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The enhanced mechanical properties of Ni-based single-crystal superalloys induced by refractory elements such as Re, Wo, Mo, etc. are essential for the manufacture of advanced aero engines' turbine blades. Whereas these elements could promote the formation of the topologically close-packed (TCP) phases, and their growth would have a deleterious effect on the creep rupture life of single-crystal superalloys. Concomitantly, the formation of the brittle phase of the internal nitrides, especially the aluminium nitride (AlN), would be detrimental to superalloys as well. Nonetheless, the formation relationship between AlN and TCP phases has maintained opaque. In this study, the relationship between the TCP phase and AlN phase was analysed by microscopic methods, such as BSE, TEM and EDS. An AlN phase located near the TCP phase in the deep surface of the superalloy was found, and its formation was suggested to be related to nitrogen penetration during the thermal exposure. This accompanying growth of the AlN phase and TCP phase in the deep surface could provide a new perspective on the formation mechanism of these brittle phases in the Ni-based single-crystal superalloys. [ABSTRACT FROM AUTHOR]

Both Sn addition and pre-ageing are known to be effective in maintaining the artificial ageing potential after natural ageing of Al–Mg–Si alloys. In this study, the combined effects of Sn addition and pre-ageing at 100 °C or 180 °C on natural secondary ageing and subsequent artificial ageing of an alloy AA6014 were investigated using hardness, electrical resistivity, differential scanning calorimetry and transmission electron microscopy characterizations. It is found that pre-ageing can suppress natural secondary ageing and improve the artificial ageing hardening kinetics and response after 1 week of natural secondary ageing in both alloys with and without Sn addition. The effect of pre-ageing at 100 °C is more pronounced in the Sn-free alloy while the combination of pre-ageing at 180 °C and adding Sn shows superiority in suppressing natural secondary ageing and thus avoiding the undesired hardening before artificial ageing. Moreover, when natural ageing steps up to 8 h are applied before pre-ageing at 100 °C, the effect of pre-ageing in Sn-added alloy can be further improved. The influence of Sn on vacancies at different ageing temperatures is discussed to explain the observed phenomena. [ABSTRACT FROM AUTHOR]

Grain boundary (GB) plasticity dominates the mechanical behaviours of nanocrystalline materials. Under mechanical loading, GB configuration and its local deformation geometry change dynamically with the deformation; the dynamic variation of GB deformability, however, remains largely elusive, especially regarding its relation with the frequently-observed GB-associated deformation twins in nanocrystalline materials. Attention here is focused on the GB dynamics in metallic nanocrystals, by means of well-designed in situ nanomechanical testing integrated with molecular dynamics simulations. GBs with low mobility are found to dynamically adjust their configurations and local deformation geometries via crystallographic twinning, which instantly changes the GB dynamics and enhances the GB mobility. This self-adjust twin-assisted GB dynamics is found common in a wide range of face-centred cubic nanocrystalline metals under different deformation conditions. These findings enrich our understanding of GB-mediated plasticity, especially the dynamic behaviour of GBs, and bear practical implication for developing high performance nanocrystalline materials through interface engineering. Grain boundary can change its structure upon deformation. Here, the authors show that during this process, grain boundary mobility can be tuned dynamically via a self-stimulated twinning process. [ABSTRACT FROM AUTHOR]

High-entropy alloys are a class of materials that contain five or more elements in near-equiatomic proportions1,2. Their unconventional compositions and chemical structures hold promise for achieving unprecedented combinations of mechanical properties3–8. Rational design of such alloys hinges on an understanding of the composition–structure–property relationships in a near-infinite compositional space9,10. Here we use atomic-resolution chemical mapping to reveal the element distribution of the widely studied face-centred cubic CrMnFeCoNi Cantor alloy2 and of a new face-centred cubic alloy, CrFeCoNiPd. In the Cantor alloy, the distribution of the five constituent elements is relatively random and uniform. By contrast, in the CrFeCoNiPd alloy, in which the palladium atoms have a markedly different atomic size and electronegativity from the other elements, the homogeneity decreases considerably; all five elements tend to show greater aggregation, with a wavelength of incipient concentration waves11,12 as small as 1 to 3 nanometres. The resulting nanoscale alternating tensile and compressive strain fields lead to considerable resistance to dislocation glide. In situ transmission electron microscopy during straining experiments reveals massive dislocation cross-slip from the early stage of plastic deformation, resulting in strong dislocation interactions between multiple slip systems. These deformation mechanisms in the CrFeCoNiPd alloy, which differ markedly from those in the Cantor alloy and other face-centred cubic high-entropy alloys, are promoted by pronounced fluctuations in composition and an increase in stacking-fault energy, leading to higher yield strength without compromising strain hardening and tensile ductility. Mapping atomic-scale element distributions opens opportunities for understanding chemical structures and thus providing a basis for tuning composition and atomic configurations to obtain outstanding mechanical properties. In high-entropy alloys, atomic-resolution chemical mapping shows that swapping some of the atoms for larger, more electronegative elements results in atomic-scale modulations that produce higher yield strength, excellent strain hardening and ductility. [ABSTRACT FROM AUTHOR]

Facet engineering to expose specific surfaces has received rapid growth attention to promote the photocatalytic performance. In this work, we reported that BaZrO3 nanocrystals with {001}/{011} facets and corresponding higher reducing capacity could effectively improve the photocatalytic hydrogen evolution in pure water. The tuned electronic band structure arising from exposed specific {001}/{011} facets and the higher surface area are the main reasons to promote photocatalytic activity. The conduction band bottom for BaZrO3 nanocrystals with {001}/{011} facets synthesized by solvothermal method (denoted as BZO-HT) is about 0.31 eV higher than that of sample prepared by hydrothermal reaction (denoted as BZO-H). During the evaluation of photocatalytic activity in pure water, the H2 production rate for BZO-HT (27.80 μmol/g/h) is 9.4 times and six times higher than BZO-H and commercial BaZrO3 (denoted as BZO-C), respectively. This work provides a reference for other facets-related photocatalysts’ design for pure water reduction or splitting. [ABSTRACT FROM AUTHOR]

Phosphorus atomic chains, the narrowest nanostructures of black phosphorus (BP), are highly relevant to the in-depth development of BP-based one-dimensional (1D) nano-electronics components. In this study, we report a top-down route for the preparation of phosphorus atomic chains via electron beam sculpturing inside a transmission electron microscope (TEM). The growth and dynamics (i.e., rupture and edge migration) of 1D phosphorus chains are experimentally captured for the first time. Furthermore, the dynamic behavior and associated energetics of the as-formed phosphorus chains are further investigated by density functional theory (DFT) calculations. It is hoped that these 1D BP structures will serve as a novel platform and inspire further exploration of the versatile properties of BP. [ABSTRACT FROM AUTHOR]

Oxidation plays a tremendous role in the long-term performance of metals. As an important lightweight metal for industrial applications, magnesium suffers from its high reactivity with oxygen and increased evaporation at high temperatures. To understand the oxidation mechanism of magnesium at elevated temperatures, in situ environmental transmission electron microscopy (E-TEM) was performed on magnesium nanoparticles. At a relatively low temperature, the growth of a MgO lamellae via the outward diffusion of bulk magnesium atoms dominated the oxidation process. In contrast, a sublimation-enhanced oxidation via gas phase reaction occurred at 200 °C, leading to the growth of MgO dendrites over the particle that finally leads to the degradation of the magnesium structure. This study provides a direct observation and model of the oxidation mechanism of a direct gas-gas reaction that improves our understanding of the oxidation mechanism at elevated temperatures. [ABSTRACT FROM AUTHOR]

Nanocrystal coalescence has attracted paramount attention in nanostructure fabrication in the past decades. Tremendous endeavor and progress have been made in understanding its mechanisms, benefiting from the development of transmission electron microscopy. However, many mechanisms still remain unclear, especially for nanocrystals that lack a permanent dipole moment standing on a solid substrate. Here, we report an in situ coalescence of Pt nanocrystals on an amorphous carbon substrate induced by electron-excitationenhanced van der Waals interactions studied by transmission electron microscopy and first principles calculations. It is found that the electron-beam-induced excitation can significantly enhance the van der Waals interaction between Pt nanocrystals and reduce the binding energy between Pt nanocrystals and the carbon substrate, both of which promote the coalescence. This work extends our understanding of the nanocrystal coalescence observed in a transmission electron microscope and sheds light on a potential pathway toward practical electronbeam-controlled nanofabrication. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

Perovskite oxides have provided magical structural models for superconducting and colossal magnetoresistance, and the search for nano-scale and/or atomic-scale devices with particular property by specific preparations in the same systems has been extensively conducted. We present here the three oxidation states of manganese (Mn3+, Mn4+, Mn5+) in the perovskite oxide, La0.66Ca0.29K0.05MnO3, which most interestingly shows the rectifying effect as atomic-scale p–n junctions (namely FY-Junctions) of single crystals and films. The family of cubic perovskite oxides were synthesised by the so-called hydrothermal disproportionation reaction of MnO2 under the condition of strong alkali media. The new concept of the atomic-scale p–n junctions, based on the ideal rectification characteristic of the p–n junctions in the single crystal, basically originates from the structural linkages of [Mn3+–O–Mn4+–O–Mn5+], where Mn3+ $$ {\left( {{\text{t}}^{{\text{3}}}_{{{\text{2g}}}} {\text{e}}^{1}_{{\text{g}}} } \right)} $$ and Mn5+ $$ {\left( {{\text{t}}^{{\text{2}}}_{{{\text{2g}}}} {\text{e}}^{{\text{0}}}_{{\text{g}}} } \right)} $$ in octahedral symmetry serve as a donor and an acceptor, respectively, corresponding to the localized Mn4+ $$ {\left( {{\text{t}}^{{\text{3}}}_{{{\text{2g}}}} {\text{e}}^{{\text{0}}}_{{\text{g}}} } \right)} $$ . [ABSTRACT FROM AUTHOR]

Shear-coupled grain boundary (GB) migration is of general significance in the deformation of nanocrystalline and polycrystalline materials, but comprehensive understanding of the migration mechanism at the atomic scale remains largely lacking. Here, we systematically investigate the atomistic migration of Σ11(113) coherent GBs in gold bicrystals using a state-of-art in situ shear testing technique combined with molecular dynamic simulations. We show that shear-coupled GB migration can be realised by the lateral motion of layer-by-layer nucleated GB disconnections, where both single-layer and double-layer disconnections have important contributions to the GB migration through their frequent composition and decomposition. We further demonstrate that the disconnection-mediated GB migration is fully reversible in shear loading cycles. Such disconnection-mediated GB migration should represent a general deformation phenomenon in GBs with different structures in polycrystalline and nanocrystalline materials, where the triple junctions can act as effective nucleation sites of GB disconnections. Shear-induced grain boundary migration at the atomic level is still not well understood. Here the authors combine in situ shear testing experiments and molecular dynamic simulations to reveal the atomistic mechanism of disconnection-mediated GB migration in different gold nanostructures. [ABSTRACT FROM AUTHOR]