Theses and Dissertations from UMD
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Item An Experimental and Theoretical Investigation of the Low Temperature Creep Deformation Behavior of Single Phase Titanium Alloys(2006-10-26) Oberson, Paul Gregory; Ankem, Sreeramamurthy; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)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.Item EFFECT OF MICROSTRUCTURE ON THE ROOM TEMPERATURE TENSILE AND CREEP DEFORMATION MECHANISMS OF ALPHA-BETA TITANIUM ALLOYS(2005-04-13) Jaworski, Allan Wayne; Ankem, Sreeramamurthy; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Two-phase alpha-beta titanium alloys are used in many applications because of their high specific strength, corrosion resistance, processability, and biocompatibility. The room temperature tensile and creep deformation mechanisms of alpha-beta alloys must be understood in order to design alloys with desired properties and improved creep resistance. There is a lack of understanding in this regard. The aim of this investigation is to systematically study the effects of microstructure, stability of the beta phase, and alloying elements on the deformation mechanisms of alpha-beta titanium alloys using Ti-6.0wt%Mn and Ti-8.1wt%V as the model systems. The tensile and creep deformation mechanisms and microstructure were studied using SEM, TEM, HREM, and optical microscopy. In addition, theoretical modeling was performed in terms of crystallographic principles and stress analysis. It was found for the first time in an alpha-beta titanium alloy (Ti- 8.1wt%V) that the alpha phase deforms by twinning and the beta phase deforms by stress induced martensite, different mechanisms than the single-phase alpha and beta alloys with similar grain size. Single-phase alpha deforms predominantly by slip, and single-phase beta deforms predominately by twinning. This is also the first time that stress induced martensite has been observed in a creep deformed alpha-beta titanium alloy. However in the case of Ti-6.0wt%Mn, where the beta phase stability is higher, stress induced martensite was not observed. The deformation mechanisms are modeled in terms of the beta phase stability and interactions between phases, including elastic interaction stresses, alpha phase templating, interactions of deformation products, and alpha-omega interactions. A model is also proposed which explains anisotropic interface sliding based on locking of growth ledges. These results are extremely valuable when designing new alloys with improved resistance to creep and other failure modes. The observed deformation mechanisms can directly affect the mechanical reliability of systems. For instance, increased creep strain can alter the dimensional tolerances of components and the observed stress induced products can act as nucleation sites for fracture initiation and stress corrosion cracking. This work was supported by the National Science Foundation under grant number DMR-0102320.