Fatigue Property And Cyclic Deformation Mechanism Of Gradient Nanograined Cu

Around 90%of service failures of metallic components and structures are caused by fatigue.The study of fatigue behaviors of materials is of vital significance for their safe services.Conventional homogeneous structured materials exhibit a trade-off trend between stress-controlled high-cycle fatigue(HCF)strength and strain-controlled low-cycle fatigue(LCF)life,which is normally ascribed to the tensile strength-ductility trade-off and plastic strain localization in fatigue.Coarse grained(CG)materials exhibit a low HCF limit but high LCF life,while ultrafine grained(UFG)materials exhibit a high HCF limit but low LCF life.Gradient nanograined(GNG)materials with a spatially graded variation of grain sizes from nano-scale in the surface to micrometer in the core exhibit a superior strength-ductility synergy and an enhanced HCF and LCF property combination.As a multi-scale heterogeneous structure,GNG materials exhibit multiple adjustable micro structure parameters,such as GNG volume fraction,microstructure in the core,gradient order,structure gradient and so on.Nevertheless,the influences of complex micro structure parameters on mechanical properties,fatigue properties and cyclic deformation behaviors of GNG materials remain scarce.Moreover,present fatigue studies on GNG materials mainly focus on symmetrical loading,the asymmetrical fatigue behaviors under realistic performance conditions remain to be explored.In this study,three pure Cu samples with quantitatively different gradient micro structures are prepared by surface mechanical grinding treatment(SMGT).The effects of the GNG volume fraction and the micro structure in the core on the fatigue properties and the cyclic deformation behaviors are systematically investigated.The asymmetric fatigue behavior of gradient nanograin/coarse grained(GNG/CG)Cu under constant load control is explored.The main results are as follows:1.Effect of volume fraction of GNG surface layer on fatigue of CuTwo GNG/CG Cu samples with different volume fractions of GNG surface layer are controllably prepared.With increasing the GNG volume fraction from 4.3%to 13%,the yield strength of GNG/CG Cu is enhanced from 123 MPa to 144 MPa,but the ultimate tensile strength keeps nearly unchanged?246 MPa.Stress-controlled HCF tests suggest with increasing the GNG volume fraction,the fatigue limit is enhanced from 88 MPa to 98 MPa.Abnormal grain coarsening accommodates cyclic plastic strain in GNG layer.Large volume fraction of GNG surface layer effectively postpones the extension of abnormal grain coarsening from a deeper subsurface layer to the topmost surface and thereby retards the initiation of surface fatigue cracks.Total strain-controlled LCF tests suggest increasing the volume fraction of high-strength GNG layer can greatly elevate the cyclic stress amplitude of GNG/CG Cu,but does not clearly influence the LCF life(about twice than that of CG Cu).Progressive homogenous grain coarsening effectively suppresses plastic strain localization in the GNG layer and surface roughening.The influences of GNG volume fraction on the coarsened grain size and the surface damage degree are minor.2.Load-controlled asymmetric tension-compression high-cycle fatigue behaviors of GNG/CG CuAsymmetric tension-compression HCF tests of GNG/CG Cu under constant load control suggest under the same stress ratio,GNG/CG Cu exhibits an enhanced HCF limit and fatigue life,compared with CG Cu.However,with increasing the stress ratio,the fatigue life of GNG/CG Cu is deteriorated.Besides,GNG/CG Cu exhibits obvious cyclic ratcheting under asymmetrical loading,a directional permanent deformation process that proceeds cycle by cycle.Compared to CG Cu,the cyclic ratcheting strain of GNG/CG Cu is apparently decreased.Different from abnormal grain coarsening under stress-or strain-controlled symmetrical HCF,obviously elevated maximum stress under asymmetrical HCF leads to entire yielding,activating normal grain coarsening.The true maximum stress gradually increases due to cyclic ratcheting,resulting in progressive migration of micro-plastic strain frontier towards the topsurface.Decreased compressive stress under asymmetric fatigue reduces cyclic plastic strain of GNG/CG Cu.3.Effect of dislocation cell in the interior on fatigue of gradient nanograined CuA GNG/DC Cu sample is obtained by SMGT on a drawing-induced dislocation cell(DC)substrate with a cell size of submicrons.The yield strength of GNG/DC Cu is 365 MPa,obviously higher than that of GNG/CG Cu,almost no work hardening.Stress-controlled HCF tests suggest GNG/DC Cu exhibits a remarkably elevated fatigue limit(at 107 cycles)of 150 MPa(53%enhancement with respect to GNG/CG Cu)and a fatigue ratio of 0.4(similar to that of GNG/CG Cu).Progressively abnormal grain coarsening from the subsurface layer to the surface accommodates cyclic plastic strain in the GNG layer.The higher-strength DC interior effectively retards the onset of abnormal grain coarsening and the initiation of surface fatigue cracks,thus enhancing the HCF limit and the fatigue life.Total strain-controlled LCF tests suggest that GNG/DC Cu exhibits cyclic softening,which is determined by homogeneous coarsening of dislocation cells in the core and gradient nanograins in the surface layer.Severe strain localization of shear handing or abnormal dislocation cell/grain coarsening is effectively suppressed.GNG/DC Cu exhibits an obviously higher stress amplitude with respect to GNG/CG Cu.The LCF life of GNG/DC Cu is slightly lower than that of GNG/CG Cu,but superior to DC Cu.Present study reveals quantitative effects of microstructure parameters of gradient nanostructures(volume fraction of GNG surface layer and different microstructures in the core)and fatigue modes(stress/strain control and symmetric/asymmetric cycling)on the fatigue properties and cyclic deformation mechanisms of gradient nanograined Cu,laying a foundation for the design of metals resistant to fatigue through tailoring gradient nanostructures.