Fracture of magnesium matrix nanocomposites - A review

As one of the lightest metallic materials, magnesium is considered as a promising replacement for aluminum, titanium, and steel. However, lightweight energy-saving magnesium and magnesium alloys suffer from poor ductility (5%–8%) due to their hexagonal closed packed crystalline structure and a limited number of easily activated independent slip systems at room temperature. This creates some challenges for magnesium and its alloys for various industrial applications. One technique to improve both strength and ductility, without any tangible weight penalty, of magnesium and its alloys is to add nanosize (less than 100 nm) reinforcements (e.g. nanoparticles) to the matrix as creat so-called magnesium nanocomposites. Various nano-reinforcing materials including oxides (Al2O3, TiO2, Y2O3, ZnO, ZrO2), carbides (SiC, B4C, TiC), nitrides (BN, AlN, TiN), borides (TiB2, SiB6, ZrB2), and carbonaceous materials (carbon nanotube and carbon nanoplatelets). The volume fraction of these nano-size reinforcing structures are normally less than 3 vol% to avoid the clustering (agglomeration) of the nano-reinforcements which can adversely affect strength and ductility. That is, depending on the matrix (magnesium and magnesium alloys) and the nanosize reinforcement, there is an optimum volume fraction beyond which the properties (strength and ductility) would decrease. Though various aspects of the microstructure/mechanical property/manufacturing process of the magnesium nanocomposites have been studied so far, the failure and, in particular, fracture of these materials have not been yet classified properly. Investigating the fracture of magnesium nanocomposites, including the type of fracture, fracture mechanism, the effect of the nano-reinforcements on the fracture of Mg nanocomposites, enables the researchers to assess the role of nanosized reinforcement on the modes of failure and mechanical properties. This paper aims at providing a review of the fractography of compressive and tensile tests of nanocomposites Mg, classifying fractography by all kinds of reinforcement and working conditions of the Mg nanocomposite. Also, the effect of test temperature, cyclic loading, and heat treatment on the fracture of magnesium -matrix nanocomposite is discussed thoroughly. Among various nanoparticles and according to the review of the available literature, Al, Y2O3, and NiTi can significantly improve the tensile strength of the Mg matrix while ZnO and Sm2O3 reinforced nanocomposites show higher compressive yield stress.