Effect of beta-Mg17Al12 phase on microstructure, texture and mechanical properties of AZ91 alloy processed by asymmetric hot rolling

In this research, the microstructure, texture, and mechanical properties of AZ91 magnesium alloy during asymmetric hot rolling were investigated. Optical microscopy (OM), scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), hardness, and tensile tests were used. The results showed that the coarse and continuous beta-Mg17Al12 phase strongly affected on twinning and basal texture. The beta phase suppressed the nucleation of extension twins. There was a strong [0001]parallel to RD texture with the intensity of 5.1 (sic) R in the as-cast AZ91 sample. With increasing the rolling reduction, the overall texture intensity decreased. This was due to the presence of both continuous beta phase along the grain boundaries and initial [0001]parallel to RD texture in the as-cast AZ91 alloy. In addition, there was a texture transition from [0001]parallel to RD to [10 (1) over bar0]parallel to RD and [11 (2) over bar0]parallel to RD during 15% thickness reduction. With increasing the strain, the average grain size increased owing to the dissolution of the beta phase in the matrix during the pre-heat treatment before rolling and also the occurrence of no recrystallization during hot rolling. It was found that the as-cast and 15% hot-rolled samples have the highest homogeneity and the 8% hot-rolled sample has the maximum inhomogeneity factor (20.87). The hardness and tensile tests indicated that with increasing the deformation, owing to the strengthening of {0002} basal texture and increasing the fraction of twins, the strength increased and the ductility decreased. Due to the weak bond between the beta phase and matrix, the micro-crack started from the alpha/beta interfaces and extended. The fracture mode of all samples was the mixture of brittle and ductile. However, with increasing the rolling reduction, the fraction of brittle fracture increased due to the accumulation of twins during deformation.