DEFORMATION AND FRACTURE OF A DIRECTIONALLY SOLIDIFIED NIAL-28CR-6MO EUTECTIC ALLOY

A directionally solidified alloy based on the NiAl-(Cr, Mo) eutectic w as examined by transmission and scanning electron microscopy to charac terize the microstructure and room temperature deformation and fractur e behavior. The microstructure consisted of a lamellar morphology with a [111] growth direction for both the NiAl and (Cr, Mo) phases, The i nterphase boundary between the eutectic phases was semicoherent and co mposed of a well-defined dislocation network. In addition, a fine arra y of coherent NiAl precipitates was dispersed throughout the (Cr, Mo) phase, The eutectic morphology was stable at 1300 K with only coarseni ng of the NiAl precipitates occurring after heat treatment for 1.8 ks (500 h). Fracture of the aligned eutectic is characterized primarily b y a crack bridging/renucleation mechanism and is controlled by the str ength of the semicoherent interface between the two phases. However, c ontributions to the toughness of the eutectic may arise from plastic d eformation of the NiAl phase and the geometry associated with the frac ture process.