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Abstract Bulk nanocrystalline alloys usually possess enhanced properties than their coarse-grained counterparts. Here, first-principles calculations and aberration-corrected transmission electron microscope (TEM) were employed to investigate the atomic-scale deformation mechanism of Cu-based alloys. The effect of alloying element concentration and temperature-induced solute distribution on the unstable stacking fault energy (γ usf), stable stacking fault energy (γ isf) and unstable twin fault energy (γ utf) were calculated using a Fermi–Dirac distribution of solutes for 42 binary Cu-X alloys. At medium temperature (>200 K) or low solute concentrations (<15 at.%), the stacking fault energies calculated from the Fermi–Dirac model accord well with the available experimental and theoretical results. The deformation mechanism was then evaluated by α = γ isf /γ usf and β = γ utf /γ usf , smaller α (β) favors an easier formability of extended dislocations (twins). Most subgroup VI-VIII metals in the periodic table can slightly increase the γ usf , γ isf and γ utf of Cu, and have almost no influence on α and β. While main group and subgroup II-V elements can decrease γ usf , γ isf and γ utf as well as the values of α and β. For alloying elements of Pd, Ag, Pt and Au, the values of α and β increase, suggesting a tendency of deformation mechanism from extended dislocations to full dislocations. Furthermore, high-resolution TEM (HRTEM) images of four representative nanocrystalline alloys (pure Cu, Cu-Fe, Cu-Ag and Cu-Zn) corroborates the prodiction with α and β as well as the empirical twinnability. The α and β remain almost the same as that of pure Cu when alloyed with Fe while they decrease with Zn, and the extended dislocations and twins were commonly observed for Cu, Cu-Fe and Cu-Zn. The α and β increased with Ag addition although the γ isf decreased, and the extended dislocations were barely observed for Cu-Ag sample. The theoretical and microstructural correlation provides insights into the deformation mechanism of Cu-based alloys. Highlights • Temperature/concentration-related stacking fault energies were calculated for Cu-based alloys. • The deformation mechanism can be predicted using the coefficients α = γ isf /γ usf and β = γ utf /γ usf. • Smaller α (β) corresponds to an easier formability of extended dislocations (twins). • HRTEM observation of four nanocrystalline alloys proves the theoretical prediction. [ABSTRACT FROM AUTHOR]

Abstract Light elements play a crucial role on the microstructure and properties of conventional alloys and steels. Oxygen is one of the light elements which is inevitably introduced into nanocrystalline alloys during manufacturing. Here, we report that severe plastic deformation can fragment the oxides formed in powder processing and eventually cause oxygen dissolution in the matrix. A comparative investigation on Cu-Fe nanocrystalline alloys generated from different initial materials, blended powders and arc-melted bulk materials which have different oxygen contents, reveals that fragmented oxides at grain boundaries effectively decrease the grain boundary mobility, markedly facilitating grain refinement. In contrast, those oxygen atoms dissolved as interstitials in the Cu-Fe matrix lead to lattice expansion and significant decrease of stacking fault energy locally as validated by density functional theory. Such oxygen-mediated microstructure gives rise to enhanced strength and superior structural stability. The remarkable tailoring effect of oxygen can be employed to engineer nanocrystalline materials with desired properties for different applications. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Usually immiscible Cu-Cr compounds in equilibrium condition were mechanically processed via high pressure torsion with large and controlled strains. A systematical investigation on 57 wt%Cu − 43 wt%Cr was carried out to get insights into the microstructural evolution of Cu-Cr nanocomposites and their dissolution process, as well as to determine the solid solubility limit of Cu and Cr elements under severe deformation. Microstructural evolution was captured with grain refinement from micron-size down to less than 20 nm as the increase of strains. A strain-saturated state in 57 wt%Cu − 43 wt%Cr bulk was achieved after 100 rotations deformation (effective strain 1360), with a stable grain size of 13.7 nm and invariable hardness of 480–495 HV. Fine scanning of X-ray diffraction and sub-nanometer scale measurements of energy-dispersive X-ray spectroscopy showed that 32 wt% Cu could be fully dissolved into Cr matrix, and conversely solubility of Cr in Cu was determined to be about 3 wt% after an enough amount of deformation. The phase fraction change associated with Cu dissolution into Cr matrix during continuous deformation was captured and accurately calculated, indicating a negative exponential phase change mode. A phenomenological intermixing mechanism based on the kinetic competition between external forcing mixing and thermal-diffusion induced decomposition was proposed, which was well accordant with the phase evolution observed from experimental results. [ABSTRACT FROM AUTHOR]

Normally immiscible 57 wt% Cu–43 wt% Cr compounds were mechanically alloyed by means of high pressure torsion with large and controllable strains. A strain-saturated state in 57 wt% Cu–43 wt% Cr bulk was achieved after 100 rotations deformation (effective strain 1360), with a stable grain size of 13.7 nm and largest solubility of 32 wt% Cu in Cr matrix. The phase fraction change between face-centered cubic and body-centered cubic due to Cu dissolution during continuous deformation was captured and accurately calculated, indicating a negative exponential phase change mode. A phenomenological dissolution mechanism based on the kinetic competition between mixing under sustained external forcing and thermal diffusion induced decomposition was proposed, which was well compliant with the phase evolution observed from experimental results. [ABSTRACT FROM AUTHOR]

A systematic investigation based on high-resolution transmission electron microscopy (HRTEM) and quantitative analysis was carried out to gain insights into the structural evolution and deformation behaviors of 57 wt%Cu - 43 wt%Cr dual-phase alloys deformed by high pressure torsion (HPT). Nanometer-sized grains were obtained, with Cu and Cr grain size being reduced from 33.7 nm to 17.4 nm and 35.6 nm to 18.6 nm respectively after deformation with 5 rotations and 25 rotations. During the early deformation stage, in a way, grain refinement in Cu was controlled by the interaction between Cr solutes (or clusters) that was forced to dissolve into Cu matrix and extended dislocations motion. At the higher strain condition, the deformation mode became more complicated and the grain refinement of Cu and Cr can be affected by solute effects and constraint effect during severe plastic deformation in this dual-phase system. In addition, inverse grain-size effect on stacking fault (SF) and twinning tendencies was observed in Cu under different deformation conditions. A critical grain size was calculated to describe the required shear stress for partial dislocation nucleation. HRTEM investigations and statistical analysis on twins observed in 25 rotations-strained sample reveal that the twins are associated with the grain boundary activities and the recovery procedure in the extremely fine grain size region. These atomic-scale observations provide new views in understanding the deformation mode, especially twinning behavior in the extremely fine grain size range below the critical grain size that are hardly obtained in experiments. • HRTEM investigation and quantitative analysis were conducted to gain insights into grain size and defect densities. • One possible grain refinement mechanism during early deformation condition in Cu was proposed. • Inverse grain-size effect on SF and DT trend was detected, and the critical grain size was calculated. • Twinning behaviors in the extremely fine size region were affected by the GB activities and recovery procedure. [ABSTRACT FROM AUTHOR]

Immiscible Cu–Fe composites with various compositions are mechanically alloyed by means of high pressure torsion directly from blended powders and vacuum arc-melted bulk materials respectively, which contain different contents of oxygen. All investigated compositions are deformed to extremely large strains reaching a saturated state consisting only of a single face-centered cubic structure. It is found that Fe alloying as well as O addition reduce the achievable grain sizes. The single phase supersaturated solid solutions show a reduced hardness compared to the Cu and Fe heterostructures with similar grain sizes, reflected in a decreasing hardness with increasing strain. In addition, oxygen introduced during powder processing has a significant influence on the microstructures, defect densities and thus properties. Samples generated from powders have smaller grain sizes and much higher dislocation and twin densities than the corresponding arc-melted bulk samples with the same nominal composition. The combined effects of Fe and O on the deformation behavior and hardness of Cu–Fe nanocrystalline alloys are discussed. The current work shows that microstructure and properties can be tuned not only by alloying, but even further by incorporating oxygen, which may offer another design strategy for nanocrystalline alloys. [ABSTRACT FROM AUTHOR]

Statement of significance This is the first transmission electron microscopic study of teenage crown dentin on the nanometer scale. Samples for TEM were prepared by mechanical thinning and chemical polishing that allowed obtaining the electron transparent foils. It was firstly shown that human dentin possesses the layered morphology: the layers are oriented normally to the main axis of a tooth and have the thickness of ~ 50 nm. HA inorganic phase of teenage crown dentin is in the amorphous state. The cellular structure, which was formed from collagen fibers (diameter is ~ 5 nm), are observed near DEJ region in teenage dentin, whereas bioorganic phase of teenage crown dentin near the pulp camera does not contain the collagen fibers. Cracks in dentin thin foils have sharp tips, but big angles of opening (~ 30 ° ) with plastic zone ahead crack tip. It means that young crown human dentin exhibits ductile or viscous-elastic fracture behavior on the nanometer scale. [ABSTRACT FROM AUTHOR]