Low temperature creep of titanium alloys: Microstructure, deformation mechanisms and modeling

Primary creep is the dominant mode of deformation during ambient temperature creep in titanium alloys. In this study the mechanisms causing this unusual creep behavior have been investigated both at the phenomenological and at the microstructural level. The main focus of this work is on a single phase a titanium alloy, Ti-6wt%Al(Ti-6Al). Creep behavior of Ti-6Al-2Sn-4Zr-2Mo(Ti-6242) was also investigated.;Neutron diffraction was used to confirm, for the first time, the presence of short-range order (SRO) in Ti-6Al. From the position of the diffuse peak, it was deduced that the SRO state is related to the ordered phase Ti 3Al.;Based on the study of both Ti-6Al and Ti-6242 it is shown that the strain rate sensitive Hollomon law, s=K3n3&d2; m , represents the constant strain rate behavior of titanium alloys reasonably well, while the transient creep behavior can be described by a power law of the form 3=Ata . A simple analytical result is derived to relate these two expressions. Using this solution the long time creep response has been predicted reasonably well from constant strain rate results. Relative to other metals it is shown that titanium alloys exhibit exceptionally low values of strain hardening.;The effect of SRO on the room temperature creep of Ti-6Al, after various heat treatments, was studied. Significant differences in the initial transient was observed, whereas the final transients were similar. Strengthening due to SRO was evaluated quantitatively using dislocation structures. Both optical microscope observations of slip line evolution and studies of dislocation structures using TEM have been used to relate the deformation mechanisms to the macroscopic behavior.;Weak-fringing faults were observed in < a> slip bands and are shown to be caused by a residual displacement of the type 1/n<112¯0>. The magnitude of the displacement was evaluated to be between 1/145 to 1/104<112¯0>.;Additional room temperature deformation anomalies like tension/compression asymmetry, room temperature recovery and negative creep have been discovered during this study. The possible sources of these anomalous behaviors have been analyzed, and future directions to improve our understanding of these effects have been identified.