| The characteristic size of microstructures of gradient nanostructured(GNS)metallic materials gradually changes from micron-scale to nanoscale spatially,spanning several length scales.Compared with homogeneous structural materials,GNS metals exhibit more outstanding mechanical properties,such as excellent strength-ductility synergy,superior work hardening and enhanced fatigue and fracture resistance,which provide an opportunity to solve the long-standing contradiction between strength and ductility and have become an edge in the field of the strengthening and toughening design of metallic materials.So far,the vast majority of GNS metals are prepared by plastic deformation.However,the nanograins fabricated by this type of ways are prone to mechanical instability(necking and grain growth,etc.)during subsequent deformation,leading to various coupling factors,which restrains us from exploring the intrinsic effect induced by structural gradient and limits the further development of associated theoretical studies.To date,GNS metallic materials have been extensively studied,but the strengthening and toughening mechanisms remains elusive.The nanotwinned(NT)structure keeps stable during plastic deformation and its structure-property relationship is clear,which could serve as an ideal structural unit to construct GNS metals for quantitatively studying the strengthening mechanism.Based on a large amount of experimental data,this work systematically studied the mechanical behavior responses and extra strengthening sources of gradient nanotwinned(GNT)Cu by developing a new gradient theory of plasticity and revealed the intrinsic deformation mechanism by molecular dynamics(MD)simulations,as follows:1.The origin of extra strengthening in GNT Cu was unraveled.The variations of corresponding components of flow stress were clarified by cyclic loading-unloading.The back stress of GNT Cu is larger than the rule-of-mixture average of that of homogeneous nanotwinned(HNT)Cu,indicating extra back stress.The extra back stress increases with the structural gradient while the effective stress keeps almost constant.Based on the experimental results,we proposed the effective configurations of geometrically necessary dislocations(GNDs)accommodating the non-uniform plastic deformation and established a new strain gradient plasticity(SGP)model by considering the effects of back and effective stress in the plastic flow resistance.The influence of structural gradient on the sample-level tensile stress,effective stress and back stress was quantitatively investigated.Simulation results well captured the overall stress-strain responses and suggested that the extra back stress almost increases linearly with the increasing structural gradient,with more room for growth compared with the HNT-induced back stress.And the two types of back stress attain saturated values at small strains.This work revealed the intrinsic plastic deformation characteristic of gradient nanostructures,i.e.progressive yielding form soft component to hard component due to the gradient strength and analyzed the nonlinear hardening in the elastic-plastic transition stage.The mechanical linkage between structural gradient,plastic strain gradient,GND and extra strength was established.2.The spatial distribution of gradient plastic deformation and associated strengthening mechanism in GNT Cu were explored.The full-field strain measurement showed that with the increase of structural gradient,the lateral strain gradient increases,but the growth rate is much smaller than the theoretical prediction from initial yield strengths;on the contrary,the maximum lateral strain difference between components is significantly reduced.Informed by the lateral strain distribution and considering the grain size dependence of the extra back stress,we derived the corresponding scaling relationship and improved the previously proposed SGP model.The spatial-temporal evolution of local plastic strain and local extra back stress was analyzed.The changes of flow stress and plastic strain difference in the three deformation stages were elucidated after the elastic-plastic transition began.Different from the conventional SGP theory,we revealed the synergistic effect of the microstructure size itself and its structural gradient on the local extra back stress,that is,under the same plastic strain gradient,the softest component with the lowest yield strength gains the largest extra strength and such gain is amplified with an increase in structural gradient.3.The plastic deformation mechanism dominated by trans-twin dislocations in NT metals was studied.Experimental results found that the effective stress is almost constant for NT Cu with twin thickness smaller than 100 nm.This implies that the effective stress is controlled by the long dislocations spanning multiple twin lamellae,termed trans-twin dislocations,rather than the commonly thought threading dislocations confined within individual nanotwin lamella.With the help of MD simulations,the nucleation,expansion and movement of trans-twin dislocations were revealed,and two formation mechanisms and corresponding dislocation configurations of trans-twin dislocations were found.MD results showed that the trans-twin dislocation lies on the corrugated {111} slip planes,which is a common deformation mechanism regardless of twin thickness and its gradient;the finite resistance to slip transmission across coherent twin boundaries(CTBs)sets a lower limit of the span length of trans-twin dislocations and thus dictates the upper limit of the effective stress.In addition,the glide resistance to trans-twin dislocations mainly originates from the dislocations at CTBs rather than CTBs themselves.In summary,this work provides an in-depth study of the deformation and strengthening mechanisms of GNT Cu,and clarifies the intrinsic relationship between structural gradient,plastic strain gradient,GND,back stress and extra strength,which advances the development of gradient plasticity theory and provides guidelines for the strengthening and toughening design of GNS materials. |
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