Local Deformation Behaviors And Micro-Mechanisms Of Ti-Al Layered Material

In the fields of scientific research and engineering application,the collaborative improvement of the strength and plasticity of metallic materials is one of the unremitting goals of material scientists.In recent years,researchers have found that the structural design by regulating microstructure can improve the strength and plasticity of materials at the same time.The current researches mainly focus on the application of structural design to different engineering materials to achieve a performance improvement,less attention has been paid to the effect of structural design on the deformation mechanism of materials and the fundamental reason for improving the comprehensive properties of materials.This thesis is based on structural design,taking layered structure as a typical model and focusing on the deformation behaviors of Ti-Al layered materials(LMs).In addition,the synchrotron radiation diffraction technology is introduced,and associated analysis software is developed.This study aims to reveal how layered structure regulates the deformation mechanisms from the view of local stress,establishing the relationship among “structural parameters,local stress,deformation behaviors,and mechanical properties.”Ti-Al LMs with different thicknesses and thickness ratios were prepared by hot pressing in a vacuum,hot rolling,and annealing with commercial pure titanium and commercial pure aluminum as raw materials.The results of mechanical properties show that there is a specific thickness ratio in Ti-Al layered materials;that is,the thickness of the Ti layer is double the thickness of Al,leading to the best comprehensive mechanical properties.The microstructure evolution and deformation behaviors of Ti-Al LMs during uniaxial tension were studied by in-situ tensile tests coupled with SEM and EBSD.It is found that the layered structure affects the deformation of the component layer in the layered material.The Al layer shows the intergranular fracture and strengthening in the layered structure,while the titanium layer shows the de-twinning and intensifying of basal texture,which deviated from the deformation behaviors of free-standing components.Further analysis on the characteristic deformation behaviors shows that the activation of slip systems of aluminum is constrained in Ti-Al LMs.The insufficient participation of slip systems decreases the deformation compatibility between Al grains and leads to intergranular fracture.On the other hand,the constraint on slip system activation promotes higher elasticity,leading to the strengthening of Al.The intensifying of basal texture in Ti can be attributed to the de-twinning of {10(?)2} extension twinning,which correlated to the local stress along ND.Layered structure affects the local stress of the component layer,result in the local stress significantly deviates from macro applied stress.The synchrotron radiation Laue diffraction analysis software,synchrotron radiation high energy X-ray diffraction radial analysis software,and synchrotron radiation high energy X-ray diffraction tangential analysis software are developed,and are used to characterize the deformation microstructure and local stress of Ti-Al LM.It is found that there are different deformation regions in the Al layer where the slip systems are distinct,indicative of multi-axial local stress in the Al layer.By resolved shear stress,the activation of slip systems in Al can be predicted.Besides,there is a local stress component along ND in the Ti layer,which promotes the de-twinning of {10(?)2} extension twinning.The calculation results of the local Schmid factor are in good agreement with the experimental phenomena.The texture evolution of Ti is basically consistent with the VPSC simulation prediction considering multi-axial stress.The thermal mismatch stress model is derived,which points out that the effect of thickness ratio on thermal residual stress is greater than that of single layer thickness.With the decrease of the ratio of aluminum layer thickness to titanium layer thickness,the multi-axial thermal residual tensile stress in aluminum layer becomes stronger.When it exceeds the yield point of aluminum layer,the initial deformed microstructure of aluminum layer is observed.The multi-axial thermal residual compressive stress in the titanium layer decreases with the thickness of the aluminum layer.When the layer thickness ratio deviates from 1:1,there is thermal residual stress component along the ND direction in the component layer.When the Al layer in Ti-Al LMs is thin,deformation can be found in the initial microstructure of the Al layers.Upon plastic deformation,numerous dislocations and substructures are formed in the thin Al layers.The thin Al layers cannot impede the propagation of local strain localization between Ti layers.On the contrary,when the Al layer in Ti-Al LMs is thick,the deformation of Al is heterogeneous,which induces local strain localization in the thick Al layers.Only when the thickness ratio of the Ti and Al layer is appropriate(2:1 in this study),the deformation of the Ti layer and Al layer has both uniformity and continuity.The results show that the adjusting of thickness ratio can realize the residual stress design to regulate the local stress.In addition,changing deformation compatibility between components is also a potential way to regulate the local stress.