First-Principles Theoretical Studies and Nanocalorimetry Experiments on Solid-State Alloying of Zr-B.

The thermodynamics and kinetics of the solid-state alloying of Zr-B, underlying a variety of synthesis processes of the ultrahigh-temperature ceramic ZrB2, are widely unknown. We investigate the energetics, diffusion kinetics, and structural evolution of this system using first-principles computational methods. We identify the diffusion pathways in the interpenetrating network of interstitial sites for a single B atom and demonstrate a dominant rate-controlling step from the octahedral to the crowdion site that is distinct from the conventional mechanism of octahedral-tetrahedral transition in hexagonal close-packed structures. In the intermediate compounds ZrBx, 0 < x ≤ 2, the diffusivity of B is highly dependent on the composition while reaching a minimum for ZrB. The activation barrier of diffusion in ZrB2 is in good agreement with nanocalorimetry measurements performed on Zr/B reactive nanolaminates.