A series of plate impact experiments have been performed on the three principal orientations of single crystal aluminium: 100, 110 and 111. Experiments were designed to probe the Hugoniot Elastic ...Limit, spall strength and Equation of state as well as understand the microstructural response, via transmission electron microscopy, Transmission Kikuchi Diffraction and mechanical testing of shocked material. Results from this work, show that orientation affects the HEL and spall strength in a similar manner to the quasi-static uniaxial stress compressive response, with 100 and 110 being similar and 111 significantly stronger. This correlates with expectations from considerations using the Schmid analysis. However we have noted that similar studies in copper yield a different ordering of the HELs. We have reconciled those differences using an analysis based on second order elastic constants. However the shocked microstructures and post shock quasi-static mechanical response are strongly influenced by orientation. The 100 orientation shows a response typical of high stacking fault face centred cubic metals, consisting of equiaxed subgrains and a strong post shock hardening during quasi-static mechanical testing, similar to results in polycrystalline aluminium. In contrast, the 110 and 111 orientations have a microstructural response more similar to polycrystalline aluminium after repeated shock loading excursions, combined with much lower post shock hardening during mechanical testing. This suggests that these orientations require much greater dislocation activity to achieve the same plastic deformation as 100. This would appear to agree with the much higher degree of work hardening observed in these orientations compared to 100.
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Grain boundaries (GBs) are often the preferred sites for void nucleation in ductile metals. However, it has been observed that all boundaries do not contribute equally to this process. We present a ...mechanistic rationale for the role of GBs in damage nucleation in copper, along with a quantitative map for predicting preferred void nucleation at GBs based on molecular dynamics simulations in copper. Simulations show a direct correlation between the void nucleation stress and the ability of a grain boundary to plastically deform by emitting dislocations, during shock compression. Plastic response of a GB, affects the development of stress concentrations believed to be responsible for void nucleation by acting as a dissipation mechanism for the applied stress.
Three 316L stainless steel materials are studied and reported upon; wrought, as-built additively manufactured (AM), and heat-treated AM material. The AM material was produced from the laser ...engineered net shaping (LENS) process. Macroscopic uniaxial compression stress-strain curves were obtained for all three materials. The curves were similar for the wrought and heat-treated AM materials but the as-built AM material demonstrated approximately 1.7 times greater flow stress at any given level of strain than the other two materials. Electron-backscatter diffraction analysis of the materials also showed that the microstructures of the three materials differed; with complex grain morphology for the as-built AM material. The mean grain size of each of the three materials also differed. The initial dislocation density was also measured with neutron diffraction and line-profile analysis for both wrought and as-built AM materials with the density in the AM material approximately 2.5 times greater. A single crystal model was proposed to represent the essential features of the three FCC materials accounting for dislocation interactions and representation of grain size via a simple Hall-Petch type term. The strength of this term is evaluated through independent experimental results on traditionally manufactured materials. The model was applied to each of the three materials by simulation of the uniaxial compression experiments by direct numerical simulation of electron-backscatter diffraction images. This allowed for representation of the size of each grain in the simulations. The results suggest that the difference in initial dislocation density of the three materials is the primary factor causing the difference in stress-strain response. Although the differences in grain size contribute to a higher stress for the as-built AM material, the effect is small. Other factors such as internal stress and grain morphology could play a role in mechanical behavior difference and these two factors are also discussed.
A new specimen geometry, the compact forced-simple-shear specimen (CFSS), has been developed as a means to achieve simple shear testing of materials over a range of temperatures and strain rates. The ...stress and strain state in the gage section is designed to produce essentially “pure” simple shear, mode II in-plane shear, in a compact-sample geometry. The 2-D plane of shear can be directly aligned along specified directional aspects of a material's microstructure of interest; i.e., systematic shear loading parallel, at 45°, and orthogonal to anisotropic microstructural features in a material such as the pancake-shaped grains typical in many rolled structural metals, or to specified directions in fiber-reinforced composites. The shear-stress shear-strain response and the damage evolution parallel and orthogonal to the pancake grain morphology in 7039-Al are shown to vary significantly as a function of orientation to the microstructure.
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Accurately representing the process of porosity-based ductile damage in polycrystalline metallic materials via computational simulations remains a significant challenge. The heterogeneity of ...deformation in this class of materials due to the anisotropy of deformation of individual single crystals creates the conditions for the formation of a damage field. In this work, a technique of soft-coupled linkage between a macro-scale damage model and micro-mechanical calculations of a suite of polycrystal realizations of a representative BCC tantalum is presented. The macro-scale model, which accounts for rate-dependence and micro-inertial effects in the material, was used to represent two plate impact experiments and predict the point in time in the loading profile when porosity is initiated. The three-dimensional loading history from the macro-scale calculation was then used to define the probable loading history profile experienced within the samples. A micro-mechanical model based on an accurate representation of single crystal plasticity is then presented. Specifically, this model is employed in performing of polycrystal calculations of statistically representative microstructures of the tantalum material subjected to the extreme loading conditions informed from the macro-scale calculations. This enables to provide local-scale stress conditions for porosity initiation within the polycrystalline network. Furthermore, the micro-mechanical model captures the non-Schmid effects that accounts anomalous motion of the dominant screw dislocations within each of the single crystals. The results of the micro-mechanical simulations suggests that the non-Schmid effects significantly influence the local stress conditions across grain boundaries and triple junctions within the polycrystalline network. The computational results also suggest that the von Mises stress conditions and triaxiality at the grain boundaries and the grain boundary triple lines are highly variable but the variability is reduced with distance to the grain center. Furthermore, we found that the stress conditions at the grain boundaries are strongly dependent on the orientation of each boundary with respect to the shock direction.
•In this work, a technique of soft-coupled linkage between a macro-scale damage model and micro-mechanical calculations of a suite of polycrystal realizations of a representative BCC tantalum is presented.•A micro-mechanical model is used for polycrystal calculations of statistically representative microstructures subjected to loading conditions from macro-scale calculations.•The computational results suggest that von Mises stress conditions and triaxiality at grain boundaries and triple lines are highly variable but variability is reduced with distance to the grain center.
We investigate the effect of grain boundary inclination with respect to the loading direction on void nucleation at a boundary, using plate impact experiments on polycrystalline copper. Examination ...of damaged specimens reveals that boundaries perpendicular to the loading direction are an order of magnitude more susceptible to failure than those parallel to the loading direction. We investigate the mechanisms and reasons behind this experimental observation through molecular dynamics (MD) simulations, as a function of loading direction, in a copper bicrystal. Two extremes of loading directions are considered, either parallel or perpendicular to the grain boundary plane, spanning the range that grain boundaries within a polycrystalline sample will ordinarily experience under uniaxial strain conditions. Using MD simulations, we demonstrate that, during shock compression, the ability of a boundary to undergo plastic deformation is altered measurably by changing the loading direction with respect to the boundary plane. This change in the plastic response of the GB affects the development of stress concentrations believed to be responsible for void nucleation. MD simulations show that boundaries perpendicular to the loading direction do not undergo as much plastic deformation, by dislocation emission, as those parallel to the loading direction. The lack of plastic deformation at the GB, in the perpendicular loading case, can decrease the stress required for void nucleation. The MD results are consistent with experimental observations, and support the contention that plastic response of a grain boundary under shock compression can be a contributing, or even dominating, factor in determining the stress for void nucleation.
A series of measurements were designed to gain confidence in the interpretation of the peak breadth in diffraction patterns collected from additively manufactured material, which has a novel ...microstructure in comparison to the well understood microstructure of wrought materials. Stainless steels made with two additive manufacturing techniques were compared to wrought material. Similar patterns observed in the scattering vector dependence for additively manufactured and deformed wrought materials suggested that the broadening in both materials was related to dislocations. This was confirmed by heat-treatment, during which both materials exhibited recovery due to the annealing of dislocations at the same temperature.
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