Notch Effects in Tensile Behavior of AM60 Magnesium Alloys

The deformation and failure behavior of an AM60 magnesium alloy was investigated using tensile test on circumferentially notched specimens with different notch radii. The strain and stress triaxiality corresponding to the failure point were evaluated using both analytical and finite element analyses. Combining with systematical observations of the fracture surfaces, it is concluded that deformation and failure of AM60 magnesium alloy are notch (constraint) sensitive. The failure mechanisms change from ductile tearing to quasi cleavage with the increase of constraint. Introduction Magnesium (Mg) alloys are attractive for applications in automobile, aerospace, communication and computer industry because of their very low density, high specific strength and good machineability and availability as compared to other structural materials. Mg has a low density of 1.74g/cm 3 , which is approximately 35% lighter than Al alloys and 65% lighter than Ti alloys. It also has good conductivity and high damping capacity. However, the disadvantages of magnesium are low elastic modulus and limited toughness due to few slip systems which are available in a hexagonal closepacked structure. Many investigations have shown that failure of metallic materials is highly dependent on the constraint condition ahead of a crack or notch. A high constraint (stress triaxiality) level can prompt brittle fracture and lead to a low fracture toughness. Normally, ductile tearing is controlled by nucleation, growth and coalescence of microvoids. It has been experimentally observed that the crack tip opening displacement (CTOD) or J-integral at the initiation of ductile fracture is higher for specimens with low constraint than those with high constraint [1-3]. Yan and Mai [4-6] indicated that ductile crack propagation and ductile-brittle transition are also strongly dependent on constraint level at the crack tip. Unfortunately, very few studies have been conducted to understand the effect of constraint on deformation and failure of magnesium alloys. In this work, the effect of notch (constraint) on deformation and failure of an AM60 magnesium alloy in tension was investigated. Experimental Procedure AM60 magnesium alloy (with 6% Al, 0.2% Zn and 0.21% Mn) was used in this work, which is one of the most popular magnesium alloys with great potential for applications in the automotive industry. The microstructure is shown in Fig. 1, which mainly consists of -Mg matrix and second phases (Mg17Al12). In order to vary the constraint condition, tests were carried out on circumferentially notched tensile bars with different notch radii, whose dimensions are shown in Fig. 2. Fig. 1 Microstructure of the Mg alloy. Fig. 2 Schematic of the circumferentially notched tensile specimen. The specimens were machined from the longitudinal direction in a cast ingot. The tests were carried out in an Instron testing machine at a crosshead speed of 1 mm/min. The axial displacement was recorded using an extensometer mounted on the specimen surface. The continuous changes in the notch profile were monitored using a digital camera connected with a video recorder. The fracture surfaces were examined using scanning electron microscopy (SEM). Finite Element Analysis Large deformation finite element analysis was carried out using MSC/MARC. Incremental plasticity theory was used for the material constitutive model. The yield function f is related to uniaxial tension by