Effect Of Thickness On The Uniaxial Tensile And Fatigue Damage Behaviour Of Ultrafine-Grained Pure Aluminum

Various mechanical behaviors of ultrafine-grained(UFG)materials produced by equal channel angular pressing(ECAP)generally exhibit distinctive characteristics from those of conventional grained materials,so that the relevant studies have received sustained attention in recent years.However,different experimental conditions will lead to the change of size and geometry of specimens,they will bring about an influence on the mechanical properties of materials.The size dependence of material properties is generally termed "size effect".In the present work,UFG pure aluminium prepared by the ECAP technique,which possess typical face-centered cubic(FCC)structure,is selected as the target material to study the size effects on the uniaxial tensile behavior and tension-tension fatigue behavior,as well as the effect of pre-fatigue on uniaxial tensile mechanical properties of UFG pure aluminium.The plastic deformation and damage behavior of UFG pure aluminium specimens with different thicknesses ranging from 0.1 mm to 1.0 mm were investigated at room temperature at a strain rate of 1.25 × 10-3 s-1 under uniaxial tension.Rt/d is the ratio of specimen thickness t to grain size d.With the increasing of Rt/d of the specimen,the yield strength,ultimate tensile strength and uniform elongation increase.When Rt/d<500,the material strength increases rapidly as the specimen thickness increases,while the change in uniform elongation is not so serious.In contrast,when Rt/d>500,the uniform elongation increases obviously as the specimen thickness increases,while the change in strength is not so obvious.With the increasing of thickness,the surface deformation characteristics of UFG pure aluminium is mainly manifested by large-scale shear deformation,meanwhile,the number and size of dimples on fracture surface increase.Since different levels of plastic deformation are induced under the process of uniaxial tension of pure aluminium with different thicknesses,the dislocation structures change from loose dislocation tangles in thinner specimens into incomplete subgrains in the specimen with an intermediate thickness,and finally the dislocation structures are evolved into the subgrains in the thickest specimen.The tensile plastic deformation behavior of UFG pure aluminium specimens,which were pre-fatigued at a stress amplitude of 50 MPa,were investigated at room temperature.As the fatigue damage grade D increases,the strength and plasticity of UFG pure aluminium decrease.The initial damage brings about a great influence on the mechanical properties of pure aluminium.The surface deformation characteristics of UFG pure aluminium is also mainly governed by the formation of large-scale shear bands.With increasing fatigue damage grade D,the depth and size of dimples on fracture surface reduce.The fatigue damage grade D show little effect on the microstructures of specimens after tension,and the microstructures in different specimens are mainly composed of loose dislocation tangles,which have little contribution to the improvement of the strength of materials.The tension-tension fatigue properties of UFG pure aluminium specimens with different thicknesses ranging from 0.1 mm to 1.0 mm were investigated at room temperature at a stress amplitude of 50 MPa.With the increasing of Rt/d of the specimen,the cyclic flow strain of UFG pure aluminium increases,meanwhile,the fatigue life of materials increases rapidly.The large-scale shear deformation is the major deformation characteristics of UFG pure aluminium after tension-tension fatigue.Thickness of specimens provides a positive effect on the surface deformation.With the increasing of thickness,the number and size of dimples on fracture surface increase,and the relevant dislocation structures change gradually from regular dislocation cells into subgrains and finally into recrystallized structures.In a word,the change in fatigue life of different-thickness specimens is closely related to the corresponding variation in microstructure.