Abstract
Oxygen contamination is a problem which inevitably occurs during severe plastic deformation of metallic powders by exposure to air. Although this contamination can change the morphology and properties of the consolidated materials, there is a lack of detailed information about the behavior of oxygen in nanocrystalline alloys. In this study, aberration-corrected high-resolution transmission electron microscopy and associated techniques are used to investigate the behavior of oxygen during in situ heating of highly strained Cu–Fe alloys. Contrary to expectations, oxide formation occurs prior to the decomposition of the metastable Cu–Fe solid solution. This oxide formation commences at relatively low temperatures, generating nanosized clusters of firstly CuO and later Fe2O3. The orientation relationship between these clusters and the matrix differs from that observed in conventional steels. These findings provide a direct observation of oxide formation in single-phase Cu–Fe composites and offer a pathway for the design of nanocrystalline materials strengthened by oxide dispersions.
Highlights
Oxygen contamination is a problem which inevitably occurs during severe plastic deformation of metallic powders by exposure to air
Stimulated by direct atomic-resolution observation, we systematically studied the thermal behaviors of oxygen in highpressure torsion (HPT) deformed Cu–Fe alloys by means of in situ spherical aberration-corrected high-resolution transmission electron microscopy (HRTEM) using a heating holder, via recording atomic-resolved images, diffraction patterns as well as capturing compositional information by electron energy loss spectroscopy (EELS) and energy dispersive X-ray spectroscopy (EDXS), supplemented by X-ray photoelectron spectroscopy (XPS), synchrotron X-ray diffraction (XRD), atom probe tomography (APT) and density-functional theory (DFT) calculations
Almost no grain coarsening is observed even when heated up to 420 °C, and careful investigation with scanning transmission electron microscopy (STEM) confirms the thermal stability of microstructures (Supplementary Figure 3 and Note 3), which has been reported in the literature and attributed to alloying of elements and the pinning effect of contaminants in nanocrystalline metals38
Summary
Oxygen contamination is a problem which inevitably occurs during severe plastic deformation of metallic powders by exposure to air. This contamination can change the morphology and properties of the consolidated materials, there is a lack of detailed information about the behavior of oxygen in nanocrystalline alloys. The orientation relationship between these clusters and the matrix differs from that observed in conventional steels These findings provide a direct observation of oxide formation in single-phase Cu–Fe composites and offer a pathway for the design of nanocrystalline materials strengthened by oxide dispersions. Some different types of carbides were detected inside annealed bulk samples, pointing out that contaminants could affect constituents of precipitates and induce discrepant properties of nanostructured alloys. Nowadays novel phase formation and transition, metal-catalyzed process, deformation twinning generation, irradiation-induced void formation as well as nanocrystal facet development have been captured in real-time observations
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