Phase Transition Unveiled in Multi-Component Ceramic

Tsinghua University Press

With the development of aerospace and nuclear industries, the performance requirements for ultra-high temperature materials are becoming more and more stringent. High-entropy carbide ceramics, as a kind of ultra-high temperature ceramics (UHTC), have not only excellent mechanical and thermal properties such as high melting point, Young's modulus, the high-temperature stability of UHTC, but also have high entropy effect brought by its complex components, which can be widely used in aerospace, nuclear industry, machinery, metallurgy and other fields.

A team of material scientists led by Yujin Wang from Harbin Institute of Technology in Harbin, China recently reported the phase decomposition induced by carbon content in multi-component (TiZrVNb)Cx ceramics. In this work, a series of equimolar (TiZrVNb)Cx ceramics with different carbon content were fabricated by spark plasma sintering (SPS) at different temperatures. The phase compositions, microstructural evolution, and mechanical properties deriving from the variation of carbon content and sintering temperature were investigated in detail. The phase decomposition in the form of a spinodal-like decomposition was discovered. An equimolar single-phase salt-rock solid solution decomposes into two different salt-rock solid solutions with alternate lamellar nanostructures during the decomposition process, namely the Zr-poor phase and Zr-rich phase, respectively. The interfaces between the Zr-poor phase and the Zr-rich phase possess a semi-coherent orientation relationship. The high dislocation densities and strain concentration at the semi-coherent interfaces can prevent the slipping and growth of dislocations effectively, and play an important role on hardening and strengthening under the synergistic effect of grain refinement.

The team published their work in Journal of Advanced Ceramics on May 28, 2024.

"In this study, we investigated the influence of carbon content on the microstructure and mechanical properties of multi-component carbide ceramics in detail. We observed that reducing the carbon content induces phase decomposition, thereby opening up a new avenue for research in multi-component carbide ceramics. Our team believes that the influence of carbon content on the microstructure of multi-component carbide ceramics is closely related to the types of metallic elements present. Therefore, discovering the specific impact of carbon content on different characteristic systems is of paramount significance. This also represents a crucial direction for our future research." said Lei Chen, senior author of the research paper, associate researcher in the School of Material Science and Technology at Harbin Institute of Technology.

"The phase separation of the system caused by the reduction of carbon content may be thermodynamically spontaneous. The formation energies of (TiZrVNb)Cx with different carbon content calculated by DFT calculations can support the inference. Besides, the apparent phase decomposition after heat treatment also supports the spontaneous phase decomposition in thermodynamics effectively.", said Lei Chen.

"The different distribution of metal elements can be attributed to the interaction coefficients among all metal elements in transition metal carbides calculated by DFT calculation. The values of the interaction coefficients ΩTi-Zr, ΩTi-V, ΩTi-Nb, ΩZr-V, ΩZr-Nb, and ΩV-Nb are 55.178 kJ/mol, 1.566 kJ/mol, -9.708 kJ/mol, 124.301 kJ/mol, 3.927 kJ/mol, and 38.134 kJ/mol, respectively. The value is lower, and the ability of the formation of solid solution is stronger. As a consequence, the Zr element makes it difficult to form a solid solution with Ti and V elements due to such high interaction coefficients, which results in the formation of the Zr-poor phase and Zr-rich phase.", said Lei Chen.

"The interface between the nanometer nodular structures of the Zr-poor phase and Zr-rich phase with the doublet diffraction spots, which suggests the occurrence of a semi-coherent orientation relationship. The continuous lattice image of the semi-coherent interface shows that there is a discrete dark contrast at the interface between the two phases because of the lattice mismatch of both sides. There are some edge dislocations at the semi-coherent interface significantly, which result in the strain concentration at the interface. The lattice mismatch is adjusted by these edge dislocations.", said Lei Chen.

"The high dislocation densities and strain concentration at the semi-coherent interfaces can prevent the slipping and growth of dislocations effectively, and play an important role on hardening and strengthening under the synergistic effect of grain refinement. The best comprehensive mechanical properties such as Vickers' hardness of 27.9 GPa, flexural strength of 473 MPa, and fracture toughness of 3.1 MPa·m1/2 were achieved in the (TiZrVNb)C0.9 sample sintered at 2300℃. Compared to single-phase high-entropy carbide ceramics, the multi-phase multi-component carbides achieved by phase decomposition have a significant advantage in hardness and strength. This work provides an important insight into the design of microstructure evolution and optimization of mechanical properties for multi-component carbide ceramics.", said Lei Chen.

The authors thank Professor Suk-Joong L. Kang (Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology) for his assistance in editing.

This work was supported by the National Natural Science Foundation of China (Nos. 52032002, 52372060, 51972081, and U22A20128), National Safety Academic Foundation (No. U2130103), National Key Research and Development Plan of China (2021YFB3701400), China Postdoctoral Science Foundation (No. 2023M730839), Heilongjiang Postdoctoral Fund (No. LBH-Z22025), National Key Laboratory of Precision Hot Processing of Metals (No. 61429092300305) and Heilongjiang Touyan Team Program.

About Journal of Advanced Ceramics

Journal of Advanced Ceramics (JAC) is an international journal that presents the state-of-the-art results of theoretical and experimental studies on the processing, structure, and properties of advanced ceramics and ceramic-based composites. JAC is Fully Open Access, monthly published by Tsinghua University Press on behalf of the State Key Laboratory of New Ceramics and Fine Processing (Tsinghua University) and the Advanced Ceramics Division of the Chinese Ceramic Society, and exclusively available via SciOpen. JAC has been indexed in SCIE (IF = 16.9, top 1/28, Q1), Scopus, and Ei Compendex.

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