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This work demonstrates the capabilities and advantages of a novel sintering technique to fabricate bulk composition gradient materials. Pressure distribution calculations were used to compare several tooling geometries for use with current-activated, pressure-assisted densification or spark plasma sintering to densify a gradient along the long dimension of the specimen. The selected rectangular tooling design retains a low aspect ratio to ensure a uniform pressure distribution during consolidation by using a side loading configuration to form the gradient along the longest dimension. Composition gradients of NixCu1−x, MoxNb1−x, and MoNbTaWHfx (x from 0 to 1) were fabricated with the tooling. The microstructure, composition, and crystal structure were characterized along the gradient in the as-sintered condition and after annealing to partially homogenize the layers. The successful fabrication of a composition gradient in a difficult-to-process material like the refractory multi-principal element alloy system MoNbTaWHfx shows the utility of this approach for high-throughput screening of large material composition spaces. 

This work demonstrates an approach using solid state electrochemical cells to study the long-term oxidation of materials at 800 °C. The capability of zirconia-based cells to control the oxygen partial pressure was first evaluated using an empty chamber. For most voltages applied to the pump cell, the steady state sensor voltage matches the pump voltage, leakage rates are low, and response times are short, allowing precise and prompt control over the chamber atmosphere. The technique was validated by measuring the oxidation of niobium and nickel. Niobium was oxidized at pump voltages ranging from 0 mV to +500 mV; decreasing the oxygen partial pressure around the specimen reduces the oxidation rate. Comparing the integrated oxidation rate with the weighed mass gain showed good agreement. Measured oxidation rates for nickel were of order 1μg h⁻¹, illustrating the sensitivity of this technique. For higher oxidation rates, a depression in oxygen partial pressure was observed around the specimen. Improved control over the oxidation potential was achieved by using a sensor cell to dynamically tune the pump voltage. Rates for both metals are compared to literature reports using other techniques. 

Abstract Multicomponent oxides have received significant recent attention due to their potential for improved property tunability. In simple structures, compositionally complex oxides can be stabilized by increased configurational entropy and are sometimes called “high entropy” ceramics. In phases with multiple cation sublattices or complex stoichiometries, it is more difficult to achieve high configurational entropy. However, there is limited knowledge about the factors influencing stability and solubility limits in many systems. This study investigated the limits on the stability of rare earth (RE) aluminates containing mixtures of RE cations including Gd, La, Nd, Yb, and Y in cases where (i) a fixed RE:Al ratio attempts to constrain the material into a single‐phase aluminate or (ii) a two‐phase aluminate, and in equilibrium with RE zirconates that readily dissolve multiple RE³⁺. The results show that it is difficult to form single‐phase, equimolar mixed‐RE aluminates encompassing a range of RE³⁺sizes. Instead, the RE³⁺selectively partition into specific phases based on RE‐size trends in the constituent binary systems. The results are discussed in terms of the phase stability and cation partition trends and potential applications.