Modeling the precipitation processes and strengthening mechanisms in a Mg-Al-(Zn) AZ91 alloy

A model is developed that couples a description of the continuous precipitation of β-Mg17Al12 laths at elevated temperatures to a summation of the different contributions to the room-temperature 0.2 pct proof strength in the commercial alloy AZ91 (Mg-9Al-1Zn (wt pct)). The central assumptions are the diffusion-controlled formation of shear resistant β-Mg17Al12 laths on the basal planes of the hcp matrix and basal slip as the dominant room-temperature deformation mode. The calculations show quantitative agreement with the evolution of both the reported precipitation kinetics and the strengthening response as a function of aging time at 200 °C. The magnitudes of the different contributions to the hardening are discussed and strategies for enhancing this response are suggested.