Creep mechanisms in titanium alloy tubing

High temperature creep studies were carried out on cp-Ti and Ti-3Al-2.5V tubing under a biaxial state of stress. A burst testing facility was employed to characterize the creep behavior of cp-Ti whereas a biaxial creep testing facility was utilized for studying Ti-3Al-2.5V.;Creep studies on cp-Ti indicated a transition in mechanism from a power law controlled creep behavior at lower stresses to a power law breakdown controlled creep behavior at higher stresses. TEM studies showed that subgrains constitute the microstructure of specimens crept at lower stresses whereas a network of random dislocations characteristic of a power law breakdown decorated the specimens tested at higher stresses.;Studies on recrystallized Ti-3Al-2.5V indicated transitions in mechanisms as a function of the applied stress and temperature. At low normalized stresses (regime I) the creep parameters suggested Coble creep as the possible rate controlling mechanism but instead on the basis of TEM observations a slip band model was invoked and was found to provide a better description of regime I than the Coble creep model.;In the intermediate normalized stress range (regime II) creep parameters suggested grain boundary sliding as the rate controlling mechanism. TEM micrographs corroborated the same.;In regime III the creep parameters suggested dislocation climb as the rate controlling mechanism. However, TEM studies indicated the presence of jogged screw dislocations along with subgrains thus suggesting the possible involvement of jogged screw dislocations as rate controlling. However, systematic studies carried out as a function of stress revealed a transition in rate controlling mechanism from dislocation climb at lower stresses to motion of jogged screw dislocations at higher stresses.;Furthermore, the effect of alloying and stress ratio was understood by studying the microstructures of specimens of cp-Ti and Ti-3Al-2.5V crept in the five power law creep regime.