Effect Of Rare Earth Yttrium On Microstructure And Mechanical Properties Of Extruded 7075 Aluminum Alloy

In this paper,aluminum alloy resources and rare earth resources,which are two characteristic resources in China,are taken as the research objects.7075 Y alloy is prepared by adding 0.2wt.% Y into 7075 aluminum alloy.The microstructure of 7075 Y alloy in extrusion state,solid solution state and aging state were studied by OM(optical microscope),SEM(scanning electron microscope),EBSD(electron backscatter diffraction)and TEM(transmission electron microscope).The mechanical properties of 7075 Y alloy were tested by microhardness tester and universal tensile machine,and the action mechanism of Y in alloy processing was discussed.The main results are as follows:1.The effect of Y on grain refinement is not obvious.The addition of rare earth Y will form eutectic products with impurity elements Fe,Si and alloy elements Mn,Zn,Cr,Ti in 7075 aluminum alloy,which can purify the matrix.In the extrusion process,these large size eutectic phases will be broken into small size,further improving the properties.At the same time,more complex precipitates are precipitated and dispersed in the matrix during extrusion and solution aging,which play the role of aging strengthening.The comprehensive mechanical properties of 7075 Y aluminum alloy after backward hot extrusion can be effectively controlled by changing extrusion and heat treatment process.2.In the solution treatment stage,the grains in the alloy will grow up under heat,but at this time,due to the distribution of the second phase containing Y at the grain boundary,it can inhibit the growth of the grain boundary.At the same time,it can promote the nucleation as the heterogeneous nucleation point,improve the nucleation rate,obviously refine the grains and reduce the grain size.Most of the alloy elements in 7075 aluminum alloy will dissolve into the matrix in this process,which will reduce the strength of the alloy matrix.However,due to the aggregation of some hard and brittle insoluble and insoluble phases,the hardness and strength of the alloy will be further improved.During plastic deformation,these second phases form crack initiation sources and microcracks,resulting in the decrease of plasticity of the alloy.These reasons make the hardness and strength of the extruded 7075 aluminum alloy increase to 113.3HV and 430.29 MPa after solution treatment,and the elongation decrease to 13.37%.The addition of Y will make the alloy form more Fe rich and Mn rich phases with larger size before solution treatment,and Cu element will concentrate to Fe rich phase.The results show that the Y-containing particles will segregate at the grain boundary,promote the nucleation of the second phase at the grain boundary,and distribute in the grain boundary as reticular or discontinuous particles,so as to enhance the brittleness,reduce the strength and plasticity,and reduce the hardness and strength to 96.7HV and 400.91 MPa respectively.However,due to the partial dissolution of part of the second phase and the uniform distribution of solute atoms in the matrix,the softening effect of the alloy plays a major role,the plasticity of the alloy will not decrease obviously,which is 14.89%.3.During single-step aging treatment,a large number of GP zones and a small amount of small size ’phase will be formed in the grains of the alloy,which will be used as heterogeneous nucleation points to promote the nucleation during the recrystallization process of the alloy structure and refine the grains.However,due to the effect of Y,7075 Y aluminum alloy will be further refined in this process.Under single-stage aging,the stress corrosion resistance of 7075 aluminum alloy decreases due to the chain like and continuous distribution of grain boundary precipitates.At the same time,the hardness,strength and plasticity of 7075 aluminum alloy decrease due to the accumulation of Cu element into Fe rich phase and the macro thickening of Fe rich phase.In 7075 Y aluminum alloy,due to the adsorption of Y,the precipitation rate of Fe,Mn,Cr,Cu and other elements will be faster,and the effect of aggregation and growth will be more obvious.Dislocations will accumulate on the boundary between particles and matrix,resulting in stress concentration.Large size of the second phase will reduce the mechanical properties of the alloy.At the same time,the decrease of Cu content in the matrix will increase the tendency of intergranular cracking and further reduce the plasticity of the alloy.4.After double-step aging treatment,the toughness of the two alloys is the highest,while the hardness and strength are the lowest;after three-step aging treatment,the two alloys have higher strength and better plasticity.5.In the process of extrusion and aging treatment,Y affects the formation of recrystallization texture in the alloy by influencing the precipitation rate of Fe and the formation of Y-containing Fe rich phase.