Elastic and plastic microscopic deformation of polycrystalline pure titanium under tension

Commercial pure titanium has been widely used in aerospace, chemical, and biomedical industries for its lightweight, high corrosion resistance, high strength, high heat resistance and good biocompatible properties. Pure titanium takes hexagonal closed-pack structure with anisotropic elasticity and plasticity, and most of the components are polycrystalline aggregate with different crystal orientations. Therefore, inhomogeneous microscopic deformation always occurs by mechanical loading from the elastic condition and the inhomogeneity brings about various damages such as inhomogeneous plastic deformation, microcracking, necking, and so on. It is therefore important to investigate microscopic inhomogeneous deformation under elastic and plastic conditions. However only a few researches deal with the microscopic inhomogeneity because it is not easy to measure a small deformation of crystal grains under elastic condition or very small load. Recently, digital holographic microscope has been developed and a high-speed measurement of surface height of materials became possible with an accuracy of less than a micron meter in a relatively wide area. In this study, a tensile test of a plate specimen of commercial pure titanium was carried out on the stage of digital holographic microscope, and the microscopic deformation of grains was observed and measured under elastic and plastic conditions. During the test, the distribution of height of grains was measured in a fixed area on the specimen surface at each tensile loading step and the height distributions at different loads were correlated. It was found from the measurement results that each grain shows different deformation even under elastic condition with a small load, and the inhomogeneous deformation expanded with increasing the load to plastic condition. Also, a strong correlation was found in the height distributions under elastic and plastic conditions. This fact suggests that the microscopic deformation under plastic condition is predictable from that under elastic condition.