Crack‐tip Degradation Processes Observed during in situ Cyclic Fatigue of Partially Stabilized Zirconia

Abstract

It is proposed that reduced transformation zone widths in Mg‐PSZ in cyclically versus critically propagated cracks are due to reductions in the crack‐tip toughness, consistent with an intrinsic cyclic fatigue mechanism. Cyclic fatigue crack growth in Mg‐PSZ was observed in situ in a SEM. Following cyclic fatigue, the samples were critically broken and the fracture surfaces observed. Extensive crack bridging by the precipitate phase was observed near the crack tip, and it is proposed that this crack bridging significantly affects the material's intrinsic toughness. Frictional degradation of the precipitate bridges occurs during cyclic loading and hence reduces the critical crack‐tip stress intensity factor for crack propagation. Reductions in the critical crack‐tip stress intensity factor also lead to reductions in the transformation zone widths during cyclic loading and hence the level of crack‐tip shielding caused by phase transformation. This appears to be the mechanism of cyclic fatigue. A degree of uncracked ligament bridging was also observed and is linked with the frequency of random large precipitates. However, analysis shows that its effect upon crack growth rates under cyclic load is limited.