Development of High Fracture Toughness Titanium Alloys

Abstract

This paper describes the relationship between composition, processing history, and microstructure of a series of α + β and metastable β titanium alloys. The relation between a wide range of microstructures and the resulting properties of these alloys has been investigated by making extensive use of electron fractography and metallographic fracture path determinations. These results were obtained in connection with an alloy development program in which two alloy compositions ultimately were selected: Ti-5Mo-4.5A1-1.5Cr and Ti-8Mo-2.5A1-4.5Cr. These alloys were processed according to various schedules in order to achieve microstructures which produce good strength-toughness combinations. In the α + β alloy (Ti-5Mo-4.5A1-1.5Cr), the optimum property combinations were achieved in β-processed material which has an acicular microstructure and α-phase at prior β-grain boundaries, although excellent properties also were obtained in equiaxed α + β processed material. In the metastable β-alloy (Ti-8Mo-2.5A1-4.5Cr), significantly better strength-toughness combinations were achieved in α + β processed material which contains globular primary α, compared to β-processed material which contains grain boundary α. The metastable β-alloy exhibits a significant degree of directionality of properties in all microstructural conditions. It is suggested that such directionality is due to the elongated β-grain structure which causes more crack branching when the crack is propagating in the transverse direction compared to the longitudinal direction. Finally, a model is presented to account for the effect of grain boundary α on fracture toughness of α + β and metastable β alloy.