Study On Size-effect Of Tensile Deformation Behavior Of Pure Titanium

With the gradual development of modern industry towards lightweight,miniaturization,Micro molding,ultra precision machining and other microfabrication technologies,the application of materials with micro size is used more widely.However,there is a size effect in micro-sized metallic materials,caused by which the properties of materials are affected by the size of sample or the size of grains,making the micro-sized metallic materials to present different property characteristics from metallic materials with common size.In this paper,industrial pure titanium TA2 was used as the material.The grain size,crystal orientation and dislocation configuration of the pure titanium sheet with thicknesses of 50 ?m,100 ?m,200 ?m and 800 ?m were controlled to almost the same by annealing heat treatment.The tensile properties of annealed pure titanium with different thicknesses were tested by conventional tensile tests.It was found that the strength and elongation decreased with the decrease of the thickness of samples.Through the in-situ stretching technique based on OM-DIC,the tensile deformation behavior of pure titanium with different thickness was studied.The strain distribution regularity of pure titanium with different thickness under different engineering strain was established macroscopically: with the decrease of the thicknesses,the strains decreased at which the strain localization phenomenon occurred.That was the smaller the uniform elongation of the material,the worse the uniformity of strain distribution,indicating that the ability of material to overall generate plastic deformation declined.The distribution of longitudinal strain was consistent with the necking degree,and the distribution of shear strain was consistent with the fracture path.The in situ tension technique based on SEM-DIC was used to study the tensile deformation behavior of grains of pure titanium sheets with different thickness.The grain deformation regularity of pure titanium sheets with different thickness was reflected by analysising the strain distribution of grains of pure titanium sheets with different thickness under different engineering strain microscopically: at the same engineering strain,with the decrease of the thickness,the proportion of grains with compressive strain distribution dereased,which means that the proportion of undeformated grians decreased;the discrepancy of the grain strain distribution enhanced,which means that the discrepancy of grain deformation degree enhanced.And it can be concluded that the micro deformation consistency became worse with the decrease of thickness of the pure titanium sheets.The microstructural observation and the macroscopic and microscopic deformation rules reveal that the fundamental reason of the influence of the thickness dimension on the tensile properties of pure titanium lies in the grain orientation in the thickness direction.For the plasticity,the thinner the thickness of pure Ti was,the fewer the number of grains in the thickness direction was,so that the role of the deformed grains at the beginning was relatively important.And this limited compatible deformation between grains,so that the degree of deformation of each grain is large,and then the strain distribution on the macroscopic was not uniform,more prone to strain concentration.In addition,when the thickness decreased to 50 ?m,the twin system began to form twins crystals which induce the stress and strain to be further concentrated,so the thinner the thickness was,the worse the plasticity was.For the strength,the thinner the thickness of pure Ti was,the fewer the number of grains was,so the weaker the total carrying capacity of the grains was.In addition,a large proportion of surface grains,dislocation can slide out the free surface,coupled with twin deformation induced stress concentration,these factors have reduced the effect of work hardening of materials,resulting in the thinner the thickness of pure Ti was,the lower the strength was.