Preparation Of Bulk Nanolaminated Structure Aluminum Alloy And Study On Its Properties

Grain size is an important microstructure parameter of metal materials,which can significantly affect a variety of properties of materials.In recent years,the preparation of ultra-fine grain and nanocrystalline materials by severe plastic deformation has attracted the attention of more and more researchers.Lu et al used surface mechanical grinding treatment with high strain rate to prepare gradient nanolaminated structures in pure nickel and pure aluminum,respectively.The nanolaminated structures in pure nickel showed high strength and good thermal stability,which has potential application value.Compared with pure nickel,pure aluminum has lower dislocation annihilation activation energy and higher recovery rate,which aggravates the difficulty of grain refinement.The addition of alloy elements(such as magnesium)can significantly reduce the steady-state grain size of aluminum alloy obtained by deformation,which provides an idea for the preparation of nanocrystalline in aluminum.In this paper,taking pure aluminum and 5052 aluminum-magnesium alloy as the research object,the bulk ultra-fine grain/nano-laminated structure were prepared in pure aluminum and5052 aluminum alloy by a two-step process:grain refinement firstly by equal-channel angular pressing and secondly by liquid nitrogen rolling.Especially in aluminum alloy,the ultra-fine grain refinement limit is broken and the nanolaminated structure is obtained.At the same time,the effects of alloy elements on the microstructure and properties of aluminum alloy during deformation are studied and the strengthening mechanism is analyzed.The main conclusions of this paper are as follows:(1)The two-step method of ECAP and LNR can break through the refinement limit of ultra-fine grains and transform the Equiaxed grains into lamellae.The nanolaminated structure with an average lamellar spacing of less than 50 nm was successfully prepared in 5052 bulk aluminum alloy,with high dislocation density(6.4×10¹⁴ m^-2)and high strength(yield strength=515 MPa).The influence of different deformation paths in the ECAP stage is greatly weakened after LNR.The lamellar spacing of the nanolaminated structure obtained by the A path is slightly smaller,and the comprehensive performance(such as hardness and thermal stability)of the nanolaminated structure obtained by the B_C path is slightly better.(2)The solute segregation is induced by the deformation.The effect of magnesium on the microstructure of ultra-fine grains/nanocrystalline is mainly divided into two parts.On the one hand,the interaction between solid-soluble magnesium and dislocations in grains hinders the movement of dislocations.On the other hand,the segregated magnesium element interacts with the grain boundary,which reduces the grain boundary mobility and grain boundary energy.The analysis shows that in the 2.695 at.%magnesium contained in 5052 alloy,0.456 at.%magnesium is segregated to the grain boundary,and the magnesium concentration at the grain boundary is up to 4-10 at.%.In the stage of ECAP,higher strain is needed to obtain uniform microstructure,and the steady-state grain size obtained by deformation is greatly reduced.At the same time,the geometric refinement efficiency of aluminum alloy is higher than that of pure aluminum in the process of LNR.Finally,the nanolaminated structure was successfully prepared.(3)After quantitatively calculating the contribution of solution strengthening,grain size strengthening and dislocation strengthening of annealed samples and deformed samples with nanolaminated structure,it is found that the calculated values of yield strength of annealed samples match well with the experimental values within the error range,while the theoretical values of deformed sample is?100 MPa smaller than the experimental values.This shows that in addition to these three strengthening mechanisms,there are solute grain boundary segregation strengthening and texture strengthening in the deformed sample.During the deformation of aluminum alloy,the alloy elements are induced to segregate near the grain boundary,forming a higher concentration of local solute atoms,hindering the movement of dislocations and forming an additional strengthening contribution.On the other hand,a typical rolling textutre for face-centered cubic metals with high stack fault energy was formed after LNR.The type and strength of the texture will cause the flow stress anisotropy of the aluminum alloy and produce strengthening in rolling directions.