Observations of multi-component boride precipitates in ultrahard boron carbide

Abstract

Boron carbide (B4C) often consists of intrinsic secondary phase precipitates that are formed during the reaction-sintering process. These precipitates not only play an important role during the densification process but also affect the mechanical properties of B4C. In the present study, the structural determination of multicomponent secondary phases that are precipitated out in monolithic B4C was investigated using atomic resolution scanning transmission electron microscopy (STEM) and atom probe tomography (APT). Here we found two types (Type-I and II) of multicomponent boride nano-precipitates in B4C i.e. (i) elongated precipitates (type-I), identified as (Cr(1-x-y)-Fex-Wy)3B4 phase, rich in chromium (Cr), Iron (Fe), tungsten (W), and boron (B) (ii) equiaxial (type-II) precipitates that are rich in titanium (Ti), Cr, and B in form of Ti(1-x)CrxB2 phase. The estimated reduced modulus and hardness using depth sensitive nanoindentation on type-I precipitates were 335 ± 6 GPa and 32 ± 2 GPa and that in type-II precipitates were 268 ± 13 GPa and 22 ± 3 GPa, respectively. These results underscore the importance of multicomponent solid solution hardening in ultra-hard B4C material.