Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.602660
Title: Formation and avoidance of surface defects during casting and heat-treatment of single-crystal nickel-based superalloys
Author: Simmonds, Scott
ISNI:       0000 0004 5353 8779
Awarding Body: University of Leicester
Current Institution: University of Leicester
Date of Award: 2014
Availability of Full Text:
Access from EThOS:
Access from Institution:
Abstract:
Nickel-based superalloys are a class of materials that have been specifically developed for high temperature applications for use in aero-engines and power generation. The evolution of superalloy chemistry and solidification structure has made the control and prevention of defect formation in single-crystal components increasingly challenging. This study examines the formation and avoidance of Surface Scale and Surface Melting, both of which are detrimental to the production cost and the performance of single-crystal turbine components. Surface Scale is a defect found on the surface of as-cast single-crystal components identified as a region of discolouration. A systematic analysis reveals that Surface Scale forms in the solid-state during casting due to the combined effects of (1) differential thermal contractions between mould and metal and (2) transient oxidation of the exposed metal casting surface at temperature below ~1300°C. The resultant transient oxide film is 0.5-1μm thick whilst the remaining surface undergoes a mould-metal reaction during the initial stages of casting, at temperatures above the liquidus, producing a 1-2μm thick protective layer of Al[subscript 2]O[subscript 3]. Surface Melting is identified on the heat-treated blade surface as an area of localised melting within the Scaled surface area. Through an in-depth analysis of the microstructural evolution it was found that Surface Melting is a consequence of elemental evaporation of γ-phase stabilisers. The subsequent change in chemistry results in the transformation of the γ phase to γ’ and TCPs and a concomitant occurrence of incipient melting at the surface during solution heat treatment. This thesis concludes that the prevention of Surface Scale is made possible by the use of an inert atmospheric gas, such as Argon, within the casting furnace. However, the subsequent formation of Surface Melting during heat-treatment can only be prevented by pre-oxidising the as-cast components, thus forming Al[subscript 2]O[subscript 3] in order to form a protective barrier to evaporation.
Supervisor: Dong, Hongbiao; Atkinson, Helen Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.602660  DOI: Not available
Share: