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

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    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.


    titanium, crack propagation, fractures (materials), deep hardenable, titanium alloys, mechanical properties, fractography, microscopy, microstructure

    Author Information:

    Froes, FH
    Manager, Titanium Research and Development, Colt Industries, Crucible Materials Research Center, Pittsburgh, Pa.

    Chesnutt, JC
    Members of technical staff, Science Center, Rockwell International, Thousand Oaks, Calif.

    Rhodes, CG
    Members of technical staff, Science Center, Rockwell International, Thousand Oaks, Calif.

    Williams, JC
    Associate professor, Carnegie-Mellon University, Pittsburgh, Pa.

    Committee/Subcommittee: E08.07

    DOI: 10.1520/STP26541S