Strain-controlled creep-fatigue interaction property of Nb521 refractory alloy: Hold time effect and life prediction

Strain-controlled creep-fatigue interaction (CFI) tests were conducted for Nb W alloy, i.e., Nb521 alloy, at 900 °C with a strain amplitude of ±0.3% under atmospheric conditions to study the effect of hold time on the CFI failure behaviour, with the results indicating that the creep-fatigue life of the Nb521 alloy decreased with an increase in the hold time. The decline rate gradually flattened, with no saturation effect observed. Furthermore, with an increase in the hold time, the hardening rate of the Nb521 alloy gradually decreased, but the softening rate remained high. Fatigue fractures can be divided into stable propagation, rapid propagation, and final fracture zones according to their morphological characteristics. Compared with the stable propagation zone of the pure fatigue fracture surface, the creep damage caused by the hold time resulted in numerous intergranular fracture features in the stable propagation zone of the CFI fracture surface, indicating that the crack propagation mode changed from a transgranular fracture to a mixed transgranular–intergranular fracture. The strain energy density exhaustion model under nonlinear damage summation better reflects the CFI damage behaviour, with the creep-fatigue life prediction result considered acceptable.