Pure titanium has good low temperature performance, excellent corrosion resistance, low density and many other excellent characteristics, and has a large number of applications in the medical field, offshore engineering, petroleum engineering and many other fields. The crystal structure of pure titanium is closely packed hexagonal, the slip system in the structure is less, and it is easy to form asymmetry when stretched. For the mechanical properties of pure titanium, Tao Zhijun et al. studied the tensile and compressive plastic deformation behavior and mechanism of commercial pure titanium, obtained the orientation distribution of pure titanium grains, and analyzed the main mechanism of tensile and compressive plastic deformation of pure titanium. Shi Xiaohui et al. studied the quasi-static tensile behavior and mechanism of coarse grain TA2 pure titanium, and the results showed that the main reason for the high deformation ability of TA2 was grain coarsing, and increasing the grain size would enhance the activity of twins during deformation. The twin boundary can divide the grain, reduce the dislocation slip distance, hinder the dislocation slip, and increase the strength of the material.
In this paper, the microstructure and macroscopic mechanical properties of the rolled and annealed TA1 pure titanium sheet were characterized and analyzed, and the corresponding relationship between the deformation process and the structure was described, so as to provide some reference for the practical application of the material.
1. Experimental materials and methods
The material selected in this experiment is TA1 pure titanium sheet after 700 °C×1 h/ air-cooled annealing provided by Xinjiang Xiangrun New Material Technology Co., LTD. The materialized material can be divided into: W (Fe) = 0.013%, w (C) = 0.017%, w (N) = 0.005%, w (H) = 0.001%, w (O) = 0.029%, allowance for Ti. The tensile sample was in the rolling direction (RD direction), and the tensile test was performed by GBT 228.1-2010 "Tensile test of Metal materials Part 1: Test method at room temperature". Metallography was observed with OLYMPUS optical microscope, electron back scattering diffraction (EBSD) and fracture morphology were measured with Zeiss SUPRA 55 field emission scanning electron microscope, X-ray diffraction was measured with Empyrean X-ray diffractometer, and high-magnification tissues were observed and photographed with Tecnaig transmission electron microscope. INSTRON testing machine was used to test tensile at room temperature.
2. Experimental results and discussion
2.1 Microstructure
Microstructure of TA1 pure titanium sheet, metallographic microstructure of the sheet has obvious characteristics of pure titanium structure, the microstructure is dominated by equiaxial α phase, and a trace of intercrystalline β phase exists between equiaxial α phase. Is the X-ray diffraction pattern, it can be found that the diffraction peaks are α phase, where (101¯0), (0002), (101¯1) three crystal plane index diffraction peak intensity is larger. For the corresponding EBSD microstructure and polar diagram, the results show that the α-titanium with close-packed hexagonal structure is 99.2%, while the β-titanium with body-centered cubic structure is 0.8%, the grain size is 50-60 μm, and there is a Burger relationship between α-titanium and β-titanium. By analyzing the polar diagram of (101¯0) and (0002), it can be obtained that the base plane texture is strong with a maximum value of 6.85, and the Angle between the ND direction (plate transverse) and the grain is small, which can be obtained by the cylindrical texture of plate {101¯0}, and the plate has a strong texture along the RD direction (rolling direction).
2.2 Tensile Analysis
The tensile process curve of TA1 pure titanium sheet can be divided into three stages. The first stage is the initial stage. At this stage, the plastic deformation of the tensile sample is mainly slip, and a small number of dislocations are generated, and the tensile curve is smooth. The second stage is the strengthening stage, in which the grain boundary stress in the structure is greater than the stress in the critical twin, and the plastic deformation is jointly involved by the twin and slip. Because the twin boundary can divide the grain, the slip length of the dislocation is reduced, the dislocation movement is hindered, and the overall strain hardening effect of the sample is increased, which is the main reason for the rapid decline of the strain hardening rate. When the tensile process enters the third necking stage, there are a large number of twins in the tissue, the strengthening effect begins to weaken, and the strain hardening effect begins to decrease. Because there are a large number of twins in the tissue at this time, the reduction rate of strain hardening is smaller than the initial stage.
2.3 Physical comparison of tensile samples
Compared with the original specimen after stretching, it is found that the tensile specimen has a significant extension of about 2.5mm, because pure titanium has good plasticity, the resulting fracture is mainly ductile fracture, ductile fracture will have obvious plastic deformation in the macro morphology before fracture, ductile fracture is a slow tearing process, which is because after the formation of cracks in the tissue, The propagation of cracks requires continuous energy consumption. Under the action of tensile stress, the micropores in the tissue will connect with each other to form micro-cracks, and then the cracks will continue to expand, resulting in the fracture of the sample.
2.4 Micromorphology of fracture location
After the tensile specimen is broken, A large number of twins appear in the microstructure and metallographic structure at the fracture position (position A), and the size of the equiaxed α phase of the organization decreases. This is because the grains in the plate structure will be broken during the stretching process. Under the action of external force, the equiaxed grains will be deformed and destroyed, and will be stretched along the direction of external force. Twinning occurs locally at first, and the twinning increases and the density increases with the increase of deformation. The formation of twins is caused by shear along the (101¯2) crystal plane when the parent grain is subjected to compression force perpendicular to the c axis or tension force parallel to the c axis, that is, the grain will rotate around the direction of the c axis <101¯0>, and then a new oriented grain will be formed.
Twin removal has a great effect on the mechanical properties of sheet metal, and also has a great effect on the grain size, which is mainly reflected in the fine grain. When the number of twins reaches a certain value, secondary twins will be formed in the earliest twin crystal. As the number of twins continues to increase, there will be delivery between the twins and the grain boundary and between the twins, and the twin boundary will have a cutting effect on the grains (position B), resulting in a decrease in the grain size and a thinning effect on the grains in the organization.
2.5 Tensile fracture microstructure
The microstructure of tensile specimen is mainly composed of dimples. During the tensile process of the sample, the mismatch caused by rapid strain or the stress concentration caused by dislocation in the tissue leads to the nucleation of micropores. With the progress of stretching, the repulsion force of the dislocation is rapidly reduced, and then it is pushed into the micropores, resulting in the reactivation of the dislocation source and the formation of new dislocation, which constantly enter the micropores and make the micropores grow. A large number of micropores eventually aggregate at the fracture and leave traces to form dimples [5]. In addition, because the TA1 pure titanium sample will have twins during the stretching process, and the twin boundary will hinder the dislocation movement, the twins in the tissue and the grains divided by the twin boundary will become new deformation units, and the new deformation units will gradually evolve into fracture units during the stretching to fracture process, which leads to smaller dimples in the fracture of TA1 pure titanium sheet. The size of the dimple is also one of the characteristics of the uniform deformation of the material.
3. Closing remarks
(1) The microstructure of TA1 pure titanium plate is dominated by equiaxed α phase, and a trace of intercrystalline β phase exists between equiaxed α phases. The diffraction peaks of X-ray diffraction are all α phase. EBSD analysis shows that the α titanium of the close-packed hexagonal structure is 99.2%, and the β titanium of the body-centered cubic structure is 0.8%, which can be obtained from the polar diagram. The Angle between the ND direction and the grain is small, and the texture of the plate is strong along the RD direction.
(2) Tensile process Tensile process can be divided into three stages: initial stage, strengthening stage, neck contraction stage;
(3) The fracture morphology of tensile specimens is mainly composed of dimples.
