Kinetics of the coherent order-disorder transition in Al 3 Zr

Within a phase field approach which takes the strain-induced elasticity into account, the kinetics of the coherent order-disorder transition is investigated for the specific case of ${\mathrm{Al}}_{3}\mathrm{Zr}$ alloy. It is shown that a microstructure with cubic ${L1}_{2}$ precipitates appears as a transient state during the decomposition of a homogeneous disordered solid solution into a microstructure with tetragonal ${\mathrm{DO}}_{23}$ precipitates embedded into a disordered matrix. At low enough temperature, favored by a weak internal stress, only ${L1}_{2}$ precipitates grow in the transient microstructure preceding nucleation of the ${\mathrm{DO}}_{23}$ precipitates that occurs exclusively at the interface of the solid solution with the ${L1}_{2}$ precipitates. Analysis of microstructures at nanoscopic scale shows a characteristic rod shape for the ${\mathrm{DO}}_{23}$ precipitates due to the combination of their tetragonal symmetry and their large internal stress.