FIB加工領域の局所格子回転評価による疲労破面解析

抄録

EBSD analysis was carried out to investigate microstructures formed below a fracture surface. A push-pull fatigue test was conducted on a polycrystalline copper strip specimen having 3×3mm² cross section. From fracture surface observation, it is recognized that a primary fatigue crack was grown from right to left side, because striations arranged parallel to the left end. After the fracture surface observation, various positions in the fracture surface were milled by a focused ion beam (FIB) to obtain small flat planes normal to loading axis. The flat planes were analyzed with the EBSD system to estimate local lattice rotation. In a grain existing just below the fracture surface, gradual orientation change was detected. To estimate the orientation change quantitatively, lattice rotation gradient was calculated at each FIB-milled plane. According to the preliminarily-obtained relationship between the lattice rotation gradient and ΔK_I value, the distribution of the ΔK_I value over the fracture surface could be drawn. At lower ΔK_I value area of the fracture surface, change in the ΔK_I value was consistent with a crack growth direction that was estimated from fracture surface topology. However, the estimated ΔK_I value decreased at the left side of the fracture surface. Although there was no significant lattice rotation, grain refinement was recognized at the left side.