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We summarize recent work on helium (He) interaction with various heterophase boundaries under high temperature irradiation. We categorize the ion-affected material beneath the He-implanted surface into three regions of depth, based on the He/vacancy ratio. The differing defect structures in these three regions lead to the distinct temperature sensitivity of He-induced microstructure evolution. The effect of He bubbles or voids on material mechanical performance is explored. Overall design guidelines for developing materials where He-induced damage can be mitigated in materials are discussed. [ABSTRACT FROM AUTHOR]

We examine recrystallization and grain growth during processing of accumulative roll-bonded (ARB) Cu-Nb and Zr-Nb composites. Throughout the ARB process, from initial millimeter thick layers down to nanometer thick layers, the mechanism for recrystallization and grain growth is the motion of high-angle grain boundaries (HAGBs). However, the driving forces for these phenomena change as the densities of different types of defects evolve during the process. The creation and redistribution of dislocations, grain boundaries, and phase boundaries has significant effects on recrystallization and grain growth and, thus, on microstructural evolution. Both Cu-Nb and Zr-Nb exhibit a distinct transition in recrystallization and growth behavior at around 500-nm average layer thicknesses. For the thicker layered materials, the microstructure evolution during recrystallization and growth is determined by the density and distribution of dislocations and HAGBs. For layers less than 500 nm, the layers are largely one-grain thick and the grains are nearly dislocation free; coarsening of grains within layers at the nanoscale is due to reduction in phase boundary energy. [ABSTRACT FROM AUTHOR]

We examine the development of stable bimetal interfaces in nanolayered composites in severe plastic deformation. Copper-niobium multilayers of varying layer thicknesses from several micrometers to 10 nanometers (nm) were fabricated via accumulative roll bonding (ARB). Investigation of their 5-parameter character and atomic scale structure finds that when layer thicknesses refine well below one micrometer, the interfaces self-organize to a few interface orientation relationships. With atomic scale and crystal plasticity modeling, we identify that the two controlling factors that determine whether an interface is stable under high strain rolling are orientation stability of the bicrystal and interface formation energy. A figure-of-merit is introduced that not only predicts the development of the prevailing interfaces but also explains why other interfaces did not develop. Through a suite of nanomechanical and bulk test results, we show that ARB composites containing these stable interfaces are found to have exceptional hardness (∼4.5 GPa) and strength (∼2 GPa). [ABSTRACT FROM AUTHOR]

3D interfaces are a new type of interface containing nanoscale crystallographic, structural, and chemical heterogeneities in all spatial dimensions. Recently, 3D interfaces have been shown to enhance strength and deformability simultaneously by frustrating shear instability under layer-normal micropillar compression in Cu/Nb nanolaminates. However, quantification of deformed microstructure and effects of loading orientation were not explored in that work. Here, we address these shortcomings by performing post mortem TEM characterization of micropillars compressed at normal and 45° inclination to layers. We find high strength and deformability in both loading geometries and show that 3D interfaces enhance mechanical behavior under multiple loading orientations. In layer-normal compression, post mortem characterization allows for quantification of key quantities correlating well to the severity of shear localization across nanolaminates with different layer thickness and interface type. In 45° compression, TEM results demonstrated no strong plastic instability. This motivated analytical computation of Schmid factors and simulation of slip system activity via crystal plasticity finite element modeling (CPFE). The CPFE model demonstrates that most slip activity occurs non-parallel to layers, indicating that dislocation-3D interface interactions must mediate the observed mechanical behavior of micropillars. This work lays the foundation for further study of 3D interface-driven deformation physics in nanostructured alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

In this work, we present a 3D microstructure-based, full-field crystal plasticity finite element (CPFE) model using a thermally activated dislocation-density based constitutive description and apply it to study the deformation of a two-phase hexagonal close packed (HCP)-body center cubic (BCC) Zr/Nb composite. The microstructure models were created using a synthetic grain structure builder (DREAM.3D) and a meshing toolset for the 3D network of grains, grain boundaries, and bimetal interfaces. The crystal orientations, grain shapes, and grain sizes for each phase were initialized based on the measured data. With this novel technique, we aspire to couple the evolution of microstructural heterogeneities with the evolution of spatially resolved mechanical fields during the deformation of complex composites. Here, we apply it to understand the role that microstructure plays in the development of the local concentrations in strain and strain rate that can trigger plastic instabilities, such as shear banding. Our chief findings are that 1) local areas of relatively high (and relatively very low) strain concentration occur at triple junctions or quadruple points and then connect via straining to create a banded configuration that extends across the polycrystalline layer, 2) this event starts in the Zr phase and not in the Nb phase, and 3) the triggering hot spots in strain occur at junctions that join grains with very dissimilar reorientation propensities and vice versa for cold spots. In order to determine how such influential localizations can be prevented during processing via application of intermediate annealing treatments, we used the model to also explore the effects of annealing-induced changes in accumulated dislocation density, crystallographic texture and grain shape on the development of strain localizations during subsequent deformation. We found that while it is difficult to avoid strain localizations at grain junctions, when provided a microstructure containing a few large grains spanning the thickness, elongated grain shapes, and reduced dislocation density, the linkage of hot spots in the form of a band can be postponed. At the end we show that when an additional softening mechanism is introduced, these localized strain concentration areas can lead to shear bands. [ABSTRACT FROM AUTHOR]

We report fabrication and nanomechanicaltension experiments onas-fabricated and helium-implanted ∼130 nm diameter Ni73P27metallic glass nanocylinders. The nanocylinderswere fabricated by a templated electroplating process and implantedwith He+at energies of 50, 100, 150, and 200 keV to createa uniform helium concentration of ∼3 atom % throughout thenanocylinders. Transmission electron microscopy imaging and through-focusanalysis reveal that the specimens contained ∼2 nm helium bubblesdistributed uniformly throughout the nanocylinder volume. In situtensile experiments indicate that helium-implanted specimens exhibitenhanced ductility as evidenced by a 2-fold increase in plastic strainover as-fabricated specimens with no sacrifice in yield and ultimatetensile strengths. This improvement in mechanical properties suggeststhat metallic glasses may actually exhibit a favorable response tohigh levels of helium implantation. [ABSTRACT FROM AUTHOR]

Abstract Nanolaminates have gained much attention due to exceptional mechanical, optical, electrical and biological properties. In this work, we explore the microstructure and mechanical properties of Al-SiC co-sputtered monolayers having different compositions. Co-sputtering enables tailoring the microstructure at an atomic level and hence is a promising route to develop new generation of materials. These co-sputtered samples were characterized through FIB/SEM, TEM and XPS. They had an amorphous microstructure, with the exception of nanocrystalline Al aggregates present in one of the compositions. The micromechanical properties were studied through nanoindentation. We observed that the modulus and hardness of the co-sputtered samples were much higher than traditional Al/SiC nanolaminate samples having the same composition. Graphical abstract Unlabelled Image Highlights • Novel co-sputtered monolayers of Al-SiC were synthesized by magnetron sputtering. • Thorough materials characterization showed a unique nanostructure that results in extremely high modulus and hardness compared to classical Al-SiC nanolaminates. • A unique nanostructure consisting of Al, SiC, Si, and C was obtained. [ABSTRACT FROM AUTHOR]