Scripta Materialia最新文献

Extended X-ray absorption fine structure (EXAFS), high-resolution transmission electron microscopy (HRTEM), and X-ray diffraction have been used to monitor the structural development, on atomic-to-nanometer scale, prior to and along with shear band initiation in a face-centered-cubic CrCoNi medium-entropy alloy (MEA) under impact punch shear loads. Our findings provide clear evidence of chemical ordering with accompanying compositional inhomogeneity, on the length scale of one nanometer at the beginning of shear banding initiation. This chemical short-range atomic rearrangement of the three constituent elemental species is a result of atomic diffusion during high-strain-rate straining. The increasing chemical/structural inhomogeneity is likely to exert perturbations to cause uneven energy dissipation and encourage dislocation slip plane softening, both promoting strain localization that may have helped to instigate shear banding. Dynamic recrystallization is observed in later mature shear bands.

It is well known that chemical strengthening of glass brings the benefit of increased fracture strength. Despite extensive research on processing and mechanics at the macroscale, the effectiveness of chemical strengthening on glass elements with all three dimensions in the micrometer regime remains largely unexplored. Here, we develop a novel process for chemical strengthening of micrometer-sized spherical glass powder particles and study the fracture behavior of these particles with in-situ particle compression tests inside a scanning electron microscope. Cross-sectional microscopy and energy dispersive spectroscopy measurements confirm ion exchange and show an increase in diffusion depth with an increase in processing time and temperature. We report a higher fracture strength for chemically strengthened powder particles compared with the as-received ones. We show that the increase in fracture strength is associated to the compressive residual stress resulting from ion exchange during chemical strengthening.

ZnO–SnO₂ branch–stem nanowires were fabricated on a Cu foil using a chemical vapor deposition system through a two-step process. Firstly, SnO₂ NWs were synthesized directly on a Cu foil substrate by evaporating SnO powder as a source material. Then, the as-synthesized SnO₂ NWs were used as templates for the growth of ZnO–SnO₂ branch–stem NWs. The effect of growth time on the growth of the SnO₂ NWs on the Cu foil was studied. The gas sensing properties of the SnO₂ NW and ZnO–SnO₂ branch–stem NW devices were studied using various toxic gases at different temperatures. Both devices exhibited high sensitivity, high selectivity, fast response and recovery times, and stability toward H₂S gas. Compared to the pristine SnO₂ NW device, the ZnO–SnO₂ branch–stem NW device exhibited higher sensitivity and faster response rate toward H₂S. Finally, the gas sensing mechanism was also discussed.

In this study, we demonstrate that the fracture toughness of amorphous silica, an electrical insulator, can be dramatically increased and restored via injecting and removing the electrical charge content. Micropillar specimens of amorphous silica were fabricated using focused ion beam machining. The specimens were charged by electron-beam irradiation (charged specimens), and the charge was removed from the specimens by exposure to atmospheric conditions and annealing (charge-removed specimens). Fracture toughness testing was conducted on non-charged, charged, and charge-removed micropillar specimens. The fracture toughness of the charged specimen was 2.4 times higher than that of the non-charged specimens. Furthermore, the fracture toughness of the charge-removed specimens was restored to a level similar to that of the non-charged specimens, but not completely restored. These results indicate that the fracture toughness of amorphous silica can be controlled by injecting and removing electrostatic charges.

The crystallography and morphology of lath martensite in a low-carbon 13Cr-4Ni stainless steel were analyzed by large-volume, high-resolution serial sectioning tomography using a combination of Xe plasma focused ion beam (PFIB) and electron backscatter diffraction (EBSD) analysis. The extracted 3D EBSD results were compared with 2D observations, and potential misinterpretations arising from 2D analyses were highlighted. 3D reconstruction of the packets revealed that the volume of each prior austenite grain (PAG) is occupied with four distinct types of packets, arranged as a specific tetrahedral pattern in 3D. Through geometrical calculations, a direct link between 3D characteristics of this tetrahedral pattern and the dominant habit plane of the microstructure, {557}γ, was established. These findings, unattainable through 2D characterizations, are expected to enhance our understanding of lath martensite formation and inform recent modeling efforts aimed at accurately representing the 3D structure and mechanical properties of these alloys.

Hydrogen-enhanced decohesion (HEDE) is a proposed mechanism of hydrogen-induced grain boundary (GB) fracture in metals and has been widely calculated from first principles over the past decade. However, the effect of GB-segregated solutes on HEDE is complex and rarely quantified. This study presents a quantitative numerical estimation method based on statistical thermodynamics using first-principles calculations of multiple hydrogen trappings at a GB and its fracture surfaces with segregated solutes. This method accurately estimates the lattice-dissolution-hydrogen-dependent HEDE, including the interactions caused by the segregated solute: the decohering or cohesion-enhancing effect of the solute itself, solute-hydrogen interaction, solute-affected hydrogen-hydrogen interaction, and mobile hydrogen effect. We present a trial calculation to examine how the attractive interaction between solute and hydrogen influences HEDE, showing that HEDE can be induced at lower hydrogen concentrations if not canceled by other interactions.

Dislocation loops are one type of irradiation defects that severely degrade the mechanical properties of nuclear materials. In this study, we found that hydrogen atoms in irradiation environment significantly modify the loop properties including loop types and shapes during in-situ hydrogen irradiation. <100> loops have been energetically stable from 300 °C in H⁺ irradiated iron whereas the stability of <100> loops is delayed until 500 °C in Fe⁺ irradiated iron. Meanwhile, both <100> loops and 1/2<111> loops exhibit polygonal shapes with sharp corners at 400 °C after H⁺ irradiation, similar to loops predicted at 900 °C without hydrogen. Our results highlight the importance of considering the hydrogen effects in dislocation loop evolution and stability.

The effect of the complex stress on the mobility of an edge dislocation on the first pyramidal plane in magnesium is investigated through molecular dynamic simulation (MDs). A novel dislocation with greatly improved mobility is obtained by applying the combined compressive normal stress and shear. The Peierls stress of the new dislocation is reduced to less than a tenth of the original and the mobility factor increases almost twice. Based on the analyses of atomic configuration and the energy barrier of vacancy formation, the mobility improvement is ascribed to the dislocation core reconstruction and its related dissociation. These new findings are also validated in other HCP metals e.g. Ti and Zr.

By using quasi-in-situ EBSD, we successfully probed the nucleation and growth of recrystallized grains during sub-solvus annealing at 1300 °C of an as-cast single-crystal superalloy after pre-compression. It clearly indicated that recrystallization nucleated in the interdendritic region through low-angle grain boundary migration under the experimental condition rather than subgrain coalescence or thermal twinning. Despite being hindered by the undissolved γ′ precipitates in the interdendritic region, these newly formed high-angle grain boundaries could still migrate rapidly. However, the migration velocity will slow quickly as the stored energy decreases. During 10.0-min annealing, the grain boundaries could migrate 14.4–27.4 μm, but the recrystallized grains were still confined in the interdendritic region without abnormal growth. These results provide novel perspective on the comprehension of recrystallization nucleation in single-crystal superalloys and the migration of newly formed high-angle grain boundaries under the hinderance of γ′ precipitates, and are valuable for developing process to control recrystallization.

During isothermal ageing of metastable β-Ti alloys decomposition of the β matrix occurs through the formation of either the ω or α phases. Recent literature has shown that the presence of ω_iso can facilitate α nucleation forming an α variant with a known orientation relationship to the β and ω phases. However, the mechanisms behind such a transformation are unclear, especially in systems with a low ω-β misfit. In this study, the effect of 450 °C ageing on a low misfit alloy, Ti-26Nb (at.%), has been studied using electron backscattered diffraction (EBSD). Isothermal ω precipitates were observed to coalesce along preferred growth directions. EBSD identified the presence of α within the core of the coalesced ω precipitates. The relationship between this α and the parent β was inconsistent with the expected Burgers orientation relationship and did not follow any known crystallographic relationship between α and ω, suggesting a novel transformation pathway.