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 (Y0.25Yb0.25Dy0.25Er0.25)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 (aa and ac) were determined to be 4.18 and 16.06 mu K-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 center dot m(-1)center dot K-1, which was lower than that of YB2C2 (19.2 +/- 0.07 W center dot m(-1)center dot K-1), DyB2C2 (11.9 +/- 0.06 W center dot m(-1)center dot K-1), and ErB2C(2) (12.1 +/- 0.03 W center dot m(-1)center dot 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.