Tensile creep and cavitation of cobaltous oxide scales
Abstract
This work was undertaken to understand cavitation process which usually accompanies high temperature creep. The test material, CoO, was chosen to model metal oxides, an important group of ceramic materials, due to its suitability for preparation of tensile specimens through the oxidation of the machined specimens of cobalt. A system to conduct constant true-stress tensile creep tests on CoO was designed and built. Creep tests were conducted in air at temperatures of 1220, 1250, and 1300 and stresses of 4.1 to 8.2 MPa, and true strains of 17 to 62% were accumulated. The creep curves show an extended primary region followed by a steady-state. The specimens crept to large strains (54-62%) exhibit a tertiary stage as well. The tertiary stage is attributed to non-uniform deformation and increased porosity at large strains. The experimentally determined values of 82.4 $\pm$ 10.0 kcal/mole for the creep activation energy and 5.6 $\pm$ 1.4 for the stress exponent suggest a dislocation creep mechanism to be dominant with the creep rate controlled by oxygen diffusion. The changes in the porosity of the specimens due to the creep tests were measured by immersion density and metallographic measurements. The specimens deformed to large strains show significant increases in their porosities. Plastic deformation appears to be the dominant mode of pore growth under the creep conditions of this study. The columnar grains on the surface of the specimens have ben found to be oriented with a $\langle$100$\rangle$ pole perpendicular to the specimen surface. Lack of nucleation in the columnar region due to the existence of low energy tilt grain boundaries in the region and slow pore growth are the two probable factors that support the extended plasticity observed in CoO scales.
Degree
Ph.D.
Advisors
Solomon, Purdue University.
Subject Area
Nuclear physics
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