SAXS from the GP Zones in Al-4mass % Cu Alloy at Initial Stages of Ageing

A series of small-angle X-ray scattering photographs with monochromated and point focussed copper Ka(1) radiation was taken of a single crystal of an Al-4mass % Cu alloy at the as-quenched state from 823K and at successive stages of ageing at 313K. The crystal was so oriented that the incident direction of the radiation was along [001] axis, and was cooled with liquid nitrogen during the exposure. The pattern of the as-quenched state was homogeneous, but each of the patterns of the aged states consisted of streaks along [100] and [010] directions and a quasi-ring around the trace of the incident beam on the film (even at the shortest ageing time. l6sec). The most intense regions were found in [100], [010] , [100] and [010] directions on the ring. With the progress of the ageing the streaks and the ring became more intense and narrower. The results can be interpreted from points of view of the formation of the GP zones parallel to {100} planes ab initio, the effect of the inter-particle interference on scattering of X-rays, and the growth of the GP zones with the ageing. Guinier radius of the GP zones at each stage can be obtained from intensity change along [110], and the values are reasonably consistent with those obtained from the cross section of the streak. The presence of multi-layer zones, besides the one-layer ones, is suggested from the humps found on several intensity curves along [100] at later stages.