Koichi Yamaguchi1 and Seiichi Nishikawa2
1Graduate School, University of Tokyo, Tokyo. Present address: Central Research Laboratory, Kyoto Ceramic Co., Ltd., Kokubu, Kagoshima
The decomposition of super saturated solid solution of aluminum alloys is generally considered to occur very rapidly even at room temperature, and so the techniques of quenching and handling a specimen bring important effects on experimental results. In the present study, a cryostat for small angle X-ray scattering has been improved in order to set a quenched specimen into the apparatus within a few seconds under 200 K. The aging processes at the vicinity of 273 K in Al-2.4 at%Ag and Al-9.4 at%Zn alloys are investigated. At an early stage of aging an increase in the integrated intensity of diffuse scattering is observed. The relation of Porod's approximation can not be applied to this stage, and a linear relation between logarithms of intensity and aging time is obtained by Rundman's method. The above results show that a spinodal decomposition occurs at this stage. The spinodal temperatures of the two alloys are 413 K and 333 K, respectively. A transient stage is observed before the normal spinodal decomposition by Cahn's theory, at which the intensity of diffuse scattering increases over the whole range of scattering angle. The transient phenomenon is considered as the results of complex interactions among the concentration fluctuations before and after aging and the rapid decay of quenched-in vacancies. The initial stage of aging of these alloys is concluded to be subdivided into the three stages; the first is a transient stage, the second is a spinodal stage, and the third is a stage of reaction saturation.
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