The growth kinetics of the discontinuous precipitation reaction in Mg−Al alloys

Abstract

The extent, the growth rate and the interlamellar spacing of the discontinuous precipitation reaction in Mg−Al solid solutions with 5, 7, 9, and 11 at. pct Al are presented and analyzed by the theories of Turnbull, Cahn and Sundquist.K ₀λD ^B-values are computed with the aid of Turnbull's formula as well as Sundquist's solution of the diffusion problem. The activation energies confirm the assumption of grain boundary diffusion to be the rate controlling process. The thermodynamic of the reaction was treated on the base of the regular solution. The “maximum growth rate” criterion yields interlamellar spacings deviating clearly from the experimental values whether Turnbull's formula or Cahn's treatment was taken as a basis. The application of Sundquist's concept provides the boundary shape as a function of interlamellar spacing. The parameter ϑ', by which the boundary shape is determined, lies in the range -0.3 ⪯ ϑ' ⪯ 2 at the experimental spacing, which is the minimum true spacing. These ϑ'-values correspond to boundary shapes with no or moderate recesses. Observed boundary shapes are not in contrast to these results. The development of recesses by increasing interlamellar spacing is observed too and confirmed theoretically. Deep recesses guarantee the creation of new lamellae which reduce the enlarged spacings to such with more stable boundary shapes. This leads to the conclusion that the concept of unique interlamellar spacing must be abandoned in favor of a distribution of spacings according to the probability of nucleation of new lamellae.