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By using the same single crystal examined by Auvray et al. [ Acta metall. 29 , 1061 (1981)], three independent investigations on the structure of Guinier-Preston (G.P.) zones have been performed by means of field ion microscopy, high resolution transmission electron microscopy and small-angle X-ray scattering measurements. The results were essentially self-consistent in terms of shape, size and number density of G.P zones and agreed with the previous result of Auvray et al . The main result is that the majority of G.P. zones are single layer, but multilayer zones co-exist, with up to 10–20% of the zones being in this form. The structure of G.P. zones and the related interpretation of experimental results are discussed.

The spin-echo spectra at 77 K for the Co-1, 3, 5 and 8% Cu alloy solid solution consist of a sharp main line at 216.9 MHz, and one or two satellite lines which are centered at 199 MHz and 180 MHz respectively. The main line corresponds to the sharply defined NMR line in the pure f.c.c. Co metal, but its center frequency changes slightly with the Cu concentration. The two satellite lines correspond to the 59Co NMR associated with perturbed Co atoms which have one and two Cu impurities in their nearest neighbor sites, respectively; resonance frequencies of these satellite Unes are in agreement with those estimated from empirical equation. The integrated intensity ratios of the satellite lines to the main line were in disagreement with the values estimated previously reported relations I 1 I 0 = 12c (1 − c) and I 2 I 0 = 66c 2 (1 − c) 2 . These intensity ratios were fairly expressed with the following relations as I 1 I 0 = 2c (1 − c) and I 2 I 0 = 11c 2 (1 − c) 2 . The concentration dependence of the resonance frequency of the main line is also discussed.

The Co 59 nuclear magnetic resonance signals in precipitated Co-rich particles have been successfully observed by a transient (spin-echo) NMR method using heat-treated Cu-3.52 at. % Co alloy powders. The observed resonance spectra consist of a main intense sharp line and a week satellite line, and the resonance frequencies of both lines at 77°K are 217 ~ 219 MHz, depending on the heat treatment, and 198 MHz respectively. The resonance frequency of the main line indicated that the crystal structure of the precipitated particles is face centered cubic as has been expected. Careful measurements of the center frequency of the main line make it possible to discuss the internal pressure, acting on the precipitated particles, as a function of the effective particle size. The satellite line is attributed to the Co atoms located on the nearest neighbor sites of the Cu atoms dissolved in the precipitated particles. From the intensity ratio of the main line to the satellite line, the Cu concentration in precipitated particles has been estimated to be an extremely small value of 0.5 % and is independent of the aging time and the particle size. This is probably the first report of the NMR study of the precipitated particles and the pulsed NMR technique is found to be one of the most effective methods to get microscopic and detailed informations of the precipitation phenomena.

The X-ray small angle scattering study through the examination of the integrated intensity and the application of Porod's law (8) indicated the occurrence of the spinodal decomposition during the initial stages of isothermal aging of a quenched Al-6.8 at. % Zn alloy. Using the amplification factor obtained by the application of the analysis of Rundman and Hilliard (3) , the spinodal temperature of this alloy has been determined to be about 120–130°C.

The X-ray small angle scattering method, measurements of electrical resistance and transmission electron microscopy have revealed the behaviour of G.P. zones on the aging, reversion, and re-aging process in Al-6.7 at.% Zn alloys with or without trace elements of Mg and Ag. The critical zone radius associated with the electrical resistance maximum has been observed to be 9–10 A on the aging process at room temperature and 10–15 A on the re-aging process at 80°C after reversion. Addition of Mg atoms retards the rate of the formation of G.P. zones, while in the Ag containing alloys two kinds of zones, either including Ag atoms or not, were observed. G.P. zones in the Al-Zn alloys, which contain trace elements such as Mg or Ag or were pre-aged for longer times, are so stable that the time for completion of reversion takes longer. A change of the shape of the G.P. zones during these processes was observed to occur in the region of a larger zone radius.