Dissolution mechanism and kinetics of β(Mg17 Al12 ) phases in AZ91 magnesium alloy

In this study, the phase transformations, crystallization kinetics and dissolution mechanism of beta phase (Mg17Al12) in magnesium alloy AZ91 were investigated by optical microscopy, X-ray diffraction, differential scanning calorimetry and differential dilatometry. The results indicate that this AZ91 alloy undergoes a phase transformation during aging, a discontinuous precipitation of the beta phase (Mg17Al12) at 150 degrees C at the grain boundaries and another continuous at 350 degrees C within the grains. The activation energy of the dissolution reaction of the beta phase (Mg17Al12) under non-isothermal conditions is 116.781 kJ/mol, while it is 129.7383 kJ/mol under isothermal conditions. The Avrami coefficient, n, relevant for the dissolution kinetics of the beta phase (Mg17Al12) is 1.152 and 1.211 in the non-isothermal and isothermal conditions respectively. The numerical coefficients m and Avrami n are 0.993 and 1.152.