Mechanical properties of titanium diboride based cermets

Mechanical properties of titanium diboride (TiB2) cermets critically depend on the composition of the binder phase. Both, fracture toughness and hardn ess are substantially increased by avoiding the formation of extremely brit tle secondary borides which form during sintering by chemical reactions bet ween TiB2 and the metallic additives. Fractographic observations of TiB2 ce rmets without secondary borides show the presence of ductile ligaments of t he binder phase bridging the advancing crack tip. The powder metallurgy pro cessing route applied to these materials allows modification of the binder phase structure from the ferritic iron-aluminium phase to Fe-Ni-Al austenit e by changing the aluminium content of the powder mixtures. The highest tou ghness values have been obtained for the TiB2 cermets with an austenitic bi nder phase. X-ray diffraction analyses of the fracture surfaces of such sam ples show that the binder phase is metastable exhibiting stress induced mar tensitic transformation during fracture. This new family of materials prese nts an outstanding combination of hardness and toughness, comparable to tho se obtained with commercial grades of tungsten carbide (WC) hardmetals. (C) 2000 Kluwer Academic Publishers.