Research On Microstructure And Mechanical Properties Of Al-Zn-Mg-Cu Aluminum Alloy Deformed By High Pressure Torsion

Al-Zn-Mg-Cu alloy is a new generation of high-quality structural material with low density,high strength,good toughness,good diffusion performances and thermal processing performances,excellent welding performances,which is considered to be the most alternative non-ferrous metal material to replace steel and iron.As a new type and high-quality structural material,ultra-high strength of Al-Zn-Mg-Cu alloy is widely used in the fields of automobile industry,ship engineering,aerospace and aviation industry,defense industry and nuclear industry.Especially in the aviation industry,the amount of Al-Zn-Mg-Cu alloy has become a significant sign to measure the advanced level of modern aviation industry.In this paper,the Al-Zn-Mg-Cu alloy was subjected to high pressure torsion experiment under different process parameters（such as deformation temperature,numbers of torsion and loading pressure）by using the limited-type high pressure torsion die according to the deformation characteristics of Al-Zn-Mg-Cu alloy.By applying a series of modern material characterization techniques（such as OM,EBSD and XRD）and related quantitative calculation theories,the microstructures,sub-structures（grain boundary characteristics and misorientation）,grain orientation,texture,microstrain,dislocation density and so on of deformed specimens were studied in-depthly and extensively.The results indicate that high pressure torsion has a great effect on the microstructure of Al-Zn-Mg-Cu alloy.The grain size is significantly reduced from 21.7μm to 0.72μm and the proportion of large angle grainboundaries increases obviously,so as the dislocation density.However,the texture features and the anisotropy of the material are weakened,which means the uniformity of the deformed material is enhanced.Al-Zn-Mg-Cu alloy has a variety of metal elements and it has a large amount of multicomponent alloy phases in microstructure.While the morphology,size and distribution of multicomponent alloy phases have great influence on the properties of the Al-Zn-Mg-Cu alloy,therefore,the multicomponent alloy phases have important research values.In this paper,by using SEM,EDS and XRD techniques,the phase composition,morphology,quantity and distribution of the multicomponent alloy phase were studied.The results indicate that the deformation temperature and the numbers of twists are the two of key factors during the re-dissolution process and have great influences on phase composition.During the process of high pressure torsion deformation,with the deformation temperature and the numbers of twists increasing,the AlCu phase disappears in the alloy and the diffraction peaks corresponding to η（MgZn₂）and θ（Al₂Cu）decrease as well.After the deformation,the alloy is mainly composed of α（Al）+ η（MgZn₂）+ θ（Al₂Cu）phase.In addition,the deformed specimens were carried out to tensile test at room temperature to analyze the influence of process parameters on the tensile strength and elongation of Al-Zn-Mg-Cu alloy.Besides,the scanning electron microscopy was used to scan the specimen fracture to study the fracture morphology,microstructure characteristics and fracture mechanism.The results show that the tensile strength of Al-Zn-Mg-Cu alloy after deformation is increased to 716 MPa,and the growth rate reaches up to 22%.Moreover,the ductility of the material has also been improved in a certain degree,the elongation increases to 14.8%.After high pressure torsion deformation,a large number of equiaxed dimples appears on the fracture surface of the sample.In addition,some of the second-phase particles are dispersed in the bottom of dimples.The fracture morphologies show that the fracture mechanism of the deformed Al-Zn-Mg-Cu alloy is mainly microporous fracture and coarse second phase cracking fracture.