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Title: High resolution characterisation of oxidation mechanisms in polycrystalline nickel based superalloys
Author: Lapington, Mark Thomas
ISNI:       0000 0004 8503 2553
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2019
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In order to withstand the demanding conditions experienced at the heart of modern jet turbine engines, nickel based superalloys must combine extreme strength with resistance to temperature-dependent degradation mechanisms such as oxidation and fatigue. This is increasingly becoming a priority, as operating temperatures continue to be pushed higher due to tighter fuel economy and emissions targets. A new generation of high temperature, damage tolerant superalloys has been developed which balances all of these properties through careful control of composition. For further alloy development, the relationship between oxidation kinetics and alloy composition must be established. Two elements in particular have been focussed on - titanium and niobium. Both of these elements are vital for maintaining high temperature strength; however Ti has been experimentally shown to increase rates of oxidation, while Nb has been implicated in accelerating dwell fatigue crack growth. In order to study these composition-property relationships, a family of near identical superalloys have been created with varying titanium/niobium ratio, and subsequently characterised. Oxidation kinetics were studied using thermogravimetric analysis, which confirmed the link between titanium and oxidation, while also creating oxide scales for further characterisation. A variety of high-resolution techniques have been used to analyse these oxide scales on the smallest scales, revealing the location of titanium - some was found dissolved in chromia oxide scales as expected - but much more was found segregated to grain boundaries within the oxide. Ion transport along these grain boundaries may explain the oxidation kinetics better than the lattice diffusion mechanism previously assumed. A number of fatigue crack tips also underwent examination, in an attempt to shed light on the mechanism by which niobium encourages dwell fatigue. While no evidence could be found for this, a number of analysis techniques have been developed and refined to hopefully aid these efforts in future.
Supervisor: Moody, Michael ; Bagot, Paul ; Reed, Roger Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Characterisation ; Materials science