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The microstructure transitions during leaching of a rapidly solidified Ni-Al alloy have been investigated by means of X-ray diffraction, transmission electron microscopic (TEM) and high resolution electron microscopic (HREM) techniques. Ni2Al3 was the main phase in the starting Ni-Al alloy. The microstructure of the Raney nickel catalyst consists of nano-scale nickel crystallites, residue of source phases surrounded by nano-scale boundary regions. The transformation during leaching of Ni2Al3 phase was an advancing interface type process. Clusters of AuCu-structure type face-centrered tetragonal Ni3Al2 as an intermediate phase seems to appear in the reaction front. Based on an analysis of the atomic configurations of phases Ni2Al3, Ni3Al2 and nickel, a reasonable explanation for the transition mechanism during leaching of Ni2Al3 phase and the arc characteristic of diffraction spots was proposed. The nickel crystallites generated during leaching obey an orientation relationship with the source Ni2Al3 phase, which is consistent with the Delannay's orientation relationship proposed for nickel and NiAl phases. The nano-scale structural characteristic of the Raney nickel catalyst, especially its porous structure at the boundary regions, provides an excellent hydrogenation catalytic activity and selectivity of the catalyst. (C) 2001 Kluwer Academic Publishers.