Ni-based superalloys used for turbine discs are typically produced via powder metallurgy, a process which introduces undesirable non-metallic inclusions. Inclusions can be regarded as fatigue crack nucleation hot-spots due to their differing mechanical properties compared with the matrix they are embedded in. In this thesis, a series of models and experiments were used to investigate the mechanistic drivers of fatigue crack nucleation in the vicinity of non-metallic inclusions.
The drivers of decohesion and fracture of inclusions, often precursors to crack nucleation, were found to be the normal stress acting on the interface and the inclusion maximum principal stress, respectively. Exact values of either criterion were found using a cohesive zone model in a crystal plasticity finite element (CPFE) model faithfully representative of a real microstructure under low cycle fatigue. Decohesion and inclusion fracture were contrasted against slip-driven nucleation by a stored energy criterion. The key finding here was that decohesion and inclusion fracture marginally reduce fatigue life.
The comparative fatigue performance of an inclusion, a twin boundary and a triple-junction were studied in a synthetic CPFE microstructure. The inclusion recorded a significantly lower fatigue life compared with the intrinsic microstructural features. Various hardening models were used to investigate cyclic decohesion in a stress-controlled regime. Under no hardening model was cyclic decohesion predicted, strongly suggesting that decohesion is purely a function of applied stress within the first cycle.
A discontinuity tolerant digital image correlation algorithm was developed to study fatigue crack nucleation near a non-metallic agglomerate at 300°C. Decohesion and fracture of inclusions occurred already within the first cycle of loading. Microcracks nucleated throughout the inclusion agglomerate after 6000 cycles. In addition, a fatigue crack nucleated adjacent a twin boundary in a coarse grain neighbouring the agglomerate. A high (angular) resolution electron backscatter diffraction (HR-EBSD) analysis and a discrete dislocation plasticity (DDP) model suggested that strong build-up of GNDs and slip near twin boundary owes to the elastic anisotropy of twin and parent.