Effects of the Fluctuation of Solute Concentration in Al-Zn Alloys on the Aging at Low Temperatures; II. Electron Microscopy and Measurements of SAXS Intensity

The effects of the fluctuation of solute concentration on the process of aging in Al-10wt % Zn alloy were studied by measurements of the intensity of small-angle X-ray scattering and by electron microscopy. Specimen was quenched from a higher temperature(T(Ql), mainly 300℃) to an intermediate temperature (T(Q2), 110~230℃), held there for a time, quenched again into iced water and aged isothermally at 0℃ or 40℃. Results obtained are as follows: (1) When the specimen was held at T(Q2) for a sufficient time and aged at relatively high temperatue(e.g. Ihr at 125℃ and aged at 40℃), a two-step increasing of both resistivity and integrated intensity was found. (2) Each of the steps has characteristics of the formation of G.P. zones by the spinodal decomposition. (3) When the maximum resistivity(p(max)) takes a minimum, Size distribution of the G.P. zones, estimated from the difference between Guinier and Porod radii, is much broader than that at p(max) in the usual direct quenching. (4) When the holding time at T(Q2) is long enough, the size distribution at p(max) is as broad as that at p(max) in the usual case. (5) After a prolonged aging under the condition that P(max) takes minimum, G.P. zones grow markedly and become oblate ellipsoidal with a major axis of about 20nm. (6) These results are consistently understood in terms of the fluctuation of solute concentration at T(Q2) and the change in the concentration of vacancies during annealing at T(Q2). That is, it is considered that these results are due to the difference in aging behavior between the regions of relatively high concentration and the ones of low concentration induced by the fluctuation.