Doctoral Dissertations

Date of Award

6-1987

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Metallurgical Engineering

Major Professor

Charlie R. Brooks

Committee Members

Robert E. C., Raymond A. P., E.E. S., C. L. White

Abstract

The intermetallic compound Ni3A offers promise as an excellent candidate material for high temperature applications. In addition to its unusual property of increasing strength with temperature (until ~700°C), it has excellent corrosion and oxidation resistance. Microalloying the alloy with boron has been shown to be dramatically effective in improving its inherent intergranular brittleness. It has also been observed that this improvement results from the strong tendency of boron to segregate to the grain boundaries of Ni3Al. This research deals with the first detailed study of the segregation behavior of this beneficial segregant. By virture of its surface sensitivity. Auger electron spectroscopy was chosen as the technique adopted to study this segregation. The strong effect of segregant level on the grain boundary strength level can be controlled by thermal history variations and by variations in the level of solute in the bulk. Cathodic hydrogen charging was shown to be a potent tool in opening up otherwise cohesive boundaries for analysis. At a more fundamental level, the effective binding energy of boron at the grain boundaries of Ni3A was calculated from experimental data; it was found to vary between 0.2 and 0.45 eV. The kinetics of segregation have been investigated; the present set of kinetic studies were shown to be inade quate to find a diffusion coefficient and that temperatures lower than those studied here need to be used. As an associated investigation, a set of elemental standards were developed for the particular scanning Auger microprobe used in this study.

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