The forming-ability, structure and mechanical properties of high entropy carbides and silicides

High entropy oxide and non-oxide ceramics, as novel materials, have demonstrated the potential to be applied in ultrahigh-temperature environment, and for electrical energy storage and catalytic applications. However, it is unclear how the structure and composition of high-entropy ceramics contributes to their performance because they are studied in a structured manner. Therefore, the aim of this project is to investigate the lattice structure of this novel type of ceramics, followed by demonstration of the possible relationship between structure and mechanical properties.

To address the aim, an approach combining experimental characterisation and theoretical calculations was employed. Characterisation techniques included X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), focused ion beam (FIB), scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy (STEM-EDS) and indentation testing. As for theoretical calculations, a thermodynamic-based methodology was proposed to predict lattice parameters and forming-ability from given compositions, which was validated by experimental results. The numerical value of ∆𝐻_𝑢, which is the total energy required to convert the binary compounds into the same structure as the final high entropy product, indicates the scale of the resistance to form a uniform lattice structure. A smaller value of ∆𝐻_𝑢 tends to form a single-phase high-entropy compound, otherwise a multi-phase one. The ∆𝐻_𝑢 cut-off for highentropy carbides was found to be a value between 43.04 and 44.49 kJ/mol.

This thesis investigates two categories of high-entropy ceramic systems from the aspects of lattice structure and mechanical properties: high-entropy carbides and high-entropy silicides, including (ZrVMoW)C, (TiZrVNbW)C, (ZrVNbTaW)C, (TiZrVTaW)C, (ZrVNbMoW)C, (TiZrHfVMoW)C, (TiZrHfVNbTaCrMoW)C (sintered at 1400 °C, 1500 °C, 1600 °C, 1700 °C and 1800 °C), (TiNbMoW)Si₂, (TiNbMoCr)Si₂, (TiVNbCrMo)Si₂, (TiVTaCrMo)Si₂, (TiVNbTaMo)Si₂, (TiNbCrMoW)Si₂, (TiNbTaMoW)Si₂, and (TiNbHfMoW)Si₂.

Eight of them, including four high-entropy carbides ((TiZrVNbW)C, (ZrVNbTaW)C, (TiZrVTaW)C, (TiZrHfVNbTaCrMoW)C sintered at 1800 °C), and four high-entropy silicides ((TiNbMoCr)Si2, (TiVNbCrMo)Si₂, (TiVTaCrMo)Si₂, and (TiVNbTaMo)Si₂), were proven to possess one solid-solution phase of the cubic structure (Fm-3m) for carbides and hexagonal structure (P6₂22) for silicides. The rest of samples consist of multiple solid-solution phases. Multi-phase carbides show 3-4 sets of cubic structures with different lattice parameters, while silicides exhibit mixed hexagonal and tetragonal structures. The investigations of (TiZrHfVNbTaCrMoW)C at varied sintering temperatures showed that intermediate phases formed under lower sintering temperatures (1400 °C to 1700 °C) and exhibited a tendency to merge into the final high entropy phase as temperature increases. The enrichment of elements in each intermediate phase was confirmed to be strongly associated with the lattice parameter of their corresponding binary carbides. The final high entropy phase was identified to start forming at 1700 °C and only the sample sintered at 1800 °C formed a single high entropy phase. The study reveals structural difference between different high-entropy ceramic systems and phase evolution of (TiZrHfVNbTaCrMoW)C system.

Mechanical property assessments showed that high-entropy silicides displayed better ductility than carbides. The high-entropy carbides and silicides demonstrated comparable mechanical performance to their corresponding binary compounds, except for (ZrVNbTaW)C, the nanoindentation hardness value (53.32 GPa) of which is superior to the corresponding binary carbides and their average performance. It was found that the Young's modulus and hardness values of most high-entropy carbides and silicides followed the rule of mixture, which can be used to estimate the properties of such type of ceramics.

The novelty of this research lies in its relatively comprehensive exploration of high-entropy ceramics, covering two high-entropy ceramic systems and involving many different compositions. This makes the findings in this research possess certain generalisability. By combining experimental characterisation with theoretical calculations, this study provides a means to analyse such complex material systems and enhances the understanding of high-entropy ceramics. Furthermore, the proposed thermodynamics-based methodology offers a novel approach to rapidly evaluate the forming-ability of high-entropy ceramics, providing valuable insights into high-entropy ceramic design and contributing to the development of its research.