Intergranular cracking under creep-fatigue deformation in lamellar TiAl alloy

Abstract

Total strain range controlled low-cycle fatigue tests ( R=−1, strain rate=4×10 −3/s) indicate that lamellar structured Ti–46.6Al–1.4Mn–2Mo (at.%) alloy shows cyclic stability behavior regardless of test conditions. Fatigue life is drastically reduced with the application of tensile hold time and this reduction of fatigue life is understood to be due to the additional creep damage occurring during tensile hold time. Microstructural analysis for continuous fatigue tests indicates that surface crack is observed to be the main defect and the transgranular fracture mode is predominant. However, in creep-fatigue tests, the initiation of internal grain boundary cracks is remarkable and the intergranular fracture mode is predominant. This implies that the introduction of creep damage during tensile hold time is responsible for grain boundary weakness and intergranular cracking. Furthermore, grain boundary morphology observation and compositional analysis indicate that the grain boundary weakness and the intergranular cracking in the creep-fatigue test are induced by α 2→γ phase transformation at the grain boundary, which controls the creep-fatigue fracture behavior.