| The ultrafine grained materials prepared by equal channel angular pressing compared with the traditional coarse grained materials:On the one hand,it has better physical properties(such as magnetism,modulus of elasticity,coefficient of expansion);on the other hand,it has higher strength and plasticity under uniaxial loading.This means that ultrafine-grained materials in engineering can be made into stronger components to ensure support safety.However,in the actual service of the material or component,most of the working conditions of the material or component will often withstand the role of cyclic load.Therefore,it is not perfect to use the mechanical properties under uniaxial loading to guide the practical application of ultrafine grained materials.It is particularly necessary to further investigate the mechanical response of ultrafine grained materials under cyclic loading.For the above reasons,the study of mechanical behavior under cyclic loading is of great significance for promoting the development of ultrafine materials and practical engineering applications.So this paper takes 6061 aluminum alloy as the research object.Firstly,8 passes extrusion was completed by equal-channel angular processing.Through metallographic observation and microhardness analysis,the grain size reached the level of ultrafine grain.Secondly,the cyclic hysteresis characteristics under different extrusion passes and different strain amplitudes are compared,and the effects of different mean stress cyclic loading on the ratcheting effect of raw coarse and ultrafine grains are studied.Finally,the microstructure and texture of ultrafine 6061 aluminum alloy before and after cyclic loading were studied by EBSD.The main findings are as follows:1?In order to obtain the ultra-fine crystal 6061 aluminum alloy material,the Equal channel angular pressing test was carried out in this paper.At the same time,the metallographic experiment,hardness test and static tensile test at room temperature were used to analyze the microstructure refining effect.The results show that:After pressing,the structure was gradually refined,and the microstructure of the extruded tissue was fine and uniform in 8 passes.The hardness increased from the original 46.2HV to 85.94HV,and the tensile strength increased from 179MPa to 292MPa,an increase of 63.1%.The effect of tissue refinement is obvious,and the strength is greatly increased.Compared with the existing research results,it is considered that the ultrafine grain range is basically reached after 8 passes of pressing.2?Through the strain control cyclic loading test,the cyclic hysteresis characteristics of different pressing passes under different strain amplitudes were studied.The results show that:The original coarse crystal hysteresis curve has good symmetry and is better wrapped under different strain amplitudes.After the pressing,the samples have different degrees of intersection hysteresis curves under different strain amplitudes.The multi-pass pressing makes the shape coefficient of the hysteresis curve of the alloy smaller,and the Bausching effect is enhanced.The original coarse crystals exhibited cyclic hardening at 0.5%and 0.6%strain amplitude,and showed cyclic softening at 0.7%strain amplitude.The ultrafine grained material has less cyclic softening at 0.5%and 0.6%strain amplitude,and exhibits significant cyclic softening at 0.7%strain amplitude.EBSD test analysis shows that the cyclic softening of ultrafine grain materials can be attributed to severe dynamic recrystallization during cyclic loading.3?Through the stress control cyclic loading test,the ratcheting effect of different average stresses on different pressing passes was studied.The results show that:The average compressive stress and the average tensile stress of the ratchet strain increase with the increase of the pressing pass.In the same pass,the ratchet strain under the average compressive stress is larger than the ratchet strain under the average tensile stress,and the ratchet strain difference increases with the increase of the squeeze pass.The EBSD test results show that the microstructure of the ultrafine grained material has a certain texture after equal channel angular pressing,and the texture produces a certain anisotropy of the material.In addition,there are obvious differences in dynamic recrystallization between the average tensile stress and the average compressive stress,which leads to the difference of ratchet strain of ultrafine grained materials under different average stresses.4?Microstructure analysis of ultrafine-grain 6061 aluminum alloy before and after cyclic loading by EBSD showed that:(1)The ultrafine grained material after pressing has an average grain size of 1.08?m and a high internal dislocation density.The improvement of the strength properties of the ultrafine grained material can be attributed to the higher dislocation density and grain size reduction.At the same time,there is a higher Schmidt factor,which is beneficial to the occurrence of dislocation slip inside the tissue during cyclic loading.In addition,a certain degree of dynamic recrystallization was observed,indicating that dynamic recovery began to occur inside the tissue after 8 passes.(2)After strain-controlled cyclic loading,the average grain size grows up to 73?m.The reason for the growth of the grain size can be attributed to the strain-controlled cyclic loading,which promotes the generation and development of dynamic recrystallization.The dynamic recrystallization ratio is from the original 25%increased to 75%.In addition,it was found that there are a large number of bow-bending grain boundaries inside the tissue after cyclic loading,which provides a basis for the nucleation and development of dynamic recrystallization.(3)After cyclic loading with different average stresses,the microstructure appeared to grow to varying degrees.Under the average compressive stress,the grain size grew to 3.60 ?m,an increase of 233.3%.Under the average tensile stress,the grain size grew to 1.26 ?m,an increase of 16.7%.It is observed that the average compressive stress has a large decrease in dislocation density after cyclic loading,and the second phase is basically dissolved,indicating that the average compressive stress cyclic loading promotes the generation of dynamic recrystallization,and the dynamic recrystallization ratio is as high as 98%,which leads to a large coarsening of the structure.After the average tensile stress is cyclically loaded,the dislocation density is slightly reduced,and the content of the second phase is basically unchanged,indicating that the average tensile stress cyclic loading has a certain inhibitory effect on dynamic recrystallization.The dynamic recrystallization ratio is only 44.91%,which is expressed as crystal.Grain coarsening is not obvious. |
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