Fatigue crack propagation in titanium single crystals

The fatigue crack growth behaviour of alpha-titanium single crystals has been investigated at room temperature in laboratory air. Four types of CT specimens with different notch orientations A, B, C and D were prepared by a strain-annealing technique. The notch plane and direction in the A-specimen were (1 (2) over bar 10) and [10 (1) over bar 0],those in the B-specimen were (01 (1) over bar 0) and [2<(11)over bar>0], those in the C-specimen were (0001) and [10 (1) over bar 0], and those in the D-specimen were (0001) and [2<(11)over bar>0], respectively. The crack in the A-specimen deflects 30 degrees to the notch direction and propagates parallel to (01 (1) over bar 0) [2<(11)over bar>0]. The crack in the B-specimen propagates along the notch direction ([2<(11)over bar>0]). Striation-like markings parallel to the [0001] direction are observed on the fatigue surfaces of both the A- and the B-specimens. The crack is, therefore,deduced to extend by alternating shear on two intersecting prismatic slip systems ((1 (1) over bar 00) [11 (2) over bar 0] and (10 (1) over bar 0) [1 (2) over bar 10]) in the A- and the B-specimens. The crack in both the C- and the D-specimens propagates roughly parallel to the (0001) plane. Coarse and fine traces which correspond to {10 (1) over bar 2} twin systems are found near the crack surfaces. Fractographic observations indicate that the crack extends across the twin bands which have been formed at the crack tip in the C- and the D-specimens.