Fracture Initiation in the Directionally Solidified NiO‐CaO Eutectic

Initiation of fracture in the directionally solidified, lamellar NiO-CaO eutectic was examined using the indentation fracture technique. Fracture could not be induced along the interphase boundary on transverse sections of the directionally solidified eutectic. Instead, radial cracks evolved at angles of approximately 35° and 55° with respect to the lamellar interface and were consistent with median/radial crack formation on (TlO) and (001) planes, respectively. Indentation of single-crystal NiO resulted in fracture only along {110} planes. Crack initiation on {110} planes in both materials was attributed to a dislocation coalescence model proposed by Keh et al. while crack formation on (001) planes in the eutectic was believed to be initiated by a Stroh-type mechanism involving dislocation pile-ups. Variations in the interlamellar spacing resulted in a Hall-Petch-type behavior for the hardness but had little effect on the fracture toughness of the eutectic.