Growth and coarsening of g. p. Zones in Al- Zn alloys
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I.
INTRODUCTION
I T is well known that spherical G.P. zones are formed during the precipitation process in A1-Zn alloys which have a relatively low solute concentration. This occurs at its early stages by a nucleation and growth process or by spinodal decomposition. The nature of this process has been extensively investigated by many authors, t~-81 However, the experimental information is still insufficient to understand the whole process of precipitation in this alloy system. The precipitation rate at the early stage is so fast that the precise change of the structure is rather difficult to be detected by conventional experimental techniques. Similarly, the theoretical viewpoint, that is, the mechanism of phase decomposition and precipitation in this alloy system has not yet been correctly solved, even though several attempts I~-4'71 have been made. Thus, it is important to re-examine the structural change during precipitation as precisely as possible and to investigate the scaling properties for comparison with the recently progressing theories. This will be attempted in the present paper. There are in general two aspects on scaling. One of them is the time scaling, from which a universal time scale may be deduced for evaluating the precipitation kinetics independently of composition and temperature as well as of the alloy system. The second one is the structural scaling, by which one can examine the mechanism of phase decomposition and precipitation. As demonstrated previously, 19j applying in situ synchrotron radiation for small-angle scattering experiments is a powerful tool to investigate the fast changes of a structure. In the present study, the aging behavior of some AI-Zn binary alloys has been investigated by using this technique. The change of structure during isothermal aging was examined with respect to the time evolution of representative structural parameters. At first, attention has been given to the time scaling. Subsequently, the precipitation process was divided into three stages. The structural and kinetic features of each stage will be discussed. KOZO OSAMURA, Professor, and HIROSHI OKUDA, Graduate Student, are with the Department of Metallurgy, Kyoto University, Sakyo-ku, Kyoto 606, Japan. YOSHIYUKI AMEMIYA is Research Associate, Photon Factory, National Laboratory for High Energy Physics of Japan. HIROO HASHIZUME is Associate Professor, Research Laboratory of Engineering Materials, Tokyo Institute of Technology. Manuscript submitted September 16, 1987.
METALLURGICALTRANSACTIONSA
II.
EXPERIMENTAL PROCEDURE
The samples investigated here were mainly three AI-X at. pct Zn alloys (X = 4.0, 6.7, and 10) in addition to some alloys with higher solute concentration. In situ smallangle scattering (SAS) measurements using synchrotron radiation were performed at BL-15A of the Photon Factory, National Laboratory for High-Energy Physics, KEK. The X-ray optics and data collection system are reported elsewhere.[9] A small furnace, kept at annealing temperature (To), was installed above two chilled copp
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