Precipitation and strengthening in selected magnesium alloys

Recent results of the characterisation of strengthening precipitate phases in magnesium alloys WE54 and AZ91 using transmission electron microscopy, identification of the formation mechanisms of the strengthening phases, and modelling of quantitative effects of the size, orientation and distribution of such phases on strengthening are reviewed. The key strengthening precipitate phase appears to be an intermediate phase Mg₃(Nd,Y), which forms as plates parallel to {11̄00}_α, in alloy WE54, and the equilibrium phase Mg₁₇Al₁₂ that forms as (0001)_α laths, in AZ91. The formation of both precipitate phases generates significant shear strain energy, and a potential mechanism to accommodate the shear strain energy involves nucleation of such phases in association with clusters of larger solute atoms and vacancies. The model that emerges suggests that, to increase the number density of precipitates of Mg₃(Nd, Y) or Mg₁₇Al₁₂, the potential microalloying elements should have atomic sizes larger than Mg and have high binding energies with vacancies. Results of modelling indicate that, for a given volume fraction and number density of precipitates which are either shearable or shear-resistant, the yield strength increment produced by precipitate plates formed on prismatic planes of the matrix phase is significantly larger than that produced by precipitate plates formed on the basal plane of the matrix phase, and increases substantially with an increase in plate aspect ratio.