Short range order and its transformation to long range order in Ni₄Mo

The short range ordered (SRO) state and the time-evolution of the long range ordered (LRO) structure of D1_a in a Ni₄Mo alloy were investigated by Monte Carlo simulation based on an f.c.c. Ising model and by transmission electron microscopy (TEM). The simulation using appropriate values for pairwise atomic interactions up to the fifth nearest neighbors bears diffuse intensity maxima at {1 1/2 0} positions in the Fourier power spectrum for the early stage of SRO, and then the maxima shift to {4/5 2/5 0} for the stable Dla structure as the long range ordering proceeds. The present results are in good agreement with the temporal change in electron diffraction due to the SRO-LRO transition in a quenched Ni₄Mo. The SRO state obtained in the simulation contains microclusters of subunit cells of D1_a, D0₂₂ and Pt₂Mo structures. If such a mixed state of microclusters is projected onto a two-dimensional {100} plane in a similar way to high resolution TEM, the mixed state exhibits locally a dot-pattern analogous to the projected N₂M₂-type (chalcopyrite-like) structure, which gives rise to superlattice reflections at {1 1/2 0}. This suggests that a statistically averaging view of the mixture of microclusters leads to the concept of static concentration waves of k = 1 1/2 0, which has been often employed to describe the SRO state. The formation of D0₂₂ and Pt₂Mo-type subunit cells in addition to stable D1_a is explained in terms of their structural relationship. The transformation from SRO to LRO is attributed to continuous growth of D1_a segments into ordered domains.