Effect of anisotropy on the mechanical testing of Ni-based single-crystal superalloys

Thesis

With the objective of generating the maximum energy possible from a given fuel supply, both the aerospace and power generation sectors are engaged in a continuous endeavour to enhance the efficiencies of gas turbine engines. Elevating the turbine entry temperature (TET) theoretically promises improved performance; but material limitations restrict the extent to which this can be achieved. Ni-based single-crystal superalloys, renowned for retaining their high yield strengths at extreme temperatures, have been created to break these boundaries. In this thesis, we investigate the mechanical properties of the Ni-based single-crystal superalloy STAL15 from three distinct angles.

Firstly, we conduct a comparative analysis of the material’s mechanical behaviour in its ⟨001⟩ and ⟨011⟩ crystallographic orientations, aiming to add to the understanding of how the material is affected by changes in temperature and strain rate.

Secondly, we explore different yield strength definitions in order to understand how they are related to the anomalous yield phenomena, and if the presence of anomalous yield is dependent on the definition used.

Finally, we employ an orthogonal dual-camera setup using multiple digital extensometers to better analyse the mechanical properties of the single-crystal STAL15. This configuration proves instrumental for characterising the elastic anisotropy of both crystallographic orientations for different temperatures and strain rates.

Our approach combines mechanical tests, Digital Image Correlation (DIC), Scanning Electron Microscopy (SEM), and optical profilometry to analyse the material’s mechanical properties and microstructural characteristics. We aim to provide the reader with an alternative perspective on how to approach not only the mechanical properties of STAL15, but all Ni-based single-crystal superalloys in general; potentially contributing to our understanding of this area.

Authors: Polatoğlu, C

• DOI: 10.5287/ora-zbemx45wp
• Type of award: MSc by Research
• Level of award: Masters
• Awarding institution: University of Oxford
• Language: English
• Keywords: Ni-based single-crystal superalloys; STAL15; tensile testing; yield strength; mechanical properties
• Subjects: Ni-based single-crystal superalloys; Tensile testing; Metallurgy; Engineering