FRACTURE-TOUGHNESS OF NIAL AND NIAL-BASED COMPOSITES

Fracture toughness of hot pressed, fully dense stoichiometric and Ni-rich NiAl was measured using chevron-notched "short-rod" specimens. Whereas stoichiometric NiAl and Ni-rich NiAl containing 60 at.% Ni exhibit K(IC) of approximately 6 MPa square-root m, toughness increases to approximately 9 MPa square-root m at a composition of 61.5 at.% Ni. This could be due to transformation toughening, arising from a stress-induced B2-L1(0), martensite transformation and/or due to the presence of a compliant layer of Ni3Al at the grain boundaries. To elucidate the mechanisms responsible for the observed toughness enhancement, hot-pressed specimens of Ni-rich NiAl (60, 61.5 and 63 at% Ni) were subjected to a variety of heat treatments to obtain certain predetermined microstructures. After identifying specific heat treatments, short-rod fracture toughness specimens from the hot-pressed material of these compositions were selectively heat treated and tested. Toughness and fracture mode were dependent on composition and heat treatment (i.e. microstructure). Finally, the role of TiB2 particulates and/or Al2O3 whiskers in affecting the toughness of stoichiometric and Ni-rich NiAl was investigated. These reinforcements were intended to improve toughness and creep resistance simultaneously in such materials although the observed toughness improvements were only marginal.