Microstructure and fracture of anomalous eutectic silicon-disilicide composites

Microstructural and mechanical investigations have been performed on a new class of silicon-disilicide eutectic composites comprising a Si matrix and reinforcing (Cr,V)Si2 disilicide phase. These composites are cast from the liquid and undergo anomalous eutectic solidification, during which loosely coupled growth of the consitutent phases results in a wide range of eutectic morphologies depending on the volume fraction (Vf) of the minor phase. As the chromium content is varied from 0 to 20 wt. %, the eutectic morphology changes from an irregular fibrous (Vf = 7%) to an irregular plate (Vf = 20%) to an irregular/complex regular plate (Vf = 24%) to a pseudo-colony (Vf = 40%) structure. Chevron-notched beam fracture toughness values for the composites were measured to be around 2–3 MPa m1/2 – more than twice that of unreinforced Si and comparable to SiC. This toughening is due to crack deflection and bridging mechanisms resulting from the weak interfaces between the Si matrix and (Cr,V)Si2 reinforcements. The experimental toughness values are reasonably well modeled using a conventional crack bridging model with some discrepancies that highlight the complexities arising from the anomalous eutectic structures.