The structure of G.P. I zones in Al-1.7 at.% Cu

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The diffuse X-ray scattering has been measured from a single crystal of Al-1.7 at.% Cu, aged to contain G.P. I zones. A new method has been employed to separate the contributions from clustering from those due to atomic displacements. Computer simulations with 108,000 atoms were employed to examine the local atomic arrangements. The zones are a mixture of single (001) planes ≈ 100% copper and multilayer regions and with the nearest-neighbor spacing near that of the matrix. There is no significant copper concentration in the matrix. The Al (100) planes near the zone collapse toward it, the displacement decreasing more rapidly (with increasing distance from the zone) than in Gerold's model. A model recently derived from EXAFS data is shown to be inconsistent with the X-ray data.