Microcrack initiation and propagation on Al2O3 inclusions contained powder superalloys studied by in situ high temperature microscopy

Traces of inclusions in powder superalloys may seriously damage the overall performance of the alloy, and the initiation and propagation of cracks in powder superalloys caused by inclusions are still the key factor affecting the overall properties of the alloy. Therefore, a systematic and nanoscale study on the mechanism of microcrack initiation and propagation induced by typical inclusions at high temperature is necessary. An in situ study was carried out at 500 °C on an Al 2 O 3 inclusion-containing powder superalloy to reveal the morphology and grain orientation variations adjacent to inclusions under different cycle times during 111333 cycles. The results show that the initial crack starts from the sharp corner interface between the Al 2 O 3 inclusion and the matrix, and then expands to form the main crack. The crystal orientation at the crack tip changes obviously, and the microcracks mainly propagate along the Σ3 twinning accompanied by large-angle grain boundaries. In the middle stage of crack development, dislocations multiply rapidly and are then blocked by the grain boundaries around the crack and at the tip. The crack propagation path coincides with the dislocation accumulation region, which is accompanied by crystal orientation variation and a significantly changed Schmid factor. After 111333 fatigue cycles, the deformation-induced defects in the γ channel far from the crack tip are mainly dislocations. High quantities of stacking fault aggregates at the crack tip, including single-layer and multi-layer atomic stacking faults along the (11-1) plane; this indicates that the deformation at the crack tip propagates and transfers through microtwins. Our in situ fatigue crack studies provide direct evidence for understanding the fatigue mechanism affected by inclusions.