An Experimental and Theoretical Investigation of the Low Temperature Creep Deformation Behavior of Single Phase Titanium Alloys
Oberson, Paul Gregory
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Titanium alloys are used for many applications due to their desirable properties, including high strength-to-weight ratio, corrosion resistance, and biocompatibility. They are used for aerospace, chemical, nuclear, industrial, biomedical, and consumer applications. Often, titanium components are subject to stresses for an extended time. It is known that single-phase hexagonally close-packed (HCP) alpha and body-centered cubic (BCC) beta-titanium alloys deform over time, or creep, at low temperatures (<0.25*Tm). However, factors that affect creep behavior including microstructure and alloy chemistry are not well understood. The aim of this investigation is to experimentally and theoretically study the creep deformation behavior of single-phase alpha and beta-titanium alloys. The first part of the investigation concerns alpha-Ti alloys. The low temperature creep behavior was studied experimentally, using alpha-Ti-1.6wt.%V as the model alloy. Creep testing was performed at a range of temperatures and slip and twinning were identified as creep deformation mechanisms. The activation energy for creep was measured for the first time for an alpha-Ti than deforms by twinning. A change in activation energy during creep is explained by a model for twin nucleation caused by dislocation pileups. The theoretical aspect of the investigation concerns the phenomenon of slow twin growth (time-dependent twinning) during low temperature creep of alpha and beta-Ti alloys. This phenomenon is unusual and poorly understood as twins in bulk metals are expected to grow very fast. It was suggested that interstitial atoms, particularly oxygen could be responsible for time-dependent twinning but there were no models to explain this. As such, crystallographic models were developed for the HCP lattice of alpha-Ti and the BCC lattice of beta-Ti to show how the octahedral interstitial sites where atoms such as oxygen can reside are eliminated by the atomic movements associated with twinning. As such, the rate of twin growth, and in turn the creep strain rate is controlled by the diffusion of oxygen away from these eliminated sites. The results of these findings are valuable when designing Ti alloys for improved creep resistance and mechanical reliability. This work was supported by the National Science Foundation under Grant Number DMR-0513751.