Correlated nucleation of precipitates in magnesium alloy WE54

Magnesium alloy WE54 has been used commercially for fabricating components for aerospace and aircraft applications. Its useful mechanical properties are achieved via a precipitation process, β₁ and β′ are the two strengthening precipitate phases when this alloy is peak-aged at 250°C. The β₁ precipitate plates often form with β′ particles attached to their end facets. The aim of this study is to understand the correlated nucleation of the β₁ and β′ phases. The equilibrium shapes of coherent β₁ and β′ precipitates are simulated using the phase field method. The preferential nucleation site of a nucleating precipitate at the interface of pre-existing particle/Mg matrix is analyzed through the calculation of the elastic interaction energy between the nucleating precipitate and the pre-existing particle. The evolution sequence of the β₁ and β′ phases are also simulated in this work. Our results indicate that the coherency elastic strain energy can be reduced significantly if a β′ particle forms at an edge facet of a pre-existing β₁ plate, and that the preferred nucleation sites of the β′ particles are at the two end-facets of the β₁ plate. Under the assumption that the β₁ phase nucleates first, the microstructure evolution sequence is proposed in this work.