Short range order and the development of long range order in Ni4Mo

The short-range ordered structure present in quenched Ni₄Mo and the initial development of the long-range-ordered structure have been studied using single crystal X-ray diffuse scattering techniques. Measurements of the diffuse X-ray intensities were made in a volume in reciprocal space for the f.c.c. alloy resulting in a quantitative determination of the three dimensional local-order parameters. The local-order parameters were used for a computer simulation of the atomic configuration.The local atomic arrangements for the quenched alloy consist of a strong preference for no molybdenum atom pairs as first neighbors. The atoms tend to satisfy the long-range-ordered structure of Ni₄Mo in which there are no Mo-Mo pairs as first neighbors, only one Mo atom as second neighbors and six or more as third neighbors. A rodlike morphology consisting of a strong preference for 200 and 400 Mo-Mo pairs exists on ordering for 5 min at 650°C. This result is similar to the rodlike morphology present in dilute Ni-W alloys and explains the [100] streaking of the short-range order peaks observed in the diffraction studies.