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Deformation behaviour of multicomponent metallic glasses are determined by the evolution/reconfiguration of the short- and medium-range order (SRO and MRO) atomic structures. A precise understanding of how different atom species rearrange themselves in different stress states is still a great challenge in materials science and engineering. Here, we report a systematic and synergetic research of using electron microscopy imaging, synchrotron X-ray total scattering plus empirical potential structure refinement (EPSR) modelling to study in situ the deformation of a Zr-based multicomponent metallic glassy alloy with 5 elements. Systematic and comprehensive analyses on the characteristics of the SRO and MRO structures in 3D and the decoupled 15 partial PDFs at each stress level reveal quantitatively how the SRO and MRO structures evolve or reconfigure in 3D space in the tensile and compressive stress states. The results show that the Zr-centred atom clusters have low degree of icosahedra and are the preferred atom clusters to rearrange themselves under the tensile and compressive stresses. The Zr-Zr is the dominant atom pair in controlling the shear band's initiation and propagation. The evolution and reconfiguration of the MRO clusters under different stress states are realised by changing the connection modes between the Zr-centred atom clusters. The coordinated changes of both bond angles and bond lengths of the Zr-centred clusters are the dominant factors in accommodating the tensile or compressive strains. While other solute-centred MRO clusters only play minor roles in the atomic structure reconfiguration/evolution. The research has demonstrated a synergetic and multimodal materials operando characterization methodology that has great application potential in design and development of high performance multiple-component engineering alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

The structures of atomic clusters and GP-zones preceding precipitation in an Al-0.70Mg-0.85Si-0.15Fe-0.25Mn (wt. %) alloy have been investigated by annular dark field scanning transmission electron microscopy imaging and density functional theory calculations. One analysed condition consisted of one-month natural aging, and another was in a pre-baked state after 24 h aging at 90 °C. To quantify the effect of clusters and GP-zones on microstructure development during a subsequent isothermal artificial aging at 185 °C, hardness evolution was measured, and precipitate statistics was performed at the peak hardness corresponding to 5 h aging. Several types of clusters and GP-zones have been observed and based on structure and visual appearance named 'Disordered Frank-Kasper', 'Square', 'Binocular', and 1β", 2β", 3β", 4β". The first four are most common in the natural aged condition and are associated with a well-known influence on the initial hardness evolution. The last three are fractions of the hardening β" phase and refer to the number of structural units they contain. These GP-zones have an increased density in the pre-baked condition, leading to a rapid increase in hardness during artificial aging and to a finer microstructure of high-density small needle precipitates at peak hardness. [Display omitted] [ABSTRACT FROM AUTHOR]

The rates of atomic clustering and precipitation hardening are closely related to the diffusivity of solutes and the concentration of vacancies during the natural aging of aluminum alloys. The measurement of the diffusivity of solutes at room temperature, especially in systems with an equilibrium vacancy concentration, is beneficial to the design of the aging process. However, this measurement has long been challenging because of the extremely low diffusion rates of solutes in aluminum at room temperature and the presence of supersaturated vacancies. In this work, we propose a method to quantify the diffusivity of solutes based on the kinetic evaluation of the spinodal decomposition process. This evaluation involves conducting atom probe tomography experiments, analyzing the radial distribution function, and modeling the phase separation process using the Cahn–Hilliard theory. The aging experiments were conducted on nanoscale samples, where excess vacancies can be eliminated at free surfaces due to a high surface-to-volume ratio. The results yielded a diffusivity of Zn in the Al-12.5 at.% Zn alloy of (1. 32 ± 0. 46) × 1 0 − 25 m 2 /s at 295 K. This work introduces a novel approach to assess the solute diffusivity under conditions of equilibrium vacancy concentration at room temperature and expands the temperature range for measuring the diffusivity in systems with spinodal decomposition, particularly in cases where kinetic data at low temperatures are scarce. [Display omitted] [ABSTRACT FROM AUTHOR]

In the past two decades, numerous relaxation or physical aging experiments of metallic glasses have revealed signatures of intermittent atomic-scale processes. Revealed via intensity cross-correlations from coherent scattering using X-ray photon correlation spectroscopy (XPCS), the observed abrupt changes in the time-domain of atomic motion does not fit the picture of gradual slowing down of relaxation times and their origin continues to remain unclear. Using a binary Lennard-Jones model glass subjected to microsecond-long isotherms, we show here that temporally and spatially heterogeneous atomic-cluster activity at different length-scales drive the emergence of highly non-monotonous intensity cross-correlations. The simulated XPCS experiments reveal a variety of time-dependent intensity-cross correlations that, depending on both the structural evolution and the q -space sampling, give detailed insights into the possible structural origins of intermittent aging measured with XPCS. [Display omitted] [ABSTRACT FROM AUTHOR]

To achieve an effective design of additively manufacturable Ni superalloys with decent service performance, a hybrid computational design model has been developed, where the strategy to tailor local elemental segregations was integrated within a scheme of minimizing the cracking susceptibility. More specifically, the phase boundary of primary NbC / γ matrix was introduced into the design routine to tune the spatial distribution of critical solutes at an atomic scale, thereby inhibiting the formation of borides and segregation-induced cracking. Based on the output of the design, new grades of Ni superalloy have been developed with excellent additive manufacturability, as confirmed by the robustness of printing parameters in fabricating low-defect-density samples. The capability of the phase boundaries to evenly distribute boron atoms was validated experimentally, and the cracking induced by uncontrolled boron segregation at grain boundaries was effectively prevented. The newly designed alloys showed good tensile properties and decent oxidation resistance at different service temperatures, which are comparable to those of conventionally produced superalloys. The finding that phase boundaries can be employed to prevent undesirable clustering of boron atoms can be extended to manipulate the distributions of other critical elements, which provides a new path for designing novel Ni superalloys with balanced printability and mechanical properties. [Display omitted] [ABSTRACT FROM AUTHOR]

The atomic structures of Cu-Zr alloys have been extensively studied due to the unusually high glass forming ability of this system. In this work, we investigate the nearest-neighbor atomic structures of several simulated disordered Cu-Zr compositions at three temperatures: 1450 K (liquid), 700 K (undercooled liquid), and 350 K (disordered solid). The results show that over 90 % of the local structures in these systems can be described by one of 12 "building-block" atomic cluster geometries, regardless of composition or temperature, and that these structures all exhibit icosahedral or quasi-icosahedral geometries. Most of the building-blocks are unique in their combination of representative Voronoi polyhedron geometry, predominant central atom type, and occupation probability for each site in the cluster. In addition to the 12 common building-blocks, 35 "outlier" structures unique to their simulated compositions and temperatures were observed. The outliers were primarily non-icosahedral. While the populations of the common building-blocks do not exhibit clear trends with temperature or composition, the overall population of outliers increased with decreasing temperature, suggestive of their role in accommodating the lack of translational order in the disordered solid. [Display omitted] [ABSTRACT FROM AUTHOR]

The development of atomic imperfections within oxide films from high-temperature oxidation of heat-resistant alloys significantly limits the self-protectiveness of the surface oxide, contributing to the failure of energy generating system components such as turbines, engines, and heat exchanges. Directly probing the dynamics of such atomic defects is challenging because of the extreme thermochemical conditions of high-temperature oxidation. Using environmental transmission electron microscopy observations, here we directly capture atomic-scale dynamics of vacancies in growing Cr 2 O 3 film during high-temperature oxidation of NiCr alloy in CO 2. Coordinated with theory modeling, we delineate the atomistic mechanisms associated with the effect of interstitial carbon derived from CO 2 on promoting the formation, migration and clustering of atomic vacancies to result in the enhanced alloy oxidation. The identified oxidation mechanism can find broader applicability in utilizing the atmosphere to tune the formation and evolution of atomic-scale defects, thereby affecting the mass transport properties of the growing oxide film. [Display omitted] In-situ ETEM combined with first-principles calculations revealed the atomistic origin of CO 2 -promoted oxidation of chromia-forming alloys [ABSTRACT FROM AUTHOR]

Excess vacancies play crucial roles in the precipitation of age-hardening precipitates in aluminium alloys, but their spatial evolution across grains during heat treatments is less known. In this work, a numerical model is developed to predict the spatial evolution of non-equilibrium excess vacancies during cooling from solution treatment and during ageing heat treatments of multicomponent aluminium alloys. A finite volume scheme is applied to derive the spatial distribution of vacancy site fraction across grains by solving the diffusion equations of vacancies under the influence of solute elements. Binding energies between solute atoms and vacancies predicted by first-principles calculations has been used to handle the trapping of excess vacancies by solute atoms and atom clusters. The annihilation rates of excess vacancies at grain boundaries and at dislocation jogs have been derived based on a rigorous description of the annihilation mechanisms of vacancies. The evolution of the density of dislocation jogs due to vacancy annihilation has been taken into account. The model is successfully applied to interpret the age hardening behaviors of experimental alloys subjected to different thermomechanical processing conditions. This model will help to reach a deeper understanding of the roles of excess vacancies in precipitation kinetics and therefore is important to further optimize thermomechanical processing parameters and alloy composition to improve the macroscopic mechanical properties of age hardening aluminium alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

The low temperature viscosities of Cu 47.5 Zr 45.1 Al 7.4 glass-forming liquid are measured by thermomechanical analysis in three-point beam bending mode. The experimental equilibrium values are fitted with the Vogel-Fulcher-Tammann (VFT) function, and the fragility parameter of the supercooled liquid is determined to be D * = 20.6. Viscosity measurements during isothermal annealing at 710 K show an initial relaxation from the glassy state into the supercooled liquid, followed by an anomalous viscosity increase of about two orders of magnitude. In-situ transmission electron microscope (TEM) and spherical aberration-corrected TEM investigations reveal that the effects of primary crystallization, nanoscale decomposition and structural ordering might be responsible for the anomalous viscosity behavior. The change in dynamic properties of the supercooled liquid, which is reflected by the larger VFT fitting parameter D * = 41.9 of the new annealed state reached before the final crystallization, is suggested to be not only related to the nanoscale structure change, but may also be caused by the change in chemical composition of the new annealed glass due to the primary crystallization. Furthermore, possible mechanisms for the nanoscale phase separation and the formation of atomic clusters are discussed based on the results of high-resolution TEM experiments. Image, graphical abstract [ABSTRACT FROM AUTHOR]

In aluminum alloys, excess vacancies often form pairs or clusters with solute atoms. This is commonly referred to as vacancy trapping by solutes. Such trapping results in a change in the number of mobile or free vacancies, which in turn affects solute diffusion and precipitate formation during heat treatment. In the current work, such trapping is investigated indirectly in the Al-Sn binary model alloy via studying the dependence of Frank loop formation on the availability of free vacancies. Current experimental results show that the addition of 100 at. ppm. Sn to aluminum leads to a significant reduction in the number density of quenched-in Frank loops. Modeling of mono- and divacancy trapping by solutes during quenching demonstrates the influences of the vacancy-solute binding energy, solute concentration, and temperature on the trapping process. The results reveal that the binding between vacancies and solutes significantly reduces the concentration of free vacancies after quenching in Al-Sn. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract It is generally recognized that single crystals exhibit orientation-dependent elastic and plastic responses. However, it is less known how the dynamic tensile, or spall, strength depends on crystalline orientation, especially for BCC materials. It has been previously shown that the dynamic tensile strength of FCC materials is highly dependent on their plastic response under compression, with a direct correlation between plastic deformation and spall strength. In BCC materials like Ta, where the primary deformation mechanism at high strain rates is a combination of slip and twinning, the quantitative dependence of spall strength on these deformation mechanisms is less understood. To fill this gap in our knowledge, a series of non-equilibrium molecular dynamics simulations are completed for six tantalum single crystal orientations: <001>, <011>, <111>, <013>, <112>, <149>, to explore the role of directional anisotropy on ductile spallation. Our results show that due to the role of non-Schmid and release effects in BCC Ta, the evolution of plasticity follows a complex trajectory through shock compression, release, and tension. Orientations that contain residual twinning deformation have a reduced spall strength proportional to the amount of twinning present. Twins and their intersections function as regions of increased stress localization within the system due to compatibility requirements and their interactions with dislocations. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Abstract Al-0.59Mg-0.79Si (wt.%) alloy was naturally aged for 2 weeks after solutionising and quenching to produce a high number density of atomic clusters. This caused an increase of both hardness and electrical resistivity. Samples were then subjected to reversion ageing (RA) by applying temperature treatments at 250 °C of 1 s–5 min duration. The associated changes of hardness, resistivity, positron lifetime and the DSC dissolution signal were measured. Subsequent secondary ageing at 20 °C was studied in an analogous way. Ideas are developed how solute supersaturation and vacancy fraction vary during RA. It is found that the most reverted state still contains enough clusters to trap positrons efficiently and to contribute to hardness. Longer ageing at 250 °C leads to the formation of coherent and later incoherent precipitates. The differences between the evolution of resistivity and hardness indicate that besides the level of solute supersaturation the Mg:Si ratio in the matrix changes during reversion. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Atomic-scale calculations indicate that both stress effects and chemical binding contribute to the redistribution of solute in the presence of vacancy clusters in magnesium alloys, leading to solute segregation driven by thermodynamics. As the size of the vacancy cluster increases, chemical binding becomes more important relative to stress. These solute–vacancy interactions also impact the diffusivity of vacancies and vacancy clusters in a solid solution. Simulations show that solute atoms accelerate mono-vacancy diffusion but decelerate the diffusion of vacancy clusters. As a result, solute atoms facilitate clustering and stabilize the resulting vacancy clusters, increasing their potential to promote solute segregation and to serve as heterogeneous nucleation sites during precipitation. Experimental observation of solute segregation in simultaneously deformed and aged Mg–Al alloys provides support for this mechanism. [Display omitted] [ABSTRACT FROM AUTHOR]

Zircaloy-4 alloy specimens consisting of α-Zr grains were irradiated on a tandem accelerator with 5 MeV He ions to a fluence of 5 × 1021 ions m −2 at different temperatures. Defect clusters and He bubbles were observed in the TEM micrographs of the irradiated microstructure. The small defect clusters and He bubbles were found to exhibit spherical shapes whereas the large defect clusters showed plate-like shapes. Molecular dynamics simulations and crystallography analyses revealed that the defect clusters are mainly composed of Zr interstitials. During He-ion irradiation, the clustering of Zr dumbbell interstitials results in the formation of small and spherical defect clusters. With further irradiation, the Zr interstitials prefer to occupy the octahedral sites that align along the { 1 2 ¯ 14 } planes with other site interstitials, forming cluster sections. These initial planar clusters then grow by extending along the < 10 1 ¯ 0 > directions and by forming more cluster sections along the < 1 2 ¯ 16 > directions, leading to large plate-like defect clusters on the { 1 ¯ 2 1 ¯ 1 } planes. He bubbles nucleate by the clustering of He atoms and vacancies that are formed by kicking out the lattice atoms, and then grow by the migration and coalescence of helium-vacancy clusters. Small He atoms have many interstitial sites to occupy in the α-Zr HCP crystal, allowing He bubbles to grow to spherical shapes. In addition, grain boundaries induce the accumulation of primary point defects and He atoms and high irradiation temperature accelerates diffusion, and hence larger defect clusters and He bubbles form in the grain boundary regions and at higher irradiation temperature. [Display omitted] [ABSTRACT FROM AUTHOR]

Different types of grain-boundary (GB) solute segregation in nanocrystalline alloys have been described in the literature as being either homogeneous or heterogeneous, which differently impacts their mechanical properties. In heterogeneous segregation, solute atoms tend to cluster along certain GB areas, while other GB regions remain solute-free. This phenomenon is evidenced with the segregation of Ni solute in nanocrystalline FCC Ag. Yet the physical origin for this segregation behavior and its existence in non-FCC alloys are not fully determined. Here, hybrid Monte-Carlo and molecular dynamics simulations were used to study the segregation of Ni solutes (4 at.%) into nanocrystalline FCC Ag, BCC Nb and HCP Zr metals at their solubility limit temperature and the same homologous temperature (0.405·T m), respectively. A range of segregation configurations was found: Fully heterogeneous segregation in Ag 96 Ni 4 at 500 K, homogeneous segregation with second-phase precipitates in Nb 96 Ni 4 at 1110 K, homogeneous segregation with dispersion of small-scale Ni clusters in Nb 96 Ni 4 at 1564 K and Zr 96 Ni 4 at 464 K, and fully homogeneous segregation with amorphous intergranular films in Zr 96 Ni 4 at 1118 K. We rationalize these changes by quantifying the Ni solute interactions at GBs. Furthermore, significant variations in mechanical behavior and associated plastic deformation mechanisms are shown for each alloy due to their different segregation behaviors. It is found that strain-localized shear banding is the most significant at interfaces with homogeneous solute segregation containing second-phase precipitates and absent with segregation leading to amorphous intergranular films. These findings underscore the importance of solute interactions in profoundly altering segregation and mechanical behavior in stable nanocrystalline alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

The structure of liquid and rapidly quenched amorphous Ag 0.74 Ge 0.26 alloy at the eutectic composition was studied by ab initio molecular dynamics (MD) simulations. The local structural properties were systematically investigated from the liquid at 1123 K to amorphous solid at 300 K. The pair-correlation function at 976 K from the MD simulations agrees well with the experimental data. The local structures were also analyzed using Honeycutt–Andersen (HA) indices, Voronoi tessellation and the atomic cluster alignment (ACA) method. The HA indices analysis reveals that there is a high population of pentagonal bipyramid structure which become more predominant upon solidification. Voronoi tessellation analysis indicates strong icosahedral short-range order (SRO) in the liquid and amorphous samples generated by the MD simulations. Using the ACA method, the development of icosahedral SRO upon cooling is further confirmed and a Bergman medium-range order is also observed. The analysis of structural properties and chemical short-range order suggests that Ag atoms tend to have a Bergman-like packing, while the dispersed Ge atoms prevent the alloy from forming a long-range order upon cooling. The Bergman medium-range order is also confirmed by the constrained reversed Monte Carlo results. [ABSTRACT FROM AUTHOR]

Due to the high solubility limit of oxygen in Zr, dissolution of oxygen in the Zr-based alloys is unavoidable. However, as an alloying element, the effect of oxygen on the microstructure and mechanical properties of monolithic Zr-based bulk metallic glasses (BMGs) is rarely reported. Here, we systematically investigated the evolution of microstructure and mechanical properties of monolithic Zr 61 Cu 25 Al 12 Ti 2 BMGs with alloying oxygen content ranging from 650 at. ppm to 5600 at. ppm. With increasing oxygen content from 650 at. ppm to 3500 at. ppm, the compressive plasticity only decreased slightly, from 2.00±0.85% to 1.60±0.91%; however, it plunged to 0.77±0.33% when the oxygen content was further increased to 5600 at. ppm. The threshold oxygen content level for the ductile-brittle transition was revealed at 3500 at. ppm, and the embrittlement at high oxygen content is closely related with a number of changes in microstructure: sharply increases in the size of shear transformation zone (STZ) and in the degree of local ordering, including the imperfect ordered packing (IOP) atomic clusters and large medium-range ordering (MRO) domains. The above findings not only advance the understanding of the effect of alloying oxygen on the microstructure and mechanical properties, but also help to design Zr-based BMGs with excellent performance, promoting their wider commercial application. [Display omitted] [ABSTRACT FROM AUTHOR]

A high strength Al-5024 alloy containing Sc and Zr with a bi-modal microstructure consisting of fine equiaxed and coarse columnar grains was successfully fabricated by laser powder bed fusion (LPBF) additive manufacturing. The formation of the bi-modal microstructure was mainly due to both the formation of primary Al 3 Sc precipitates that act as nucleation sites and the steep temperature gradient during LPBF. By simulating the thermal field of a single melt pool, the formation mechanism of the bi-modal microstructure was explained. It was found by simulation that a solidification interface velocity less than 110 mm/s was beneficial to the nucleation of Al 3 Sc precipitates and, hence, facilitated the formation of a fine grain microstructure. Applying different heat treatments revealed a trade-off trend between yield strength and ductility as a function of the heat treatment time, and a correlation in fatigue life and yield strength was observed, both of which were closely related to the status of the secondary Al 3 Sc precipitates. The highest ultimate tensile strength of 450 MPa and corresponding 107 cycle fatigue strength of 105 MPa were achieved after hot isostatic pressing for 4 h at 325 °C with 100 MPa pressure. Dynamic strain aging was found to occur in both as-built and some heat treated samples, which was related to magnesium (Mg) solute atom clustering attributed to: (i) the formation of a diffuse "Mg wall" due to the repetitive melting and rapid cooling in LPBF, and (ii) the growth of intragranular (Al 3 Sc) and intergranular precipitates (Fe-, Mn-rich) during subsequent heat treatment, thereby leading to an increasing number of misfit dislocations that promote the formation of Mg atom clusters. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: The present work develops a physical model of metallic glass structure that gives a reasonable estimate of density. The efficient cluster-packing model is used as a starting point, and is refined by a high-fidelity estimate of the size of structure-forming clusters and cluster–cluster separations. These are predicted as continuous functions of composition and relative atom radii. Predicted densities are all are within ±10% of measured densities for 200 binary metallic glasses, representing a precision in cluster–cluster separations of ±3%. New structural insights from this work include the importance of acknowledging the unique cluster topologies to estimate cluster–cluster separations; an improved ability to estimate the higher packing efficiency of unlike atoms in the first coordination shell of atomic clusters; and an improved estimate of metalloid–metalloid separations. The unusual, bilinear influence of composition on density in Fe–B glasses is explained by considering the sizes of β and γ sites in different metallic glass structures. Global atom packing fractions derived from measured densities range from about 0.62 to 0.76, and the most stable binary glasses all have packing fractions in excess of 0.70, supporting the idea that atom packing efficiency influences glass stability. [Copyright &y& Elsevier]