The size of a platelike G.P.zone in an Al-4%Cu alloy has been more successfully determined by the measurement of the X-ray diffuse scattering intensity distribution around (110) in the reciprocal lattice, than the small angle scattering photographic method reported previously by the present authors.
The formation, growth and reversion of G.P.zones have been investigated by this method together with the measurement of electrical resistivity.
G.P.(1) zones of the specimens quenched from 450°C grow with ageing time at the early stage of isothermal ageing at 40, 80 and 110°C, and the zone diameters reach constant values of approximately 58, 75 and 85 Å respectively. However, the zone radius of the specimen aged at 135°C stayed constant at a value of about 85 Å, but increased again 300 min after the subsequent ageing. This was probably due to the formation of G.P.(2) zones. The upper temperature limit for the formation of G.P.zones was determined to be approximately 200°C, and no G.P.zones were detected at temperatures above 205°C by use of X-ray analysis. The value of electrical resistivity of the reverted specimens was found to be always 5% lower than that of the as-quenched specimens. This should be explained in terms of the exsistence of solute clusters in the as-quenched specimens.