Study On The Micro-mechanical Behavior Evolution Of Pure Titanium And TB6 Alloy

Titanium and titanium alloy has been widely used in the fields of aviation,aerospace,ships and so on for its low density,high specific strength and corrosion resistance,high and low temperature resistance and other excellent properties,therefore it is always a research focus in the field of metal materials at home and abroad.Due to different loading rate will have influence on macroscopic and microscopic mechanical behavior of materials.At the same time different degree of deformation will take place inside the materials during machining process due to the impact by friction,temperature and its shape and other factors and result in activating the corresponding slip system and dislocation accumulation in every grain inside the material which will eventually affect the macroscopic mechanical properties and long-term application of material.Therefore,to make clear of microscopic stress state and the microscopic mechanical behavior evolution of the material has an important guiding significance.In this thesis,TA2 pure titanium target and TB6 titanium alloy forging as the objects are studied based on the combination of macroscopic and microscopic methods.The basic microstructure is obtained by microtructure observation and at the same time in-situ synchrotron radiation of high energy X ray diffraction technique is carried out to research on the effect of tensile speed on the micro-mechanical behavior evolution of TA2 pure titanium and the micro-mechanical behavior evolution features in different regions of TB6 titanium alloy forging during the deformation process.The main conclusions in this dissertation are shown as follows:The research on TA2 pure titanium finds that TA2 pure titanium has significant stain rate strengthening and strain hardening in the quasi-static tensile tests in the room temperature.At the same time,the higher of the tensile speed,the greater the external stress required for each crystalline plane to yield and the microscopic yield stress of the material is less than the macroscopic yield stress.The micro-mechanical behavior evolution of per crystal plane is remarkably different between the transverse direction and loading direction which shows the significant anisotropy of pure titanium with hcp structure.In the plastic deformation process,the lattice strain increasing amplitude of per crystal plane improves and the disparity becomes larger in the transverse direction while the orientation tends to be consistent with each other among different crystal planes in the loading direction with increasing tensile speed which shows the stronger of interactions between grains of different orientations with increasing tensile speed.The initiation of the slip mechanism and the generation of twinning cause the lattice strain mutations of some crystal planes and the difference of required stress for mutations shows the different magnitude of CRSS to initiate twinning of different crystal plane.In the plastic deformation process,the shifting trend of intensity in each crystal plane is different because the stress redistribution due to the initiation of slip mechanism and twinning which leads grains to rotate to the orientation advantageous to initiate slip or twinning.At the same time,the width increase of diffraction peaks due to the decrease of the grain size and the increase of the dislocation density in the deformation process.In addition,the study finds that by sampling from five different areas in TB6 titanium alloy forging the microstructure of different areas is primary alpha phase and transformed beta phase and there is just a difference of morphology and volume fraction of primary alpha phase.For the initial state of forging,there is no texture in 2#sample while 3#and 5#sample has typical tensile texture and typical compressed texture.All of these show that there exists a certain influence on the microstructure and preferred orientation of materials due to the five samples are located in the region suffered different stress field and temperature field during forging process which causes the significantly different mechanical performance.In the elastic stage,the lattice strain increases linearly of each crystal plane in five samples.In the plastic deformation stage,the lattice strain increasing amplitude of(200)crystal plane is the largest which is the softest orientation(except for 3#)and the(0002)crystal plane increases fast which is the softest orientation in alpha phase.At the same time the lattice strain of crystal plane in beta phase is bigger than the crystal planes in alpha phase and only the lattice strain of(10-11)decreases while others increase with increasing stress becoming the hard orientation and bearing the biggest stress.In the same range of stress,the growth of lattice strain of crystal planes in beta phase is similar to the crystal planes in alpha phase in 2#sample which probably because the microstructure of 2#sample is lamellar,so the stress which the two phase suffered is uniform.The FWHM magnitude of all crystal planes of alpha and beta phase increases in five samples which is due to grain size decreases and dislocation density increases gradually during the deformation process.