EFFECT OF CRYSTALLOGRAPHIC ORIENTATION ON FATIGUE-CRACK GROWTH OF AN FE-3 MASS-PERCENT-SI SINGLE-CRYSTAL

The fatigue crack growth behavior of Fe-3 mass%Si single crystals has been investigated in the laboratory air at room temperature. Three typ es of SEN specimens A, B and C with different notch orientations were prepared. The notch plane and direction in the A-specimen were (110) a nd [001], and those in the E-specimen and in the C-specimen were (110) and [1(1) over bar0$], and (001) and [100], respectively. A definite dependence of crystallographic orientation on the fatigue crack growth behavior is obtained. The crack. in the A-specimen deflects approxima tely 30 degrees from the notch plane and subsequently changes its grow th direction into (110) [001]. Striations aligned parallel to [1(1) ov er bar0$], [1(1) over bar1$] and [131] are observed on the fatigue sur face. These results indicate that the crack extends by alternating she ar on two intersecting slip systems {11(2) over bar}[111] and {01(1) o ver bar}[111], and that the crack planes are to he {110} which is a bi sector of the activated slip planes, The crack in the B-specimen grows parallel to the [1(1) over bar0$] notch direction. Cleavage-like feat ures with lines parallel to the [1(1) over bar3$] direction which is t he direction of the intersection between ((1) over bar 2 (1) over bar) and (2(1) over bar1$) are observed on the fatigue surface. Ii two set s of slips(((1) over bar 2 (1) over bar)[111] and (2(1) over bar1$[<(1 1)over bar 1>]) are activated at the crack tip and the dislocation rea ction occurs to form sessile dislocations, cracking may occur on the ( 110) which is the maximum tensile stress plane due to stress concentra tion of dislocation pile-up during cyclic loading. The fatigue crack p ropagates along (100)[001] in the C-specimen. Cross-hatch patterns com posed by two intersecting [011] furrows are observed on the fatigue su rface.