THE SHORT-RANGE ORDER STRUCTURE OF A WATER-QUENCHED NI-12.5 AT PCT SI ALLOY - A SYNCHROTRON X-RAY DIFFUSE-SCATTERING STUDY

A wide-angle X-ray diffuse scattering measurement of a water-quenched Ni-12.5 at. pct Si alloy single crystal was performed using synchrotron radiation. The Cowley-Warren short-range order (SRO) parameters and static displacement terms between atom pairs were obtained. The first seven SRO parameters were fitted in an 8000-atom model, from which it was deduced that no ordered gamma' domains existed in the water-quenched structure, but an enhancement of the C16 and C17 nearest-neighbor configurations did occur as compared to a completely random model. The C16 configuration is the basic structural unit of the L1(2) ordered structure, and the C17 may be thought of as a faulted variant thereof. These results were confirmed by performing equivalent SRO modeling in a 140,608-atom model. Silicon-silicon nearest neighbors show a repulsive strain of approximately 12 pct, while Ni-Ni pairs contract by less than 1 pct.