The relationship between microstructural and plastic instability in AI-4.0 Wt Pct Cu alloy
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I.
INTRODUCTION
THE traditional heat-treatment procedure of a precipitation-hardened alloy comprises an initial solution heat treatment at a temperature at which the precipitating elements are brought into solid solution followed by quenching, usually to room temperature, and a final annealing treatment at a temperature at which precipitates can nucleate, grow, and coarsen to some extent. The whole procedure contains a series of steps and the necessary time to obtain the desired properties of the material is in most cases several hours. The industrial process is, thus, rather complex and costly and a modification of the heat-treatment schedule with a resulting reduction in time would be of considerable interest. Most of the time is spent on the process of nucleating the strengthening precipitates and their subsequent growth and coarsening to an optimum dispersion. It is well known that these reactions (nucleation, growth, and coarsening) can be strongly influenced by plastic deformation of the material. Most investigations on the effect of plastic deformation have been related to the static conditions, i.e., nucleation, growth, and coarsening in a plastically deformed material,11.2.3] while the dynamic conditions, in which these processes take place simultaneously to the plastic deformation, have been much less studied. The intention of the present study was to investigate whether controlled straining during aging could be used to reduce the time needed to obtain acceptable mechanical properties of precipitation-hardened alloys. However, the basic understanding of such dynamic reactions is much less developed than that of the static ones. Apart from studies of dynamic precipitation in steels and particularly in high-strength lowalloy (HSLA)-steels,[4] very little work has been done on E. CERRI, Assistant Professor, and E. EVANGELISTA, Professor, are with the Department of Mechanics, Ancona University, 60131 Ancona, Italy. N. RYUM, Professor, is with the Department of Metallurgy, The Norwegian Institute of Technology, N-7034 Trondheim, Norway. Manuscript submitted September 12, 1995. 291~-VOLUME 27A, OCTOBER 1996
other alloy systems. Quite recently, Blaz et al.[51 have investigated dynamic precipitation in an Cu-Si alloy and Evangelista et aL t61 have made similar studies in some A1 alloys. It thus appeared that a more systematic approach to the general problem of dynamic aging was needed. When precipitation takes place during plastic straining, the straining will affect all three stages of the precipitation process: nucleation, growth, and coarsening. In order to characterize the dynamic precipitation process in general, it seems reasonable to try to investigate each of these reactions separately. The stages of the precipitation process that seem to be simplest to investigate are the growth and coarsening stages, because the straining will affect the reactions mainly through the effect on the magnitude of the diffusion constants. The present investigation reports the results of an investigation intending to stud
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