Novel approaches for the characterisation of spinodal alloys

The introduction of compositional heterogeneity is one of the most widespread strategies for strengthening alloys. In spinodal alloys, this is achieved by diffusion-controlled elemental clustering, known as spinodal decomposition, during ageing, which produces a compositionally modulated microstructure. This typically confers a large strengthening increment and is therefore exploited in several industrially important alloys with load‑bearing applications. However, spinodal decomposition is an inherently metastable phenomenon and is often accompanied by the formation of more thermodynamically stable phases, through processes such as discontinuous precipitation. This generally leads to softening and embrittlement and has thus precluded more widespread use of spinodal alloys. These limitations have motivated efforts to improve the microstructural stability of spinodal alloys, primarily through trace element additions and thermo‑mechanical processing. For reliable assessment of the evolution and stability of spinodal alloy microstructures, the use of advanced characterisation techniques is critical. Conventional microscopy and diffraction‑based methods often provide valuable insights. However, the former are often too localised, while the latter tend to lack the necessary resolution to distinguish between features arising from spinodal decomposition and those from competing phase transformations. There is therefore a need to explore novel approaches for characterising these alloys. The work presented in this thesis explores the utility of two techniques, resonant ultrasound spectroscopy and high‑resolution synchrotron X‑ray diffraction, for investigating the phase transformations that operate in spinodal alloys. Both techniques were employed to characterise the ageing response of the industrially important Cu‑15Ni‑8Sn (wt%) alloy, with results complemented by microstructural analyses obtained via electron microscopy. Resonant ultrasound spectroscopy measurements revealed that systematic variations in elastic moduli occurred during ageing. These were correlated with observations of spinodal decomposition, ordering, and discontinuous precipitation, demonstrating the potential of this technique for detecting microstructural changes in spinodal alloys. Meanwhile, using high‑resolution synchrotron X‑ray diffraction, it was possible to identify phase transformations that were not detectable using standard laboratory X‑ray sources. The short acquisition time also enabled time‑resolved data to be collected in situ during ageing, allowing the concurrent phase transformations to be monitored simultaneously. High‑resolution synchrotron X‑ray diffraction was subsequently utilised to investigate the microstructural evolution of the developmental Au‑34Pt‑24Pd (at.%) alloy, which was designed to have a low solvus temperature to ensure good processibility. Results revealed the relative rates of spinodal decomposition and discontinuous precipitation across a range of ageing temperatures, which were used to identify heat treatment parameters for achieving desirable mechanical properties. Due to the temporal resolution achieved in the diffraction studies of both alloys, it was possible to verify and quantitatively assess the power‑law coarsening behaviour of the spinodally modulated microstructures and the adherence of discontinuous precipitation to the Avrami model for nucleation and growth. Collectively, the work presented in this thesis provides novel insights into the microstructural evolution of both existing and newly developed spinodal alloys, as well as their corresponding effects on mechanical properties. It is hoped that the characterisation techniques employed here will be applied in future work to gain a deeper understanding of spinodal decomposition and related phenomena, thereby supporting the development of new high‑performance spinodal alloys for demanding structural applications.

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

• Hogg, James Morgan

Advisor dc:contributor.advisor

• Stone, Howard

• Diffraction
• Discontinuous precipitation
• Phase transformations
• Resonant ultrasound spectroscopy
• Spinodal decomposition

Licence

• https://www.repository.cam.ac.uk/bitstreams/9d252ab9-a68b-4d18-a4ee-c2e0540f99ec/download
• http://purl.org/NET/rdflicense/allrightsreserved

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.122796

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/391803