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1. Direct evidence of the shockley tetragonal L1’ phase in a bulk Fe-Pd alloy

   https://doi.org/10.1016/j.scriptamat.2023.115540  

   Savovici, A; Soffa, WA; Floro, JA (May 2023, Scripta Materialia)

   Direct evidence is provided for the existence of the tetragonal L1’ phase, first predicted by Shockley in 1938, in bulk Fe - 62 at% Pd alloys aged at 525 deg C. L1’ existence as the dominant phase is supported by quantitative x-ray diffraction analysis. This is combined with transmission electron microscopy of the polytwinned microstructure, examining the diffracted intensities in specific superlattice reflections where the complete extinction in L10 is relaxed in L1’. Ordering to L1’ appears to occur directly from the A1 parent phase at 525 deg C, while aging at 650 deg C only produces L10. The possibility of L1’ ordering may have consequences for the ferromagnetic properties of classic and important binary alloy systems where L10 is the assumed equilibrium phase. 

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2. Ordered-Phase Equilibria in the Eutectoid Region of Bulk Fe-Pd

   https://doi.org/10.1007/s11661-024-07577-4  

   Savovici, A; Jensen, W; Soffa, W A; Floro, J A (September 2024, Metallurgical and Materials Transactions A)

   Abstract This report is the first analysis of the coexistence and microstructure of the equilibrium phases in the Fe-Pd L1₀ + L1₂eutectoid region. Coexistence of L1₀ + L1₂is observed at higher temperatures (650$$^\circ {\text{C}}$$ $_{}^{\circ }\text{C}$), resulting in L1₀polytwin plates with internal boundaries that are decorated by L1₂. For higher Pd content, the L1₀plates are embedded in an extended L1₂matrix, but the L1₂wetting layers still persist. For aging at low temperatures (525$$^\circ {\text{C}}$$ $_{}^{\circ }\text{C}$), L1’ + L1₂coexistence is observed, but the microstructure is essentially similar, except that L1₀is replaced by L1’. The two-phase region is found to be much narrower than reported in published phase diagrams, of order 0.6 to 1 at pct in extent. There may be a further re-entrant narrowing below the L1’ formation temperature. This work establishes L1’ as a phase distinguishable from both L1₀and L1₂, but does not yet prove that L1’ is an equilibrium phase. The preferred formation of L1’ at lower temperatures may relate both to stability conferred by overall ferrimagnetic interactions, and perhaps by kinetics, where L1’ should have a reduced nucleation barrier from A1 relative to L1₀. 

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3. Microstructural Impacts on the Oxidation of Multi-Principal Element Alloys

   https://doi.org/10.1007/s11085-024-10225-2  

   Pavel, Michael J; Weaver, Mark L (June 2024, High Temperature Corrosion of Materials)

   The impacts of thermal treatment on the precipitate morphology and oxidation behavior of a dual-phase (FCC + L12) multi-principal element alloy (MPEA), Ni45Co17Cr14Fe12Al7Ti5, was studied at 1000 °C via isothermal and cyclic testing. Thermogravimetric analysis and subsequent characterization revealed that smaller precipitates had an increased capacity to form protective sub-surface oxide layers which mitigated total mass gain. The smaller-precipitate-containing samples exhibited a decrease in thickness of the primary Cr2O3 scale and parabolic growth rate. Mechanistically this behavior is believed to stem from the increased growth rate of initial Al2O3 nuclei and decreased inter-precipitate spacing which results in faster lateral diffusion and agglomeration. 

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4. Solidification Behavior and Microstructure Evolution in Dissimilar Electron Beam Welds Between Commercially Pure Iron and Nickel

   https://doi.org/10.1007/s11661-024-07375-y  

   Hochanadel, Joris; Panton, Boyd; Fink, Carolin; Lippold, John (June 2024, Metallurgical and Materials Transactions A)

   Electron beam welding processes have highly accurate control of both spatial and temporal heating profiles which provide unique capabilities in dissimilar metals joining. In this work, electron beam welds were made between commercially pure nickel and iron to determine the effect of fusion zone composition on solidification behavior and microstructure evolution. The weld was made with a beam deflected in a circular pattern to enable joining and promote mixing. The beam traveled at a shallow angle of approximately 1 deg to the joint interface starting in the nickel and finishing in the iron. The shallow angle created a weld with a composition gradient along its 110 mm length. The solidification behavior and final weld microstructure were characterized using both light optical microscopy and scanning electron microscopy. Electron backscatter diffraction was used to determine the phase fractions in the fusion zone. A change in solidification mode from face-centered cubic austenite to body-centered cubic ferrite was observed as a function of fusion zone composition. Weld cross-sections containing 65.5 wt pct Fe and 76.9 wt pct Fe had a two-phase fcc + bcc microstructure. Using the compositions and phase fractions, the two-phase region was estimated to be between 56.4 and 79.7 wt pct Fe. Martensite was observed in cross-sections containing between 76.9 wt pct Fe and 98.1 wt pct Fe, which was confirmed using hardness measurements. 

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5. High-throughput strategy to design high entropy alloys with an FCC matrix, L12 precipitates, and optimized yield stress

   https://doi.org/10.1016/j.matdes.2024.113173  

   de_Araujo_Santana, Diego; Ellyson, Benjamin; Clarke, Amy; Clarke, Kester; Schell, Norbert; Kaufman, Michael; Shyinti_Kiminami, Claudio; Gil_Coury, Francisco (August 2024, Materials & Design)

   This work proposes a methodology for designing high-strength precipitation-hardened high entropy alloys (HEAs) with an FCC matrix and L12 precipitates. High-throughput solidification calculations were conducted using the CALPHAD method, evaluating 11,235 alloys in the Cr-Co-Ni-Al-Ti system under specific boundary conditions. The acquired information was used to filter the alloys, focusing on alloys exhibiting an FCC+L12 phase field at 750 °C, a solidification interval narrower than 100 °C, and a solvus temperature under 1100 çC. The filtered alloys were analyzed to estimate their solid solution and precipitation hardening contributions to yield strength, with antiphase boundary energy (APB) assessed using two models. Three alloys were selected for testing the proposed strategy, including one with the highest yield stress and others for comparison. These alloys were produced, processed, and characterized using DSC, synchrotron XRD, SEM, and TEM. The results showed that the desired microstructure was achieved, with the alloys consisting of an FCC matrix and a high-volume fraction of L12 precipitates. Tensile tests at room temperature, 650 °C, 750 °C, and 850 °C demonstrated that the proposed model predicts well the yield strength trends, demonstrating the potential of the proposed approach for accelerating the discovery and development of novel HEAs with tailored properties. 

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