Processing and deformation mechanisms of boron carbide titanium diboride directionally solidified eutectics

Current military personnel armor solutions are generally monolithic ceramics including boron carbide (B4C), silicon carbide (SiC), titanium diboride (TiB2), and aluminum oxide (Al2O3). Recent work by the US Army Research Lab indicates that nanoscale ceramic composites are of interest for the next generation of armor ceramics. In this work, research on processing, properties, and deformation mechanisms of a micro/nano-scale composite of B4C and TiB2 is presented.;A high power laser (500-1000W) is used to melt an resolidify a psuedobinary eutectic mixture of 75 mol% B4C and 25 mol% TiB2. The resulting microstructure is of the lamellar-type eutectic: a matrix of B 4C with well-ordered TiB2 lamellae reinforcing phase throughout. The scale of the microstructure (the interlamellar spacing) is found to decrease with increasing laser scan rate, consistent with theories of eutectic growth. Use of the high power laser allows for eutectic growth rates up to 42 mm/s, which results in an interlamellar spacing of approximately 180 nm.;Residual stress distribution throughout the eutectic microstructure is calculated with finite element modeling. The boron carbide matrix is found to be in compression and the TiB2 phase in tension, as predicted by analytical calculations. Strain energy and principal stress concentrations are found at the tips of lamellae, corresponding to enhanced microcracking at lamellae tips during deformation.;Vickers indentation of eutectics results in hardness as high as ≈31 GPa at indenter loads of 10 N when the interlamellar spacing is 1 microm or smaller. Conversely, monolithic boron carbide (grain size 6 microm) was found to have a Vickers hardness of ≈27 GPa at 10 N indenter loads. Indentation fracture toughness is measured to be 1.66--2.80 MPa m in B4C-TiB2 eutectics, with no clear dependence on interlamellar spacing, and ≈1.95 MPa- m in B4C. Shear banding and fissuring is evident in deformed monolithic B4C and in some cases dislocations were observed in the TiB2 reinforcing phase of deformed eutectics. In some cases, the presence of the TiB2 reinforcing phase in the eutectic mitigates shear banding and fissuring found in monolithic B 4C, potentially leading to increased hardness.