Fatigue crack propagation behavior in magnesium single crystals

The fatigue crack growth behavior of magnesium single crystal was investigated in order to clarify the orientation dependence of fatigue crack propagation mechanisms. Four types of compact specimens with respect to active slip and twin systems at a crack tip were prepared from magnesium single crystals grown by the Bridgman technique. In the case of the plane and the direction of notch are (1(2)over bar 1>0) and [1 0(1) over bar 0] respectively, fatigue crack propagated parallel with the notch, while the fatigue crack propagated inclined 30 degrees from the initial notch in the case of (1 0(1) over bar 0) [1(2) over bar 10] notch. In both cases, the growth directions of their cracks were [1 0(1) over bar 0] and the configurations of their crack surfaces were similar. From TEM observation, it was found that pyramidal slip systems with (c + a) Burgers vector were activated at the crack tip. From these results, a fatigue crack propagation mechanism was proposed. In the case of (1(2) over bar 10) [0001] notch, fatigue crack propagated to [0001] at low stress intensity factor range, DeltaK and the growth direction was deflected in parallel to basal plane at high DeltaK. In the case of (0001) [1(2) over bar 10] notch, fatigue crack propagated along the basal plane being accompanied by {1 0(1) over bar 2} twins. The fatigue crack growth rate of the specimen with (1 0(1) over bar 0) [1(2) over bar 10] notch was the lowest in the four type specimens. It is concluded that the fatigue crack propagation behavior of magnesium changes greatly with crystallographic geometry of notch.