查看更多>>摘要：We present a fast and accurate method for predicting the thermodynamics of hydrogen solubility and trapping in alloys, which is two orders of magnitude faster than conventional ab-initio approaches. The model hinges on the finding that the solubility of H is dominated by its nearest neighbour environment. We apply the method to the problem of hydrogen redistribution in Nb-containing Zr nuclear fuel cladding, and validated it against brute-force ab-initio approaches. We find that hydrogen preferentially dissolves into the beta-Zr phase found in as-manufactured 2.5%Nb alloys, and H is likely to redistribute into the a phase following the irradiation-induced decomposition of beta-Zr. beta-Nb particles found in as-manufactured 1%Nb alloys may act as weak sinks for H, however irradiation-induced change of composition of beta-Nb particles increases their hydrogen-trapping strength. Nano-platelets formed under irradiation in these alloys are potentially even stronger hydrogen sinks, especially if induced by proton irradiation rather than neutrons.
查看更多>>摘要：Control of thermal expansion is of great fundamental and technological significance for functional materials. Here, we have achieved the thermal expansion controllable from giant negative, zero to positive in Ni2P2O7-based materials through doping Mn. A joint study of XRD and Raman was conducted to reveal negative thermal expansion originates in the change of phase proportional. This work also provided new insight into to improve negative thermal expansion in phase transition type materials.
查看更多>>摘要：The tensile creep of 3 mol% Y2O3 stabilized tetragonal ZrO2 ceramics under a DC field was systematically investigated. The results show that the deformation mechanism of the material strongly depends on the current density and the applied stress. Exceptionally large uniform elongation can be obtained only when the creep is dominated by dislocation accommodated grain-boundary sliding. A maximum uniform elongation of ~& nbsp;300% is achieved at a strain rate close to 10(-3) s(-1). At an even higher strain rate of about 10(-2) s(-1), uniform elongation is still near 100%. The current results suggest that high strain rate superplasticity can be induced by applying a strong electric field at proper stress and current density.
查看更多>>摘要：Single-phase high entropy fluorite (Ce0.2Zr0.2Y0.2Gd0.2La0.2O2-delta) samples were synthesized by co-precipitation and consolidated by ultrafast high-temperature sintering (UHS) in less than 2 min. The chemical homogeneity of the sintered materials was confirmed by X-Ray Diffraction (XRD), Energy Dispersive X-ray Spectroscopy (EDXS), high-resolution EDXS using Transmission Electron Microscopy (TEM), and Raman spectroscopy. Compared to conventional sintering, UHS of high entropy ceramics was a hundred times faster and it resulted in highly dense microstructures (relative density > 93%) with nanometric grains. Ce0.2Zr0.2Y0.2Gd0.2La0.2O2-delta & nbsp;densified under an UHS current of 20-25 A. An optimized step-wise UHS schedule was proposed to obtain dense, single-phase pellets with a reduced amount of defects. The extreme heating rates were found beneficial to limit grain coarsening and to obtain a single phase.
查看更多>>摘要：Grain boundaries (GBs) of nanograined metals are prone to migrate when subjected to mechanical loading. In this study, nanograined pure nickel samples with an average grain size of about 70 nm were deformed under pressure of 20 GPa and 43 GPa using a diamond anvil cell. It was found that the average grain size decreased slightly after high pressure compression, which meant high pressure suppressed the nanograin boundaries migration. Extensive extended dislocations with large dissociation width (averagely 5-6 nm) as well as LomerCottrell locks were observed in the high pressure deformed nanograined Ni, indicating that partial dislocation activities dominated the plastic deformation, which may facilitate to relax GBs and thus suppress GBs migration. The finding of this study may help to fabricate stable nanocrystalline metals.
查看更多>>摘要：Researchers attributed the orderly arranged nanoscale phases observed in many multi-principal element alloys (MPEAs) to spinodal/spinodal-mediated phase transformation pathways. However, spinodal decomposition is not well understood in multicomponent systems. Although the theoretical background is available, CALPHAD databases were not used to explore the miscibility gap in MPEAs. In this work, we develop a CALPHAD framework utilizing the Hessian of free energy to study the stability of solid solutions in MPEAs. In particular, we utilize the geometry of higher dimensional Gibbs simplex in conjunction with the Hessian to calculate concentration modulations in early stages of spinodal decomposition. We apply this framework to a diverse set of multiphase MPEAs that have been studied in the literature including TiZrNbTa (BCC/BCC), Fe15Co15Ni20Mn20Cu30 (FCC/FCC), Al0.5NbTa0.8Ti1.5V0.2Zr (BCC/B2), AlCo0.4Cr0.6FeNi (BCC/B2) and Al0.5Cr0.9FeNi2.5V0.2 (FCC/L1(2)). We show that the MPEA systems are unstable only to certain concentration modulations which could be further explored to design microstructurally engineered alloys.
查看更多>>摘要：Grain refinement increases yield strength, but usually at the cost of ductility reduction. In the present work, we explored the strategy of grain refinement and composition tuning in CoCrNi alloys to optimize mechanical properties. Grain refinement was found to strongly suppress deformation-induced martensitic transformation in metastable CoCrNi alloys, which was utilized to modulate the transformation-induced plasticity (TRIP) effect in combination with composition tuning guided by theoretical calculations. We demonstrated in a non-equiatomic CoCrNi TRIP medium-entropy alloy (MEA) that our approach resulted in an excellent combination of strength and ductility. The proposed strategy is expected to be useful in exploring superior mechanical properties of MEAs and high-entropy alloys in varying systems.
查看更多>>摘要：Electron beam welding of the medium-entropy alloy (MEA) (NiCoCr)(94)Al3Ti3 to 304 stainless steel (SS) has been undertaken. Defect-free joints were obtained with coarse columnar grains: the columnar grains on the 304 SS side of the fusion zone (FZ) were finer (94 mu m, length-to-width ratio-2.7) than the grains on the MEA side (204 mu m, 2.9), a difference that probably resulted from a higher cooling rate after welding due to the higher thermal conductivity of the 304 SS compared to the MEA. Mixing of the two materials in the FZ led to lower vol.% (14%) and larger diameter (44 nm) of L1(2) nanoparticles compared to the 23 nm dia. and 37% vol.% in the aged MEA, a result consistent with CALPHAD analysis. The joint showed a good combination of yield strength (323 MPa), ultimate tensile strength (670 MPa), and elongation (24%). Failure occurred by ductile fracture in the FZ.
查看更多>>摘要：This study reports the one-directional growth of single crystalline nickel oxide nanostructures that is facilitated by the oxidation of nickel nanoparticles. Layer-by-layer growth at the buried NiO/Ni interface was directly observed by in-situ high resolution transmission electron microscopy at 650 degrees C in an oxygen partial pressure around 4 x 10(-4) Pa. Individual layers of NiO grow by ledge movement, i.e., disconnection migration along the oxide/metal interface plane. Oxidation at interfacial steps is governed by oxygen vacancy migration along the interface plane, while the junction between the oxide/metal interface and the gas phase serves as nucleation site. The results of this study demonstrate the applicability of the terrace-ledge-kink crystal growth model for reactive crystal growth processes at internal heterophase interfaces.
查看更多>>摘要：Metastable beta titanium alloys with the incorporation of multiple beta-stabilizing elements could exhibit unique mechanical and functional properties. In the present study, a Ti-Nb-O metastable beta titanium alloy with a high oxygen content of 5.2 at.% was prepared, and the microstructural evolution and dynamic mechanical behavior were systematically characterized. It was found that strain glass transition occurs in the Ti-Nb-O alloy, which is responsible for the invariant elastic modulus over a wide temperature range. The interstitial oxygen atoms dissolved in the Ti-Nb-O alloy suppress beta-alpha ''& nbsp;long-range martensitic transformation during cooling, which softens the elastic modulus of the alloy, while promote strain glass transition characterized by the formation of nano-domains, which hardens the elastic modulus. As the temperature decreases, the modulus softening of beta matrix compensates for the modulus hardening caused by nano-domains due to anharmonic atomic vibration, thus giving rise to the temperature-independent elastic modulus, i.e. Elinvar effect.
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Elsevier