The Physics and Mechanics of Creep Cavity Nucleation and Sintering in Energy Materials

Lead Research Organisation: The Open University
Department Name: Faculty of Sci, Tech, Eng & Maths (STEM)


The research project will study the physics and mechanics of creep cavity nucleation and the reverse process of healing by sintering in polycrystalline materials for energy applications using both modelling and experimental approaches. The experimental work will focus on a model single phase material (commercially pure Nickel), a simple particle strengthened material (Nickel with addition of Carbon), a commercial austenitic stainless steel (Type 316H), a superalloy (IN718) and a martensitic steel P91/92. An array of state-of-the-art experimental techniques will be applied to inform the development of new physics-based cavity nucleation and sintering models for precipitation hardening materials. Once implemented in mechanical analyses, and validated, such models will form the basis for development of improved life estimation procedures for high thermal efficiency power plant components.

Planned Impact

The life of modern (and legacy) power generating plant is limited by the high temperature performance of the construction materials. But our continuing lack of understanding of the underlying processes controlling nucleation of creep cavities (i.e. damage) means that empirical models fitted to macroscopic data are currently employed by industry to assess creep failure and define safe operating life. The reverse process of cavity healing by sintering has received even less attention than nucleation, but is equally important in developing mechanistic understanding. The fundamental insights, knowledge and models arising from the proposed research programme will allow more physically based design and assessment procedures to be developed for high temperature power generating plant. This will help to underwrite life extensions of legacy power generating plant that are limited by the high temperature performance of the construction materials, as well as supporting future designs of power generating plant that must exceed a 60 year life specification. Moreover a deeper understanding of cavities sintering opens up new opportunities for designing components and thermomechanical histories that promote self-healing in-service and extending a component's life. The research project will have far reaching national and international academic impact because of the fundamental nature of the proposed modelling and experimental studies, widespread industrial impact owing to the potential for improving design methods and lifetime assessment procedures, societal benefits through improved assurance of fail-safe operation of power plant, substantial economic benefits arising from life extension of legacy power plant (and longer design life for new power plant), as well as the training of 3 post-doctoral researchers, 4 PhD students and the strengthening of leading UK research groups working in the high temperature materials field.
Description A powerful macroscopic method of testing materials in vacuum conditions under variable stress (using an hourglass sample design) with digital image correlation strain monitoring at elevated temperature has been developed and used to study creep deformation and damage development in copper and Type 316H stainless steel. Small angle neutron scattering (SANS) has been successfully applied to quantify the volumetric size and number distributions of nano-sized cavities and the results compared with published models and new approaches being developed within the project. A novel miniaturised cantilever bending test rig in a vacuum tube has been designed and used to study creep cavitation under stress relaxation conditions. Plus new correlative microscopy approaches (using AI segmentation) have been pioneered and applied to creep test samples that have revealed links between creep cavity initiation and the local microstructure in copper and stainless steel. The approach has been particularly successful in identifying the role of grain boundary ferrite evolution and thermal ageing on creep cavitation of type 316H austenitic stainless steel. The classical model of cavity nucleation has been extended to include the contribution of dislocation structures to the energetics.The data and insights acquired from test programme have been used to inform the extension of physically based crystal plasticity models, developed in a linked industrial funded PhD project, to include models for the nucleation and growth of grain boundary cavities using new interface type elements within the finite element formalism.
Exploitation Route The macroscopic and miniature creep testing methods developed combined with the volumetric (SANS) and the planar correlative microscopy creep cavitation damage measurement approaches and state of the models for creep deformation and damage in energy materials, will be taken up and further developed by other academic research groups, RTOs (like CCFE and TWI), power generation operators (for example EDF Energy) and developers of new reactor concepts.
Sectors Aerospace, Defence and Marine,Energy

Description The outputs of the project are contributing to the development of the R5 Assessment Procedure for the High Temperature Response of Structures that is maintained, developed and used by EDF Energy to underwrite for lifetime extension safety cases for the UK's Advanced Gas Cooled Reactors and thereby helping to maintain diverse, clean energy benefiting the UK economy and society. The Culham Centre for Fusion Energy (CCFE) has expressed strong interest in the Project research outputs as the high temperature response of the materials studied (pure copper, Type 316 stainless steel and martensitic steels) are of direct relevance to the design and assessment of fusion reactor structures (e.g. vessel, divertor and blanket).
First Year Of Impact 2023
Description Creep cavity initiation - early growth and closure in a model polycrystalline material: copper
Amount £48,501 (GBP)
Funding ID COL061-22 (19 December 2022) awarded to the University of Bristol 
Organisation Culham Centre for Fusion Energy 
Sector Academic/University
Country United Kingdom
Start 01/2023 
End 04/2023
Description EPRI agreement number 10010187
Amount $130,000 (USD)
Organisation Electric Power Research Institute (EPRI) 
Sector Charity/Non Profit
Country United States
Start 09/2019 
End 08/2024
Title Cavitation under stress relaxation 
Description A novel stress relaxation test rig has been developed by applying a fixed displacement at the end of a cantilever beam under vacuum at 250 deg C. 2,000 our creep tests have been performed and the shrinkage/closure for cavities owing to stress relaxation investigated. 
Type Of Material Improvements to research infrastructure 
Year Produced 2021 
Provided To Others? No  
Impact The novel test method is providing a greater understanding of creep cavity initiation, early growth and closure under complex stress conditions including residual stress relaxation. 
Title Correlative cavity quantification methods 
Description By using Dragonfly (software) AI segmentation of SEM images of creep damaged metallic samples, we have been able to discriminate a population of creep cavities in a region of interest and correlate the cavitation with EBSD analysis of the local microstructure. 
Type Of Material Data analysis technique 
Year Produced 2021 
Provided To Others? No  
Impact This new data processing approach will help develop understanding of the relationships between cavity nucleation and the local microstructure including grain boundaries, misorientations, Schmid factors, strain distributions and dislocation densities. 
Description Beihang University 
Organisation Beihang University
Country China 
Sector Academic/University 
PI Contribution Preparation of crept stainless steel samples containing nucleated cavities that have been characterised by correlative microscopy methods.
Collaborator Contribution Beihang University undertook some limited cavity closure testing on austenitic stainless steel specimens containing pre-induced creep cavities by applying hydrostatic pressure.
Impact Covid-19 severely delayed experimental work in the UK and delivery of specimens to China. . More significantly Covid-19 non China prevented planned sintering work.
Start Year 2018
Description Copper bi-crystal preparation 
Organisation Academy of Sciences of the Czech Republic
Country Czech Republic 
Sector Academic/University 
PI Contribution Provision of pure copper bar and technical exchanges.
Collaborator Contribution Members of the Czech Academy of Sciences, Prague have developed a technique to produce copper bi-crystals with the boundary normal to the long axis of the specimen. The Czech partners have provided the project with a total of 10 bi-crystal specimens having a combination of misorientations across the normal boundary. Each specimen is worth about £5k. The specimens are for testing at elevated temperature using a novel cantilever testing rig that has been developed by the University of Bristol.
Impact Testing is on-going beyond the end of the project by the University of Bristol (owing to the impact of Covid-19) with further funding from CCFE (UKAEA).
Start Year 2019
Description EDF Energy 
Organisation EDF Energy
Department EDF Energy Nuclear Generation
Country United Kingdom 
Sector Private 
PI Contribution The Open University, the University of Bristol and the University of Oxford University have hosted 3 PhD studentships co-funded by EDF Energy that are associated with this project.
Collaborator Contribution 1. EDF Energy co-funded 3 PhD studentships associated with the project at the Open University, the University of Bristol and the University of Oxford. 2. EDF Energy has provided test materials as in-kind contributions. 3. EDF Energy has actively contributed to studentships' supervision providing an industry perspective and steer. 4. EDF Energy provided technical background information and specialist advice on the project aims, execution and outputs
Impact Markian Petkov, PhD Thesis (Oxford University), Modelling Microstructure Evolution, Creep Deformation and Damage in 316H Stainless Steel, June 2020. Johannes Nicol, PhD Thesis (The Open University), Effect of Prior Plastic Strain on the High Temperature Creep Deformation and Damage Response of Type 316H Stainless Steel, Jan 2022. Siqi He, PhD Thesis, (University of Bristol), New Correlative Microscopy Approaches to Understand the Link between Local Microstructure and Creep Cavity Initiation, Sept 2022.
Start Year 2018
Description EPRI Collaboration 
Organisation Electric Power Research Institute (EPRI)
Country United States 
Sector Charity/Non Profit 
PI Contribution 1. PhD studentship registered and supervised by Project staff at the University of Bristol. 2. The PhD student started in December 2020 and is co-supervised by EPSRC Project Partner University of Oxford.
Collaborator Contribution 1. EPRI is co-funding and supervising a PhD student associated with the project based at the University of Bristol in collaboration with the University of Oxford 2. Jonathan Parker from EPRI attends the 6-monthly EPSRC Project progress meetings (in person or virtually) and joins regular PhD supervisor meetings in the role of industrial sponsor. Jonathan Parker is a renowned world leader in the high temperature behaviour of steels and actively critiques the project.
Impact EPRI agreement number 10010187 (amendment no 1), (22 July 2019, amended February 2021) with EPSRC Project Partners (University of Bristol and University of Oxford). Title: Characterization and modelling of creep cavity nucleation in martensitic 9Cr1Mo steels.
Start Year 2018
Description Creep Cavity Initiation Workshop 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Workshop on Creep Cavity Initiation and Early Growth Leading to Failure in Metal Alloys organised by the UK Forum for Engineering Structural Integrity (FESI).The workshop communicated the outcome of research funded by the EPSRC Grant EP/R02607/1 conjoint between the Open University, the University of Oxford and the University of Bristol.The workshop closed with a lively discussion around key messages and future challenges. A report on this workshop, which includes key findings, can be found in FESI Bulletin Number 26, Volume 16 - Issue 2: Winter 2022.
Year(s) Of Engagement Activity 2022
Description Project Technical Review Meetings with Industry Partners 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Industry/Business
Results and Impact The purpose of the biannual review meetings with the project collaborators (industry/business partners, CCFE, EPRI and Beihang University) was to report on academic progress at the 3 universities and provide a platform for the collaborators to critique and question the work, set direction of the next period and comment on the projects relevance to industry and international context.
Year(s) Of Engagement Activity 2019,2020,2021,2022