The discontinuous precipitation reaction is a solid-state reaction taking place at grain boundaries. This reaction, which relates one mother phase and two daughter phases, proceeds by two different mechanisms at the same time: (i) a decomposition of the supersaturated mother phase α0 and (ii) grain-boundary (GB) migration. The reaction front consists of the GB separating the depleted mother phase α and the supersaturated matrix α0, as well as the interface between the latter and the solute-rich β daughter phase. These two phenomena are controlled mainly by surface or interface diffusion  . A previous 2D investigation of the discontinuous precipitation, with the phase field method [2, 3], has shown the effect of both bulk and surface diffusivities on the initiation and on the stability of growth for this reaction. In particular, when bulk diffusion is the rate limiting process, the β precipitate is observed to have the behavior of a finger or a crystal growing in a channel. It has also been shown  that bulk diffusion plays a crucial role on the initiation of the reaction. Here, we investigate three-dimensional (3D) effects on the initiation of the reaction as well as the behavior of the precipitate as a finger in a 3D channel. A critical size of the β nucleus is observed, below which dissolution takes place instead of growth. Above this size, the β nucleus is observed to develop or to transform into a lamella « the β precipitate » and to grow in a steady-state manner later on. The effect of the simulation’s box width in the z direction is examined as well.
 I. Manna, S. K. Pabi, and W. Gust, Int. Mat. Rev., Vol. 46, p. 53 (2001)
 L. Amirouche and M. Plapp, Acta Mater. 57, 237 (2009)
 L. Amirouche and M. Plapp, Solid State Phenomena Vols.172-174, 549, (2011).