Abstract
A cyclic tensile test was carried out using a plate specimen of commercial pure titanium on a digital holographic microscope stage. Microscopic deformation of the grains was observed, and their height distribution was measured on the specimen surface. Each grain showed nanoscopic movement up and down, as well as reverse movement corresponding to specimen loading and unloading. We suggest that the different grain-specific changes in height were caused by microscopic inhomogeneities in the material, such as differences in the crystal orientation and geometries of both the surface and subsurface grains. Changes in grain height increased with tensile load, and a strong relationship was found between the height changes that occurred under elastic and plastic conditions. This suggests that microscopic plastic deformation is predictable from microscopic elastic deformation. In order to investigate the plastic deformation of grains in more detail, slip-line angles were measured after the tensile test. We found slip lines with similar angles in neighboring grains, suggesting that the plastic deformation of grains was not independent, but rather was related to that of surrounding grains and influenced by the deformation of subsurface grains.
Highlights
Because industrial metals are usually composed of a large number of crystal grains with anisotropic elasticity and plasticity, an inhomogeneous microscopic deformation occurs as a result of external mechanical loads and internal thermal stresses
We found that the microscopic inhomogeneity of surface grains appeared as a small change in the height distribution, and that this shift in height distribution under plastic deformation was strongly correlated with that under elastic deformation
With respect to the change in vice versa) during loading and unloading, respectively, and this continued under the macroscopic height between loading and unloading, all grains showed opposite movements. The extent of these movements increased substantially at the beginning of sinking or vice versa) during loading and unloading, respectively, and this continued under the the plastic deformation stage
Summary
Because industrial metals are usually composed of a large number of crystal grains with anisotropic elasticity and plasticity, an inhomogeneous microscopic deformation occurs as a result of external mechanical loads and internal thermal stresses. Various methods for minimization of image distortion and noise reduction have been proposed, which have greatly improved accuracy Both plastic and elastic strain have been evaluated on the surface of a nickel-based superalloy [12]. It was found that grain size and the distribution of flow stress strongly influenced the development of surface roughness In this case, the drastic simplification of the shapes and constitutive equations of grains made it possible to carry out simulations of whole polycrystalline specimens. Recent studies on the inhomogeneous deformation and surface roughness of polycrystalline metals indicate that it is still difficult to investigate grain deformation when considering the three-dimensional geometries and crystal orientation of subsurface and interior grains. Grain slip lines were observed after the test, and we discuss the overall plastic deformation behavior of surface grains
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