Fracture mechanism of Ni3Al alloys and their composites with ceramic particles at elevated temperatures

Ni3Al alloys and their matrix composites reinforced with fine ceramic particles have been successfully fabricated by reactive hot-pressing. This paper investigates the embrittlement mechanism of these materials at intermediate temperatures using a mechanical fracturing technique, i.e. a single edge chevron-notched beam method with variation of loading rate. In the case of monolithic alloys, extrinsic embrittlement originating from diffusion of atomic oxygen into plastic deformation zone coincides with the inherent brittleness connected with deterioration of grain boundary cohesion and unique dislocation motion at 673-1073 K. Oxygen embrittlement predominates over other mechanisms at 673 K, because significant loading rate dependence of fracture toughness is observed in air. The fracture toughness of the alloys intrinsically decreases at 873-1073 K. However, the mechanical behavior of their matrix composites is quite different, depending on the kind of reinforcement particles. Although the composites with TiN particles have high strength and ductility, their fracture toughness decreases at intermediate temperatures, in a similar manner to the monolithic alloys. The fracture toughness of TiC particle reinforced composites is exceptionally constant between 300 and 900 K. (C) 2000 Elsevier Science Ltd. All rights reserved.