An Electron Microscopy Study of Modulated Microstructures in Beta Nickel-Aluminum and Other Alloys
Robertson, Ian Miles
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https://hdl.handle.net/2142/71801
Description
Title
An Electron Microscopy Study of Modulated Microstructures in Beta Nickel-Aluminum and Other Alloys
Author(s)
Robertson, Ian Miles
Issue Date
1983
Department of Study
Metallurgy and Mining Engineering
Discipline
Metallurgical Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Metallurgy
Abstract
The tweed contrast in an as-quenched, beta phase 63% nickel-aluminum alloy has been studied by conventional transmission electron microscopy. Supplemental investigations have been carried out on the 3R, 7R, Ni(,5)Al(,3) and Ni(,3)Al phases in the same alloy in the as-quenched condition or after aging at temperatures in the range 300(DEGREES)-850(DEGREES)C, and on Cu-Be, Cu-Zn and Cu-Al-Ni alloys.
It has been demonstrated that the tweed contrast in beta nickel-aluminum behaves analogously to stacking fault contrast, the fringe spacing varying as a function of foil orientation, the extinction distance for the operative reflection and the deviation from the Bragg condition. Tweed microstructures which behave in the same way were found in all phases and alloys indicated above.
Aging experiments, a survey of the literature and comparison of the tweed microstructures of the beta and 3R phases indicate that tweed is a property of the solid solution itself, rather than the result of fine scale precipitation or ordering, or a pre-martensitic phase.
The diffraction evidence points to {110} shear strains distorting the lattice. Kinematic image calculations based on a model of transverse displacement waves reproduce the experimentally observed behavior of the tweed contrast. The displacement waves are considered to constitute the elastic response to a spatially varying stress field. In the beta phase, the stress probably arises from a tetragonal distortion around substitutional nickel atoms on the aluminum sublattice. The high anisotropy of beta alloys is an important factor in the elastic response. Elasticity calculations have been performed confirming the hypothesis.
The results of aging thin foils of 63% nickel-aluminum and copper-38.9% zinc are discussed. What was previously considered to be a spinodal decomposition microstructure in the aged Cu-Zn foils has been shown to more closely resemble the tweed mirostructure, and has been termed the "grid microstructure".
Also reported is the first determination of the region of stability of the Ni(,5)Al(,3) phase in the nickel-aluminum binary phase diagram, by means of powder X-ray diffraction and optical microscopy.
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