Investigations On Macro-micro Deformation Behaviors And Mechanisms Of Layered Heterostructured Aluminum

Collaborative enhancement on strength and ductility is one of the eternal themes in the field of metallic structural materials.In recent years,researchers have found that heterogeneous structured metals,which regulate microstructures to form particular architecture,can break the bottleneck of strength-ductility trade-off.The present investigations have developed a series of strengthening and toughening mechanisms,such as back stress strengthening,geometrical necessary dislocation pile-ups,etc..However,as a key deformation factor,the “local stress/strain” and its mechanisms are not fully understood yet.Focusing on the local stress/strain,this dissertation takes the layered structured aluminum as a model,and charaterizes the distribution and evolution of local stress/strain during the tensile deformation by utilizing advanced synchrotron radiation X-ray diffraction and digital image correlation techniques.And the essential deformation behaviors are studied,including the slip system mechanisms,dislocation structures and lattice rotation.The traditional crystal plasticity theories are optimized by introducing the local stress/strain,revealing the relationship between the local stress/strain and the essential deformation behaviors.And then the structure-activity relationship between “layered design-local stress/strain-deformation behaviors-macroscopic mechanical properties” is established,forming the local stress/strain mechanism in heterogeneous layered structure.Based on the structure-activity relationship,the influence of layered configuration parameters,such as layer thickness,layer thickness ratio and microstructural variations,on the local stress/strain mechanisms and mechanical properties were also investigated.The optimal design of layered configuration parameters for a superior strength-ductility synergy was pointed out.This work is expected to provide a theoretical guidance for designing configuration parameters and improving mechanical properties of heterogeneous structured metals,and a new insight into the strengthening and toughening mechanism for new generation heterogeneous structured metals with high performance.Specifically,we utilized commercial pure aluminum(AA1060)foils as raw materials,and fabricated AA1060/AA1060 fine-/coarse-grained layered aluminum with different layer thickness ratios by vacuum hot-pressing-hot-rolling.And then taken the aluminum alloys(AA3003)and commercial pure aluminum(AA1060)sheets as raw materials,accumulative rolling bonding-annealing method was performed to fabricate AA3003/AA1060 fine-/coarse-grained layered aluminum with different layer thicknesses.Based on the classical synchrotron radiation X-ray diffraction techniques,the high-energy X-ray diffraction-rotation method is developed,realizing the local stress characterization on constituent layers of layered aluminum;a diffraction pattern peak analysis method of Laue micro-diffraction is established to describe the distribution characteristics of local dislocation structures;this provides advanced and effective characterization methods for local deformation analysis.A typical AA1060/AA1060 layered aluminum was investigated.The synchrotron radiation Laue micro-diffraction experiment found that the classical Schmid factor cannot predict the dominant slip systems of coarse grains effectively during the early yielding deformation stage.Combined with local stress in layered structure,the classical Schmid law was optimized,proving that the the local stress state tailored by layered structure is the key to changing the deformation mechanisms in the coarse-grained layers.Through digital image correlation technique and in-situ EBSD experiment,the regulation of layered structure on local strain state and dislocation structures was investigated.Combined with the local strain results,the classical Taylor model was supplemented to reveal the correlation between local strain state and dislocation structures in heterogeneous layered aluminum.The relationship between essential deformation behaviors related to local stress/strain and macroscopic mechanical properties was discussed,forming the local stress/strain mechanism in heterogeneous structured metals.The effects of layer thickness parameters on the mechanical properties of AA3003/AA1060 layered aluminum were investigated.The synchrotron radiation white-beam Laue micro-diffraction experiment found that there are dislocation interface affected zones near the coarse-grained layer interface in layered aluminum samples with different layer thicknesses.Combined with newly-developed synchrotron radiation high energy X-ray diffraction-rotation method and optimized Schmid factors,it was found that the local stress state on the constituent layers is the main reason for the formation of dislocation interface affected zone.Within the dislocation interface affected zone,multiple slip systems are more likely to be activated and the relative dislocation density is obviously higher than that in the central region of coarse-grained layer,providing extra strain hardening capacity for layered aluminum.Moreover,the range of interface affected zone is ～4 μm width,independent of the layer thickness.Therefore,the layer thickness parameter regulates the range proportion of the dislocation-interface influence zone associated with the local stress mechanism,thus affecting the work hardening ability of layered aluminum.The effects of layer thickness ratio parameter on the plastic deformation capacity of AA1060/AA1060 layered aluminum were also studied.Based on digital image correlation method,the effects of layer thickness ratio parameter on the strain delocalization and interfacial strain gradient in the layered aluminum were investigated,explaining that the layer thickness ratio parameters have an important influence on work hardening ability of layered aluminum.Combined with multi-dimensional observation on fracture tomography,a mechanic model of interfacial micro-crack metastable propagation was established.This model revealed the influence of layer thickness ratio parameter on the fracture mechanisms of layered aluminum,and predicted the optimal layer thickness ratio in heterogeneous layered aluminum for superior strength-ductility synergy.A further comparing investigation between the AA1060/AA1060 and AA3003/AA1060 layered aluminum with different microstructural variations was performed.The effects of microstructural variations in layered aluminum on the local strain evolution and fracture behavior were investigated,revealing the local strain mechanisms that the microstructural variations of constituent layers affect the mechanical properties of layered aluminum.The optimization direction of microstructural design in constituent layers is also discussed and summarized.