Small-angle neutron scattering by Guinier-Preston zones in Al-Mg alloys

The small-angle neutron scattering from G. P. zones in aged Al-Mg alloys has been measured by the difference in intensity between the scattering before and after an anneal at 100°C sufficiently long for dissolving the zones. No zones have been found by this method in Al-7% Mg alloys. The scattering by zones found in Al-11.5% Mg alloys has been analysed with the help of a radial pair-correlation function written in the form of a sum of Gaussian distribution functions. Best fitting is obtained with a rather smooth function. The mean size of the zones has been determined: mean diameter of about 70 Å for an alloy aged one year at room temperature, and 110 Å, for an alloy aged the first three months at 40°C and then nine months at room temperature. The corresponding mean distance between the zones is found to be equal to about 130 and 200 Å, respectively. The concentration in Mg of the zones has been found to be of the order of magnitude of 20 to 25%.