Research On Deformation Mechanism Of Layerd Titanium With Alternating Coarse-and Fine-grain Layers Based On Analysis Of Local Stress/strain

Until today,metallic materials are still the major workhorse of engineering structural materials.The aim of researchers is to obtain the synergy of enhanced strength and ductility.The bottleneck of the development of metals are low strength/weight ratio and the trade-off of strength and ductility.Currently,the most common method to improve the mechanical properties of metals is alloying.However,alloys with complicated compositions depend more on resource and are difficult to be recycled,which impedes the sustainable development of metals.Crystaline defects(such as dislocations,vacancies and grain boundaries)have potential to change mechanical properties of metals instead of alloy elements.However,it is hard to control defects in crystal due to the instability.Recently,Layered structural design exhibits its advantage in solving the trade-off of strength and ductility by tunning the distribution of defects.Therefore,model materials are needed ugently to deeply investigate the deformation mechanism in layered materials.In this paper,layered model material has been designed and fabricated,which aim to exclude the influence of thermal expansion coefficient and elastic modulus difference between component layers and investigate the mechanism how the yield strength difference between component layers insert influence on the deformation mechanism of layered materials independently.Advanced characterization technologies were used to in-situ characterize the distribution and evolution of local strain/stress and dislocations in order to establish the correlation effect among local strain/stress and the evolution of dislocation structure.So as to reveal the response relationship between deformation mechanism and mechanical properties of layered materials.Layered pure titamium with alternating coarse-and fine-grain layers(C/F-Ti)as the model material which match the requirements was fabricated by hotpressing.Pure titanium with only coarse-grain layers(C-Ti)and fine-grain layers(F-Ti)respectively were also prepared in the same fabrication process.C/F-Ti exhibits nearly the same yield strength with that of F-Ti with only 11.5vol% of fine grains and improved uniform elongation and fracture elongation compared to F-Ti.Hence,C/F-Ti obtain the synergy of enhancement of strength and ductility successfully.There are only one or two grains along normal direction(ND)of single layer in layered pure titanium.Therefore,the investigation of deformation behavior of single crystal is necessary.Analysis of early deformation stage is also important to reveal the strengthening mechanism.Several methods are available to analyze the activation of slip systems of grains,including Synchrotron 3D X-ray diffraction,in-situ characterization of TEM and Synchrotron Laue Microdiffraction.The first method could predict the main slip system activated in the grain through the average information of the grain.The information in the grain with the grain size of lower than hundreds micrometers is not available.The second method could observe the dislocation activity directly.However,the size effect may influence the deformation mechanism due to the small sample size.Lattice strain/stress,crystal orientation and dislocation density can be obtained simultaneously through Sychrotron Laue Microdiffraction.The method developed based on this technology can predict the activation of slip system only at the situation of single slip system activated and only for edge dislocations.In this paper,a new method is proposed to analyze the activaton of slip systems at various positions in the grain during tensile test by analyzing the distribution and evolution of resolved shear stress of different slip systems as well as rotation of grain or crystal directions.The yield strength of C/F-Ti does not satisfy Hall-Petch relationship,nor rule of mix.The main reason for the strengthening is the pile-up of <a> dislocations at layered interfaces due to the constraint of layered structure,which increase the necessity of the activation of <c+a> dislocations next to the layered interface so as to coordinate local deformation.The critical resolved shear stress of <c+a> slip is much higher than <a> slip,which leads to the enhacement of yield strength.The influence of crystal orientaion,interaction between neiboring grains and distribution of grain size on the mechanical properties are excluded through analysis.Local strain evolution of layered pure titanium during the plastic deformation was analyzed through DIC technology.It is found that stain localization in C/FTi is the worst compared to C-Ti and F-Ti.However,effective relief of strain localization appears in C/F-Ti due to strain transfer behavior,which is related to the early deformation mechanism and different work hardening rate between component layers.The main reason for the difference of work hardening rate among three material systems has been revealed through the analysis of dislocation structure evolution.The influence of yield strength difference between component layers on the relationship between deformation mechanism and mechanical properties of C/FTi has been revealed based on the analysis of response relatonship between local strain/stress and the activation and evolution of dislocations.This study provide theoretical guidance for the design of layerd materials,expecially for hcp metals.