Improvement on dynamic fracture properties of magnesium alloy AZ31B through equal channel angular pressing

Abstract

In this work, Mode I dynamic fracture experiments are conducted on pre-cracked three point bending specimens by using modified Split-Hopkinson pressure bar. The microstructure of magnesium alloy AZ31B is modified through equal channel angular pressing (ECAP). The two sets of specimens with different initial textures are considered here: one set of the specimens are machined from a hot rolled AZ31B Mg alloy plate with a bigger grain size; the other set are machined from the billet with material treated by four pass of ECAP after they are cut from the hot rolled plate and they are with the finer grain size. Critical crack tip opening displacement (CTOD) is invocated to determine the dynamic fracture property. Digital image correlation (DIC) technique is used to determine the strain contours around the crack tip and electron back scatter diffraction (EBSD) is employed to analyze the texture evolution after tests. It is found that the dynamic fracture property of fine grain specimen is better than that of coarse grain specimen. The fracture property of both sets of specimens is enhanced by increasing the loading rates. Texture analysis shows the formation of tensile twinning in the ligament ahead of the crack tip in the coarse grain specimen but no sign in fine grain specimen. The brittle features e. g. cleavage planes and twinning lamellas are observed on the fracture surface of coarse grain specimen by scanning electron microscope (SEM). However, the relative ductile features such as micro-voids surrounding by tear ridges present on the fracture surface of fine grain specimen. The improvement on dynamic fracture properties of magnesium alloy through ECAP has important significance in the aerospace and automobile industry.