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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. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: The microstructure and texture variations near adiabatic shear localization regions in as-received and pre-shocked 304 stainless steels were investigated using electron backscatter diffraction (EBSD). Shear bands were developed in both steels during a forced dynamic shear deformation produced on a split-Hopkinson pressure bar. EBSD examination revealed large intragranular misorientation gradients toward the shear bands. Taylor factor and misorientation analyses were applied to explain deformation characteristics. Shear-induced deformation structure was observed to be sensitive to both initial microstructure and local orientation. Comparison of as-received and pre-shocked steels indicated that the pre-shocked condition yielded sharper shear band boundaries but moderate misorientation gradients, while the as-received condition led to substantial misorientation adjacent to the band. Shear-induced texture evolution generally resulted in the reorientation of 〈110〉 toward the shear direction for both cases. Grains with slip systems favorably oriented with respect to the applied shear path displayed less intragranular misorientation development compared to unfavorably oriented grains. [Copyright &y& Elsevier]

The banding in a friction stir weld was determined to correspond to a periodic textural variation between the component and a mixture of and C components of the simple shear texture. These textures have similar deformation reference frames, indicating that the banding arises from a periodic variation in the strain magnitude or temperature during welding. [Copyright &y& Elsevier]

Abstract: The microstructure of AL-6XN, a commercial super-austenitic stainless steel, was investigated using novel two- and three-dimensional (3D) analysis and characterization techniques. The austenite matrix, the second-phase σ precipitates, and the relationships between them have been analyzed, with particular emphasis on the true 3D microstructure of the material, including grain boundary character, grain morphologies and connectivity. A combination of serial sectioning with electron backscatter diffraction (EBSD) analysis allowed for reconstruction of individual grains, and definition of all five degrees of freedom of the grain boundary planes. Second-phase σ particle size and morphology, crystallography, composition, potential formation mechanisms, orientation relationships, and coherency with the matrix have been analyzed. These results provide boundary conditions for atomistic calculations of specific grain boundary structures as well as the basis for mesoscale image-based models of mechanical behavior of the microstructures. [Copyright &y& Elsevier]

A stop-action friction stir weld has been prepared in Al–Li 2195 to “freeze in” the dynamic deformation field surrounding the FSW tool. Analysis of a plan-view section of this weld reveals important new details of the grain structure evolution and texture development occurring around the FSW tool. [Copyright &y& Elsevier]

The local orientation features in as-cast α-uranium were investigated in order to ascertain the influence of texture and heterogeneity in the microstructure on hydride initiation and growth. Several samples were interrogated via automated electron back-scattered diffraction measurements in the post-hydride state. Characterization included orientation mapping, deformation twin identification, and grain boundary analysis. It was found that preferential hydride initiation sites were associated with general high-angle grain boundaries, low-angle boundaries, and twin boundaries of two systems. Linear segregation or precipitation features suggest the existence of unresolved twins that could be correlated with many of the remaining hydrides. The tendency of certain orientations to exhibit differing hydride potentials based on atomic density in the habit plane was also considered. The results suggest the dominant role of enhanced sub-surface hydrogen diffusion at misoriented regions in controlling hydride nucleation and growth. [Copyright &y& Elsevier]

A forged and round-rolled pure tantalum bar stock was observed to exhibit large asymmetry in bulk plastic flow response when subjected to large strain Taylor cylinder impact testing. This low-symmetry behavior was analyzed experimentally investigating both the initial stock and the impact-deformed material via x-ray crystallographic texture measurements and automated electron back-scatter diffraction scans to establish spatial microstructural uniformity. Polycrystal simulations based upon the 110 measured duplex texture and experimentally inferred deformation mechanisms were performed to project discrete yield surface shapes. Subsequent least squares fitting and eigensystem analysis of the resulting quadratic fourth-order tensors revealed strong normal/shear stress coupling in the yield surface shape. This mixed-mode coupling produces a shearing deformation in the 1–2 impact plane of a Taylor specimen whose axis is coincident with the compressive 3-axis. The resultant deformation generates an unusual rectangular-shaped impact footprint that is confirmed by finite-element calculations compared to experimental post-test geometries. [Copyright &y& Elsevier]

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 °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 × 1014 m-2 to 1 × 1014 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. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Abstract: The microstructural evolution around the tool during friction stir-welding of aluminum is described. The rotating tool induces a gradual rotation of the crystal lattice with a concomitant development of elongated subgrains. Additional small subgrains form within this structure by continuous dynamic recrystallization, gradually increasing in misorientation, eventually developing high-angle (>15°) grain boundaries. The development of texture and similarities to friction stir welds in titanium alloys are also discussed. [Copyright &y& Elsevier]

Abstract: Commercial purity aluminum AA1050 billets with similar submicrometer grain sizes but distinctively different boundary structures were obtained by the use of different equal channel angular extrusion (ECAE) routes. Route “A” is defined as no rotation between extrusion passes, while route “C” is rotated 180° between extrusion passes. The microstructure processed by route “A” consists mainly of high-angle boundaries (HABs) while route “C” has primarily low-angle boundary (LAB) structures. Tensile tests conducted on these two microstructures at intermediate and dynamic strain rates at both 77 and 298K were compared with the results of material tested at quasi-static strain rates. The boundary structure was found to play an important role in the mechanical properties at low temperature. [Copyright &y& Elsevier]

At approximately 13GPa, body-centered cubic alpha-iron undergoes a fully reversible, pressure-induced phase transition into hexagonal close-packed epsilon-iron. Microstructural evidence of this phase transition has been identified in the fully reverted alpha-iron as a large number of {332} twins found primarily as secondary twins within {112} primary twins. The {332} twins were produced during high-pressure shock-loading of 1018 steel at a peak pressure above the alpha-epsilon phase transition pressure. The twins were identified using electron backscattered diffraction and transmission electron microscopy. [Copyright &y& Elsevier]

Abstract: This paper provides a brief overview of a multidisciplinary effort at the Naval Research Laboratory aimed at developing a computationally-based methodology to assist in the design of advanced Naval steels. This program uses multiple computational techniques ranging from the atomistic length scale to continuum response. First-principles electronic structure calculations using density functional theory were employed, semi-empirical angular dependent potentials were developed based on the embedded atom method, and these potentials were used as input into Monte-Carlo and molecular dynamics simulations. Experimental techniques have also been applied to a super-austenitic stainless steel (AL6XN) to provide experimental input, guidance, verification, and enhancements to the models. These experimental methods include optical microscopy, scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction, and serial sectioning in conjunction with computer-based three-dimensional reconstruction and quantitative analyses. The experimental results are also used as critical input into mesoscale finite element models of materials response. [Copyright &y& Elsevier]

Significant enhancements in critical current densities in rolled multifilamentary Bi-2223 HTS composite conductors have been achieved using the powder-in-tube (PIT) technique. At 77 K and self field, oxide critical current densities (J/sub c/) of 55 kA/cm/sup 2/, overall or engineering critical current densities (J/sub e/) of 15 kA/cm/sup 2/, and critical currents (I/sub c/) of 125 A have been achieved in different rolled multifilamentary composites. Progress in achieving such high electrical performance is believed to stem in part from an improvement of grain connectivity by reducing weak links. The J/sub c/ dependence on magnetic field (B) and the degree of c-axis texture of these high quality conductors have been investigated at various temperatures. Our results also demonstrate that the critical current retention in magnetic field can be independently controlled from the self field critical current density, suggesting that flux pinning improvements and weak link reductions can be separately engineered into Bi-2223 composites fabricated using manufacturable processes. [ABSTRACT FROM PUBLISHER]