The phase transition of multi-component (TiZrVNb)C ceramics

Part Ⅱ: From single phase to multiple phases via adjusting V content.

In recent years, high-entropy carbide ceramics have received extensive attention and become another research focus in the high entropy materials field, which are also known as multi-component carbide ceramics. The multi-component carbide ceramics not only inherit the special properties of high-entropy materials brought by complex compositions, but also keep the advantages of transition metal carbide ceramics as a kind of ultra-high temperature ceramics (UHTCs), such as high melting point, high-temperature stability, high Young’s modulus, high hardness, and so on. Hence, multi-component carbide ceramics have potential applications in the aerospace industry, nuclear industry, machinery, and metallurgy field, etc.

A team of material scientists led by Yujin Wang from Harbin Institute of Technology in Harbin, China recently reported the transition from single phase to multiple phases via adjusting V content in multi-component (TiZrV_xNb)C_0.8 ceramics. In this work, A series of (TiZrV_xNb)C_0.8 ceramics with different V content were fabricated by spark plasma sintering (SPS) at 2200 ℃. The phase composition, microstructure evolution, and mechanical properties caused by the variation of V content were investigated in detail. The transition behavior from single phase to multiple phases caused by the variation of V content is discovered and discussed. The obtained nanometer lamellar structure with a semi-coherent interface via in-situ decomposing is reported for the first time in multi-component carbide ceramics, which can be attributed to the increase of lattice distortion and mixed enthalpy by adding the V element. The semi-coherent interfaces with the high dislocation density and strain concentration have an effective influence on the improvement of mechanical properties as well as grain refinement and multi-phase formation.

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

“In this study, we investigated the influence of V content on the microstructure and mechanical properties of multi-component carbide ceramics in detail. We observed that the addition the V content induces the transition behavior from single phase to multiple phases. Our team believes that changes in the composition of metal elements have a significant impact on the microstructure and mechanical properties of multi-component carbide ceramics. Compared with equimolar multi-component carbide ceramics, non-equimolar multi-component carbide ceramics have a broader space for composition design and performance optimization, which gives full play to the multi-component effect of high-entropy materials. At present, there are few studies on non-equimolar multi-component carbide ceramics. The changes in structure and properties caused by the composition differences of different metal elements are not yet known. There is a huge space for exploration. This is also an important 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.

“We observed that the addition of the V content induces the transition behavior from single phase to multiple phases. With the increase of V content, the microstructure evolution of phase decomposition is first observed in the (TiZrV_0.15Nb)C, which can be regarded as the critical point of the transition. This result is in good agreement with the calculated phase diagram, which shows the cutting point of the V content for phase transition is 0.17 at 2200 ℃. This is effective evidence that the phase transition is thermodynamically spontaneous. Besides, the apparent phase decomposition after heat treatment also supports the spontaneous phase decomposition in thermodynamics effectively.”, said Lei Chen.

“The lattice distortion (δ) and mixed enthalpy (∆HmixΩA-B ) are widely recognized as important parameters on the criterion of single-phase formation for high-entropy alloys and ceramics. The lattice distortion and mixed enthalpy value of the whole system increase with the increment of V content, illustrating that increasing the V content has a negative effect on the formation of single-phase solid solution for the (TiZrV_xNb)C system. Those results can also provide the explanation for the phase transition.”, said Lei Chen.

The nanometer lamellar structures in the (TiZrV_0.15Nb)C sample show the same orientation. In consideration of the complete grain boundary of lamellar structures, the special structures are formed originating from the phase decomposition, and inherit grain orientation of parent phase. The same grain orientation of the Zr-rich phase and Zr-poor phase are obtained consequently. The strain of this phase decomposition area is much higher than that of other single-phase areas significantly. With increasing of the V content, this system is more inclined towards forming two carbide phases. There are some lamellar structures with the same orientation existing in the (TiZrV_0.20Nb)C sample. However, when the V content increases up to 25 mol.%, the Zr-rich phase and Zr-poor phase have no orientation relationship nearly, and form two phases completely. The strain in the lattice also disappears meanwhile.”, said Lei Chen.

“The strain of the phase decomposition area can be attributed to the edge dislocations at the semi-coherent interface, which can adjust the lattice mismatch between the Zr-rich phase and Zr-poor phase to keep the semi-coherent interface. The strain concentration caused by those edge dislocations at the interface is in the form of a discrete dark contrast. Besides the interface of two phases, the dislocations can be observed apparently in the inner of grains. The high dislocation density and strain concentration can restrain the growth and slide of dislocations, and then improve the hardness effectively. The dislocation pileup can be observed at the interface of the Zr-rich phase and Zr-poor phase clearly, which can verify the hardening and strengthening effect of the semi-coherent interface with high dislocation density and strain concentration.”, said Lei Chen.

“Those hardening mechanisms also apply to the (TiZrV_0.20Nb)C sample with fine grain sizes and existence of semi-coherent interfaces. Hence, the optimal mechanical properties of hardness (26.3 GPa), strength (369 MPa), and indentation fracture toughness (3.1 MPa·m^1/2) are achieved in the (TiZrV_0.20Nb)C sample. The enhancement of mechanical properties could be attributed to the grain refinement, the semi-coherent interfaces with high dislocation density and strain concentration.”, 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. The support of thermodynamic calculations with Factsage® software given by Professor Yudong Fu (College of Material Science and Chemical Engineering, Harbin Engineering University) are also acknowledge.

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.

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Journal: Journal of Advanced Ceramics
DOI: 10.26599/JAC.2024.9220889
Article Title: Phase transition of multi-component (TiZrVNb)C ceramics—Part II: From single phase to multiple phases via adjusting V content
Article Publication Date: 28-May-2024

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