Microstructural and mechanistic study of low temperature creep and dwell fatigue in single colony alpha/beta titanium alloys

Commercial alpha(hcp)/beta(bcc) titanium alloys benefit from having a high specific strength, modulus and fracture toughness and are used in a variety of aerospace applications up to moderate temperatures (∼600°C). However, they are observed to exhibit relatively large primary creep strains at low homologous temperatures and at stresses below the yield strength. These alloys have also been observed to have over an order of magnitude decrease in fatigue life during dwell cycle fatigue testing. This dwell effect has been attributed to several mechanism, including the accumulation of significant creep strain during dwell at maximum stress, which leads to initiation of basal cracking in suitably oriented neighboring alpha/beta colonies. To understand this effect, investigations into the dwell fatigue, constant strain rate and room temperature creep behavior of the commercial aeroengine alloy Ti-6Al=2Sn=4Zr-2Mo-0.1Si have been performed. Detailed Transmission Electron Microscopy (TEM) investigations into the crystallography, morphology and orientation relationship (OR) between the alpha and beta phases in single colony structures has been performed. Comparisons of the deformation microstructures of materials subjected to dwell and low cycle fatigue to failure demonstrate the presence of planar a-type (a3⟨-2110⟩) deformation modes on both basal and prism planes, similar to observations in room temperature creep and constant strain rate deformation. Due to the nature of the Burger's OR between the alpha and beta phases, the critical resolved shear stress for the three a/3⟨11-20⟩ basal and prism slip systems exhibit strongly anisotropic behavior. A strong tension/compression asymmetry has also been observed for the basal slip systems. Similar anisotropy has been observed in the room temperature primary creep response of single colonies oriented for basal slip in compression. Significant room temperature recovery was observed upon a 24 hour unload of one of the basal slip oriented creep samples, resulting in a two order of magnitude increase in creep rate upon reloading. Detailed TEM investigations into the mechanisms of dislocation slip transmission through the alpha/beta interface have been performed for a-type slip on basal planes, where the strongest anisotropy was observed. This anisotropy has been explained in light of the Burger's OR and the observed dislocation mechanisms active during room temperature deformation.