High Temperature X-Ray Decomposition Studies of Alpha Phase Nickel-Silicon and Beta Phase Copper-Aluminum-Nickel Alloys
Dvorack, Michael Albert
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https://hdl.handle.net/2142/71803
Description
Title
High Temperature X-Ray Decomposition Studies of Alpha Phase Nickel-Silicon and Beta Phase Copper-Aluminum-Nickel Alloys
Author(s)
Dvorack, Michael Albert
Issue Date
1984
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
In-situ high temperature X-ray diffraction studies have been performed in order to study the transformation mechanisms and associated kinetics regarding the decomposition and precipitation processes of two metallic alloys of commercial significance: Ni-12.5 at% Si and Cu-28.15 at% Al-3.88 at% Ni. Earlier resistivity measurements of the Ni-Si superalloy of the same composition yielded what was thought to be an apparent order-disorder instability temperature, below which formation of an ordered (gamma)' Ni(,3)Si phase occurred by a continuous transformation, and above which precipitation occurred by nucleation and growth processes. The X-ray results, however, have shown that the precipitation is actually dependent upon the thermal history of the specimen. In addition, cyclic heating and cooling of the specimen at elevated temperatures alters the growth rate of the ordered phase, thereby giving reason to believe that, for the particular composition studied, the growth of the ordered phase is simply based upon classical nucleation and growth phenomena.
The Cu-Al-Ni shape memory alloy was studied with respect to both its thermal diffuse scattering (TDS) and decomposition behavior. It was shown during this study that diffuse streaks observed in diffraction patterns of this alloy are due to the presence of a thermally activated soft phonon TA(,2) mode. Decomposition studies of this alloy have revealed that the (gamma)(,2) brass phase appears from the initial (beta)(,1) phase matrix at a peritectoid transition temperature of 275(DEGREES)C. In addition to the (gamma)(,2) structure, the fcc (alpha) phase and orthorhombic (gamma)' martensite phases also appeared. Of particular interest with regard to the (gamma)(,2) phase is the existence of an unusual reversible behavior of intensity with respect to temperature at certain superlattice reflections. This particular effect is believed to be due to preferential growth of the (gamma)(,2) phase due to the anisotropy of the (beta)(,1) parent phase.
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