Fatigue deformation in beta titanium (titanium-15vanadium-3aluminum-3tin-3chromium)

This work examines the fatigue behavior of the metastable beta titanium alloy Ti-15V-3Al-3Sn-3Cr, commonly referred to as Ti-15-3. A wide variety of mechanical property combinations are available within this alloy because the distribution and morphology of strengthening precipitates can be altered through processing and heat treatment. Fatigue resistance of several microstructural variations of Ti-15-3 is compared under a variety of loading conditions and environments. Deformation structures developed during fatigue are examined by transmission electron microscopy. Experimental crack growth rates are then compared to growth rates predicted by a ductility exhaustion model that has successfully predicted crack growth in a series of aluminum alloys.; Multiple step aging has been found to enhance fracture toughness of Ti-15-3 compared to the industry standard single step aging process. The effect of these various procedures on fatigue crack growth (FCG) through the beta/alpha microstructures is measured and reported. Increased uniformity in size and distribution, as well as decreased particle spacing of the alpha precipitate, result in degradation of FCG resistance when multiple aging steps are employed. Better resistance to FCG is offered by Ti-15-3 in the beta quenched condition where deformation concentrates in well defined bands.; Dislocation structures resulting from testing under low cycle fatigue (LCF) and fatigue crack growth conditions are examine by transmission electron microscopy. Using a focused ion beam (FIB) to thin the TEM foils allowed precise location of the area to be examined, with respect to the distance above or below the crack plane of FCG specimens. A successful schedule of progressively decreasing FIB beam currents and spacing parameters is presented.; LCF deformation structures are related to applied stress intensity during FCG and distance from the FCG crack plane. Slip band spacing is inversely proportional to the applied strain amplitude during LCF testing. This dependence on strain amplitude is reflected in the FCG specimen cross section: fine spacing develops in the highly strained region adjacent to the crack, transitioning to wider spacing as the matrix strain falls, away from the fracture surface.; Air is an aggressive environment for these Ti-15-3 materials. FCGR through both aged and single phase microstructures is found to be much faster in air than in vacuum. Single phase Ti-15-3 exhibits a plateau in the fatigue crack growth rate curve that is attributable to environmental effects. This transition, present only when the material is tested in air, is found to occur at a crack growth rate of ∼3 x 10-8 m/cycle, similar to the dimension reported by other researchers on many different materials.; Modeling using parameters determined from LCF testing and predicted mean free path for slip produced inconsistent prediction of FCGR. However, good correlation was found between the elastic-plastic threshold value determined during LCF testing to the threshold stress intensity measured in vacuum on the single phase material.