Crystallographic fatigue crack growth in titanium single crystals

The crystallographic dependence of fatigue crack growth in titanium single crystals is characterized by striation formation, herringbone-pattern formation and twin-related basal plane separation. In crystals that are oriented favourably to prismatic slip, i.e. soft-oriented crystals, the crack grows parallel to a prismatic plane while forming striations. The crack is thought to extend by alternating shear on two intersecting planes composed of prismatic and pyramidal slip systems at the crack tip. When the crack grows along a direction within ∼40° of [0 0 0 1], herringbone patterns are formed by complementary operation of two sets of pyramidal slip systems containing the 〈c + a〉 slip directions. In crystals oriented unfavourably to the glide, i.e. hard-oriented crystals, the activation of { 0   1   1 ¯   2 } micro-twins participates in the crack growth parallel to the basal plane. The resistance to fatigue crack growth is higher in the hard-oriented crystals than in the soft-oriented crystals.