High-entropy materials have attracted considerable interest due to the combination of useful properties and promising applications. Predicting their formation remains the major hindrance to the discovery of new systems. Here we propose a descriptor-entropy forming ability-for addressing synthesizability from first principles. The formalism, based on the energy distribution spectrum of randomized calculations, captures the accessibility of equally-sampled states near the ground state and quantifies configurational disorder capable of stabilizing high-entropy homogeneous phases. The methodology is applied to disordered refractory 5-metal carbides-promising candidates for high-hardness applications. The descriptor correctly predicts the ease with which compositions can be experimentally synthesized as rock-salt high-entropy homogeneous phases, validating the ansatz, and in some cases, going beyond intuition. Several of these materials exhibit hardness up to 50% higher than rule of mixtures estimations. The entropy descriptor method has the potential to accelerate the search for high-entropy systems by rationally combining first principles with experimental synthesis and characterization.

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High-entropy high-hardness metal carbides discovered by entropy descriptors

Bulk equiatomic (Hf-Ta-Zr-Ti)C and (Hf-Ta-Zr-Nb)C high entropy Ultra-High Temperature Ceramic (UHTC) carbide compositions were fabricated by ball milling and Spark Plasma Sintering (SPS). It was found that the lattice parameter mismatch of the component monocarbides is a key factor for predicting single phase solid solution formation. The processing route was further optimised for the (Hf-Ta-Zr-Nb)C composition to produce a high purity, single phase, homogeneous, bulk high entropy material (99% density); revealing a vast new compositional space for the exploration of new UHTCs. One sample was observed to chemically decompose; indicating the presence of a miscibility gap. While this suggests the system is not thermodynamically stable to room temperature, it does reveal further potential for the development of new in situ formed UHTC nanocomposites. The optimised material was subjected to nanoindentation testing and directly compared to the constituent mono/binary carbides, revealing a significantly enhanced hardness (36.1 ± 1.6 GPa,) compared to the hardest monocarbide (HfC, 31.5 ± 1.3 GPa) and the binary (Hf-Ta)C (32.9 ± 1.8 GPa).

/pdf/processing-and-properties-of-high-entropy-ultra-high-2vhnga7e8s.pdf

Processing and Properties of High-Entropy Ultra-High Temperature Carbides

https://escholarship.org/content/qt2px2805c/qt2px2805c.pdf?t=px97vh

Phase stability and mechanical properties of novel high entropy transition metal carbides

A comprehensive review of the thermodynamic properties of the oxide compounds of the lanthanide and actinide elements is presented. The available literature data for the solid, liquid, and gaseous state have been analysed and recommended values are presented. In case experimental data are missing, estimates have been made based on the trends in the two series, which are extensively discussed.

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The Thermodynamic Properties of the f-Elements and their Compounds. Part 2. The Lanthanide and Actinide Oxides

Thermodynamic modelling of advanced oxide and carbide nuclear fuels: Description of the U–Pu–O–C systems

TaC, HfC and their solid solutions are promising candidate materials for thermal protection structures in hypersonic vehicles because of their very high melting temperatures (>4000 K) among other properties The melting temperatures of slightly hypostoichiometric TaC, HfC and three solid solution compositions (Ta1−xHfxC, with x = 08, 05 and 02) have long been identified as the highest known In the current research, they were reassessed, for the first time in the last fifty years, using a laser heating technique They were found to melt in the range of 4041–4232 K, with HfC having the highest and TaC the lowest Spectral radiance of the hot samples was measured in situ, showing that the optical emissivity of these compounds plays a fundamental role in their heat balance Independently, the results show that the melting point for HfC098, (4232 ± 84) K, is the highest recorded for any compound studied until now

/pdf/investigating-the-highest-melting-temperature-materials-a-4xmths72fn.pdf

Investigating the highest melting temperature materials: A laser melting study of the TaC-HfC system

Reassessing the melting temperature of PuO2

Commercial ZrC(0.96) powder (ABCR, Karlsruhe, Germany) was densified by spark plasma sintering to greater than 96% relative density at temperatures of 1900-2180 degrees C, applied pressures of 40-100 MPa, and soak time of 6-30 min. Effects of process parameters on microstructure were assessed by ceramography. High temperature (>2000 degrees C) was more instrumental in full densification than was high pressure, and excessive ramp rate resulted in high residual porosity. Grain coarsening was promoted by prolonging the isothermal dwell.

Laser heating was used to melt sintered ceramics, as part of a novel thermal analysis technique for probing extremely high temperature phase transformations. Temperatures well in excess of the expected melting temperature of ZrC and up to 4000 K were achieved. The feasibility of the technique for detecting melting transitions in zirconium carbide was demonstrated, and solidus and liquidus temperatures within 50-80 K of predicted values were measured. Post-melting analysis of laser-melted specimens revealed dendritic microstructure and composition consistent with single phase ZrC.

Konstantinos Boboridis

Papers

Investigating the highest melting temperature materials: A laser melting study of the TaC-HfC system

Reassessing the melting temperature of PuO2

Laser melting of spark plasma-sintered zirconium carbide: thermophysical properties of a generation IV very high-temperature reactor material