Unraveling the Role of Interfaces on the Spall Failure of Cu/Ta Multilayered Systems.

Molecular dynamics (MD) simulations are carried out to investigate the effects of the type and spacing of FCC/BCC interfaces on the deformation and spall behavior. The simulations are carried out using model Cu/Ta multilayers with six different types of interfaces. The results suggest that interface type can significantly affect the structure and intensity of the incoming shock wave, change the activated slip systems, alter dislocation slip and twinning behavior, affect where and how voids are nucleated during spallation and the resulting spall strength. Moreover, the above aspects are significantly affected by the interface spacing. A transition from homogeneous to heterogeneous dislocation nucleation occurs as the interface spacing is decreased to 6 nm. Depending on interface type and spacing, damage (voids) nucleation and spall failure is observed to occur not only at the Cu/Ta interfaces, but also in the weaker Cu layer interior, or even in the stronger Ta layer interior, although different mechanisms underlie each of these three distinct failure modes. These findings point to the fact that, depending on the combination of interface type and spacing, interfaces can lead to both strengthening and weakening of the Cu/Ta multilayered microstructures

Mapping Structural Heterogeneity at the Nanoscale with Scanning Nano-structure Electron Microscopy (SNEM)

Here we explore the use of scanning electron diffraction coupled with electron atomic pair distribution function analysis (ePDF) to understand the local order as a function of position in a complex multicomponent system, a hot rolled, Ni-encapsulated, Zr$_{65}$Cu$_{17.5}$Ni$_{10}$Al$_{7.5}$ bulk metallic glass (BMG), with a spatial resolution of 3 nm. We show that it is possible to gain insight into the chemistry and chemical clustering/ordering tendency in different regions of the sample, including in the vicinity of nano-scale crystallites that are identified from virtual dark field images and in heavily deformed regions at the edge of the BMG. In addition to simpler analysis, unsupervised machine learning was used to extract partial PDFs from the material, modeled as a quasi-binary alloy, and map them in space. These maps allowed key insights not only into the local average composition, as validated by EELS, but also a unique insight into chemical short-range ordering tendencies in different regions of the sample during formation. The experiments are straightforward and rapid and, unlike spectroscopic measurements, don't require energy filters on the instrument. We spatially map different quantities of interest (QoI's), defined as scalars that can be computed directly from positions and widths of ePDF peaks or parameters refined from fits to the patterns. We developed a flexible and rapid data reduction and analysis software framework that allows experimenters to rapidly explore images of the sample on the basis of different QoI's. The power and flexibility of this approach are explored and described in detail. Because of the fact that we are getting spatially resolved images of the nanoscale structure obtained from ePDFs we call this approach scanning nano-structure electron microscopy (SNEM), and we believe that it will be powerful and useful extension of current 4D-STEM methods

Heterostructured materials: superior properties from hetero-zone interaction

Heterostructured materials are an emerging class of materials with superior performances that are unattainable by their conventional homogeneous counterparts. They consist of heterogeneous zones with dramatic (>100%) variations in mechanical and/or physical properties. The interaction in these hetero-zones produces a synergistic effect where the integrated property exceeds the prediction by the rule-of-mixtures. The heterostructured materials field explores heterostructures to control defect distributions, long-range internal stresses, and nonlinear inter-zone interactions for unprecedented performances. This paper is aimed to provide perspectives on this novel field, describe the state-of-the-art of heterostructured materials, and identify and discuss key issues that deserve additional studies

Extreme shear-deformation-induced modification of defect structures and hierarchical microstructure in an Al–Si alloy

Extreme shear deformation is used for several material processing methods and is unavoidable in many engineering applications in which two surfaces are in relative motion against each other while in physical contact. The mechanistic understanding of the microstructural evolution of multi-phase metallic alloys under extreme shear deformation is still in its infancy. Here, we highlight the influence of shear deformation on the microstructural hierarchy and mechanical properties of a binary as-cast Al-4 at.% Si alloy. Shear-deformation-induced grain refinement, multiscale fragmentation of the eutectic Si-lamellae, and metastable solute saturated phases with distinctive defect structures led to a two-fold increase in the flow stresses determined by micropillar compression testing. These results highlight that shear deformation can achieve non-equilibrium microstructures with enhanced mechanical properties in Al–Si alloys. The experimental and computational insights obtained here are especially crucial for developing predictive models for microstructural evolution of metals under extreme shear deformation.This article is published as Gwalani, Bharat, Matthew Olszta, Soumya Varma, Lei Li, Ayoub Soulami, Elizabeth Kautz, Siddhartha Pathak et al. "Extreme shear-deformation-induced modification of defect structures and hierarchical microstructure in an Al–Si alloy." Communications Materials 1, no. 1 (2020): 85. doi: https://doi.org/10.1038/s43246-020-00087-x. © The Author(s) 2020. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/)

Grain refinement in bulk pure tantalum using equal channel angular extrusion

Due to the character of the original source materials and the nature of batch digitization, quality control issues may be present in this document. Please report any quality issues you encounter to [email protected], referencing the URI of the item.Includes bibliographical references (leaves 127-131).Issued also on microfiche from Lange Micrographics.For this study, the effectiveness of equal channel angular extrusion (ECAE) to improve the material properties and processing characteristics of vacuum arc remelted (VAR) pure tantalum was determined. The primary objectives were: 1) determination of recrystallization temperatures for processed material 2) determination of the grain refinement potential of ECAE and 3) determination of the ability of ECAE to produce a homogeneous grain structure. The effects of initial grains size (as-cast: [] 5 mm, large: 500 [u]m - 2 mm, medium: 20 [u]m - 100[u]m) and morphology, annealing temperature (23°C - 1370°C) and level of strain (one, two, or four extrusions) and extrusion route (C and E) on the recrystallized grain size, percent recrystallization, microstructural uniformity, grain morphology and Vickers microhardness were investigated. All extrusions were performed at room temperature in a 90° die using a punch speed of 5 mm/sec. Microstructural uniformity and morphology were observed and characterized using an optical metallograph equipped with a polarizing filter. Grain size measurements were made using the linear intercept method on optical micrographs. Four consecutive passes without intermediate annealing show that pure VAR tantalum is very workable when subjected to ECAE processing. The initial grain size and processing route have little if any effect on the workability or recrystallization temperature after one extrusion pass. Microhardness values are similar to published literature values produced by conventional deformation methods at equivalent strains and tend to increase significantly during the first two extrusions. Routes 2C and 4C result in fine (<22 [u]m), uniform grains after annealing for the large and medium initial grain size materials but do not for the as-cast initial grain size. Route E results in fine-grained, homogeneous, equiaxed microstructures for all initial grain sizes with ~11 [u]m being the smallest produced. The microstructural homogeneity and uniformity, and the fine-grain size resulting from ECAE processing may be advantageous to those produced by the conventional methods such as rolling, swaging, forging and wire drawing, with the added advantage of being a bulk product. The Hall-Petch relationship is found to be valid for ECAE processed tantalum over a grain size range of 10 [u]m to 100[u]m

Amorphous Hf-based foams with aligned, elongated pores

Warm equal channel angular extrusion is used to consolidate a blend of amorphous Hf44.5Cu27Ni13.5Ti5Al10 and crystalline W powders. Dissolution of the W phase results in ∼60% aligned, elongated pores within an amorphous Hf-based matrix exhibiting ductility in compression, but at lower strengths than similar amorphous Zr-based foams due to incomplete bonding between Hf-based powders

Unraveling the Role of Interfaces on the Spall Failure of Cu/Ta Multilayered Systems.

Molecular dynamics (MD) simulations are carried out to investigate the effects of the type and spacing of FCC/BCC interfaces on the deformation and spall behavior. The simulations are carried out using model Cu/Ta multilayers with six different types of interfaces. The results suggest that interface type can significantly affect the structure and intensity of the incoming shock wave, change the activated slip systems, alter dislocation slip and twinning behavior, affect where and how voids are nucleated during spallation and the resulting spall strength. Moreover, the above aspects are significantly affected by the interface spacing. A transition from homogeneous to heterogeneous dislocation nucleation occurs as the interface spacing is decreased to 6 nm. Depending on interface type and spacing, damage (voids) nucleation and spall failure is observed to occur not only at the Cu/Ta interfaces, but also in the weaker Cu layer interior, or even in the stronger Ta layer interior, although different mechanisms underlie each of these three distinct failure modes. These findings point to the fact that, depending on the combination of interface type and spacing, interfaces can lead to both strengthening and weakening of the Cu/Ta multilayered microstructures

Atomic mixing mechanisms in nanocrystalline Cu/Ni composites under continuous shear deformation and thermal annealing

Molecular dynamics (MD) simulations are used to reveal the mechanisms of defect substructure evolution and atomic mixing in nanocrystalline Cu/Ni composites under severe shear deformation and subsequent thermal annealing. A continuous shear scheme of MD simulation utilizing an on-the-fly periodic boundary adjustment approach enables it to reach any large shear strain. The comprehensive evaluation on the evolution of dislocation structures at various strain states indicates partial dislocation-mediated plastic deformation and triple junction slide via partial dislocation nucleation and emission from triple junctions and grain boundaries. The analysis of atomic structures in unique mixing regions suggests that triple junction sliding can result in a long-range region of atomic mixing facilitated by net dislocation flux, which is the key mechanism of atomic mixing in nanocrystalline Cu/Ni composite under severe shear, while dislocation emissions at interfaces result in short-range mixing. It is also found that thermal annealing causes the evolution of non-equilibrium defect substructures which assists atomic mixing