Date of Award
Doctor of Philosophy
Materials Science and Engineering
Hahn Choo, George Pharr, John Landes
Nondestructive evaluation is a useful method for studying the effects of deformation and fatigue. In this dissertation I employed neutron and X-ray diffraction, nonlinear resonant ultrasound spectroscopy (NRUS), and infrared thermography to study the effects of deformation and fatigue on two different nickel based superalloys. The alloys studied were HAYNES 230, a solid solution strengthened alloy with 4% M6C carbides, and secondarily HASTELLOY C-2000 a similar single phase alloy.
Using neutron and X-ray diffraction, the deformation behavior of HAYNES 230 was revealed to be composite-like during compression, but unusual in tension. The carbides present in this alloy do not provide strengthening in tension as would be expected or finite element modeling predicted. The carbides provide strengthening until just after the macroscopic yield strength and then they begin to debond and crack, creating a tension-compression asymmetry that is revealed clearly by in situ diffraction. HASTELLOY C-2000, a similar alloy without carbides, shows typical anisotropic load sharing between differently oriented grains.
In fatigue of HAYNES 230, the hkl behavior as revealed by neutron diffraction showed that the elastic strain changes very little in tension-tension fatigue. However, in situ tension-compression studies showed large changes over the initial stages of fatigue. There was slight evidence for changes in elastic modulus as fatigue progressed.
The HAYNES 230 samples studies had two distinct starting textures, measured by neutron diffraction. Some samples were texture free initially and deformed in tension and compression to fiber textures. Other samples started with a bimodal texture due to cross-rolling and incomplete annealing. The final texture of these bimodal samples is shown through modeling to be a superposition of the initial texture and typical FCC deformation mechanisms. The effect of these different textures on the macroscopic and internal-elastic stress-strain curves are shown. The texture-free samples deformed significantly more macroscopically and in internal elastic strains than the samples with the cross-rolled texture.
In contrast to the relative insensitivity of neutron diffraction to the effects of tension-tension fatigue, NRUS revealed large differences between as-received and progressively fatigued samples. This showed that microcracking and void formation are the primary mechanisms responsible for fatigue damage in tension- tension fatigue. NRUS is shown to be a useful complimentary technique to neutron diffraction to evaluate fatigue damage.
Finally, infrared thermography is used to show temperature changes over the course of fatigue in HASTELLOY C-2000. Four stages of temperature are shown over the course of a single fatigue test: an initial temperature rise, followed by an equilibrium region, a sharp increase of temperature at failure and, finally, a cooling back to room temperature after fracture. Both empirical and theoretical relationships between steady state temperature and fatigue life are developed and presented.
Saleh, Tarik Adel, "Nondestructive Evaluation of Loading and Fatigue Effects in Haynes® 230® Alloy. " PhD diss., University of Tennessee, 2006.