Font Size: 

To evaluate the effect of soft zones along β grain boundaries on crackpath during fatigue crack growth and on the onset of unstable cracking (KIc) of βtitanium alloys, this comparison study included large grained β annealed and βprocessed as well as fine grained α + β processed microstructures with extremely fineand coarse α plate sizes, and, thus, with very high (σ0.2 = 1500 MPa) and low yieldstress (1050 MPa). When increasing yield stress level from 1050 to 1500 MPa,basically, fatigue crack growth threshold level of ~6 MPa√m is reduced to a level of ~4MPa√m, whereas fracture toughness values of 48 MPa√m (α + β processed) and ~70MPa√m (β annealed or β processed) both dropped by 50 %. Differences in fatiguecrack growth behavior are mainly attributed to the effect of lamellar matrix on crackfront profile. While part of the increase in fracture toughness is attributed to theincrease in plastic deformation of the lamellar matrix before onset of unstable crackadvance occurs within the soft zones at the grain boundaries, crack growth retardingsteps in the crack front lead to an increase in the ductile transgranular fracture withdecreasing strength level, and, thus, also contribute to the observed increase in fracturetoughness.