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

Tension tests were carried out at 20, 704, 760, and 815 degreesC on sheet s pecimens 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 gamm a structure obtained by annealing at 1200 degreesC to the fine-grained, ful ly lamellar structure obtained by a short-term solution treatment at 1320 d egreesC followed by air cooling to 900 degreesC. The yield strength and the tensile strength of specimens containing the equiaxed, primary gamma micro structure were consistently low at all test temperatures, while the strengt h properties of duplex and fully lamellar structures were comparatively qui te high. The tensile ductility values showed a complex dependence on the mi crostructure and the test temperature. Fully lamellar structures exhibited poor ductility at all temperatures up to 815 degreesC. Fractography and met allography of the tensile test specimens revealed evidence of a mixture of failure mechanisms: cleavage cracking, ductile failure by microvoid coalesc ence, and intergranular fracture by cavitation and wedge cracking. The rela tive contributions of these mechanisms varied with the initial microstructu re and the test temperature. Both the lamellar grains and the gamma grains became significantly elongated near the fracture zone, particularly in spec imens deformed at 760 and 815 degreesC. The yield and tensile strength of T i-45.5Al-2Nb-2Cr alloy sheets in all microstructural conditions were signif icantly higher than those reported for a majority of alloys containing 47-4 9% aluminum. However, the ductile-to-brittle transition temperatures of the Ti-45.5Al-2Nb-2Cr alloy were somewhat higher than those obtained for the m ajority of gamma titanium aluminides. (C) 2001 Elsevier Science B.V. All ri ghts reserved.