Influence of crystallographic orientation on fatigue crack growth in strongly textured Ti--6Al--4V

Fractographic features of fatigue crack growth in a strongly textured Ti--6Al--4V bar were studied in order to correlate these features with the orientation of crystallographic deformation modes. The observations fell into two groups: flat fracture surfaces and striations normal to the overall crack growth direction when m values in the Paris equation were low (approximately 2.5); and ridged fracture surfaces and striations at various angles to the overall crack growth direction when m values were high (3.1--4.1). A consideration of the active deformation modes, [1010] slip and [1121] twinning, showed that only when deformation occurred symmetrically about the tip of the crack plane, and thus satisfied the continuum flow field set up in plane strain, did stable fatigue crack growth occur (low m values). Higher growth rate dependences on ΔK occurred as a result of asymmetric deformation about the crack tip by fatigue crack growth on a number of different fronts, sometimes arising due to (1123) slip, and monotonic fracture processes