Date of Award
Doctor of Philosophy
Materials Science and Engineering
Peter K. Liaw
David Joy, Charles Feigerle, Claudia Rawn
Lifetime-prediction models are useful for simulating a material’s in-service behavior or outcome. Perhaps the greatest advantage of these models is the ability to use the predicted results to help optimize engineering designs and reduce costs. The Hastelloy® C-2000® superalloy is a single-phase material and face-centered cubic in structure at all temperatures. The C-2000® alloy is a commercially designed alloy manufactured to function in both reducing and oxidizing solutions. C-2000® is used as a fabrication material for heat exchangers, piping for chemical refineries, and storage repositories. The corrosion properties of C-2000® are excellent, and the ductility and fatigue properties are good. In this study, C-2000® is used to verify the theoretical basis of an electrochemical-micromechanical crack-initiation corrosion-fatigue model for materials under passive electrochemical conditions. The results from electrochemical and mechanical experiments, along with the findings from the conventional electron microscopy and a laser interferometer will be presented.
A nanocrystalline Ni-18 weight percent (wt.%) Fe alloy is examined to investigate its electrochemical behavior in a 3.5 wt.% NaCl solution. Three Ni-18 wt.% Fe samples were annealed at 400ºC for 3, 8, and 24 hours (hrs.) to study the effects of grain sizes on the electrochemical properties of bulk Ni-18 wt.% Fe. The electrochemical results from the annealed samples are compared with those measured for the as-received Ni-18 wt.% Fe material consisting of an average grain size of 23 nanometers (nm). The samples annealed for times longer than 8 hrs. appear to have undergone an abnormal grain growth, where nanometer and micrometer (μm) grain sizes are present. Unlike the electrochemical results for the as-received material, the annealed nanocrystalline materials appear to be susceptible to localized corrosion. Consequently, these larger grains within the nanoncrystalline-grain matrix are preferentially attacked during electrochemical corrosion. Of the four materials studied, the as-received nanocrystalline alloy possesses the best corrosion properties, and the nanocrystalline material coarsened to an average grain size of 23 μm has the poorest electrochemical properties.
Steward, Rejanah Vernice, "A Lifetime-Prediction Approach to Understanding Corrosion: The Corrosion-Fatigue and the Corrosion Behavior of a Nickel-Based Superalloy and a Nanocrystalline Alloy. " PhD diss., University of Tennessee, 2006.