| Abstract Scope |
Crack growth characteristics under dwell-fatigue loading are investigated in this paper for a new PM nickel superalloy developed by Rolls-Royce plc. In order to achieve a good balance of mechanical properties, it is critical that an appropriate heat treatment be defined for the alloy. Here, the influences of cooling rate after solution heat treatment on γ' size and distribution, grain boundary serration and dwell-fatigue crack growth resistance are first investigated. A medium cooling rate has been found to produce good dwell-fatigue crack growth resistance and has been carried forward to a comprehensive matrix of dwell-fatigue growth testing in air with varied temperatures (700 and 760 C), initial K values, and testing procedures (constant amplitude loading and load shedding). Tests have also been interrupted to allow detailed examination of crack tips using scanning electron microscopy. Notably the dwell time is 120 s (positioned at maximum load), and the applied stress ratio is fixed at 0.1. Significant variations in behaviour are observed at both temperatures. It is concluded that the variation in crack growth rates results from the selection and interactions between two different time-dependent mechanisms: environmentally assisted oxide forming and cracking, and environmental independent creep deformation and creep crack growth. When the environmentally related mechanism operates alone, the fastest crack growth rates are obtained. This study demonstrates that the environmentally assisted, sustained fast crack growth can be inhibited with a combination of an appropriate microstructure and a defined range of mechanical driving force. |