The local atomic order of a Ni-12.7 at.% Al alloy quenched from 1323 K

High-angle X-ray diffuse scattering has been measured in a volume element in reciprocal space for a Ni-12.7 at.% Al alloy single crystal quenched from 1323 K. The Cowley-Warren order parameters were determined after separation of the first- and second-order size effects, using a procedure based on the Borie-Sparks quadratic approximation of atomic displacements. The experimental order coefficients were used to reconstruct the real-space locally ordered distribution of atoms, from which the spectrum of nearest-neighbor atomic configurations was determined. Some 61% of the Al atoms were found to be in a planar ordered configuration consisting of a nickel atom and four nearest-neighbor aluminum atoms, the basic structural unit of Ni₃Al. These planar elementary ordered units were, of course, distributed on the three equivalent {100} planes. While most of the ordered 'domains' consisted of from one to three Ni₃Al rings, a few domains were found to contain on the order of a hundred such elementary ordered units. The larger ordered domains may be thought of as embryos of the ordered Ni₃Al phase which have formed during quenching of the bulk alloy single crystal. There is evidence that the spatial distribution of ordered domains is not random.