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dc.contributor.authorG. Kelekanjeri, V. Siva Kumaren_US
dc.date.accessioned2007-05-25T17:30:13Z
dc.date.available2007-05-25T17:30:13Z
dc.date.issued2007-04-06en_US
dc.identifier.urihttp://hdl.handle.net/1853/14561
dc.description.abstractIn this research, controlled Waspaloy microstructures were produced with the objective of studying microstructural evolution in this alloy via electrically-based ac/dc non-destructive techniques. Correlations were developed between electrical measurements and alternate characterization techniques such as Ultra Small Angle X-ray Scattering (USAXS), Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) to gain a complete understanding of the microstructural transformations and the associated mechanisms. Three different sets of controlled microstructures were produced in this research. In Set I microstructures, matrix (gamma) grain sizes of 13, 52 and 89 micrometers were obtained after solution-treatments at 1045 and 176;C, 1090 and 176;C and 1145 and 176;C respectively. A vacancy stabilization treatment at 1045 and 176;C followed after which, the specimens were aged at 800 and 176;C for times ranging from 0.1 hrs to 100 hrs to vary the gamma prime precipitate size distribution. In Sets II and III, the solution-treatment was only conducted at 1145 and 176;C, with the stabilization treatment conducted only in Set II. Subsequently, aging experiments were conducted at 725 and 176;C (or 700 and 176;C in Set II), 800 and 176;C and 875 and 176;C for times up to 100 hrs. DC four-point probe resistivity of specimens increased to a maximum upon initial aging from the solution-treated condition and showed a decreasing trend thereafter with successive aging. This, in addition to complementary evidence from SEM and USAXS, led to the conclusion that gamma prime nucleation-growth was complete by the time the resistivity maximum was observed. Resistivity variations that ensued upon successive aging after the maximum were attributed to microstructural/compositional changes due to gamma prime coarsening. The height of the maximum decreased drastically with increase in aging temperature from 725 and 176;C to 800 and 176;C, while the resistivity did not increase from the solution-treated condition upon aging at 875 and 176; C. Coarsening studies based on USAXS analysis indicated an LSW type volume diffusion mechanism of coarsening in Waspaloy, with an average coarsening rate constant of 3.25x10-29 [m3/sec] for Set I specimens aged at 800 and 176;C. Analytical and Finite Element (FE) models of two-probe impedance and dc four-point probe resistivity methods were developed to gain insight into the measured response and the accurate determination of material properties. AFM-based localized electrical examination of sub-grain Waspaloy microstructures was successfully conducted using electrostatic force microscopy (EFM), scanning Kelvin probe microscopy (SKPM) and current-AFM (I-AFM) electrical modes. I-AFM experiments revealed that the conductivity of the gamma prime phase was lower than that of the gamma phase.en_US
dc.publisherGeorgia Institute of Technologyen_US
dc.subjectNondestructive evaluationen_US
dc.subjectNickel-base superalloysen_US
dc.subjectCoarseningen_US
dc.subjectHeat-treatmenten_US
dc.subjectMicrostructureen_US
dc.subjectElectrical resistivityen_US
dc.subject.lcshNickel alloysen_US
dc.subject.lcshHeat resistant alloysen_US
dc.subject.lcshMicrostructureen_US
dc.titleNon-destructive Electrical Characterization of Controlled Waspaloy Microstructuresen_US
dc.typeDissertationen_US
dc.description.degreePh.D.en_US
dc.contributor.departmentMaterials Science and Engineeringen_US
dc.description.advisorCommittee Chair: Dr. Rosario A. Gerhardt; Committee Member: Dr. Arun M. Gokhale; Committee Member: Dr. Laurence Jacobs; Committee Member: Dr. Stephen D. Antolovich; Committee Member: Dr. Thomas Sanders, Jr.en_US


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