Effect Of High Pressure Torsion On Microstructure And Stress Corrosion Resistance Of Al-Zn-Mg-Cu Alloy

Super-strength aluminum alloys such as Al-Zn-Mg-Cu are widely used in aerospace and other fields due to their low density,high strength and excellent fatigue resistance.They are considered to be the most likely non-ferrous metal materials to replace steel and iron.However,due to the high degree of alloying,the stress corrosion resistance of the alloy is poor,and it is difficult to improve the mechanical and stress corrosion resistance of the alloy at the same time by heat treatment process.In this paper,the effects of high pressure torsion test under different deformation parameters on the mechanical and stress corrosion resistance of Al-Zn-Mg-Cu alloy were studied,and its mechanism was analyzed.The microstructural evolution,mechanical properties and stress corrosion resistance of Al-Zn-Mg-Cu alloy under different deformation conditions were studied by OM,TEM,EBSD,electrochemical corrosion,normal temperature tension and slow strain rate tension.The test results show that the high pressure torsion has remarkable refining ability for the second phase and grain of the alloy.The microstructures of 0.5 turn sample are non-uniform lamellae distribution,i.e.the coarse-grained lamellae and the fine grain lamellae overlap each other,and the coarse grain lamellae and the fine grain lamellae have obvious differences in grain orientation and local orientation.In addition,with the decrease of deformation temperature,the grain size of 0.5 turn sample decreases and the inhomogeneous lamellar structure characteristics gradually disappear.The results of mechanical properties test show that the tensile strength and elongation of the initial sample are 426Mpa and 4.6%.The strength and plasticity of the deformed alloy are significantly improved due to the remarkable grain refinement and second phase refinement under high pressure torsion.At the same time,the tensile strength and elongation of 0.5 turn sample increased to 781 Mpa and 8.5%respectively,which were higher than 701 Mpa and 7.8%of 2 turns sample due to the influence of heterogeneous lamellae structure.The results of electrochemical corrosion and immersion corrosion show that the polarization resistance of as-cast alloy is 93.6 KΩcm~2,the corrosion current density is0.21 m A/cm~2,and there is obvious re-passivation phenomenon in the polarization curve.The corrosion type is obvious intergranular corrosion.After 0.5 cycles of HPT deformation,the polarization resistance of the sample decreases slightly by 80.9 KΩcm~2and pitting occurs in some areas.The corrosion current density decreases to 0.068m A/cm~2,and the corrosion type becomes weak intergranular corrosion and pitting corrosion.When the number of torsion cycles increases to 2,the corrosion current density does not change much,but the polarization resistance decreases to 34.6 KΩcm~2,and the corrosion type changes into pitting corrosion.In addition,with the decrease of deformation temperature,the polarization resistance of 0.5 turn sample decreases gradually and the corrosion degree tends to increase.The results of slow strain rate tensile test show that the stress corrosion susceptibility of as-cast alloys decreases significantly after HPT deformation.After 0.5cycles of deformation,the stress corrosion susceptibility coefficient of the specimen decreases from 0.46 to 0.30 of the original sample.When the number of torsion cycles is increased to 2 cycles,the stress corrosion sensitivity coefficient of the specimens increases slightly due to the increase of hydrogen induced cracking and other phenomena.