum, 2006
A two-level homogenisation approach is applied to the micro-mechanical modelling of the elasto-plasticity of polycrystalline materials during various strain-path changes. The model is tested by simulating the development of intragranular strains during different complex loads. Mechanical tests measurements are used as a reference in order to validate the model. The anisotropy of plastic deformation in relation to the evolution of the dislocation structure is analysed. The results demonstrate the relevance of this approach for FCC polycrystals.
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.
Mobility of grain boundary dislocations during the conservative untwisting of
Physical review. B, Condensed matter, 1996
We modeled the mobility of grain boundary dislocations ͑GBD's͒ during the untwisting of the ͓001͔ twist boundaries. Instead of assuming two semi-infinite crystals in calculating the grain boundary energy ͑i.e., the Read-Shockley approach͒ and therefore the driving force for untwisting, we assume equally spaced GBD's moving in the ͑001͒ boundary plane with the dislocations closest to the surface being pulled out by the image force. Experimental results from crystallite rotation in fcc gold were used to investigate the mobility of the GBD's. Two types of GBD motion were tested: viscous and thermally activated. The observed motions of the GBD's during untwisting can be described only as thermally activated. The Hirth-Lothe approach, which involves a thermally activated process overcoming the Peierls barrier, was applied to describe the mobility of GBD's during untwisting into the ⌺5 cusp/minimum ͑⌺ is the reciprocal of the density of the lattice sites in coincidence between two lattices at a misorientation͒ and the mobility of lattice dislocations ͕100͖ ͗110͘ during untwisting into the ⌺1 cusp/minimum. The Peierls barrier for GBD motion confined to the glide plane of the boundary ͑001͒ is significantly higher than that for lattice dislocations glide on ͕111͖ planes. From the untwisting rates, we estimate the energy barriers for GBD motions as 1.69 eV for ⌺1 and 1.84 eV for ⌺5 ͓001͔ twist boundaries. These results can explain the high yield stress and its sharp temperature dependence during plastic deformation of nanoparticle compacts of fcc metals. These results can also be used to estimate the largest size of crystallites that will rotate.