| Magnesium alloys are known as green materials in the 21st century.As components and parts materials in the field of automobile and aerospace,magnesium alloys are of great significance for energy saving and emission reduction.However,the low strength and poor corrosion resistance of magnesium alloys hinder their wide application in various fields.In recent years,many scholars have done a lot of research on magnesium alloys,especially Mg-Al alloys,in order to improve their strength without sacrificing plasticity.Until now,this is still one of the hot spots in magnesium alloy research.Based on the composition of commercial AZ31 magnesium alloys,the as-cast and as-extruded magnesium alloys containing 0%,0.5%,1%and 2%VN,AZ31+0.5%,AZ31+1%VN and AZ31+2%VN were prepared.After the preparation of alloys,the microstructures of as-cast and as-extruded alloys were observed by optical microscope(OM)and scanning electron microscope(SEM).The phase detection and micro-zone composition analysis of as-cast and as-extruded alloys were carried out by XRD and EDS respectively.Tensile tests of as-cast and as-extruded alloys were carried out by universal electronic material testing machine.The conclusions are as follows:(1)The grain size of as-cast AZ31+wt.%VN(=0,0.5,1,2)alloy decreased first and then increased with VN content.With the addition of VN particles,the size of the coarse second phase along the grain boundary in as-cast alloys decreased significantly,and changed to the rod-like second phase and the segregated irregular particles uniformly distributed within and along the grain boundary.XRD and EDS results show that the coarse second phase is Mg17A112,and the rod-like second phase is Mg17Al12,Al-Mn,AIN and VN,and the segregated irregular particles are Al-V-N.Thermodynamic analysis showed that Al atoms reacted with VN in the VN containing magnesium so that AlN was in-suit formed in the melts.(2)After adding VN particles,the mechanical properties of as-cast alloys have been significantly improved.The as-cast alloys with VN content of 0.5%own the best comprehensive mechanical properties.However,the elongation decreased with the increase of VN content.(3)The crystal planar disregistries of alpha-Mg,AIN and VN are calculated by Bramffit formula.Results show that the minimum crystal planar disregistriy of AIN and alpha-Mg on the low index plane is 3.24%,and the minimum crystal planar disregistriy of VN and alpha-Mg on the low index plane is 6.85%.Experiments show that some AIN and VN with small grain size are uniformly distributed in grains.Therefore,both AIN and VN can be act as heterogeneous nucleation substrates of alpha-Mg during solidification.It is concluded that duplex heterogeneous nucleation is the main reason for grain refinement of as-cast magnesium alloys.(4)The grain size of extruded AZ31+wt.%VN(=0,0.5,1,2)alloy decreased first and then increased with the increase of VN.Microstructure analysis shows that the complete dynamic recrystallization of hot extruded AZ31 magnesium alloy occurs at high temperature.But after adding VN particles,some areas of incomplete dynamic recrystallization appear,so the addition of VN hinders the dynamic recrystallization of AZ31 magnesium alloy in hot extrusion to some extent.In addition,EDS surface scanning results show that some of the particles of AIN,VN and Al-V-N migrated during extrusion process,and flow-type distribution occurs along the extrusion direction in the central region of the alloy.(5)With the addition of VN particles,the yield strength and ultimate tensile strength of as-extruded alloys increased.The yield strength and ultimate tensile strength of as-extruded alloys with 0.5%VN increased without sacrificing plasticity.However,with the increase of VN content,the plasticity of as-extruded alloys decreased.Fracture analysis of extruded alloy in ED direction shows that the fracture mechanism of AZ31+wt.%VN(=0,0.5,1)is ductile fracture,and AZ31+2wt.%VN is a mixture of ductile fracture and brittle cleavage fracture. |
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