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Title:Effects of Surface Oxidation on Vacancy Defects in Beta Nickel-Aluminum Alloys
Author(s):Parthasarathi, Arvind
Department / Program:Metallurgy and Mining Engineering
Discipline:Metallurgical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Engineering, Metallurgy
Abstract:Transmission Electron Microscopy and Auger Electron Spectroscopy has been used to make observations of the oxidation process in (beta)-NiAl alloys annealed under controlled conditions of temperature and oxygen partial pressure. Binary Ni-Al alloys, with nominally 47.5, 50.4 and 52.5 at .% Al, were examined. The Auger spectroscopy analysis showed that the predominant oxide formed on these alloys is Al(,2)O(,3), with a possible thin intermediate layer of spinel. Bulk oxidation experiments indicate that the oxide grows by outward diffusion of cations to the oxide/gas interface.
As a result of this diffusion, a counter-flow of vacancies occurs, and these vacancies are injected into the substrate. The vacancy injection results in formation and growth of interfacial voids and growth of existing vacancy dislocation loops and internal voids in thin foil samples. Vacancy loop growth rates increase with annealing time and also increase as the oxygen partial pressure is increased - in accordance with a vacancy injection model. This vacancy injection takes place in the alloys tested here, at temperatures above about 600(DEGREES) C, even in ultra high vacuum ( 850(DEGREES) C in UHV. In these samples the oxidation mechanism seems to change from cation diffusion to anion diffusion and as a result, vacancy defects shrink, rather than grow, in these samples.
In addition to vacancies injected by oxidation, vacancy defects are also formed from retained thermal vacancies. From quenching and ageing experiments, and using electron microscopy, an estimation of 1.45 (+OR-) 0.2 eV was made for the formation energy of a pair of vacancies, one on each sublattice. Large concentrations of thermal vacancies can be generated as a result of this low vacancy formation energy, and can be retained in the alloy even during slow cooling due to the impervious oxide formed.
Issue Date:1980
Description:146 p.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1980.
Other Identifier(s):(UMI)AAI8018197
Date Available in IDEALS:2014-12-14
Date Deposited:1980

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