Conclusions
In the present study, the ordering processes in fcc-based Ni-Mo and Ni-V alloys were investigated by TEM observation and the kinetic Monte Carlo simulation to clarify the atomistic ordering mechanism in Ni-based 1 1/2 0 alloys. The following conclusions were drawn. 1.
It was experimentally confirmed that the ordering processes are described as 1 1/2 0 type SRO → D1a and/or Pt2Mo type LRO for the Ni-Mo alloy, and 1 1/2 0 type SRO → D022 and/or Pt2Mo type LRO for the Ni-V alloy. The D022 type ordering occurs before the development of Pt2Mo type LRO even in the stoichiometric Ni2V alloy.
2.
The SRO structure in Ni-based 1 1/2 0 alloy is commonly described as a mixed state of D1a, D022 and Pt2Mo type microclusters. The formation of the mixed microclusters is due to the similar short-rage atomic arrangements of D1a, D022 and Pt2Mo structures. The fractions of D1a, D022 and Pt2Mo type microciusters change with the alloy system and composition.
3.
The SRO-LRO transition proceeds by the selected growth of D1a, D022 and Pt2Mo type microclusters into LRO domains, depending on the alloy system and composition. The selection of the LRO structures is largely influenced by the effective pairwise atomic interactions in larger distances than the fourth coordination distance. The dependence of the effective pairwise interactions on alloying elements in Ni-based 1 1/2 0 alloys can be rationalized in terms of the concentration of free electrons in the alloys.