Atomistic microstructures in short-range ordered alloys

Short-range order (SRO) in Ni-Mo alloys and their relatives has been controversial for decades, since it causes clearly diffraction intensity maxima at positions which do not coincide with the superlattice reflections in the long-range order (LRO) state. This paper gives an overview of recent studies on the structure of SRO and the transition from SRO to LRO in Ni-Mo alloys, including our results obtained in atomic level by combination of kinetic Monte Carlo simulation and semi-quantitative high-resolution transmission electron microscopy. It is rationalized in our results that the SRO state is set up by local ordering of A4B, A3B and A2B types in sub-unit cell scale. The dispersed mixture of the sub-unit cell clusters gives diffraction intensity maxima at the particular positions. An LRO state is formed by selected growth of the A4B, A3B and A2B type clusters into LRO domains depending on alloy-composition. (author)