Influence Of RE Mischmetal On The Microstructures And Mechanical Properties Of AZ61 Magnesium Alloy

Magnesium alloys containing rare-earth (RE) are among the best light weight structural materials with a good capability, significant latent plastic deformability high strength-to-weight ratio and excellent mechanical properties. The microstructure and mechanical properties of AZ61 magnesium alloys through semi-continuous casting, homogenization heat treatment and hot-extrusion with different RE contents were investigated.The microstructure of AZ61 magnesium alloys with different RE contents through semi-continuous casting were analyzed By means of optical microscopy(OM), X-Ray Diffraction(XRD), Scanning Electron Microscope(SEM), the effect of RE on microstructure of casting ingots were systematically studied. Tensile strengthσb, yield strengthσ0.2 and elongation of above alloys with different chemical constitution and working were determined. Microstructure evolution of AZ61 Magnesium alloys with different RE contents via homogenization heat treatment and hot extruding were analyzed, strengthening mechanism, the effect of treatment and hot extrusion on mechanical properties of AZ61 Magnesium alloys with different RE contents, fractography of the alloys subjected to homogenization and hot extruding, was evaluated.The research results showed that with the addition of rare-earth in the AZ61 magnesium alloy,the microstructure of the semi-continuous casting alloys could be refined, and mechanical properties was increased at room temperature and 150℃. The morphology and distribution of theβ-phase were gradually changed from irregularly network to semi-continuous reticular and to spot distribution. Acicular-shape RE rich phase dispersed on grain boundary and rent the basal body of the alloy. The addition of RE strengthens the alloy by grain refinement and precipitation. However, it has no obvious effect on the plastic property. The alloy with 2% Re addition presents the best over-all tensile properties.The defects, such as cavity, shrinkage porosity and void, etc, can be eliminated by hot extrusion. The microstructures of the extruded alloys possess the features of dynamic recrystallization, the grain size was significantly reduced, and the tensile properties were increased obviously as well, the max value of the tensile strength, the yield strength and the elongation are 301MPa, 226.67MPa, 22.39% respectively. With the increase of Re addition, both transverse microstructure and longitudinal were refined obviously.Hardness measurement shows that the alloys subjected to hot-extrusion have the highest value as compared to the semi-continuous casting and homogenization heat treatment. The hardness increases firstly, reaches its peak with 2% Re addition and, then, reduces in all three processing conditions, the highest values are HRE61.04,HRE61.15, HRE71.4 respectively.At room temperature, the fractures of the alloys in their casting condition demonstrate brittle failure in nature, while in their hot-extrusion condition, the alloys fail in mixed mode and the fractures were decorated with ample dimples, cleavage steps and tearing edges. The more addition of RE, the more and deeper of the dimples, and the more enhanced ductile rupture in nature.