Topologically close-packed phase development during solution heat treatment of rhenium containing single crystal nickel-based superalloys

Harrison, John (2024). Topologically close-packed phase development during solution heat treatment of rhenium containing single crystal nickel-based superalloys. University of Birmingham. Ph.D.

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Abstract

Superalloys, aptly named for their high-temperature performance, are at the forefront of materials technology for applications such as turbine blades in jet engines for both power and propulsion. Their complex chemistry, comprising often over a dozen elements, unique production methods, such as single-crystal casting, on top of their manufacturing complexity, with preformed cooling channels and coatings enable these materials to function in extreme circumstances.
However, their ever-growing complexity creates equally complex problems that are ever more unique to the alloys and harder to separate from all the different processes involved in the manufacture and operation.
Of these, topologically close-packed phases, or TCP phases, stand out as they are poorly understood in both why and how they form. It is known they are typically enriched in refractory elements, the same elements that provide the alloy with its superior high-temperature properties. However, the growing complexity of these alloys now see TCP phases occurring not just in-service conditions, but during manufacturing, in the as-cast state, heat-treatments and the focus of this work; the solution heat-treatment stage. The occurrence of these phases during solutioning is particularly problematic, as this stage is intentionally included to homogenise the alloy into a single phase, not introduce additional detrimental phases in a heterogeneous manner.
This work seeks to review past work on the study of nickel-based superalloys, in particular high refractory content alloys prone to TCP phase precipitation. As well as other works carried out to understand the occurrence of TCP phases. Through the casting of single-crystal alloys containing varying levels of Re; CMSX4, CMSX10K and CMSX10N, where Re is an element strongly associated with TCP phase precipitation, experiments

are carried out on the standard solutioning regime of these alloys to better understand the microstructural evolution during the solutioning process.

Through these experimental results, supported with modelling experiments primarily carried out using ThermoCalc software, efforts are undertaken to better the understanding of when; if at all, which TCP phases occur during the solutioning process. Calculation of volume fractions present, where in the microstructure they occur and expansion of the understanding of their initial formation, and why in some cases they appear to eventually redissolve or never form in the first place. The aspiration of this work is to inform future alloy development of the risk certain aspects such as composition and processing contribute to TCP phase precipitation during manufacturing and how, if at all, it can be avoided or minimised.
Through the experiments and analysis undertaken during this thesis, it has been shown that TCP phases; namely

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):