Study of fatigue crack initiation and propagation mechanisms in an advanced Ni-based superalloy: effects of microstructures and oxidation

Abstract

Low Solvus, High Refractory (LSHR) alloy is a latest generation of turbine disc alloy which contains relatively high Co and Cr levels compared with incumbent alloys, this provides a low solvus temperature for enhanced high temperature processing versatility, and the alloy is designed to operate at higher temperatures to achieve higher thrust-to-weight ratio and higher fuel efficiency. Understanding the fatigue performance of the LSHR alloy along with the coupled influences of deformation and oxidation damage is important for both its practical application and in further development of turbine disc alloys.

Short crack tests have been conducted on coarse grained (CG) and fine grained (FG) LSHR alloys by uninterrupted and interrupted three-point bending with a replication procedure at room temperature, 650 and 725 oC in air and vacuum under a sine waveform (20Hz) and/or 1-1-1-1 trapezoidal waveform to investigate fatigue crack initiation and early propagation. The results show that fatigue lives in the LSHR alloy are shortened by high temperature, high oxygen partial pressure and low frequency loading waveforms which are associated with more intergranular fracture mechanisms. At room temperature, where the effect of oxidation is absent, twin boundary (TB) cracking in relatively large grains dominates the crack initiation process along with occasional crack initiation due to slip band cracking. Activation of the primary slip systems parallel to the TB at matrix and twin and high resolved shear stress associated with high Schmid factor (SF) are required for TB crack initiation. Cracks preferentially propagate along slip bands associated with high SF slip systems after initiation. But cracks also propagate along slip bands associated with slip systems with lower SF if the inclination angle between the slip band ahead of the crack tip and the crack segment of the crack tip is small enough to enable a steady transition (or non-deflected growth) of cracks across the grain boundary (GB).

At elevated temperatures (i.e. 650 and 725 oC), fatigue crack initiation and early short crack propagation behaviour are closely related to coupled deformation-oxidation damages. Plentiful crack initiation mainly occurs at GBs and/or ?/?? interfaces (in the FG variant) with bulged Ni/Co-rich oxides and Cr/Ti/Al oxide intrusion due to oxide cracking. Cracks subsequently propagate along oxidised GBs at the surface and exhibit significant crack coalescence in the final stages of fatigue life. In terms of crack propagation in the depth direction, this is a consequence of the competing effects between mechanically-driven and oxidation-assisted crack propagation, and exhibits transgranular or intergranular or mixed inter-transgranular propagation.

Formation of bulged Ni/Co-rich oxides and Cr/Ti/Al oxide intrusion along grain boundaries is accompanied by dissolution of ?? precipitates, and is closely related to the strain localisation which is associated with grain orientation and applied stress. The boundaries of high/low SF grains are preferential sites for bulged Ni/Co-rich oxides and Cr/Ti/Al oxide intrusion formation. Strain-assisted formation of internal GB Cr/Ti/Al oxide intrusion can facilitate coupled formation of GB bulged Ni/Co-rich oxides by providing abundant Ni and Co. Apart from the significant oxidation at grain boundaries and ?/?? interfaces, uniform surface oxide scale along with internal Al oxide particles form within grains and primary ??.

Studies of long fatigue crack growth (FCG) behaviour in LSHR alloy at 650 and 725 oC in air and vacuum under trapezoidal waveforms of 1-1-1-1 and 1-20-1-1 indicate that a CG structure possesses better FCG resistance due to the enhanced slip reversibility promoted by planar slip as well as the reduction in grain boundary area. The fatigue performance of the LSHR alloy is significantly degraded by the synergistic oxidation effect brought about by high temperature, high oxygen partial pressure and dwell at the peak load, associated with increasingly intergranular fracture features and secondary grain boundary cracking. Secondary cracks are observed to be blocked or deflected around primary ??, carbides and borides, and their occurrence closely relates to the roughness of the fracture surface, FCG rate and grain boundary oxidation. The apparent activation energy analysis provides a further insight into the underlying mechanism of the FCG under dwell-fatigue testing conditions, and confirms that oxidation fatigue is the dominant process contributing to the intergranular failure process. At high enough crack growth rates and at lower temperatures, mechanically-driven fatigue crack growth processes can outstrip crack-tip oxidation processes.

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ORCID for John McBride: orcid.org/0000-0002-3024-0326

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