Ultra–precision cutting has its own problems that are not taken into the consideration in ordinary cutting. One of such problems is grain boundary step （GBS） formation. The GBS is a step forming along a grain boundary on ultra–precision finished surface of polycrystalline metals. The height of GBS can be higher than that of cutting marks, resulting in reducing the quality of the finished surface. Although grain boundary sliding is considered as the origin of GBS formation, the growth mechanism of GBSs is still not clear. In the studies of grain boundary sliding in creep phenomenon, the apparent activation energy was calculated from the temperature dependence of creep strain rate, and the mechanism of grain boundary sliding was proposed based on the calculated activation energy. The apparent activation energy will be effective measure to study the growth mechanism of GBSs. In this study, in order to calculate the apparent activation energy of GBS growth process, the change in the height of GBS at －30°C, 25°C, 60°C and 90°C was measured. The height of GBS saturated in shorter time at higher temperature. The height increasing with time was higher at higher temperature. The value of the apparent activation energy was about 20 kJ/mol, which was significantly smaller than the apparent activation energy of the grain boundary diffusion （104 kJ/mol）. Such difference in the apparent activation energy indicates that the growth of grain boundary steps on ultra–precision finished surface originates from mechanisms different from the grain boundary diffusion.
The relationship between the grain boundary steps on the specimen surface developed by cyclic bending and fatigue crack generation on the grain boundaries inclined at about 45 degree to the principal stress axis has been discussed for 4N-aluminum wire. The crystal lattice rotation near the grain boundary induced by slip bands in a grain resulted in a strain incompatibility with the neighboring grain. Due to the shear deformation along the grain boundary, the individual grain boundary steps coalesce and form a grain boundary microcrack, resulting in the fatigue crack generation and propagation. The strain incompatibility is developed at the grain boundary between the grain accompanied with crystal rotation over 10 degrees in the vicinity of the boundary and the grain with almost no crystal rotation. Based on the approximation that only the primary slip system is operative, the step height due to the crystal slip deformation is estimated from the lattice rotation near the grain boundary.
The strain measurement by conventional two-dimensional digital image correlation (DIC) method was applied to the strain inhomogeneous in the vicinity of grain boundaries of pure aluminum polycrystals. Even if a polycrystal is deformed uniformly in macroscopic view, nonuniform deformation occurs in microscopic scale. It is important to investigate local plastic strain concentration at grain boundaries in metals and alloys because the accumulation of dislocations near the grain boundaries can generate microcracks. Therefore, the purpose of this study is to measure the strain distribution in such microscopic dimension using digital image correlation method, which is a non-contact strain measurement by means of digital image processing. It was found that DIC measurement in submicron region is possible by attempting to refine the random pattern speckle. Similarly, indentation hardness near grain boundary after plastic deformation was examined by using micro-load indentation testing with a Berkovich indenter.