Effect of crystallographic orientation on fatigue crack growth of an Fe-3 mass%Si single crystal

Abstract

The fatigue crack growth behavior of Fe-3 mass%Si single crystals has been investigated in the laboratory air at room temperature. Three types of SEN specimens A, B and C with different notch orientations were prepared. The notch plane and direction in the A-specimen were (110) and [001], and those in the B-specimen and in the C-specimen were (110) and [110], and (001) and [100], respectively. A definite dependence of crystallographic orientation on the fatigue crack growth behavior is obtained. The crack in the A-specimen deflects approximately 30° from the notch plane and subsequently changes its growth direction into (110) [001]. Striations aligned parallel to [110], [111] and [131] are observed on the fatigue surface. These results indicate that the crack extends by alternating shear on two intersecting slip systems {112}〈111〉 and {011}〈111〉 and that the crack planes are to_be {110} which is a bisector of the activated slip planes. The crack in the B-specimen grows parallel to the [110] notch direction. Cleavage-like features with lines parallel to the [113] direction which is the direction of the intersection between (121) and (211) are observed on the fatigue surface. If two sets of slips ((121)[111] and (211) [111]) are activated at the crack tip and the dislocation reaction occurs to form sessile dislocations, cracking may occur on the (110) which is the maximum tensile stress plane due to stress concentration of dislocation pile-up during cyclic loading. The fatigue crack propagates along (100)[001] in the C-specimen. Cross-hatch patterns composed by two intersecting 〈011〉 furrows are observed on the fatigue surface.