A study on the phase transformation of γ2-Al8Mn5 to LT-Al11Mn4 during solutionizing in AZ91 alloy

Abstract γ 2 -Al8Mn5 intermetallic particles are generally present in commercial Mg-Al-Zn alloys. The present study is concerned with the effect of solutionizing on the morphology and phase stability of the γ 2 -Al8Mn5 particles in an as-cast AZ91 alloy. Analysis using energy dispersive spectroscopy (EDS) and electron back scattered diffraction (EBSD) in a scanning electron microscope (SEM), transmission electron microscopy (TEM) and atom probe tomography (APT) suggests that solutionizing resulted in uphill diffusion of Al from the α-Mg matrix resulting in the phase transformation of γ 2 -Al8Mn5 to Low Temperature (LT) -Al11Mn4 phase. This leads to a shell type structure of LT-Al11Mn4 encapsulating γ 2 -Al8Mn5 and the formation of cracks in the LT-Al11Mn4 phase. These can propagate during external loading, affecting the mechanical properties. Additionally, the content of Fe and Si decreased in the LT-Al11Mn4, which could potentially decrease locally the corrosion resistance.