Microstructure-fatigue relations in titanium alloys
File(s)
Author(s)
Lord, Hannah
Type
Thesis
Abstract
Materials with good mechanical properties at elevated temperatures and environmental resistance are necessary in developing lighter, more efficient aero-engines with increased operating temperatures. Titanium is ideal due to the high strength to weight ratio and lack of inclusions. Ti-64 has good corrosion resistance and fatigue life properties, but has been in use for several decades, prompting the idea for the development of a new alloy. In this study, microcantilever high cycle fatigue (HCF) testing was utilised to characterise isolated αp grains, and alloy development aimed to increase the αp strength and improve the fatigue properties, including HCF and dwell, through compositional and microstructural changes.
HL1 alloy, a variant of RR11, was developed. The increased transus temperature, increased molybdenum diffusion rate and thus increased plate growth rate resulted in coarser αs forming, giving HL1 a lower strength.
Titanium alloys with different compositions and processing histories were examined for fatigue testing of microcantilevers, and for use in comparative macro-scale fatigue work.
Microcantilever HCF samples were developed and tested in αp grains, examining aluminium content and orientation. The hard orientation resulted in a higher HCF strength than the soft, and an increased aluminium content resulted in a higher HCF strength. The soft orientation showed a flat failure with striations and the hard a textured failure, reinforced by macro-scale fracture morphology.
This thesis explored the mechanical properties of titanium alloys on different lengthscales. The strength of the αp was targeted through both the alloy development and the microcantilever HCF projects. The results can continue to influence alloy variants to maximise the strength of the αp component in the microstructure. Future work to develop stronger microstructures in fatigue could include microcantilever HCF testing of further changes to the αp, as well as altering the αs lamellae orientation and width in transformed β.
HL1 alloy, a variant of RR11, was developed. The increased transus temperature, increased molybdenum diffusion rate and thus increased plate growth rate resulted in coarser αs forming, giving HL1 a lower strength.
Titanium alloys with different compositions and processing histories were examined for fatigue testing of microcantilevers, and for use in comparative macro-scale fatigue work.
Microcantilever HCF samples were developed and tested in αp grains, examining aluminium content and orientation. The hard orientation resulted in a higher HCF strength than the soft, and an increased aluminium content resulted in a higher HCF strength. The soft orientation showed a flat failure with striations and the hard a textured failure, reinforced by macro-scale fracture morphology.
This thesis explored the mechanical properties of titanium alloys on different lengthscales. The strength of the αp was targeted through both the alloy development and the microcantilever HCF projects. The results can continue to influence alloy variants to maximise the strength of the αp component in the microstructure. Future work to develop stronger microstructures in fatigue could include microcantilever HCF testing of further changes to the αp, as well as altering the αs lamellae orientation and width in transformed β.
Version
Open Access
Date Issued
2023-09-15
Date Awarded
2024-03-01
Copyright Statement
Creative Commons Attribution NonCommercial NoDerivatives Licence
Advisor
Dye, David
Rugg, David
Martin, Nigel
Moschini, Jim
Sponsor
Engineering and Physical Sciences Research Council
Rolls-Royce Group plc
Grant Number
2018 NPIF grant EP/S515085/1
Publisher Department
Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)