Effect Of Duplex Aging On The Initiation And Propagation Of Fatigue Cracks In The Solute-Rich Metastable Beta Titanium Alloy Ti 38-644

Aging of highly beta-stabilized titanium alloys commonly leads to the formation of precipitate-free zones being susceptible to fatigue crack initiation. Duplex aging improves the fatigue properties of metastable beta titanium alloys by enhancing a homogeneous alpha phase formation. In this study a duplex-aging cycle was designed for Ti 38-644 (beta-C). Depending on the prior processing history heat treatment parameters were adapted on the basis of microstructure studies, hardness measurements and comparative tensile tests. The fatigue limit and fatigue crack growth threshold were determined for duplex-aged beta-C. The results indicate that duplex aging promotes a homogeneously precipitated alpha phase providing excellent values of the fatigue limit. Surface-related fatigue crack initiation was observed. Comparing the fracture surfaces of direct- and duplex-aged beta-C a transition of the tensile fracture mode from intergranular to predominantly transgranular was observed accompanied by a gain in ductility at comparable yield strengths. This was assumed to be the reason for the slightly improved fatigue crack growth behavior of duplex-aged as compared to direct-aged beta-C. Along the entire heat treatment cycle the microstructure response was evaluated with regard to the particular effects on the fatigue properties. The results indicate clearly that key to success is a completely recrystallized beta microstructure and the reasonably controlled aging response.