Transformation of chemical ordering and configuration entropy in quaternary CrxTi0.75Mo0.75V1.5−xAlC2 MAXs system

As a promising material family that integrates the advantages of both alloys and ceramics, the MAX phase ceramics have attracted wide attention around the world. At the same time, improving the physicochemical performance of the MAX phase is a constant pursuit of the field. However, although the abundant chemical composition of the transition metal (M) in MAXs endows them with a promising prospect for properties regulation, the combination of multiple metals in a single MAX phase remains a challenge, let alone illuminating the relationship among the composition, structure, and performance.

Recently, a team of material scientists led by Cheng-Feng Du from Northwestern Polytechnical University, China first reported a series of Cr_xTi_0.75Mo_0.75V_1.5−xAlC₂ MAXs with quaternary transition metals. Interestingly, an unprecedented structure transition from out-of-plane order to disorder is observed along with the alteration of metal composition, which also increases the configurational entropy from medium- to high-entropy. By combining the experimental observation and theoretical simulations, the mechanical, electrical, and thermal properties of the new MAXs are evaluated, and the influence of the order-disorder structure transition on their properties is analyzed.

The team published their work in Journal of Advanced Ceramics on October 6, 2024.

“In this report, we synthesized Cr_xTi_0.75Mo_0.75V_1.5−xAlC₂ by hot pressing sintering. According to the HAADF-STEM analysis under double Cs corrected TEM, the out-of-plane ordered atomic occupancy gradually disappears in the system along with the changed M-site constituents. The out-of-plane ordering in Cr_1.25Ti_0.75Mo_0.75V_0.25AlC₂ can be described as the preferential occupation of Cr, Mo, and V in the 4f site while the Ti and V reside at the 2a site. While with the reduced Cr and increased V content, the equimolar introduction of the Ti, V, Cr, and Mo atoms in Cr_0.75Ti_0.75Mo_0.75V_0.75AlC₂ form solid solution in both 4f and 2a sites.” said Cheng-Feng Du, distinguished research fellow at School of Materials Science and Engineering at Northwestern Polytechnical University (China), a senior expert whose research interests focus on the field of the synthesis, structure, and tribological behaviors of MAX and MXene based materials.

“As calculated, along with the change in Cr and V content in the lattice, the quaternary Cr_xTi_0.75Mo_0.75V_1.5−xAlC₂ MAXs not only present a transition in atomic occupancy but also demonstrate an evolution from medium-entropy to high-entropy structure.” said Cheng-Feng Du.

The Cr_1.25Ti_0.75Mo_0.75V_0.25AlC₂ shows a high Vickers hardness of 6.99 ± 0.22 GPa, which is close to that of the Cr_0.75Ti_0.75Mo_0.75V_0.75AlC₂ and Cr₁Ti_0.75Mo_0.75V_0.5AlC₂. Compared to Cr₂TiAlC₂ (4.98 ± 0.30 GPa), the Vickers hardness of three quaternary MAXs is about 40% higher, which might be associated with the strengthening effect caused by the solid solution lattice structure.

The thermal conductivities (κ) of Cr_xTi_0.75Mo_0.75V_1.5−xAlC₂ (x = 1.25, 1, 0.75) and Cr₂TiAlC₂ are 20.53, 13.49, 13.85 and 14.90 W m⁻¹ K⁻¹, respectively. “The low κ detected in Cr₁Ti_0.75Mo_0.75V_0.5AlC₂ and Cr_0.75Ti_0.75Mo_0.75V_0.75AlC₂ can be ascribed to the enhanced scattering of both electrons and phonons dominated by the solid solution structure”.

However, more delicate research works are still needed to explore the suitability of Cr_xTi_0.75Mo_0.75V_1.5−xAlC₂ as a new material. In this regard, Du also put forward three major development directions that may be pursued in future works including the high-temperature lubrication and wear resistance, antioxidant properties, and structural stability of the material during performance.

Other contributors include Yaqing Xue, Hong Yu, Hongwei Liang, Xiaomei Wang, Lili Xue, Shiyao Lei, Conghui Meng, and Long Wang, from the School of Materials Science and Engineering at Northwestern Polytechnical University in Xi’an, China.

This work was supported by the National Natural Science Foundation of China (No. 52275212), the “Special Lubrication and Sealing for Aerospace” Shaanxi Provincial Science and Technology Innovation Team (No. 2024RSCXTD-63), the Fundamental Research Funds for the Central Universities (No. D5000230047), and the Aeronautical Science Foundation of China (2024Z046053001). The authors thank the support and expert advice of Professor Zhang Qichun’s group (City University of Hong Kong) on the calculations.

About Author

Cheng-Feng Du is currently a distinguished research fellow in the School of Materials Science and Engineering at Northwestern Polytechnical University (NPU). He received his Ph.D. degree in 2016 from the Fujian Institute of

Research on the Structure of Matter, Chinese Academy of Sciences, and then worked at Nanyang Technological University as a research fellow from 2016 to 2018. Since 2018, he joined NPU, affiliated to the Center of Advanced Lubrication and Seal Materials. His current research interests focus on the synthesis, structure, and tribological behaviors of MAX and MXene-based materials.

Yaqing Xue received her M.S. degree from Northwestern Polytechnical University (NPU) in 2023. She is pursuing her Ph.D. degree in the School of Materials Science and Engineering at NPU. Her current research interests mainly focus on the synthesis, structure, and tribological behaviors of MAX-based materials.

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