High-entropy rare-earth diborodicarbide: A novel class of high-entropy (Y_0.25Yb_0.25Dy_0.25Er_0.25)B₂C₂ ceramics

A novel class of high-entropy rare-earth metal diborodicarbide (Y_0.25Yb_0.25Dy_0.25Er_0.25)B₂C₂ (HE-REB₂C₂) ceramics was successfully fabricated using the in-situ reactive spark plasma sintering (SPS) technology for the first time. Single solid solution with a typical tetragonal structure was formed, having a homogeneous distribution of four rare-earth elements, such as Y, Yb, Dy, and Er. Coefficients of thermal expansion (CTEs) along the a and c directions (α_a and α_c) were determined to be 4.18 and 16.06 μK⁻¹, respectively. Thermal expansion anisotropy of the as-obtained HE-REB₂C₂ was attributed to anisotropy of the crystal structure of HE-REB₂C₂. The thermal conductivity (k) of HE-REB₂C₂ was 9.2±0.09 W·m⁻¹·K⁻¹, which was lower than that of YB₂C₂ (19.2±0.07 W·m⁻¹·K⁻¹), DyB₂C₂ (11.9±0.06 W·m⁻¹·K⁻¹), and ErB₂C₂ (12.1±0.03 W·m⁻¹·K⁻¹), due to high-entropy effect and sluggish diffusion effect of high-entropy ceramics (HECs). Furthermore, Vickers hardness of HE-REB₂C₂ was slightly higher than that of REB₂C₂ owing to the solid solution hardening mechanism of HECs. Typical nano-laminated fracture morphologies, such as kink boundaries, delamination, and slipping were observed at the tip of Vickers indents, suggesting ductile behavior of HE-REB₂C₂. This newly investigated class of ductile HE-REB₂C₂ ceramics expanded the family of HECs to diboridcarbide compounds, which can lead to more research works on high-entropy rare-earth diboridcarbides in the near future.