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We report on the observation of a novel hot cracking mechanism in a direct laser deposited nickel-based superalloy, using joint electron, X-ray, and atom microscopy. The majority of the observed cracks were related to Ni and Mo oxides formed in grain interiors as well as at grain boundaries. Using atom probe tomography, we detected an intermediate zone between oxide particles and the base alloy, where oxygen existed in solid solution and chemical compositions varied due to the oxide formation. The variation in composition lowered the melting point of the matrix while the oxides promoted stress concentration and crack nucleation, thus causing liquation cracking during reheating.

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Short-range order (SRO) strengthening is one of the major strengthening mechanisms in alloys. Generally it is attributed to the increased resistance to dislocation slip caused by breaking favorable bonds. This contribution may be strong for the first dislocation slipping in a slip plane, but it is certainly less strong for the following next dislocations. Present work discusses SRO strengthening with a different optic: with the focus on the dissipated energy due to the formation of a diffuse antiphase boundary (APB). In this sense, SRO strengthening becomes important to define the friction stress on gliding dislocations. Cluster Variation Method calculations of APB energies are used to estimate the strengthening effect in BCC alloys. The results are illustrated using VNbTaMoW and VNbTaWAl high-entropy alloys, with ab-initio derived parameters. The results show that the strengthening is intense in the range of concentrated solid solutions, and also, that this effect is strongly composition-dependent.

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Almost no studies have been carried out on the roles of reinforcements in the microstructural evolution of the titanium matrix composites (TMCs) during high-temperature fatigue. In this paper, short TiB fibers and TiC particles enhanced the dislocation accumulation in the primary α and β phases, and we found a new phenomenon in which {10-12} deformation twins nucleated at two interfaces: the α/β interface and the reinforcement/matrix interface. Dislocations can decompose into twinning dislocations due to the local stress concentration and result in twin nucleation. Moreover, under thermal activation, dislocations act as channels for rapid diffusion of elements (V, Al and O), resulting in numerous nano-α phases precipitating in the β phase, increasing the local stress concentration at primary α/β interface, which was conducive to twin nucleation. Our work helps further clarify the roles of reinforcements and to supplement the mechanism underlying TMC fracture during high-temperature fatigue.

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Carbide reduces the ductility of tungsten and leads to brittle intergranular fractures. These fractures are inevitably formed in tungsten by carbon diffusion owing to graphite mold during spark plasma sintering. Here, we introduce protective foils made up of two different carbide-forming elements, molybdenum and tantalum, to minimize the carbide formation. Cross-sectional elemental mapping shows that the tantalum foil suppresses carbon diffusion into tungsten, while the molybdenum foil is not an effective diffusion barrier. Thermodynamic-kinetic simulations demonstrate that the suppressed carbon diffusion in tantalum is attributed to high solubility and low diffusivity. Furthermore, the thermodynamically stable tantalum carbide prevents further carbon diffusion at the tantalum/tungsten interface. For the opposite reason, carbon diffuses faster not only in the molybdenum, but also at the molybdenum/tungsten interface. This study provides strategies to minimize the carbon diffusion during spark plasma sintering as well as an intuition into developing structural materials for extreme carbonaceous environments.

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A novel high magnetic induction Fe-6.5%Si alloy with B800 that reached up to 1.77 Telsa was prepared by twin-roll strip casting. Electron backscattered diffraction (EBSD) investigations demonstrated that Σ3 grain boundaries formed in the body-centered cubic (BCC) metal solidified microstructure without phase transition where the ratio of grain boundaries was greater than 20%. Transmission electron microscopy (TEM) revealed that the partial Σ3 grain boundaries are BCC structured twinning. A certain amount of Σ3 boundaries were inherited from the process of warm rolling, cold rolling and annealing, which acted to stabilize the matrix grain boundaries, resulting in the formation of a perfect Goss microstructure with excellent magnetic properties.

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Sn coatings were electrodeposited at four different temperatures (24 ?C, 47 ?C, 70 ?C, and 80 ?C) over mild steel. Electrochemical impedance spectroscopy revealed the highest and lowest corrosion resistance for Sn coating fabricated at 70 ?C (Sn3 coating) and 80 ?C, respectively. Electron backscattered diffraction technique was utilized to correlate the electrochemical properties with the coating texture. A significant increase in the low angle grain boundary fraction was noticed with an increase in the electrodeposition temperature. Five-parameter grain boundary character distribution analysis revealed that the highest fraction of (031)[01-3] twin boundaries at 62.8?/[100] misorientation and a high fraction of exposed low energy {100} planar orientations led to the highest corrosion resistance for Sn3 coating. Whereas a low fraction of (031)[01-3] twin boundaries with a high fraction of exposed high energy ‘near-(110)’ planar orientations led to the poor corrosion resistance of Sn coating fabricated at 80 ?C.

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A novel strategy combining laser surface treatment, rolling and annealing (LST-RA) to produce gradient equiaxed grains is proposed and verified after applying to pure Zr sheet. Dense high angle boundaries (HABs) are first produced by the laser-induced rapid β→α transformation in the surface layer of pure Zr while a considerable amount of stored energy is then introduced by 50% rolling. During subsequent annealing, the preexisting denser HABs facilitate generating more recrystallization nuclei in the surface layer than in the matrix through the strain induced boundary migration mechanism. This eventually results in a gradient equiaxed grain structure from surface to interior of the pure Zr sheet. Furthermore, the fine-grained surface layer is found to have a weakened texture compared to the coarse-grained matrix.

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We explore the deformation features and mechanisms of bulk gradient aluminum. A powderassembly approach was employed, for the first time, to fabricate bulk aluminum with finelytuned gradient cycles. We found that the deformation behaviors of individual components are affected by the gradient structure, which is attributed to the hetero-deformation induced internal stress. Our experimental and modelling results further reveal that the hardening of each component is gradient magnitude dependent, and is governed by the distribution of geometrically necessary dislocations. Our work provides new insights into the understanding and development of gradient metals with superior properties.

SCRIPTA

While the isothermal omega-assisted mechanism of homogeneously distributed fine scale alpha precipitation has been well-documented in several titanium alloys, including Ti-19at%V (Ti-20wt%V), this paper reports that a similar type of alpha precipitation may occur in the absence of any direct or indirect influence of omega precipitates. Rapid heating of a beta solution-treated and quenched Ti-19at.%V alloy to 500 °C results in destabilization of the congruent athermal omega precipitates, followed by the development of compositional fluctuations in the form of Ti-rich or V-depleted pockets during isothermal holding, as revealed by atom probe tomography. These pockets possibly result from beta phase separation and may subsequently act as homogeneously distributed alpha nucleation sites.

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The effect of welding temperature on thermal stability of friction-stir welded (FSWed) 2519 aluminum alloy was investigated. A lowering of the welding temperature below the particle dissolution threshold was shown to be a very effective way for suppression of abnormal grain growth during post-weld solution treatment. This effect was attributed to a prevention of the welded material from precipitation of fine dispersoids during subsequent annealing which resulted in relatively low Zener pressure and thus provided an activation of the apparently-normal grain-growth mechanism. This phenomenon was demonstrated for the first time and could be used as a processing strategy to improve thermal stability of FSWed heat-treatable alloys.