Microstructural Evolution in Refractory Metal Multi-Component Alloy Systems

Aeroengine and power-generation efficiency targets demand higher operating temperatures than those tolerated by current Ni-based superalloys, motivating the search for alternative high temperature materials. Refractory superalloys based on AlTaTiZr and AlMoNbTaTiZr have attracted particular interest, as early studies indicate that the essential attributes for elevated temperature service are accessible within this class. The principal challenge is that a single composition combining high-temperature strength, creep resistance, and oxidation resistance with room-temperature ductility has yet to be realised. This thesis addresses two linked objectives. First, it clarifies the microstructural formation pathway in RSA system by applying an in situ synchrotron diffraction diffracted-spot tracing approach to follow phase evolution during controlled thermal histories. Across AlTaTiZr and the canonical AlMoNbTaTiZr alloy, the method resolves the sequence of phase separation, ordering and subsequent transformations, provides transformation temperatures, and establishes a reliable, reproducible technique for deciphering the microstructural formation pathway. Second, an investigation addresses a persistent alloy-design constraint. Experience with Al-Zr-containing RSAs shows that Al-Zr intermetallic phases are difficult to supress while maintaining the desired high-temperature phase balance, which in turn compromises room temperature ductility. This motivates exploration of alternative systems that avoid the simultaneous presence of Al and Zr while retaining access to designable two-phase architectures. Cr-Mo alloys were selected since the literature frequently proposes a wide miscibility gap in the binary, implying a tunable window for phase separation and the construction of controlled dual-A2 microstructures. A systematic study combining X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and in situ synchrotron measurements shows no evidence of a miscibility gap within the explored compositional and temperature ranges of Cr-Mo binary system. This provides a corrective experimental anchor for phase-diagram assessments that have relied predominantly on thermodynamic calculations and indirect indicators. Although phase separation was not observed in Cr-Mo, the system displays a strong tendency for carbide formation ,with minor C uptake during high-temperature processing potentially arising from graphite hot-zone components. Guided by this behaviour, compositional modifications were assessed to promote controlled precipitation for strengthening. V and Ti were selected as strong carbide formers with comparatively low density among the refractory elements. Furthermore, Ti also presents a potential secondary strengthening mechanism through Laves-phase formation when realised as a fine, intragranular dispersion. In CrMoV, M23C6 and M2C are prevalent, with M23C6 contributing the more effective strengthening response under the conditions examined. In CrMoTi, MC carbides dominate and commonly act as brittle phases, yielding limited hardening; Laves-phase strengthening was not achieved within the present processing window. These outcomes delineate the practical boundaries of carbide-based reinforcement in Cr-Mo-based alloys and highlight the need to balance precipitate identity, morphology and distribution against the risk of embrittlement. Collectively, the work advances methodology by validating diffracted-spot tracing for elucidating transformation sequences, corrects a long-standing assumption regarding Cr-Mo immiscibility by providing direct experimental evidence of its absence within the studied window, and offers actionable guidance on precipitation strategies in Cr-Mo-based alloy systems.

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

• Yang, Shang-Te

Advisor dc:contributor.advisor

• Jones, Nicholas

• Cr-Mo-based alloy systems
• Microstructural formation pathway
• Refractory superalloys
• Spinodal decomposition
• Synchrotron diffraction

Licence

• https://www.repository.cam.ac.uk/bitstreams/4e09c406-dac3-4f36-b381-21d17155ee22/download
• http://purl.org/NET/rdflicense/allrightsreserved

Language dc:language

eng

DOI dc:identifier.doi

https://doi.org/10.17863/CAM.126015

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/396742