Microstructures and tensile properties of Ti–45.5Al–2Nb–2Cr rolled sheets

Tension tests were carried out at 20, 704, 760, and 815°C on sheet specimens machined from pack rolled and heat-treated sheets of a multi-phase gamma titanium aluminide alloy, Ti–45.5Al–2Nb–2Cr. The microstructures of the heat-treated sheets ranged from equiaxed, (recrystallized) primary gamma structure obtained by annealing at 1200°C to the fine-grained, fully lamellar structure obtained by a short-term solution treatment at 1320°C followed by air cooling to 900°C. The yield strength and the tensile strength of specimens containing the equiaxed, primary gamma microstructure were consistently low at all test temperatures, while the strength properties of duplex and fully lamellar structures were comparatively quite high. The tensile ductility values showed a complex dependence on the microstructure and the test temperature. Fully lamellar structures exhibited poor ductility at all temperatures up to 815°C. Fractography and metallography of the tensile test specimens revealed evidence of a mixture of failure mechanisms: cleavage cracking, ductile failure by microvoid coalescence, and intergranular fracture by cavitation and wedge cracking. The relative contributions of these mechanisms varied with the initial microstructure and the test temperature. Both the lamellar grains and the gamma grains became significantly elongated near the fracture zone, particularly in specimens deformed at 760 and 815°C. The yield and tensile strength of Ti–45.5Al–2Nb–2Cr alloy sheets in all microstructural conditions were significantly higher than those reported for a majority of alloys containing 47–49% aluminum. However, the ductile-to-brittle transition temperatures of the Ti–45.5Al–2Nb–2Cr alloy were somewhat higher than those obtained for the majority of gamma titanium aluminides.