In-service parameters influencing gamma prime (\(\gamma\)’) evolution and its impact on dwell fatigue behaviour in a nickel based disc alloy

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Schulz, Fiona (2018). In-service parameters influencing gamma prime (\(\gamma\)’) evolution and its impact on dwell fatigue behaviour in a nickel based disc alloy. University of Birmingham. Ph.D.

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Abstract

The evolution of strengthening precipitates (y') under different conditions is detrimental to the performance of nickel based superalloys. Especially for engine disc applications, such y' characteristics in isothermal and stressed conditions and their implications on mechanical behaviour are important for lifing predictions. This project investigated the coarsening behaviour of y' in coarse grained (CG) RR1000 after isothermal heat treatments for various times (50 to 500 h) at temperatures ranging from 700 to 800°C. The experimental findings were compared with current precipitation models. Subsequently, the influence of different applied stresses under isothermal conditions was studied with particular emphasis on the tertiary L' precipitates. Finally, dwell crack growth tests were carried out at 700°C to investigate the impact of overageing on dwell fatigue
crack growth resistance. Methodical image analysis was applied to systematically study tertiary (5- 100 nm) and secondary (75 - 500 nm) r' precipitates. In addition to SEM, TEM analysis was performed on selected microstructure conditions to confirm trends observed. Overageing as a thermodynamic process has a greater impact on tertiary y' precipitate coarsening, especially with increasing temperature and treatment time. Externally applied stress fields, however, seem to influence secondary y' evolution to a greater extent. Isothermal heat treatments applied prior to dwell fatigue growth testing were found to noticeably affect dwell fatigue crack growth resistances after only 100 h at 700°C. Inadvertently, it was found that grain size distributions have a significantly greater influence on dwell fatigue crack growth behaviour, while overaging leads to continuous trends in dwell fatigue crack growth resistance improvement.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):