High-temperature rotary-bending fatigue characteristics of a high Nb-containing beta-gamma TiAl alloy

The rotary-bending fatigue tests of a high Nb-containing beta-gamma TiAl alloy based on microstructure control have been conducted at 800 °C. A fine nearly lamellar microstructure with the lamellar colony of 80 µm and slight content of B2 phase of 2% is obtained by an improved two-step heat treatment. The rotary-bending fatigue results perform the scattered S-N data, flat S-N fitting curve and an excellent fatigue limit reached at 355 MPa. SEM results exhibit an obvious transition between stable crack growth and fast fracture surface. Massive cracks initiate at lamellar interface, colony boundary and B2 phase on the specimen surface, which coalesce and propagate from the margin to the center of specimen. TEM analysis indicates that dislocation glide and mechanical twinning are the mainly deformation mechanism. The interlamellar and translamellar fracture result from dislocation pile-up and the inclined slip band or mechanical twinning, respectively.