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Title: Modelling (D022) Precipitate nucleation, growth and coarsening in the nickel-base superalloy 625
Author: Moore, Ian
ISNI:       0000 0004 6351 915X
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2017
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Although typically thought of as a solid solution strengthened material, Alloy 625 is susceptible to significant precipitation hardening through the formation of γʺ (D022 ) precipitates. These particles can form both during manufacture and in high temperature service and, consequently, the accurate prediction of their behaviour is crucial. To this end, a model is developed in this work which describes γʺ precipitation in Alloy 625, encompassing the concurrent homogeneous nucleation, growth and coarsening of different particles and allowing for them to be shape changing. Based on the Svoboda-Fischer-Fratzl-Kozeschnik (SFFK) framework, the model is calibrated with respect to experimental measurements of aspect ratio evolution at 650°C, with the statistics were acquired through a novel analysis methodology. The outputs generated by the model for interfacial energy, particle size distribution and number density are in excellent agreement with experimental data for a simulation of 1000 hours at 650°C, however beyond this point a divergence is introduced by the phenomenon of precipitate “encounter" (described by Davis et al. [1]). Following suitable changes for descriptions including precipitation driving force and material elastic constants, application of the model to the evolution of γʺ precipitates in Alloy 718 at 700°C measured by Han et al. [2] reveals reasonable agreement. Whilst the results presented in this paper are predicated on a homogeneous microstructure and the formation of exclusively γʺ precipitates, the underlying frame work outlined is flexible enough for inhomogeneous and competitive precipitation to be included at a later date. Furthermore, through the application of an addition class management mechanism the model is likely applicable to a large range of ageing temperatures and durations.
Supervisor: Palmiere, E. J. ; Burke, M. G. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available