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The influence of parent Nb-alloy grain morphology on the layer formation of Nb₃Sn and its pinning characteristics was clarified using a fundamental method. Nb-Ta-Hf was used in this work, given its high recovery temperature of more than 700 °C, which is higher than the Nb3Sn-phase formation temperature. First, a Cu/Nb-4at%Ta-1at%Hf single-core composite wire (outer: Cu) was prepared. Subsequently, two pieces were cut off, one of which was intermediately annealed at the recrystallization temperature. Thereafter, Sn was attached onto the surface of both the samples by electroplating, and the samples were heat-treated for Nb₃Sn layer formation. Evidently, a significantly finer grain morphology appeared on the as-drawn fine parent Nb-alloy layer, leading to a much better pinning force property.

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The purpose of this study is to identify the high-temperature oxidation mechanisms of a Ti₂AlNb-based alloy via detailed analyses of oxide scale/substrate interface using transmission electron microscopy, X-ray diffraction and thermodynamic calculations. Oxygen is first adsorbed on the alloy surface, reacting with B2 phase to generate TiO₂. The depletion of titanium in B2 phase drives the remaining aluminum and niobium to form AlNb₂, which prevents further oxidation reaction of B₂ phase. α₂ phase is easily oxidized to form TiO₂ and Al₂O₃. The oxidation products of O-phase are TiO₂ and AlNbO₄, which are formed by further reaction of Nb₂O₅ and Al₂O₃. Via the first-principles calculations based on density-functional theory, the possibility of the reactions is confirmed, and two transition paths of Al2O3 from metastable state to steady state are established as: δ-Al₂O₃→θ-Al₂O₃→α-Al₂O₃ and κ-Al₂O₃→α-Al₂O₃.

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Nanoscale solid-state transformation of metals and alloys during electrochemical processes are critical for applications including corrosion, electrochemical machining and alkali metal batteries. However, these nanoscale phase and chemical transformations at metal surface and interface cannot be probed by conventional characterization techniques lacking of site-specific information with high spatial resolution. Here, we use in situ transmission electron microscopy to demonstrate a unique phase transformation upon electrochemical alloying of Ag nanowire with Li. It features a surface reaction of electrochemical lithiation of Ag followed by a fast redistribution of Ag into small nanoparticles embedded in Li₂O. The Ag-Li alloying reaction proceeds through a surface reaction mode with a strong dependence of surface facets. The weak bonding between AgLi and the presence of residual O₂ in TEM column causes the as-formed Ag-Li alloy instantly transform to a Li₂O shell with well-dispersed Ag nanoparticles.

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A L1₂-strengthened multicomponent Co-rich high-entropy alloy with superior microstructural stability was developed in this study. High-density cuboidal γ′ particles were found to embed in the matrix for the alloy subjected to isothermal aging at temperatures ranging from 800 to 1100 °C without the formation of brittle intermetallic phases neither at grain boundaries nor grain interior after long-term thermal exposure. Atom probe analyses indicated that Ta and Nb elements preferentially partitioned to the γ′ intermetallic phase, which is found to be substantially important for stabilizing the L1₂ ordered crystalline structure. More importantly, the present alloy exhibited a good combination of both high γ′-solvus temperature (1125°C) and low mass density (8.28 g cm^-3), together with superb high-temperature tensile strength (755 MPa at 800 °C and 664 MPa at 900 °C). The present findings offer an essential paradigm for designing advanced L1₂-strengthened high-temperature alloys with broader temperature capabilities based on Co.

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It was usually believed that the formation of intragranular κ-carbide in γ-austenite was attributed to spinodal decomposition followed by ordering reaction. In this work, near-atomic scale characterization of an austenite-based Fe-20Mn-9Al-3Cr-1.2C (wt. %) low-density steel, using (high-resolution) scanning transmission electron microscopy and atomic probe tomography, reveals that the initially-formed κ-carbides (2-3 nm in particle size) are featured with an ordered L′1₂ structure but without detectable chemical partitioning. The Gibbs energy of the FCC phase obtained by thermodynamic calculations always shows a positive curvature (i.e. (d²G/dx²)< 0) with the variable contents of Al and C in the temperature range 400-800 °C. Both the results demonstrate that the ordered nuclei of κ-carbide can form directly in the disordered γ-austenite rather than through the well-known spinodal decomposition-ordering mechanism. The extremely low nucleation barrier is due to the similar lattice structure, same composition and complete coherency between the γ-austenite matrix and κ-carbides.

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In the last four decades, the structure of pre-critical solid nuclei within a liquid phase of single-component face-centered-cubic (FCC) materials has been extensively investigated. These studies demonstrated that formation of equilibrium-FCC is mediated by metastable-body-centered-cubic (BCC) structured pre-critical nucleus. However, it is unknown if such nucleation mechanism is applicable to multi-component structural alloys that forms FCC and BCC structures as equilibrium phases. We have investigated this matter using Ni-35at.%Cr and Ni-50at.%Cr binary alloys via molecular-dynamics simulations. Our results indicated that the pre-critical and critical nucleus comprised of coupled Cr-rich BCC and Ni-rich closed-packed (FCC and hexagonal-closed-packed) structures. Formation of such multi-structured pre-critical nucleus was facilitated by prior phase-separation of alloy-melt into Cr-rich and Ni-rich liquid-pockets. BCC and closed-packed regions inside the nucleus formed with non-equilibrium compositions that evolved over time. Thus, a non-conventional nucleation mechanism in concentrated Ni-Cr alloys allows them to solidify into FCC-BCC microstructures.

Vol. 199, 1 July. 2021, 113863

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