The nucleation and growth of precipitates in aluminium alloys

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Abstract

The aim of the work described in this dissertation was to
gain an improved understanding of the basic factors controlling the
nucleation, growth and stability of precipitates in age-hardening
alloys. The stimulus for the work was provided by the current use of
complex ageing heat treatments for modifying (normally with the object
of improving) the mechanical properties of alloys. The research was
confined to aluminium-base alloys, but it is believed that many of the
ideas and concepts yielded by the work will be equally applicable to
other alloy systems.
Two aluminium-base alloys were studied in detail: one
composition of aluminium-magnesium-silicon alloy and three
compositions of aluminium-zinc alloy. The experimental heat treatments
were carried out on thin foil samples (~0.01 cm. thickness) and the
resulting precipitation was studied by means of thin foil transmission
electron microscopy. The low temperature precipitation sequence of aluminium-zinc
alloys has been investigated extensively by other workers and their
results are reviewed in Chapter 3. During this investigation, some
samples of aluminium-zinc, alloy were quenched rapidly to a high ageing
temperature arid, under these special ageing conditions, a new type of
precipitate was observed. The morphology and crystal structure of
this precipitate are described in Chapter 3. Also described in
Chapter 3 are the results obtained from an extensive electron
diffraction investigation into the crystal structure of the small
needle-shaped precipitates which are formed, under suitable ageing conditions, in aluminium-magnesium-silicon alloys.
The types of heat treatment that were studied may be broadly
classified into two categories: (1) single-step ageing treatments,
and (2) two-step ageing treatments. A large number of single-step and
two-step ageing treatments were investigated, with the general objective
of obtaining a comprehensive idea of the overall response of the two
alloys to the heat treatments. These experimental results provided
data, for the development of a theoretical model to explain the basic
processes affecting the response of both alloys to two-step ageing
treatments. It is emphasized that the basis of this model had already
been detailed by Dr. D. W. Pashley, F. R. S. to explain the extensive
microstructural observations obtained, at T. I. Research Laboratoriesq
with an aluminium-magnesium-silicon alloy, during an earlier research
programme (the salient points of this model are discussed in Chapter 5
of this dissertation). Many aspects of this model are extended and
amplified in Chapters 4 to 7 and it is shown that there is an excellent
qualitative agreement between the predictions of the model and the
numerous experimental results which have been obtained. This applies
not only to the aluminium-magnesium-silicon system but also to the
aluminium-zinc system.
The model has been particularly successful for aiding the
semi-quantitative explanation of the observed marked dependence on
heat treatment conditions of the width of grain boundary. precipitate-free
zones in aluminium-zinc alloys. A full account of this aspect
of the work is presented in Chapter 6. The model also provides a new
insight into the basic processes controlling the phenomenon of
reversion in aluminium-zinc alloys, and this is described in Chapter 7 together with in account of the experimental results obtained from
"reversion" studies with this alloy.
Finally, in Chapter 8, the theories and concepts developed
in this dissertation are compared and contrasted critically with those
of other workers.
For brevity, the alloys are denoted throughout this
dissertation by their chemical symbols. Except where otherwise
state; all alloy compositions are, given in weight per cent.

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