2D and 3D characteristics of intermetallic particles and their role in fracture response of AZ91 magnesium alloy

Highlights: • 2D and 3D microstructures of intermetallic particles in as-cast Mg-Al-Zn have been characterized. • Correlation between microstructure-solidification process has been established. • Nano-sized γ₂-Al₈Mn₅ resulted in a bimodal grain size distribution of α-Mg matrix. • Damage of the as-cast alloy depends predominantly upon the Mg₁₇Al₁₂ precipitates. The presence of intermetallic constituent particles strongly influences the deformation and fracture characteristics of as-cast magnesium alloys. The present study investigates the two-dimensional (2D) and three-dimensional (3D) microstructure of these intermetallic particles in a direct chill as-cast AZ91 alloy and their effect on the tensile deformation of the alloy. Electron backscattered diffraction (EBSD) was employed to characterize the non-equilibrium eutectic β-Mg₁₇Al₁₂ phase, which further assisted in hypothesizing the solidification process of the alloy. Transmission Kikuchi diffraction (TKD) indicated the small-sized spherical Mg₁₇Al₁₂ precipitates, formed adjacent to the non-equilibrium eutectic precipitate, did not exhibit any orientation relationship reported in the literature. Further, micron-sized AlMn inclusions were observed to be single phase γ₂-Al₈Mn₅ particles exhibiting a cyclically twinned structure with $\left(20\overline{2}1\right)$ habit plane. Additionally, optical microscopy (OM) in the dark-field (DF) and differential interference contrast (DIC) mode, in assistance with high resolution – transmission electron microscopy (HR-TEM) confirmed that the nano-sized γ₂-Al₈Mn₅ particles resulted in a bimodal grain size distribution of the alloy. The three-dimensional (3D) spatial distribution of these intermetallic constituent particles along with fractography facilitated in understanding the damage process of the alloy in uniaxial tension.