Investigation of new synchrotron X-ray and neutron techniques for local crystallographic analysis of Ni-based single crystal superalloys

Nickel-based single crystal superalloys exploit a γ/γʹ microstructure, providing exceptional high- temperature strength and environmental stability, outperforming other known alloy systems. These properties make them useful for gas turbine applications. However, enhancing gas turbine efficiency requires higher operating temperatures, which place greater temperature and stress demands on these materials. One critical factor influencing their mechanical properties is the lattice misfit between the γ and γʹ phases. Consequently, extensive research has been dedicated to optimising lattice misfit values to achieve the desired mechanical performance. Accurate determination of lattice misfit is challenging due to the similar cubic crystal structures of the two phases. Conventional diffraction analyses are complicated by peak overlap of these phases, making precise peak position measurements difficult. To address this, a series of synchrotron X-ray, and neutron diffraction studies were conducted to explore the possibility of more reliable lattice misfit determination and how it varies across the microstructure as well as during deformation. Firstly, a monochromatic synchrotron X-ray source was used to investigate a high misfit Ni-based single crystal superalloy, which involved rocking the sample to ensure the Bragg condition was maintained during in situ loading at elevated temperature. Whilst this technique highlighted difficulties in fitting diffraction peaks where the Ewald sphere was not perfectly intersecting the reciprocal lattice point, it also revealed substantial differences in the lattice parameter when sampling regions across the γ/γʹ interfacial region and the bulk material. This suggested that further exploration of the interfacial region and the effect it has on the overall mechanical properties of the material is required. For this reason, a novel X-ray diffraction (XRD) approach utilising a synchrotron X-ray nanoprobe was developed to analyse local lattice parameters in the γʹ and γ phases. As a result of current technological constraints, and challenges in aligning single crystal superalloys to satisfy the Bragg condition, there were complications in accessing the diffraction signal. However, the method yielded reasonable results and demonstrated capabilities beyond those of conventional techniques that only probe larger volumes of material. Local misorientations and significant lattice parameter variations were observed, demonstrating the heterogeneity of the microstructure at smaller length scales. As such, future in situ studies using this technique appear promising. Whilst in situ heating experiments were unsuccessful in yielding diffraction data, the X-ray fluorescence (XRF) collected in conjunction demonstrated the potential for compositional analysis at elevated temperatures, particularly for polycrystalline or thermally unstable alloys. Additionally, pulsed source neutron diffraction was used to track lattice rotations during tensile deformation, employing a multi-peak 2D Gaussian fitting approach. While the measurements from this experiment do not directly indicate how slip progressed the extent of misorientation occurring during slip was successfully quantified. Furthermore, conventional fitting techniques were used to assess the evolution of lattice misfit during tensile deformation, corroborating the measurements obtained in previous studies. This research demonstrates both the challenges and potential of diffraction techniques for enhanced characterisation. While the deconvolution of γʹ and γ phase peak positions remains a significant challenge, valuable insights have been gained into stress gradients at interfacial regions and local misorientations within the microstructure. Furthermore, a foundation for methods to probe local microstructural changes and track lattice rotation during deformation have been established.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2025

Author dc:creator

• Pitchforth, Jessica

Advisor dc:contributor.advisor

• Stone, howard

• Neutron diffraction
• Ni based superalloys
• single crystal
• Synchrotron X-ray diffraction
• XRD

Licence

• https://www.repository.cam.ac.uk/bitstreams/84522916-fb29-466f-9f9b-cb95f2336f32/download
• http://purl.org/NET/rdflicense/allrightsreserved

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.126253

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/397039