X-Ray Study of the Local Atomic Arrangement in Partially Ordered Cu3Au

Abstract

The atomic arrangement present in Cu3Au in a state of equilibrium partial long-range order has been investigated by measuring with x-ray techniques the short-range order existing below the critical temperature. A single crystal of Cu3Au, cut parallel to a (100) face was quenched into water from 380°C, just below the critical temperature for ordering. Using monochromated Cu Kα radiation and a Geiger counter spectrometer, the diffuse scattering from this crystal was measured in the (h1h20) plane of reciprical space. From these data, short-range order parameters and a first-neighbor size effect coefficient were obtained. The measurements show that there is a marked short-range order among the wrongly occupied sites below the critical temperature. It is shown that the measured short-range order parameters can be predicted by assuming that the wrongly occupied sites coalesce into tiny antiphase domains, and that these domains are one atomic plane in thickness, lie parallel to any one of the three sets of (100) planes in the crystal, and contain, on the average, about nine atoms each.