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

Abstract

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 202¯1 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.