Effect of alloying elements and heat treatment on the fracture toughness of Ti-Al-Nb alloys

The fracture toughness and toughening mechanisms of Ti3Al based alloy compositions covering a large range of Nb, small variations in Al and quaternary substitutions of Nb have been studied in a variety of heat treated conditions designed to vary the volume fractions of the constituent phases. It was found that the B2 phase of these alloys failed by cleavage in a coarse grained condition but in a ductile manner when fine grained. A higher Nb and a lower Al content improved the cleavage fracture stress of the BZ phase while replacement of a part of Nb with Mo or Ta had no significant effect. Heat treatments which result in a two phase microstructure (alpha 2 + beta/B2) exhibited a trend of increasing fracture toughness with increasing volume fraction of beta/B2 up to about 60-80 volume fraction of beta/B2. This behaviour was largely explained by quantifying the role of crack tip blunting. The effect of alloying elements on fracture toughness in two phase microstructures was similar to that observed in the coarse grained BZ condition. (C) 1997 Acta Metallurgica Inc.