Effect of phase morphology on the mechanical behavior of two titanium aluminide composites

Two binary titanium aluminide alloys, Ti-43A1 and Ti-47A1 (atomic percent), were discontinuously reinforced with 6 vol pct titanium diboride, resulting in a two-phase Ti₃Al and TiAl (α₂ and γ, respectively) matrix with a dispersion of TiB₂ particulate. Cast material was successfully ex-truded and subjected to a series of single-step and duplex-step heat treatments. Thermo-mechanical processing was correlated with microstructural changes, and the ambient temperature mechanical properties were measured for the various heat-treated conditions using tensile and hardness testing. Yield stress and plastic elongation to failure and hardness were found to cor-relate well with the fraction of proeutectoid, or primary, TiAl formed during heat treatment within the α/γ phase field. Precipitation of y within proeutectoid α grains during subsequent aging treatments within the α₂ phase field was seen to increase the room-temperature ductility with negligible debits in yield stress. Enhanced ductility and decreases in yield stress and hard-ness are associated with morphologically large regions of the TiAl phase. Incompatibility of slip systems across γ/α₂ and the inherent resistance to slip in hyperstoichiometric Ti₃Al are suggested as possible explanations for the observed phenomena. Formerly with Martin Marietta Laboratories