Oxidation Damage at a Crack Tip and Its Significance in Crack Growth under Fatigue-Oxidation Conditions

Lead Research Organisation: University of Southampton
Department Name: Faculty of Engineering & the Environment

Abstract

Nickel-based alloys are widely used in power generation, nuclear and aerospace industries due to their superior mechanical properties at high temperature. As structural materials, a strong resistance to crack initiation and propagation is particularly required for safe-life design and assessment of their components. At elevated temperature, crack growth rates in such alloys exposed to air can be drastically accelerated, by two and even three orders of magnitude, due to the attack of oxidation. Over time, significant effort has been made to investigate the crack tip oxidation mechanism in order to provide a basis for the development of quantitative models that predict crack growth under operational temperatures and loading conditions. However, this problem has been neither fundamentally nor fully understood, and current lifing practice in industries is still predominantly empirical and relies on expensive and extensive experimental data on crack growth.

This research aims to investigate the physical process of oxidation damage at a crack tip and the associated crack growth behaviour for nickel alloys, which will provide a direct insight, for the first time, into the oxidation-embrittlement phenomenon at crack tip. Oxidation damage at a crack tip is a combined effect of time, temperature, local deformation and material microstructure. Knowledge of this process is vital to assess crack propagation behaviour under the attack of oxidation. In the proposed work, single crystal, directionally solidified and polycrystal nickel alloys will be used for crack growth testing under fatigue-oxidation conditions in controlled environments (vacuum, air, oxygen-18). Advanced microscopy analyses will be carried out to characterise and measure the oxygen penetration and microstructural damage at a crack tip, and the results will be used to calibrate important diffusion and damage parameters during oxidation. Numerical analyses will be carried out to model such processes at a microscopic scale using a coupled mechanical-diffusion model. Effects of loading condition and grain boundary character on oxygen diffusion will be fully investigated, especially the connection between oxidation damage and crack growth. A crack propagation model will be ultimately developed and validated for accurate fatigue-oxidation life prediction.

The work draws together three established groups to tackle these fundamental problems in a collaborative, systematic and multi-scale manner. Interaction between oxidation damage and crack tip deformation requires carefully designed specialist testing on fatigue crack growth in a controlled environment, which is the expertise of UoS. The problem also requires advanced microscopy characterisation and physical measurements of the phenomena using the established techniques at IC. The new models will be developed, with validation against these experimental results, by UoP who has a strong background in material and crack growth modelling. Owing to our complementary skills, this joint project should establish a physically based connection between oxidation damage and crack growth for fatigue design and safe life prediction of nickel alloy components.

The research will generate unique and practically-useful data and models which can be quickly exploited through our committed industrial collaborators including E.On, Alstom, NASA and Dstl. The results will also be of generic use to other industries striving to achieve maximum service life and temperature capabilities of critical high-temperature components. Researchers and academics working on high-temperature materials and related areas will also directly benefit from our targeted dissemination activities including workshops, conferences and journal papers. A wider audience will be reached via specially designed public engagement programmes and continuously updated web sites.
 
Description Effects of oxygen-related damage (i.e. oxidation and dynamic embrittlement) on fatigue crack propagation behavior in an advanced disc alloy have been assessed in air and vacuum under dwell-fatigue conditions at 725 °C. The enhanced fatigue crack propagation is closely related to oxygen-related damage at/ahead of the crack tip, which is determined by the testing environment, the dwell period and the crack propagation rate itself based on two dimensional (2D) observation of the crack tip in an optical microscope and scanning electron microscope. X-ray computed tomography has also been employed to examine the differences between three dimension (3D) crack morphology in air and vacuum conditions, and the crack features have been quantified in terms of crack opening displacements, secondary cracks and uncracked bridging ligaments. The results show that the fatigue crack propagation rate is related to the amount of secondary cracks, and the crack length increment in a loading cycle is related to the breaking/cracking of the uncracked bridging ligaments within the discontinuous cracking zone ahead of the crack tip as oxygen-related damage preferentially occurs in these highly deformed regions. By combination of 3D X-ray computed tomography and traditional 2D observation, a deeper understanding is provided of the mechanisms of oxygen-enhanced fatigue crack propagation behavior. The role of oxygen in enhanced fatigue cracking in an advanced Ni-based superalloy for turbine disc application has also been evaluated at fatigue crack initiation and propagation stages along with static oxidation tests. It is found that the grain boundary oxide intrusion has a layered structure. The microstructure- and deformation-dependent grain boundary oxidation dominants the fatigue crack initiation and early propagation processes. As the crack propagates, this contribution arising from oxidation damage may gradually be overtaken by dynamic embrittlement processes until the mechanical damage outstrips the oxygen-related damage, resulting in a transition from intergranular to transgranular crack propagation.
Exploitation Route This data and mechanism assessment supports mechanistically informed modelling approaches of relevance to the industrial collaborators of the work
Sectors Aerospace, Defence and Marine,Energy,Manufacturing, including Industrial Biotechology,Transport

 
Description Alstom power 
Organisation Alstom
Department Alstom UK
Country United Kingdom 
Sector Private 
PI Contribution Experimental and computational study of fatigue-oxidation damage and crack growth in single crystal and directionally solidified turbine blade superalloys for gas turbines in power generation applications.
Collaborator Contribution Supply of materials and technical advice; attendance at project review meetings.
Impact Conference and journal publications
Start Year 2013
 
Description E.ON 
Organisation Uniper SE
Country Germany 
Sector Private 
PI Contribution Provide a fundamental understanding of fatigue-oxidation damage mechanism for gas turbine blades and discs
Collaborator Contribution Supply of technical advice and attendance at project review meetings. Support of accompanying CASE award student
Impact Journal and conference publications
Start Year 2010
 
Description NASA 
Organisation National Aeronautics and Space Administration (NASA)
Country United States 
Sector Public 
PI Contribution Experimental and computational study of critical fatigue-oxidation damage processes of NASA superalloy LSHR for application in gas turbines.
Collaborator Contribution Supply of materials and technical advice.
Impact Journal and conference publications.
Start Year 2010
 
Description Swansea 
Organisation University of Rwanda
Country Rwanda 
Sector Academic/University 
PI Contribution We are working on fatigue testing at high temperatures - we provide crack propagation results
Collaborator Contribution Swansea have great expertise on strain controlled testing, something we cannot contribute
Impact joint publications are planned
Start Year 2014
 
Description dstl 
Organisation Defence Science & Technology Laboratory (DSTL)
Country United Kingdom 
Sector Public 
PI Contribution Provide guidance for life assessment of gas turbines
Collaborator Contribution Providing technical advice and attending project review meetings.
Impact Journal and conference publications
Start Year 2013
 
Description Big Bang Solent event 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact developing engineering displays around nickel base superalloys and their use in turbine applications

this was mostly focussed on local Southampton schools and families
Year(s) Of Engagement Activity 2014
 
Description Cheltenham Science Festival 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? Yes
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact We were part of the University of Southampton exhibition at this major science festival

Developing and expanding our show to communicate more effectively about the fundamentals of materials science in turbine applications
Year(s) Of Engagement Activity 2014
 
Description Presentation at the Thomas Hardye School, Dorchester 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Schools
Results and Impact presentation at school based fair

general raising of awareness of materials applications in turbine materials
Year(s) Of Engagement Activity 2013
 
Description Science and Engineering Day, UOS 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact We ran a stand at our University's award winning Science and Engineering Day on 22nd March 2014, a large number of people from the local region attended. We ran it again in March 2015

increased visibility of materials science applied to turbine systems for the general public
Year(s) Of Engagement Activity 2014,2015
 
Description Tomorrow's World (Science Tent), Bestival, Isle of Wight 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? Yes
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Part of the University of Southampton roadshow activity

good attendance and lots of discussion with a range of school age children
Year(s) Of Engagement Activity 2014,2015
URL http://www.campbestival.net/line-up/2014-line-up/science-tent/
 
Description Winchester Science Festival 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? Yes
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact part of the University of Southampton exhibition

general public outreach about materials in turbine applications
Year(s) Of Engagement Activity 2014,2015
URL http://winchesterscifest.org/