The nucleation and growth of precipitates in aluminium alloys

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