Project Title: A Microstructurally Informed Structural Integrity Analysis of Austenitic Alloys For Nuclear Power Applications

ICO-CDT in Nuclear Energy

Funded by the EPSRC and Rolls-Royce

Academic Supervisor: Dr. Ben Britton (Imperial College)

Industrial Supervisor: Dr. Andrew Barrow (Rolls-Royce)

Duration: 01/10/2018-31/09/2021

 

Stress Corrosion Cracking (SCC) is a type of cracking that occurs only when three factors are present, setting it apart as unique to corrosion. These factors are, tensile stress, environment and material where each material will have specific and generally unique environmental conditions that are required. A microstructural feature that has been shown to impact both the sensitivity to general corrosion and resistance to SCC is the distribution of chromium carbide precipitates. A proposed mechanism for this SCC resistance is crack tip blunting from inter-granular carbides (Bruemmer, Charlot and Henager, 1988). Figure 1 shows one such carbide. The goal of this project to; develop an understanding of the distribution of these carbides in Inconel Alloys, and elucidate the relationship between microstructure and SCC sensitivity. The focus will be on the application of Electron Microscopy based techniques to quantitatively characterize the type, shape and distribution of the carbides with an understanding of local strain. The primary techniques involved will be HR-EBSED for strain, STEM based TKD and EDX for analysis of spatial distribution and type of precipitate.

Investigator: Dafni Daskalaki-Mountanou

Project Title: Microstructural Sensitivity of Stress Relaxation Cracking

Supervisor: Dr Ben Britton, Prof Mary Ryan, Prof Alex Porter

Duration: March 2017-March 2020

Funding: Shell Global Solutions

 Stress relaxation cracking (SRC) is a degradation mechanism which occurs in stainless steels and nickel alloys between 550° C and 750°C operation temperature. SRC is a failure mode which can occur during post-weld heat treatment, within 1 to 2 years of service. Cracked regions are located in the heat treatment affected zone (HAZ) or in cold deformed areas subject to long term annealing. An intergranular crack is developed due to high temperature relaxation of internal stress causes local deformation within the microstructure, i.e. a creep failure mechanism.

The project will decouple the impact of microstructural history, chemical composition, temperature and loading conditions using bespoke mechanical testing and characterisation. This will enable us to explore the fundamental mechanisms of creep, stress relaxation, and the role of chemistry in the formation and propagation of microstructurally sensitive cracks during service.

 

Title: strain partitioning in dual phase titanium alloys for aerospace applications

Investigator: Xinping Fan

Supervisors: Ben Britton (primary), Fionn Dunne (secondary)

Duration: 2017 Sept 30 – 2020 Sept 29

Description:

Dwell fatigue was one of the major causes for crack initiation on aeroengine discs, which lead to the reduction of service life. Cold dwell fatigue (CDF) was a complex deformation process that can take place at room temperature. This project is aiming to understand the role of microstructure on damage accumulation in Ti-624x series, with controlled microstructures, and understand how they perform in complex cyclic loading regimes. Major characterisation techniques will be high spatial resolution digital image correlation and high resolution electron backscatter diffraction. Experimental results combined with crystal plasticity modelling will be used to understand the strain partitioning in Ti-624x alloys.

Alexander Foden – Improving Techniques for High Resolution Electron Backscatter Diffraction

Investigator: Alexander Foden

Supervisor: Dr. Ben Britton

Duration: 3/10/16 – 3/10/19

Description: Electron backscatter diffraction (EBSD) is a well-established technique used to probe samples with scanning electron microscopy, where an electron beam is fired at a crystalline material and diffracted from crystal planes to form a Kikuchi pattern. In conventional EBSD, images are processed to extract crystal orientation and maps are formed with systematic mapping of a sample surface.

In this project, I will generate new analysis approaches to improve the precision of the EBSD data obtained and wealth of information probed. I will develop these algorithms using dynamical simulations and use them to probe unknown phases, measure orientation with higher precision and understand deformation in engineering materials.

Project title: Experimental Micromechanical Characterisation of Solder Alloys

Investigator: Dr Tianhong Gu

Supervisor / Line Manger: Dr Ben Britton, Finn Giuliani

Description:

This project aims to measure individual microstructural units in solder joints (e.g. a βSn grain or embedded Cu₆Sn₅ crystal) and extract knowledge and understanding of their thermal, mechanical and fatigue response. The experimental programme will combine the precise measurement of individual microstructural units (e.g. βSn critical resolved shear stresses as a function of strain rate using micropillar compression), together with the entire engineering response in well-defined geometries with loading and environments representative of solder in-service conditions. This will utilise new experimental characterisation approaches, based upon in-situ loading & heating at the sub-μm scale as well as of whole solder joints, and will utilise: live imaging to track failure and microstructural evolution; high (angular) resolution electron backscatter diffraction (HR-EBSD) to measure local stress heterogeneity and defect content; and high (spatial) resolution digital image correlation (HR-DIC) to measure permanent shape change.

A Project Title – H₂S Corrosion of High Strength Steels

 

Investigator: Jim Hickey

Supervisors: Dr. Ben Britton, Prof. Mary Ryan, Dr. David Payne

Duration: Sep 2015 – March 2019

Funding:  EPSRC & Shell UK

 

Description: In the presence of aqueous H₂S and stress, high strength steels (HSSs) (those with yield strengths > 700 MPa) embrittle and can fail catastrophically. This phenomenon is termed sulphide stress cracking (SSC). Understanding this is crucial since H₂S containing oil and gas wells (sour gas wells) are now routinely exploited. HSSs are desirable, if not necessary, for some of the infrastructure of oil wells with extreme environments. The aim of the project is to simulate conditions found in down-well environments and characterise the effect of the environment and load on a series of HSSs to guide mechanistic understanding of the effect of H₂S on HSSs. 

Title: The effects of microalloying on the deformation of hexagonal closed packed alloys

Investigator:  Ning Fang

Supervisor:  Dr Ben Britton and Dr Finn Giuliani (Co-supervisor)

Duration: 20-11-2019 - 19-11-2023

Description:

Zircaloy-4 (Zr4) is a vital engineering material used in nuclear power reactors because of its low neutron capture cross section and excellent mechanical properties. Deformation behaviour is important for the nuclear safety as it has the possibility to cause failure. Despite previous experiments done on the low strain rate deformation, there is still a lack of research in the area of high rate deformation. Deformation at high strain rates can happen in extreme conditions. These conditions are difficult to simulate because of the inertial contributions and the stress wave under large-scale tests.
In my project, micromechanical testing will be performed using focus ion beam (FIB) machined mechanical test specimens, which are subsequently tested in the scanning electron microscope (SEM). To aid understanding of the deformation performance, surface strain analysis will be performed using digital image correlation (DIC) to provide 2D maps of the surface strain during the tests. Tests will be performed at a range of strain rates (from 10-4 to 102) using an Alemnis system. These experiments will be modelled using crystal plasticity finite element modelling (in collaboration with Professor Fionn Dunne) to extract micromechanical properties and calibrate experimental models.

Investigator: Dr Thibaut Dessolier

Supervisor: Dr Ben Britton

Co-supervisor:  Dr Finn Giuliani

Funding: Shell Global Solutions

Duration: 03/01/2019 – 31/12/2020 (Postdoctoral researcher)

 

Resume of the project:

HP series alloys centrifugally cast are generally used in petrochemical plants as reformer tubes for hydrogen production by steam reforming. During the gas process, reformer tubes are exposed to high temperature (range between 700 to 1000°C) while steam flow inside of it at a pressure around 5 MPa. Creep damage may occur in these tubes due to a combination of temperature, time and stress solicitation which can lead to tube failure. In this situation, HP series alloys (≈ Fe - 35Ni - 25Cr – 1.5Nb - 0.4C wt% plus minor other elements) are used as they have a good resistance against high temperature creep and carburization. Tubes are formed using centrifugal casting and this results in a complex through-wall microstructure, with equiaxed grains at the tube inside diameter (ID) and columnar grains towards the outside diameter (OD) as Figure 1 shown it.

The main purpose of this project is to assess the effect of creep deformation on the localisation of the plastic strain heterogeneities inside the microstructure of a HP40Nb alloy reformer tube. To do so, high temperature mechanical tests will be performed while plastic strain heterogeneities will be capture digital images correlation (DIC) thus high resolution EBSD. Figure 2 and Figure 3 show an example of result from DIC and HR-EBSD technique for strain assessment. Based on the experimental result, plasticity simulation could be used in order to predict creep behaviour and improve tube life management.

 

 

 

 

 

 

 

References:

[1]        J. Jiang, F. P. E. Dunne, and T. Ben Britton, “Toward Predictive Understanding of Fatigue Crack Nucleation in Ni-Based Superalloys,” J. Miner. Met. Mater. Soc., vol. s7-IV, no. 82, pp. 863–871, 2017, doi: 10.1007/s11837-017-2307-9.

 

Title: Mechanism of deformation and failure in Nickel base superalloy

Investigator: Yi Guo

Supervisor: Dr Ben Britton

Funding: Beijing Institute of Aeronautical Materials (BIAM)

Description: Ni superalloy carries safety critical responsibility while operating in a most harsh environment in gas turbine. The ability to predict the deformation and lifetime of this material is important for accurate mission control. In this project, I study the deformation heterogeneity at casting pores and cracks, in a second-generation superalloy ‘DD6’, using high resolution EBSD (HR-EBSD), digital image correlation (DIC), crystal plasticity finite element (CPFE) simulation. The project aims to gain a fundamental understanding of the void growth, crack nucleation, and the competition between crack growth and dislocation slip in the context of dislocation theories.