The spatial distribution of nucleation sites and its effect on recrystallization kinetics in commercial aluminum alloys
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10/30/03
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The Spatial Distribution of Nucleation Sites and its Effect on Recrystallization Kinetics in Commercial Aluminum Alloys K. MARTHINSEN, O. DAALAND, T. FURU, and E. NES Particle distributions play a major role in the processing response of aluminum alloys. While large constituent particles play an important role in the nucleation of recrystallization, small particles may heavily restrict the growth of recrystallized grains. In the present investigation, a two-dimensional (2-D) tesselation procedure has been used to characterize the particle distribution in commercial aluminum alloys and its relevance to nucleation of recrystallization. This procedure enabled the quantification of the degree of particle clustering in samples rolled to different strain levels. A characteristic aspect seems to be a transition from a rather nonuniform spatial distribution at low rolling strains, toward a more or less random distribution at high strains. Nucleation kinetics has been found to be site saturated, indicating that all nucleation events effectively occur at the start of recrystallization. A simple model is proposed, which explains the development of the spatial particle distribution as a function of rolling strain.
I. INTRODUCTION
FOR metals that do not undergo any allotropic transformation and that are not processed by rapid solidification, recrystallization after deformation is the only method of achieving grain sizes of 25 m or less. The fact that small grains enhance a variety of material properties such as bendability, formability, superplastic forming, and strength implies that it is of considerable importance to establish appropriate mechanical-working and heat-treating procedures to control grain size and obtain a fully recrystallized material. Nonheat-treatable sheet aluminum alloys, which are used in large tonnages for a range of applications, are examples of such materials. Recrystallization of a cold-worked metal is the thermally activated process of nucleation and growth of new strainfree grains, and their gradual consumption of the deformed matrix. The reaction is driven by the stored energy in the cold-worked matrix, and # the reaction kinetics is a function of the nucleation rate N and the growth rate G. The transformation kinetics is in most cases conveniently and accurately described by the following simple relation between the volume fraction of transformed material XV and the annealing time t: XV 1 exp (kt n)
[1]
This is the so-called Johnson–Mehl–Avrami–Kolmogorov (JMAK) equation,[1–4] and n is usually referred as the JMAK exponent. In the special case of constant growth rate and a random spatial distribution of nucleation sites, Eq. [1] is exact with n equal to 4 when also the nucleation rate is constant (Johnson–Mehl nucleation kinetics) and n equal to 3 when the nucleation rate decreases so rapidly that all nucle-
K. MARTHINSEN and E. NES, Professors, are with the Department of Materials Technology, Norwegian University of Science and Technology, N-7491 Trondheim
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