Disclosing small scale length properties in core-shell structured B4C-TiB2 composites

Highlights: • Densification of WC-doped B₄C-TiB₂ composites occurred by Ostwald ripening and diffusion. • High-energy milling resulted in harder ceramics than those processed by soft milling, 33 vs 28 GPa, respectively. • TEM investigations revealed the formation of core/shell morphology in TiB₂ grains. • Nanoindentation was used to measure the properties of different (Ti_1-xW_x)B₂ solid solutions. • Hardness of (Ti_1-xW_x)B₂ solid solution increased from 41 GPa for x = 0 to 30 GPa for x = 0.27. This work explores the fine microstructural features of B₄C-TiB₂ composites and suggests an overall microstructure evolution from powder processing to sintering. In addition, small-scale grain structures are correlated to local properties measured by nanoindentation, thus providing trends in mechanical behavior. Dense ceramics were typified by development of a core/shell structure of the boride grains, with the shell comprising a (Ti,W)B₂ solid solution with different assemblage and variable amount of W guest cation depending on the processing route. TEM analyses revealed chemical and morphological differences that were associated to the presumed densification mechanisms. Nanoindentation was used to extract the overall and single phase properties of TiB₂ core, shell and B₄C phase highlighting for the first time hardness and modulus variation as a function of the lattice perturbation, i.e. of nominally pure core boride and shells region. This work provides new experimental findings fundamental for the development and synthesis of high-performance structural materials starting from a small-length scale perspective.