Ultrafine grained (UFG) metals and alloys typically exhibit mechanical and thermal instabilities, partially due to the high density of lattice defects, which limits their engineering applications. Annealing represents a simple and effective way to regain strain hardening, ductility and thermal stability, and stabilize the UFG structures. In this study, we systematically investigated the mechanical behavior, microstructural evolution, fracture and deformation mechanisms of UFG Al during recovery via low-temperature annealing. More specifically we report that low-temperature annealing at 250 degrees C for 20 min increased the ultimate tensile strength by 10% from 190 to 208 MPa and tensile ductility by 50% from 4.5 to 6.8% without any changes in yield strength (180 MPa). Microstructural analyses indicate that the annealing increased the average grain size from 740 to 840 nm, dislocation density decreased from 5 x 10(14) m(-2) to 1 x 10(14) m(-2), while the nature of the grain boundaries and associated precipitated phases remained unchanged. Moreover, annealing led to modification of statistically stored dislocations into low-energy dislocation walls (sub-grain boundaries). Results from the fracture surface morphology indicated that the enhanced ductility of the annealed sample was related to the activation of numerous homogeneous micro shear bands, which were controlled by cooperative grain boundary sliding. These observations suggest that the dislocation walls formed during recovery promoted the formation of micro shear bands/cooperative grain boundary sliding and thereby enhanced the ductility.

We report on the microstructural features associated with the formation of incipient spall and damage in a fully recrystallized, high purity copper sample. Before and after ballistic shock loading, approximately 0.8 mm3 of the sample's crystal lattice orientation field is mapped using non-destructive near-field High Energy Diffraction Microscopy. Absorption contrast tomography is used to image voids after loading. This non-destructive interrogation of damage initiation allows for novel characterization of spall points vis-a-vis microstructural features and a fully 3D examination of microstructural topology and its influence on incipient damage. The spalled region is registered with and mapped back onto the pre-shock orientation field. As expected, the great majority of voids occur at grain boundaries and higher order microstructural features; however, we find no statistical preference for particular grain boundary types. The damaged region contains a large volume of Σ–3 (60°〈111〉) connected domains with a large area fraction of incoherent Σ-3 boundaries.

We investigate the temperature and rate dependence of slip, twinning, and secondary twinning in high-purity hexagonal close packed α-Zr over a wide range of temperatures and strain rates (from 76K to 673K and 0.001s−1 to 4500s−1). To reliably identify the dominant deformation mechanisms for each condition, we employ electron-backscattered diffraction (EBSD), dislocation theory, multi-scale polycrystal constitutive modeling, and a thermally activated dislocation density evolution based hardening law. We demonstrate with direct comparison with measurement that the constitutive model, with a single set of intrinsic material parameters, can predict the underlying texture evolution, primary and secondary slip and twin activity, and twin volume fraction associated with the different loading orientations and applied temperatures and strain rates. We find that the {101‾2}〈101‾1‾〉 twin is the preferred tension twin, either as a primary or secondary twin depending on the sample orientation, over the broad temperature and strain rate range tested. In contrast, we show that the preferred contraction twin, whether {112‾2‾}〈112‾3‾〉 or {101‾1}〈101‾2‾〉, is sensitive to temperature but insensitive to strain rate and whether it is a primary or secondary twin. Based on the concomitant changes in the dominant slip mode predicted by the model and revealed by the texture development, we rationalize that the temperature-induced transition in contraction twinning is due to the increased predominance of basal 〈a〉 slip at high temperatures (>673K). Last, our analysis implies that all twin modes studied are rate insensitive and so the strong influence of strain rate and temperature on twinning is due to the rate-sensitivity of slip.

The convergence of multiple characterization tools has been applied to investigate the relationship of microstructure on damage evolution in high purity aluminum. The extremely coarse grain size of the disc-shaped sample provided a quasi-two dimensional structure from which the location of surface-measured features could be inferred. In particular, the role of pre-existing defects on damage growth was accessible due to the presence of casting porosity in the aluminum. Micro tomography, electron backscatter diffraction, and digital image correlation were applied to interrogate the sample in three dimensions. A recently developed micro-bulge testing apparatus was used to deform the pre-characterized disc of aluminum in biaxial tension, and related analysis techniques were applied to map local strain fields. Subsequent post-mortem characterization of the failed sample was performed to correlate structure to damaged regions. It was determined that strain localization and associated damage was most strongly correlated with grain boundary intersections and plastic anisotropy gradients between grains. Pre-existing voids played less of an apparent role than was perhaps initially expected. These combined techniques provide insight to the mechanism of damage initiation, propagation, and failure, along with a test bed for predictive damage models incorporating anisotropic microstructural effects.

The dynamic deformation of metallic polycrystalline materials leading to ductile damage and failure events involves a complex series of physical processes which are poorly understood. This lack of understanding prevents us from properly formulating and offering the appropriate physically based theories for accurate and robust representation of the ductile damage and failure response of ductile materials. This paper briefly describes and illustrates a coupled experimental and computational methodology to develop greater physical insight linking the structural details of the material to its formation of damage sites. Results from examinations of both tantalum and copper are presented to illustrate the types of methodologies that will be needed in the future to better understand the critical physical processes occurring in polycrystalline metallic materials leading to their catastrophic failure.

The emerging characterization technique of high-energy diffraction microscopy (HEDM) was used to investigate ductile dynamic damage evolution in a Cu polycrystal. Experimental efforts were undertaken with the goal of elucidating correlations between microstructural features with preferred damage nucleation sites and the progression of damage at the localization stage. HEDM was used to microstructurally map the initial volume of a 1.2 mm-diameter Cu sample. HEDM in the near-field mode collects diffraction information from high-energy synchrotron radiation to non-destructively probe microstructure and orientation in three dimensions in volumes approaching the bulk scale. The Cu sample was subsequently planar shock-loaded in a plate-on-plate geometry and soft-recovered, using an assembly specially developed for sub-size samples. The ex situ shocked sample was then re-characterized by HEDM, providing data on the location of incipient spall voids with respect to the local microstructural neighborhood. In addition, diffraction quality and misorientation gradient data provide qualitative measures of the spatial distribution of stored work and indicate regions of plastic localization. This provides the potential for unprecedented insight as to the relative preference of spall nucleation sites and correlations between microstructure, damage, and plastic flow.

Numerous studies have examined the microstructural evolution of adiabatic shear bands through the utilization of the forced shear or "tophat" test specimen. While the geometry of this specimen does not allow for the microstructure to play a dominant role in the location of a shear band, the forced shear specimen has been shown to be particularly useful for characterizing the influence of parameters such as strain rate, temperature, strain, and load on the microstructural evolution within a shear band. Additionally, many studies have also utilized this geometry to advance the understanding of shear band development in a number of materials.In this study we systematically examine the influence of integrated loading states on the dynamic shear localization response of high-purity Fe by varying the geometry of the forced shear specimen. Post-mortem characterization was performed to quantify the width of the localizations and to examine the microstructural and textural evolution of shear deformation in a body centered cubic (BCC) metal. Increased instability in mechanical response is strongly correlated with development of enhanced intergranular misorientations and high angle boundary evolution. Stress state was also critical to the localization process. Single-component, simple shear configurations were found to promote instability over multi-component stress states. In addition, these geometries resulted in traditional BCC deformation shear textures, while multi-component stress states led to less developed textures. Published by Elsevier Ltd.

Hydride nucleation and growth has previously been studied in uranium with an air-formed oxide. Preferred directional growth of uranium hydride has not been observed, presumably due to the constraint of the oxide layer and/or the presence of a surface layer distorted by mechanical grinding and polishing. Instead, hydrides typically first form as subsurface blisters that do not exhibit preferred growth directionality. By eliminating the strained surface layer through electropolishing, removing the natural oxide through ion sputtering, avoiding exposure of the uranium to air, and then exposing uranium to high purity hydrogen in an environmental cell, hydride growth patterns emerge that correspond to defect structures within the microstructure. These hydride growth patterns are similar to filiform corrosion, a type of corrosion that frequently forms under thin protective films. This work describes the first reported observation of filiform-like corrosion in uranium. The uranium hydride initiates at defects, but grows into filaments up to 20μm wide, and tends to form in straight lines, largely propagating along twin boundaries. Propagation is driven by hydrogen reaction at the filament head, promoted by more efficient delivery of reactant. However, this phenomenon does not involve an electrochemical process associated with conventional filiform corrosion and is therefore described as filiform-like. Hydride growth was observed using optical microscopy for a period of nearly three years. Sample characterization included automated electron backscatter diffraction (EBSD) measurements to determine growth directions. Observation of this anomalous hydride growth provides clues as to the mechanisms operating in uranium hydriding for more conventionally prepared sample surfaces.

Most structural materials are polycrystalline aggregates whose constituent crystals are irregular in shape, have anisotropic mechanical properties and contain a variety of defects, resulting in very complicated damage evolution. Failure models of these materials remain empirically calibrated due to the lack of a thorough understanding of the controlling processes at the scale of the materials’ heterogeneity, i.e. the mesoscale. This paper describes a novel formulation for a quantitative, microstructure-sensitive three-dimensional mesoscale prediction of ductile damage of polycrystalline materials, in the important void growth phase of the process. Specifically, we have extended a formulation based on fast Fourier transforms to compute growth of intergranular voids in porous polycrystalline materials. In this way, two widely used micromechanical formulations, i.e. polycrystal plasticity and dilatational plasticity, have been efficiently combined, with crystals and voids represented explicitly, to predict porosity evolution. The proposed void growth algorithm is first validated by comparison with corresponding finite-element unit cell results. Next, in order to isolate the influence of microstructure on void growth, the extended formulation is applied to a face-centered cubic polycrystal with uniform texture and intergranular cavities, and to a porous material with homogenous isotropic matrix and identical initial porosity distribution. These simulations allow us to assess the effect of the matrix’s polycrystallinity on porosity evolution. Microstructural effects, such as the influence of the Taylor factor of the crystalline ligaments linking interacting voids, were predicted and qualitatively confirmed by post-shocked microstrostructural characterization of polycrystalline copper.

We present results from a modeling effort that employs detailed non-destructive three-dimensional microstructure data obtained from X-ray based High Energy Diffraction Microscopy (HEDM) experiments. The emphasis is on validating models that capture microstructural sensitivities so that these models can then be employed in rapid certification procedures. By focusing validation efforts on models that connect directly to experimentally measurable features of the microstructure, we can then build confidence in use of the models for components prepared under different processing routes, with different chemical compositions and attendant impurity distributions, or subjected to different loading conditions. The computational model makes use of a crystal mechanics based constitutive model that includes porosity evolution. The formulation includes nucleation behavior that is fully integrated into a robust numerical procedure, enhancing capabilities for modeling small length scales at which nucleation site potency and volume fraction are more variable. Three-dimensional experimental data are available both pre-shot and post-shot from the same volume of impact-loaded copper. Crystal lattice orientation and porosity data are obtained, respectively, from near-field HEDM and tomography techniques. The availability of such data serves as a primary motivation for the model effort at the microstructural scale.

Damage in plate impacts of 50 kbar and 30 kbar Cu, has been simulated with the traditional square wave loading profile of plate impacts and a triangular loading profile to study the time dependence of the damage evolution. At 30 kbar loading the square wave loading produces complete fracture while the triangular wave loading does not, similar to experiments by Koller et al. [1]. The calculated stress and strain loading histories are compared to provide insight for this result. The damage process in the triangular loading simply stops before fracture due to lack of kinetic energy.

Deformation twinning resulting from high explosive-driven shock and associated plasticity was investigated in high-purity b.c.c. tantalum. Post mortem characterization of samples shocked at relatively higher and lower pressures showed significant {112}〈111〉 twin activity. Further analysis of the lower shock pressure sample showed twins to be spatially clustered at the mesoscale, indicating the role of twin termination at grain boundaries to produce requisite twin initiation stresses in neighbor grains. In addition, analysis of electron backscatter diffraction data suggests that twin propagation across boundaries does not require minimal misorientations between the active variants of the twins in adjacent parent grains. A minimum threshold grain size of approximately 25μm was determined for both samples, below which twinning was suppressed. Finally, the observation of spall voids at twin intersections implied that twinning increases the density of preferred damage initiation sites during the shock deformation process. Overall, twinning was shown to play a significant role in the deformation and damage evolution of shock-loaded tantalum.

We present the free surface response of 2, 5, and 8 μm aluminum films to shocks generated from chirped ultrafast lasers. We find two distinct steps to the measured free surface velocity that indicate a separation of the faster elastic wave from the slower plastic wave. We resolve the separation of the two waves to times as short as 20 ps. We measured peak elastic free surface velocities as high as 1.4 km/s corresponding to elastic stresses of 12 GPa. The elastic waves rapidly decay with increasing sample thickness. The magnitude of both the elastic wave and the plastic wave and the temporal separation between them was strongly dependent on the incident laser drive energy.

A fast and accurate method has been developed for measuring crystalline texture in homogeneous materials. The method uses a conventional powder x-ray diffractometer capable of θ scans. Two scans are recorded from the sample: first, a high resolution θ-2θ scan is obtained of a Bragg peak whose diffracting planes are normal to the preferred orientation direction; second, a θ scan is obtained using this peak. The η scan contains the required texture information, but the intensities must be corrected for defocusing and absorption to obtain the texture profile. The θ-2θ scan of the Bragg peak is used to make the defocusing correction, and first principles calculations are used to correct for absorption. The theory behind these corrections is presented here. The validity of the technique has been verified by making measurements on untextured alumina. Data obtained from Bi2Sr2Ca2Cu3O10 superconducting tape specimens with this technique are compared with texture data obtained with a four-circle diffractometer.