Investigation Of Grain Refinement And Property Strengthening In Nanostructured Al-4% Cu Alloy

Large plastic deformation can refine grains and produce nanostructured metals with significantly higher strength than coarse-grained materials.However,when the strain reaches a certain level,the high deformation energy stored in the nanostructure will lead to dynamic recovery,which will retard the structural refinement induced by deformation until it reaches saturation,the grains will no longer refined with continuous deformation.Previous studies show that the structure of materials produced by large deformation is lamellar nanostructure,and its dynamic recovery is dominated by the migration of Yjunction.Therefore,inhibiting the migration of Y-junction during large plastic deformation is of great significance for the grain refinement and strength improvement.In this study,Al-4% Cu alloy melted by adding 4 wt.% Cu element to ultra-high purity aluminum(99.999%)was used as experimental material.The original material is supersaturated solid solution,and large strain rolling was used as plastic deformation method.The precipitation and segregation of alloy elements are regulated by introducing appropriate intermediate annealing in the rolling process,so as to realize the further refinement and strengthening of lamellar nanostructured Al-4%Cu alloy.The direct rolled samples and the rolled samples with different intermediate annealing processes were characterized by using the mechanical property test methods such as microhardness test and tensile test,and the characterization techniques such as scanning electron microscope and transmission electron microscope.The effects of intermediate annealing process on the microstructure refinement and mechanical properties optimization of lamellar nanostructured Al-4%Cu alloy were compared.The conclusions are as follows:(1)The lamellar structure of Al-4% Cu alloy gradually forms with the increase of strain.The average boundary spacing of lamellae decreases with the strength increases,but the rate of them gradually decreases without intermediate annealing.When the reduction is 98.6%,the average boundary spacing of the Al-4%Cu alloy is 80 nm,the yield strength is 417 MPa,the tensile strength is 442 MPa and the hardness is 137 HV.At this time,the effect of deformation on the microstructure refinement and strength improvement of the material tends to be saturated.(2)Different intermediate annealing parameters were explored,that is,annealing at different temperatures(75 ～150 °C)for 30 minutes under different reductions(75% and94%),and then rolling.The results show that under the same conditions of the final total reduction(98% and 98.6%),the optimal microstructure refinement and material strengthening are obtained under the conditions of intermediate annealing at 100 °C for30 min with 75% reduction.Finally,for the sample rolled to 98.6% reduction,the lamellar boundary spacing is refined to 50 nm,and the yield strength reaches 497 MPa.The introduction of intermediate annealing process enhances the refinement effect of large strain rolling on lamellar structure and makes it play a better strengthening effect.(3)The microstructure analysis shows that when the sample is rolled to 75%reduction,the lamellar structure has formed initially,and the deformation leads to the segregation of Cu elements on the lamellar boundary.After annealing at 100 °C for 30 min,the Cu element is segregated and precipitated on the lamellar boundary.These second phase particles precipitated on the lamellar boundary play a good role in hindering the migration of Y-junction in the subsequent deformation process and inhibiting the dynamic recovery.This discovery makes it possible to further refine and strengthen the material by regulating the large plastic deformation and heat treatment process.