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Title: Multiscale modelling of deformation in Co-Al-W superalloys
Author: Hasan, Hikmatyar
ISNI:       0000 0004 7963 7804
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2019
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Since their discovery nearly thirteen years ago, Co-Al-W superalloys have emerged as the frontrunner materials to replace the ubiquitous Ni superalloys used in gas turbines, where temperatures can reach up to 1500°C. Studying the link between composition and deformation mechanisms in these alloys is important in accelerating the identification of optimal alloys. The high-temperature strength of these alloys can be attributed to their unique γ' microstructure. Dislocation configurations can feature a variety of possible planar fault structures and their associated energies play a key role in defining the observed mechanical properties. To capture this complexity, a robust relaxation scheme has been implemented to calculate a range of Γ-surfaces associated with Co-Al-W and Ni-Al superalloys using density-functional theory. Also known as Generalised Stacking Fault energies, these 2D energy surfaces describe the energy cost of associated local atomic displacements at the dislocation core. Subsequently, the first principles data was incorporated into a Phase Field Dislocation Dynamics model to investigate the critical stress required to shear various types of microstructure. The model found that Co-Al-W superalloys were generally more resistant to shear by dislocations than Ni-Al superalloys which was due to the higher planar fault energies of the Co system. Recently, solute segregation has been experimentally observed in planar fault regions within Co-Al-W superalloys. As such, diffusion is believed to play an important role in the deformation of the material. The Phase Field model was extended to simulate diffusion concurrently with deformation. In order to do so, a novel method of calculating off-stoichiometric Γ surfaces for the Co3(Alx,W1−x) compound was also implemented. The effect of diffusion was found to be discernible at low stresses and had caused the shearing rate to decrease. This application of multi-scale modelling can be used to further the understanding of plastic deformation in superalloys and other alloys that exhibit complex microstructures.
Supervisor: Vorontsov, Vassili ; Haynes, Peter Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral