High Cycle Fatigue Behaviors at High Temperatures of a Ti2AlNb-Based Alloy

High cycle fatigue (HCF) behaviors of the bimodal size lamellar O phase microstructures of Ti-22Al-25Nb alloy are studied at the potential service temperatures. The axially-tensile-compressing HCF strength limits of this alloy are measured to be 470MPa at 650 degrees C and 400MPa at 700 degrees C via the small sampling up-and-down method (SSUDM). The ratio of the maximum stress and the minimum stress (R=sigma(max)/sigma(min)) is -1 while the run out fatigue life is 10(7) cycles. It is found that transformation B2+O occurs in the mode of cellular precipitations at initial B2 grain boundaries during the HCF tests. The transformation results in inhomogeneous regions in the microstructure. In those transformed regions, the acicular O phases are clearly coarser than in the normal regions. The HCF micro cracks preferentially initiate in the inhomogeneous regions along the slip lines in phase or at the O/ boundaries perpendicular to the slip lines. The cleavage fracture characteristic of the bimodal size lamellar O phase microstructure of this Ti2AlNb-based alloy has significant effect on the HCF crack propagation. Flat fracture surface will form on cleavage planes in B2 grains. Moreover, different directions of cleavage planes in different B2 grains will deflect the straight HCF crack at B2 grain boundaries. It is the B2 matrix rather than the lamellar O phase in the bimodal size lamellar O phase microstructure that provides the crack-tip shielding effect. B2 grain boundaries are also beneficial to increase the inhibition of HCF crack propagation.