Study On Deformation And Fracture Behavior Of Layered Ti/Al Metal Composites

Layered metal structural material can combine mechanical properties of component layers to achieve a good match between strength and plasticity.In recent decades,scientific interest focused on the deformation and fracture mechanism of layered metal composites,and based on dislocation and shear theories,a series of strengthening models were proposed to reveal the mechanisms of strengthening and toughening.However,their plastic deformation theories were relatively scattered,and the influence of macroscopic layered structure on its deformation behavior was neglected.Why can layered structure design simultaneously improve strength and ductility of metal structural material? In addition to layer thickness effect,interface effect and mechanical properties of component layer,the essential factors are still unclear,and a more detailed research is required.The aim of the present work is to clarify the influence mechanism of layered structure on strengthening and toughening of layered composites from local stress/strain distribution of view,and reveal the essence that layered structural parameters affect mechanical behaviors of layered Ti/Al metal composites(LMCs).The LMCs with different structural parameters were designed and fabricated by hot rolling and annealing of industrial pure Ti foils and pure Al foils,and then they were used as the model material of research.For the scientific problem of strengthening and toughening of layered structure,in-situ EBSD technique,digital image correlation technique,synchrotron radiation tomography,in-situ neutron diffraction technique and in-situ high-energy X-ray diffraction technology technique were used to investigate the correlation response mechanism among ?macro plastic deformation-local stress/strain distribution-deformation microstructure? in the LMCs,and reveal strengthening and toughening mechanisms of layered structure.Then,in this premise,the influence of structural parameters on the mechanical properties of layered materials is investigated to optimize the layered structural parameters,and reveal the influence essence of structural parameters.Analysis of mechanical properties of LMC(A100)show that its yield strength generally meets the rule of mixture(ROM),but its ductility(43%)is higher than that of any component layer.The stress/strain evolution and fracture behavior of LMC during tensile deformation were analyzed by in-situ observation.It is found that the layered structure can change stress state during deformation,and the deformation process is divided into three stages: elastic deformation stage,elastic-plastic deformation stage and plastic deformation stage.Stress partitioning behavior in elastic-plastic stage improves a strain balance,but leads to an internal stress accumulated at the interface,which affects its plastic deformation behavior.Based on the distribution and evolution of local strain,it is found that the high ductility of LMC derives from the strain non-localization by strain transfer from Ti layer to Al layer,and strain transfer behavior effectively alleviates the strain localization of Ti layer,which improves the ductility of Ti layer,changes the deformation behavior of Ti,and suppresses the twinning of Ti.The deformation texture evolution shows that under constraint effect of layer structure,grain rotation and detwinning behavior of Ti layer are beneficial to alleviate the strain localization of Ti along and perpendicular to the tensile direction.In addition,layered structure affects deformation behavior of Al layer through the internal stress distribution,so that the Al layer exhibits no obvious orientation correlation,which improves uniform deformation ability of Al layer.The in-situ observation of crack initiation and propagation based on SR-CT shows that the accumulation of internal stress and strain localization can promote crack nucleation of the LMC at a low macro strain.However,layered structure can inhibit the crack propagation to some extent.On the one hand,the crack tips are blunted by the plastic deformation of component layer,which reduces the driving force of crack propagation;on the other hand,layer structure changes the stress state of the crack tips,which enhances the resistance of crack propagation.In addition,a large number of microcracks are found in Al layers,and layered structure can effectively constrain the propagation of these microcracks.These constrained microcracks alleviate the stress concentration of the Al layer and improve the ductility of the Al layer,so the crack indirectly affects the deformation behavior of LMC,and thus improves its entire ductility.Mechanical behavior analysis of LMCs with different layer thickness ratios shows that layer thickness ratios can redistribute the initial residual stress and the internal stress during deformation process.On the one hand,the redistribution of residual stress changes the initial microstructure of the Al layer.On the other hand,the redistribution of internal stress changes their deformation stages,local strain transfer and crack initiation of LMCs,which further affects their mechanical properties.Under residual stress and applied stress,there is an interface affected zone(IAZ)of stress,and the width of the IAZ(~17?m)remains unchanged during deformation process.When the thickness of the Al layer(35?m)is close to twice the width of the stress IAZ,the stress IAZ and the Al layer overlap exactly,which can ensure complete deformation of the entire Al layer and generate high GNDs dislocation density.Thus,it can provide sufficient back stress to coordinate the deformation of Ti layer,prevent its strain localization and ensure its good work hardening ability.As a result,the LMC exhibits an optimum mechanical property,particularly a uniform elongation.In general,this paper reveals the strengthening and toughening mechanism of layered structure from local stress/strain distribution of view,clarifies the influence essence of layered structure parameters on the mechanical behavior of layered materials,and reveals the relevance between optimal layer thickness and stress IAZ of layered materials,which provides theoretical guidance for the design of high-performance layered structural materials.