Relationship of Fracture Toughness and Ductility to Microstructure and Fractographic Features in Advanced Deep Hardenable Titanium Alloys

Abstract

This paper describes the results of a program conducted to develop a metastable β titanium alloy with deep hardening characteristics and with a yield strength of 1165 MPa (170 ksi), a fracture toughness of 66 MPam (60 ksiin), and tensile ductility of 12 percent elongation and 18 percent reduction of area. After a screening study, three compositions were chosen: Ti-10Mo-6Cr-2.5A1 (Alloy 334), Ti-7Mo-4Cr-2.5A1 (Alloy 227), and Ti-10Mo-8V-2.5A1 (Alloy 253). Both β-working and β followed by α-β working were investigated as means of controlling the primary α distribution. Three distinct microstructures were subjected to extensive testing together with microstructural and fractographic analysis. It was found that grain boundary α was detrimental to properties in most cases. Stringered α, resulting from β-working, caused a directionality in mechanical properties with longitudinal values being higher than those in the transverse direction. Lenticular primary α was generally beneficial to toughness, whereas equiaxed or globular primary α was beneficial to tensile ductility. The predominant role of secondary (aged) α was in influencing strength level. The causes for these property variations with α phase morphology were analyzed using metallographic fracture face profiling and scanning electron fractography. It was found that the α-β interfaces initiate fracture and thus have a large influence on fracture related properties. Finally, the properties of the three metastable β alloys were compared to those of high strength, deep hardening α/β alloys. More attractive strength-toughness combinations are achievable in the metastable β alloys. The tensile ductilities of the α-β alloys were somewhat higher than those of the metastable β alloys. However, ductilities of the latter class of alloys were acceptable.