The effect of hold time on the microstructure-based creep-fatigue interaction failure mechanism of Nb-W refractory alloy

Strain-controlled creep-fatigue interaction (CFI) tests were carried out at 900 ℃ under different hold times to investigate the structure-environment-fatigue failure mechanism of Nb-W refractory alloy. The results show that the grains of Nb-W alloy are equiaxed with no obvious texture. Within the grains, there are many uniformly distributed ZrO₂ second phase particles entangled by dislocations, which can readily lead to stress concentration under alternating loading. The creep-fatigue life of the Nb-W alloy consistently decreases with increasing hold time, showing no saturation phenomenon. In addition, the hold time has a significant effect on the area and position of the hysteresis loop. The fatigue fracture surface can be divided into three distinct zones based on morphological characteristics, namely the stable propagation zone (SPZ), rapid propagation zone (RPZ), and instantaneous rupture zone (IRZ). Compared with the transgranular fracture in the SPZ under pure fatigue, the creep damage caused by hold time under CFI condition makes the transgranular-intergranular mixed fracture happen in SPZ. Additionally, some deep-long cracks with intergranular fracture mode are observed near the IRZ. Three-dimensional fractographic analysis under different hold times (0 s, 10 s, and 120 s) indicates that the IRZ height is 972 μm, 1162 μm, and 5406 μm, respectively, showing a positive correlation with hold time. Electron back scatter diffraction analysis demonstrates that the hold time significantly influences damage progression. In particular, the investigation of the mean core misorientation and the geometrically necessary dislocation can provide more fundamental information on fracture physics. In particular, the CFI crack initiation-propagation mechanism of the Nb-W refractory alloy is summarized.