Accelerated design and testing of new nickel-based superalloys

Thesis

Nickel-based superalloys have emerged as the materials of choice for high-temperature jet propulsion and industrial gas turbine applications due to their unique combination of resistance to loading under static, fatigue, and creep conditions, as well as to environmental degradation. The specific fuel consumption and CO2 emissions of turbines can be significantly reduced by increasing the operating temperature, thus providing a continued drive within the materials research community to develop new alloy grades with improved properties.

However, design and qualification activities are difficult and costly, which explains why the time needed to insert novel materials into industrial applications can be notoriously long. The research of the present work was carried out with the aim of accelerating the overall design—make—test—analyse development cycle for new superalloys, with a focus on the design and testing of single-crystal compositions with improved creep properties.

First, a novel mechanical testing system using miniaturised testpieces is introduced. Rapid testing methods involving temperature ramping and stress relaxation are employed to extract key high-temperature strength parameters, with the quantitative data measured then compared to conventional tests.

Second, operating creep damage mechanisms during such tests are investigated with cutting-edge electron microscopy characterisation techniques and dislocation dynamics modelling. This combination of experiments and simulations yields an improved understanding of damage evolution and of the roles played by alloy composition and microstructure in imparting creep strength.

Third, these new insights into damage mechanisms inform a novel physically based model for the creep resistance of single-crystal superalloys. A comprehensive critical assessment of previous modelling methods is carried out in an effort to approach best practice. Predictions are tested against a large experimental data set and it emerges that the present model is more accurate for alloy design purposes than existing merit indices from the literature.

Last, a set of trial alloys with carefully controlled chemistries is extensively analysed to reveal the complex influence of composition on physical properties. Experimental results provide an important benchmark for thermodynamic calculations and help derive a set of guidelines for alloy design within this particular compositional space.

Authors: Sulzer, S

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: mechanical modelling; superalloy; creep; miniaturised testing; alloy design
• Subjects: Materials; Engineering