The kinetics of the internal nitriding of Fe-2 at. pct Al alloy

The kinetics of the precipitation of aluminum nitride on internal nitriding the Fe-2 at. pct Al alloy was investigated for cold-rolled and recrystallized specimens exhibiting “ideally weak” interaction behavior of the solutes Al and N. The kinetic analysis was performed using mass-increase data obtained for thin foils (thickness ⪯0.1 mm) upon nitriding in a NH₃/H₂ gas mixture at temperatures in the range 803 to 853 K. Activation-energy analysis revealed that precipitation of AlN in the recrystallized specimens is associated with a Gibbs free energy barrier for the formation of a precipitate of critical size; the precipitation rate is controlled by both nucleation and growth. On the other hand, precipitation of AlN in the cold-rolled specimens occurs without a Gibbs free energy barrier for formation of a precipitate of critical size; the precipitation rate is controlled by growth with kinetics governed by volume diffusion of alu-minum. Analysis of the total Gibbs free energy of formation of AlN in the α-Fe matrix showed that in the case of the recrystallized specimens, the formation of incoherent AlN precipitates with a hexagonal crystal structure is favored. In the case of the cold-rolled specimens, containing a high dislocation density, the formation of coherent AlN precipitates with cubic crystal structure is favored, at least in the beginning of precipitation.