Crack growth thresholds in nickel-based superalloys under dwell-fatigue loading.

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Civil aviation requires the development of nickel-based superalloys to reduce life cycle cost. A higher operating temperature will generate a greater volume of thrust which increases efficiency. The development of RR 1073 is designed to increase the entry temperature to 800 degrees Celsius (from 727 degrees Celsius in previous alloy RR 1000). Resistance to dwell fatigue crack growth is now regarded as one of the primary parameters against which the discs have to survive to achieve an increase in service life. Discs are subjected to dwell fatigue at elevated temperature, which consists of long-term stress cycles for a hold time at maximum stress. A mechanism change from cyclic dependent transgranular fracture to time dependent intergranular fracture can occurs at longer dwell times. This project will study underlying micro-mechanisms of dwell crack growth and in particular will define conditions under which intergranular crack growth occurs in terms of crack growth threshold behaviour across a range of dwell times, temperatures and testing procedures. Insight into this phenomenon will be hugely beneficial to predictions of life in-service.