Dislocation structures around propagating fatigue cracks in iron

Abstract

Dislocation structures around the fatigue cracks for known stress intensity factor ranges were observed through an electron microscope operating at 2000 kV. To prepare thin foils for this purpose, a jet electropolishing technique has been developed, and regions in the vicinity of crack tips, as close as 0.1 μm from the sides, could be preserved in these foils. There were found 1) in front of the crack tip, substructures characterized by considerably dense dislocations in the cells just ahead of the tip and the formation of a lot of cells elongated in one direction, and 2) in the vicinity of the crack sides, a very fine cell structure with the interior of the cells showing relatively low density of dislocations. Besides cracks growing along the boundaries of cells or grains, some cracks penetrating into cells were observed. However, any crack supposed to be formed by linking of voids was not found. The misorientation between adjacent cells formed near the crack was generally a few minutes of arc, but the value as high as about 10° was sometimes observed immediately ahead of the crack tips. From the difference between the structures ahead of the tip and those in the vicinity of the sides, a brief discussion is made about structural changes produced when the crack tip has passed.