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

Lead Research Organisation: 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.


10 25 50
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

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 is undertaking cavity closure tests on austenitic stainless steel specimens containing pre-induced creep cavities by applying hydrostatic pressure.
Impact None yet owing to Covid-19 delays to experimental 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 bi-crystal specimens having a (111)+(111) combination of misorientations across the normal boundary. These are being tested at elevated temperature using a novel cantilever testing rig developed by the University of Bristol. Other planned combinations (100)+(100); (111)+(100) with 10, 15 and 20 debgree designed micro-rotations are still under preparation by the Czech Academy of Sciences.
Impact Testing is on-going (owing to the impact of Covid-19).
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 is co-funding 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 continues to provide technical background information and specialist advice on the project aims, execution and outputs
Impact The PhD thesis of Johannes Nicol (The Open University)
Start Year 2018
Description EPRI Collaboration 
Organisation Electric Power Research Institute (EPRI)
Country United States 
Sector Charity/Non Profit 
PI Contribution 1. EPSRC Project partner the University of Bristol is co-funding a PhD studentship with EPRI. 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 PhD studentship contract between the University of Bristol and EPRI
Start Year 2018
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