Title: Development of Non-Destructive Ultrasonic Detection Methods of Macrozones within Titanium Alloys

Investigator: David Shepherd

Supervisors: Prof Fionn Dunne and Prof Mike Lowe

Collaborators: Mr Koichi Inagaki, IHI Corporation

Duration: 03/09/2018-03/09/2022

The project aims to establish ultrasonic wave speed measurement methods in order to extract information about macrozones in aero-engine titanium compressor discs. Macrozones are relatively large regions of uniform crystallographic grain orientation which are believed to lead to a life debit in engine components through dwell fatigue leading to crack nucleation. Hence a non-destructive method using ultrasound waves to detect their presence in engineering components is eagerly sought.

The project aims to investigate ultrasonic normal and shear wave speed measurements in titanium alloys for this purpose. Because titanium grains are elastically anisotropic, reflecting their orientations, wave speed methods offer the potential to detect large regions of uniform crystallographic orientation by inferring their presence from measured wave speed changes.

Title: Microstructures and fatigue behaviour of Ni single crystals made by casting and additive manufacturing

Investigator: Alessandro Piglione

Supervisor: Dr. Minh-Son Pham

Co-supervisor: Prof. Fionn Dunne

Industrial partner: Beijing Institute for Aeronautical Materials (BIAM)

Abstract: Nickel single-crystal components play a key role in the operation of modern jet engines. Understanding their fatigue behaviour at elevated temperatures leads to significant improvements in their reliability, thereby enabling the full exploitation of their potential for an improved efficiency. Whilst most of the previous studies focus on uniaxial fatigue testing, fundamental studies will be carried out in this project in order to understand the fatigue behaviour of nickel single-crystals in complex multi-axial loading conditions. In particular, the main efforts will be directed towards (1) understanding cyclic deformation mechanisms in high-temperature rotating-bending fatigue conditions, and (2) assessing the influence of surface notches on the components’ fatigue life. In addition, this project will explore Ni single-crystal fabrication via additive manufacturing. Solidification microstructures and deformation behaviours of additive manufactured parts will be investigated and compared with those of the components fabricated by casting.

Title: Experimental and Modelling Methodologies for Investigation of Fatigue Crack Nucleation in Agglomerate-containing Poly-crystalline Nickel Superalloys Fabricated by Powder Metallurgy.

Investigator: Alexander Bergsmo

Supervisor: Prof Fionn Dunne

Duration: 36 Months

Powder metallurgy has recently become the preferred method for producing nickel-superalloy turbine disks. An unavoidable side-effect of powder metallurgy is the propensity to include non-metallic agglomerates in the nickel matrix. These inclusions may create a locally active stress field which may drive the nucleation of cracks. Recent studies have investigated different criteria for crack nucleation at room temperature. The real working conditions of a jet engine turbine are at high temperature and work remains to be done on crack nucleation in such an environment. This project aims to investigate the criteria for which cracks appear around an inclusion subject to high temperature conditions by using experimental and modelling techniques.

TITLE:  Armin Daszki - Nucleation kinetics of Lead-free solder joints

Project title: Nucleation kinetics of Lead-free solder joints

Investigator: Armin Daszki

Supervisors: Dr. Christopher Gourlay

Duration: 01/10/2016 – 30/09/2020

Project Description:

Lead-free solders, which have been developed to substitute lead based alloys, suffer greatly from anisotropy compared to the original tin-lead alloys due to the tetragonal crystal structure of the majority component, βSn. Additionally βSn suffers from nucleating difficulties usually leading to a single nucleation event which results in a single βSn grain, cyclic-twinned βSn grains, and/or interlaced-twinned βSn grain morphology. This nucleation-difficulty phenomena is seen clearly when repeatedly melting and solidifying Sn-based alloys and measuring the nucleation undercooling. The stochastic value of the nucleation undercooling can vary (depending on sample purity and size) from 45K to 80K, this is in stark comparison to the melting which is seen to always happen at a fixed repeatable value. Nucleation kinetics will be studied in lead-free solders, with and without soldering substrates, in order to understand fundamental nucleation laws with experimental data, as well as the implication of nucleation onset on the resulting microstructure. By understanding the nucleation phenomena of βSn in lead free solders we seek to control the microstructure sufficiently to alter the mechanical, and electrical properties of solder joints to improve reliability of electrical devices. Finally, the gained knowledge of nucleation in the studied system will be generalised and extended to any system. 

Project title: The Temperature Sensitivity of Galling in Hard-facing Alloys

Investigator: Benjamin Poole

Supervisors: Prof. Fionn Dunne and Prof Daniele Dini (Mechanical Engineering)

Collaborators: Dr David Stewart, Rolls Royce plc.

Duration: 01/10/2016 – 30/09/2020

Funded by the ICO CDT in Nuclear Energy (EPSRC) and Rolls Royce plc.

Description: Hard-facings are hard surface coatings applied to components to provide resistance from wear and corrosion. Within a pressurised water reactor, hard-facings are typically applied to the internal surfaces of primary loop control valves. These coatings are crucial in order to resist galling, a severe wear process associated with large amounts of plastic deformation and leading to operability issues. Cobalt based Stellite alloys provide excellent galling resistance but are easily activated by neutron fluxes within the core. Work has been proceeding on the development of Co-free, Fe-based hard-facings for nuclear applications but most have been shown to display unacceptable and poorly-understood temperature sensitivity

Through the use of representative crystal plasticity finite element models and elevated temperature mechanical testing, this project aims to study the plastic deformation of Fe-based hard-facings at elevated temperatures. This understanding will feed into the development of future Co-free hard-facing alloys.

Title: Use of Machine Learning to design alloys for additive manufacturing

Investigator: Bogdan Dovgyy

Supervisor: Dr. Minh-Son Pham

Co-supervisor: Prof. David Dye

Abstract: Additive manufacturing (AM) has unique characteristics (e.g., rapid solidification and thermal cycles) that are very different to other processes. Most of existing alloys were initially designed for slow solidification processes, not for AM. This is partly responsible for unsolved metallurgical and mechanical performance challenges in metal additive manufacturing, increasing the barrier for the full realisation of AM. To search for alloys suitable for AM, we have developed key criteria that successfully assisted the selection of a highly printable high entropy alloy. This PhD study project aims at developing an algorithm that integrates the criteria into a platform of machine learning (ML) and thermodynamics phase diagram to assist the search for new alloys for AM process. The printability (in particular, the microstructure and mechanical properties) of a selected alloy manufactured by selective laser melting to verify the effectiveness of the developed ML platform. Subsequently, the relationships between the alloy microstructures and process parameters will be studied to improve the ML platform and enable the effective tailoring of microstructures.

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.

Research Project Title: Microstructure Effects on Fatigue in (New) Titanium Alloys

Investigator: Hannah Lord

Supervisors: David Dye (Imperial), David Rugg (Rolls-Royce Plc)

Duration: 1/10/18 - 1/10/22

Description: The aim of this project is to study current and new titanium alloys to see the effect that various microstructure changes will have on fatigue in titanium. Altering the microstructure of the titanium alloys may help to stop fatigue crack growth initiating or propagating in the material, increasing the lifetime that the material can be used in jet aero-engines without failure. The project will involve mechanical testing of samples, as well as using various microscopy techniques to look at the alloys.

Investigator: Dr Ioannis Bantounas

Supervisors: Prof. David Dye

Collaborators: Dr Mark Hardy, Rolls-Royce

Duration: 01/08/2015 – 01/08/2017

The discovery of the gamma prime L1₂ phase in the Co-Al-W ternary system has enabled the development of Co-based alloys with increased high temperature mechanical performance. Cobalt exhibits a higher melting point than nickel, the base material of current high temperature engineering alloy applications. Co-superalloys thus promise a higher upper limit for high temperature mechanical performance.

Applied to an industrial context, development of these alloys could realise increase gas turbine inlet temperatures. This in turn would result in a positive impact on engine efficiency, reducing fuel consumption and CO₂ emissions.

The current project is aimed at understanding the relationship between microstructure and mechanical performance of this new class of alloys. A particular focus is placed on the high temperature dwell fatigue crack growth behaviour and the micro-mechanisms governing crack advance under such loading and environmental conditions.

Title: Understanding the role of hydrogen-dislocation interactions in the corrosion and hydrogen uptake of irradiated zirconium fuel cladding alloys

Supervisor: Mark Wenman

Co-supervisor: Andrew Horsfield, Adrian Sutton

Industrial partner: Carrie Miszkowska (Rolls Royce) 

Abstract: Zirconium alloys are predominantly used in nuclear fuel cladding. The lifetime of these alloys is limited by the pickup of hydrogen from the surrounding water coolant, and subsequent formation of hydrides. In addition to the alloy composition, the defects, dislocations and dislocation loops caused by radiation damage affect the hydrogen pickup fraction and corrosion rate. The mechanistic understanding of the interactions between hydrogen and radiation damage (especially dislocation loops) requires computational modelling techniques able to simulate thousands of atoms. The empirical potentials available at the moment for the Zr-H system (most notably EAM) do not provide sufficient accuracy, and DFT calculations are too slow for use on the system sizes required. The aim of this project is a development of DFTB (Density Functional Tight Binding) potential for the Zr-H system, where electronic structure is included explicitly. This should provide a model much faster than DFT codes, but more accurate and more transferable than empirical potentials. This will allow for modelling of hydrogen in irradiated zirconium alloys.

Project Title                       Multiscale hierarchical lattices: Crystal plasticity-based FEM modelling

Investigator                       Jedsada Lertthanasarn

Supervisor                          Dr. Minh-Son Pham

Co-Supervisor                   Prof. Fionn Dunne

Duration                              36 months

Abstract                              

The advent of additive manufacturing technology enables the fabrication of intricate lattices of a wide range of materials from polymers, metals to ceramics. Tailoring lattice structures can lead to the generation of lightweight materials with improved functionality. Previously reported studies on lattice materials had only focused on lattices with single orientations (analogous to single crystals). Such single-oriented lattices suffer substantial drops in load-bearing capacity. Inspired by the hardening mechanisms by tailoring crystal microstructures (such as grains, precipitates and phases) in metallurgy, we design macro lattices (i.e. meta-crystals) that mimic crystal microstructures to bring the rich knowledge in metallurgy to lattice design. The application of this approach to metals leads to the generation of multi-scale hierarchical lattices over wide length scales: from Å up to mm and beyond. This PhD study project focuses on using crystal plasticity-based finite element methods (CP-FEM) to simulate the deformation behaviour of hierarchical lattices. Together with experimental study, this CP-FEM approach will offer an integrated platform to develop hierarchical lattice materials with designed properties specifically tailored for desired energy absorption and load transfer.

Investigator: Liuqing Peng

Supervisors: Dr. Christopher Gourlay

Title: Reactions between liquid AZ91 and mild steel crucibles

Duration: 01/12/2016 – 30/11/2020

Project Description:

In the Mg industry, liquid Mg-Al alloys are commonly melted, ladled and/or poured from steel containers because the ceramics used by the Al industry would react with liquid Mg and because there is a low solubility for Fe in liquid Mg. However, Fe reacts with the Al and Mn in AZ91 when the molten alloy is held above the liquidus temperature, e.g. for many hours before handling in high pressure die casting (HPDC). This can lead to the formation of Al-Mn-(Fe) intermetallic particles that build up as die-casting sludge, block filters in DC casting, and affect the corrosion performance of Mg components. In this study, electron backscatter diffraction (EBSD) and energy dispersive X-ray spectrometry (EDX) are utilized to understand the nucleation and growth mechanisms of Al-Mn-(Fe) intermetallics in liquid AZ91 in contact with mild steel, at a range of temperatures and holding times. The quantitative results are used to discuss (i) the build-up of die-casting sludge and methods to minimize this and (ii) the impurity pick-up from the steel crucibles.

Title: Rapid solidification microstructure formation in cubic and non-cubic alloys fabricated by selective laser melting

Investigator: Minsoo Jin

Supervisor: Dr. Minh-Son Pham

Co-supervisor: Dr. Chris Gourlay

Rapid solidification in additive manufacturing (AM) process results in unique microstructures such as fine cells, epitaxial crystal growth and meta-stable phases. Such microstructures often lead to undesired mechanical properties of AM alloys (e.g., anisotropy, strong but less ductile and shorter fatigue/creep lives), making alloys fabricated by AM process less favourable in load-bearing application compared to other manufacturing processes. Understanding how crystals grow in rapid cooling and how they follow up with sudden changes in solidification direction will enable an effective control of microstructure to obtain desirable mechanical properties for high load-bearing application. The aim of the PhD project is to study the crystal formation in rapid cooling with sudden changes in solidification direction and subsequent microstructure evolution of both cubic and non-cubic alloys. Cubic crystals (namely, a Nickel superalloy and CoCrFeMnNi high entropy alloy) and a non-cubic alloy will be printed with different scanning paths by selective laser melting and examined by high resolution scanning/transmission electron microscopy to gain in-depth understandings of microstructure formation in AM.

Name: Peilong Dong

Title: Atmospheric Stress Corrosion Cracking of Low Carbon Austenitic Stainless Steels used in Dry Storage Casks for Interim Storage of Spent Nuclear Fuels.

Supervisor: Dr Mark R Wenman

Collaborator: EDF

Duration: 01/10/2016 – 01/10/2019 (PhD studentship)

Description: One of the major issues with nuclear power generation is the disposal of nuclear waste. In the UK, it has been decided that the ultimate disposal route for high level nuclear waste would be deep geological, however such a facility has not been sited yet and it will be decades before one is constructed and in operation. Previously PWR reactors, for example Sizewell B, used wet storage in the form of fuel ponds, however, these have since reached capacity and alternative methods need to be employed. EDF have decided that further high level waste such as spent nuclear fuels generated at Sizewell B would be stored in austenitic stainless steel dry storage casks.

Austenitic stainless steels are known to fail via a localised corrosion mechanism known as stress corrosion cracking (SCC). In low carbon steels, transgranular SCC may occur when a susceptible material is exposed to a specific chemical environment and a tensile stress. This is an issue as dry store facilities are located coastally (on site of nuclear power stations) exposing canisters to an aggressive chloride atmosphere. Residual tensile stresses will also be present in the canisters due to welding. These conditions make failure by SCC a possibility. However, as these canisters have an expected service time of 50-100 years, it is important to find out the SCC susceptibility and the extent of SCC cracking in these materials as a part of the safety case.

This project looks at the effect of salt deposition and different type of salts (MgCl₂, Synthetic Sea-salt, actual sea-salt composition at Sizewell) as well as different compositions of steels (304L, 316L weld material from an actual Holtec MPC canister, and additively manufactured 316L ). Analysis will be done primarily using optical microscopy, SEM, EBSD and EDX for crack counting, grain and texture information, and qualitative chemical analysis. Atom probe tomography will also be used to try and identify the presence and role of chlorine at/ahead of crack tips using atom probe tomography. 

Title: Alloy Conceptual Design and the Fundamental Mechanisms of Galling & Wear in the PWR Environment

Investigator: Samuel Rogers

Supervisors: Prof. David Dye and Prof. Fionn Dunne

Collaborators: Dr. David Stewart, Rolls Royce plc.

Duration: 9/2017 - 3/2021

Project Description:

Hardfacing alloys have been used for many years in components where surface degradation is of concern, particularly when coupled with extreme environments; be it high wear, high temperature, corrosive or erosive environments or a combination of these. Of particular interest is the use of hardfacings in valve seatings within the PWR environment. This is a high temperature, highly corrosive environment in which wear is of critical concern. The wear mechanism of most concern is galling which can result in catastrophic surface degradation and valve seizure. Galling is characterised by plastic deformation of surfaces resulting in the formation of protrusions on the contact surfaces. Although this much is known, the exact mechanisms by which it may occur are little known, as are the factors affecting it.

Although further research into the mechanisms of galling are necessary, some alloys are known to have greater galling resistance than others. One such alloy is Stellite 6, a cobalt-based alloy containing carbides. From a mechanical point of view, Stellite 6 is a very good candidate for valve seatings, however, in the PWR environment, cobalt activates, meaning that Stellite 6 should not be used. Thus the scope of this project is to understand the mechanisms of galling, including those in Stellite 6 which result in its galling resistance, and to produce a galling resistant alloy suitable for the PWR environment.  

Title: The role of pores with regards to plastic deformation and fatigue crack initiation in single crystal nickel superalloys.

Investigator: Tom Bellamy

Supervisor: Professor Fionn Dunne

Industrial Partner: Beijing Institute of Aeronautical Materials (BIAM)

Abstract: For a gas turbine engine, the demands of the turbine blades require the high temperature stability that single crystal nickel alloys can bring. The high service temperature required of turbine blades, along with large stresses and a corrosive operating environment result in complex fatigue behaviours. The aim of this research is to help provide new understanding into the mechanisms of plastic deformation for single crystal nickel superalloys and hence the mechanistic drivers for fatigue crack nucleation and growth. In particular, the role of notches to represent both internal casting defects and surface damage will be addressed. Initial work will be carried out to analyse the effect of notches on the fatigue performance of small cylindrical samples. Appropriate modelling methods will be used in combination with independent experimental studies to understand the deformation and degradation processes taking place around the notch.

Title: Deformation mechanisms in Co/Ni-base superalloys

Investigator: Tom McAuliffe

Supervisor: Prof David Dye

Collaborator: Rolls-Royce plc

Funding: Rolls-Royce plc, CDT Advanced Characterisation of Materials

Duration: 02/10/2017 - 30/09/2021

Since the discovery of the Co₃(Al,W) L1₂ phase, Co-base superalloys have undergone significant development. The addition of Ni has enabled the realisation of polycrystalline Co/Ni-base superalloys with a wider ɣ’ phase field, higher solvus temperature, and oxidation resistance comparable to Ni-base superalloys. This project aims to characterise plasticity over micro and nano length scales in order to open up new avenues for alloy design.

At intermediate temperatures, primary creep progresses via a variety of phenomena including stacking faults, antiphase boundaries, and microtwins. Optimisation of creep behaviour requires further understanding of the rate limiting mechanisms, and the effect of stress, temperature and composition. These factors will be investigated principally using TEM. Additionally, the evolution of strain at and around grain boundaries warrants further characterisation. Grain boundary sliding and microstructural changes will be investigated via EBSD, SEM and DIC. This is with an aim to assessing the impact of grain boundary strengthening carbides and borides.

Investigator: Tom Whiting

Supervisors: Dr. Mark Wenman, Prof. Robin Grimes

Collaborators: Rolls-Royce

Duration: 03/10/2016-03/10/2019

Description: The lifetime of nuclear power plants is limited by the integrity of the reactor pressure vessel (RPV), that is often constructed out of low-alloy steel. Over time, the mechanical properties of the steel RPV degrade due to neutron irradiation and temperature effects which in particular lead to embrittlement and increase its ductile-to-brittle transition temperature. One of the major contributions to embrittlement of the RPV steel is from clustering of impurities such as silicon, manganese and nickel that could potentially cause a "late-blooming phase"; this would lead to an exponential increase in embrittlement of the steel after around 20-30 years of use.

Computational modelling using density functional theory will be performed to investigate how clustering of the impurities occurs, with particular emphasis on the effect of strain fields caused by dislocation loops and solute interactions in different formations. The overall aim of the project is to examine the extent of the damage caused by neutron irradiation and elevated temperature on steel with the goal of extending the lifetime of RPVs and hence nuclear power plants.

Vassilios Karamitros – Modelling of Microstructure Sensitive Short Crack Growth in Gas Turbine Alloys

Investigator: Vassilios V. Karamitros

Supervisor: Prof. Fionn PE Dunne

Collaborator: Dr. Duncan Maclachlan, Rolls – Royce plc. / Royal Society

Duration: 01/10/18 – 30/09/22 (PhD Studentship)

Description:

Short Crack Growth (SCG) in metals and alloys refers to the process of crack growth from nucleation of a small crack approximately the size of a single grain, up to the size of some tens of grains, typically 100 to 200 microns. The process of short crack growth depends on the local microstructure which is significant in determining the useful service life of aero-engine components. In this research project new methods for modelling the growth of short cracks in advanced nickel based superalloys are to be developed. The project combines 3D crystal plasticity modelling and crack growth rate calculations with explicit modelling of crack propagation using the eXtended Finite Element Method (XFEM). Micro-mechanical parameters that are affected by the material’s local microstructure such as the stored elastic energy will be explored to attempt to quantify SCG. Through state of the art modelling techniques and characterization methods, effect of grains, the grain orientations and grain boundaries on crack growth rates, crack branching and crack arrest can potentially be decoded.

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.

Investigator: Yitong Shi

Project Title: Environmental Effects on Cracking in Titanium Alloys

Supervisors: Prof. David Dye, Prof. Trevor Lindley

Collaborators: Mr. Edward A. Saunders and Dr. Rebecca S. Sandala, Rolls Royce plc.

Funding: Rolls Royce plc.

Duration: 01/03/2017-01/03/2020

Description: Titanium alloys are widely used as the structural components in gas turbine engines, owing to its low density, excellent specific fatigue strength and corrosion resistance at high temperature. Ti-6Al-2Sn-4Zr-6Mo is a good candidate material for compressor discs. A ‘blue spot’ region was initially found at crack initiation sites in the component tests, which was identified as corrosion by NaCl assisted by stress and pressure at intermediate temperature (>300). Recently, it was reported that similar failure appeared in service resulting from hot salt stress corrosion cracking by AgCl. One of the aims in this project is to demonstrate the mechanism for silver chloride corrosion reactions.

Fatigue striations are important features for characterizing fatigue crack growth and one striation normally form in one cycle at certain stress intensity range. However, it has been found that striations disappear in vacuum. The other part of this project is to investigate the environmental effects on fatigue striations. Various atmospheres such as oxygen, nitrogen and hydrogen will be examined to understand the effects on striation profile.

Investigator: Jingwei Xian

Supervisor:  Dr Chris Gourlay

Duration: 5/11/2012 - 4/11/2015 (PhD Studentship)

Description: Cu₆Sn₅ crystals are commonly found in Pb-free solder joints and their size, shape and volume fraction influence the reliability of joints (e.g. during thermomechnical fatigue). This project aims to control the Cu₆Sn₅ size and morphology by building a fundamental understanding of the nucleation and growth of Cu₆Sn₅ in both the bulk solder and the interfacial layer. The project uses analytical electron microscopy to investigate the 3D growth morphology, and stable isotope SIMS to study diffusion mechanisms in interfacial layers.

Investigator: Abigail Ackerman

Supervisors: Dr David Dye

Collaborators: Prof. David Rugg, Rolls-Royce plc.

Duration: 10/04/2014 - 10/01/2017

Description:Ti-6Al-2Sn-4Zr-6Mo (Ti 6246) is currently used in the HP compressor of gas turbine engines. Secondary alpha is the phase of the alloy that nucleates from primary alpha, growing within the beta grains and additionally at the grain boundaries. The presence of secondary alpha gives the alloy it’s strength, however little is known about its nucleation, growth and behaviour.

By investigating how the phase grows under differing processing conditions, it is hoped some insight will be gained into the development of secondary alpha in Ti6246. The project will include analysis of chemistry, morphology, grain orientations and behaviour under deformation.

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.

Student: Bo Chen

Supervisors: Prof. Fionn Dunne, Dr Jun Jiang

Funding:  IC-CSC, Rolls-Royce

Abstract

Nickel based alloys have been increasingly used in aero industries because of their good in-service mechanical properties compared to other alloys, such as high strength, resistance to oxidation, corrosion and creep under high strength conditions. This material is some of the toughest material available. But these components will experience repeated loading and unloading during service which may result in highly localized plastic strains within materials and then lead to the uncontained failure which is potentially catastrophic. In this case, fatigue crack nucleation accounts for a great part of the total fatigue life of this component. However, the development of quantitative understanding of fatigue crack nucleation in Nickel superalloys has been a bit limited. As a result, it is crucial to develop understanding the mechanic basis of fatigue crack nucleation process, which is fundamentally microstructure sensitive, and hence to enable predictive modelling techniques to be developed to facilitate analysis of the key microstructural features (such as grain size, crystallographic orientation combination, twinning etc.) on lifetime.

Student: Chong Zhao

Supervisor: Prof. Fionn Dunne, Dr.Jun Jiang

Funding: Rolls Royce (55%), Imperial College London (45%)

Abstract:

Hardfacing alloys have excellent corrosion and wear resistance. Cobalt base hardfacing alloys have good hard facing capacity and are widely used in nuclear applications. However, radiation from Cobalt is hazardous for both workers and plant materials so a non-radiative iron base hardfacing alloy is designed to replace traditional cobalt base alloys. This project will observe the sliding wear behaviour in this material. Mechanical properties, such as shear strength, elastic strain and galling resistance in increasing temperatures will be studied. These mechanical properties shall be compared with other hardfacing alloys and the relationship between chemical compositions and mechanical properties shall be investigated.

Chris Collins - The effect of nitrogen on Ti-6Al-4V

Investigator: Chris Collins, Rolls-Royce plc

Supervisor: Professor David Dye

Collaborators: Professor David Rugg, Rolls-Royce plc

Duration: 01/02/2016 - 01/02/2019 (Part-time MPhil)

Description: 

Although the effects of oxygen on titanium alloys have been well characterised; the effects of another alpha stabilising interstitial, nitrogen, have not been researched as throughly. Samples will be taken from bars manufactured in the laboratories at Imperial, with particular care taken over reproducing microstructures and mechanical properties relevant to industrial applications.

This project aims to quantify the effects of a range of nitrogen contamination levels on the general properties of Ti-6Al-4V moving onto study the effects under simple LCF loading.

Student: Claire F. Trant

Supervisors: Prof. David Dye, Prof. Trevor Lindley

Funding:  EPSRC, Imperial College and Rolls Royce MMRE P61357   

Abstract

The limited ductility and relatively fast crack propagation of gamma TiAl make its use as a material for highly stressed components particularly challenging. The fatigue crack growth threshold has been identified as a key material property for design, supported by an improved understanding of the deformation mechanisms associated with a crack growing near threshold. In this project, the influences on the crack growth threshold will be investigated, followed by the effect of overloading, and applying an over and under-temperature. Further investigations will then be carried out with gamma TiAl depending on previous findings. Current ideas for further investigation include looking at air vs. vacuum dwells, forging vs. casting, or surface residual compressive stresses. Rolls Royce will supply components for this project, with potential future use in turbine blades.

Investigator: David Wilson

Supervisors:  Prof Fionn Dunne and Dr Ben Britton

Collaborators: Michael Martin (Rolls-Royce)

Duration: 01/10/1015 to 01/10/2018

Abstract

Microstructure-sensitive computational modelling techniques will be established, with which safety cases may be justified for in-service components. This is to be achieved initially through the development of crystal plasticity techniques for Zirconium alloys and will utilize data from a range of micromechanical test techniques including both high resolution electron backscatter detection (ebsd) and digital image correlation (DIC) which provide quantification of grain-level stress and plastic strain respectively. Appropriate mechanistic models are to be developed for defect nucleation which are to be tested by microstructure-level comparison with experimental observations. The success of the project would provide the underlying mechanistic basis, and the tools for the prediction of safety-critical component life, thus supporting the necessary safety justifications required by the aero-engine and nuclear industries.

Project details:

Investigator: Felicity Dear

Supervisors: Prof David Dye, Dr Vassili Vorontsov

Collaborators: Rolls-Royce plc

Funding: Rolls-Royce plc, CDT Advanced Characterisation of Materials

Duration: 03/10/2016–30/09/2020

Project title:

Fundamental mechanisms in titanium alloys

Project description:

Titanium alloys such as Ti–6 Al–4 V (wt. %) are widely used in aero engines, providing lightweight yet strong materials that can withstand elevated temperatures. This project will use advanced characterisation techniques to gain understanding of fundamental behaviours in these materials, aiming to better inform alloy design and usage.

Factors controlling and contributing to dwell fatigue are of primary interest, including slip behaviours and solute site partitioning within ordered Ti₃Al precipitates. Atomic-scale structure and chemistry will be observed using techniques such as atom probe tomography and ALCHEMI, while dislocation behaviours will be studied using mechanical testing and TEM. Control of texture forms another aspect of the project, to be addressed with EBSD and fatigue testing.

 ‌

Fig 1. Diffraction pattern and dark-field image of a Ti–11.7 Al–0.7 O–1.0 V (at. %) alloy after ageing, containing ordered Ti₃Al precipitates within an α-Ti matrix.

Intermetallic nucleation and growth in Magnesium alloy solidification

Investigator: Dr Guang Zeng

Supervisor: Dr Chris Gourlay

Duration: 07/03/2016 – 31/10/2019 (PDRA)

Funding:  EPSRC (Future LiME hub)

Description:  Solidification of magnesium engineering alloys involves the formation of intermetallic compounds (IMCs) throughout the solidification sequence. Impurities such as Fe, Cu and Ni can have determintal effects on the corrosion resistance of Mg alloys, often by forming IMCs with a different cathodic potential to (Mg). This study aims to understand the nucleation and growth mechanisms of intermetallics in Mg castings and explore ways to harness impurities. It involves controlled solidification experiments, X-ray video microscopy, X-ray diffraction and analytical electron microscopy. This project is the Imperial spoke of the Future LiME hub [http://www.lime.ac.uk/], based at Brunel University, which is working towards full metal circulation in which the global demand for metallic materials is met by the circulation of secondary metals.

Primary Al₈Mn₅ intermetallic in AZ91 magnesium alloy

Investigator: Hikmatyar Hasan

Supervisors: Dr. Vassili Vorontsov, Prof. Peter Haynes & Prof. David Dye

Funding: ESPRC (via TSM CDT)

Duration: 01/10/2015 – 01/10/2018

Since their discovery nearly ten years ago, Co-Al-W-based superalloys have emerged as the frontrunner materials to replace the ubiquitous Ni-based superalloys used in gas turbines. The modelling of deformation mechanisms in these alloys is of paramount importance for accelerating the identification of optimal alloy compositions, saving both time and money during the development process.

The dynamics of dislocations within these superalloys and the effects of diffusion will be studied using the Phase Field Model for Dislocations. Prior to this, a Generalised Stacking Fault Energy Surface will need to be calculated, as an input, at a much smaller length scale using Density Functional Theory.

The aim of the project is to characterize the effect of composition on the mechanical deformation of Co-Al-W-based superalloys using the above models.

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. 

Investigator: Lucy R Reynolds

Supervisors: Prof. David Dye, Dr. Vassilli Vorontsov

Duration: October 2015 – March 2019

Funding: EPSRC & Rolls-Royce plc

Description: The drive for aerospace engine efficiency is increasing, and demand for materials able to withstand the higher turbine entry temperatures is rising. Promising current candidates are Cobalt based Superalloys, with an ordered L1₂ phase being discovered in 2006 by Sato et al.

Through tweaking alloying additions and processing, a family of new stable high temperature superalloys with excellent resistance to oxidation have been developed. This project will focus on anti-phase boundaries and stacking faults within the alloys, including energy determination and assessment of the effect varying composition has on their mechanical properties. A number of analysis techniques will be used, for example, SEM, TEM, EDX, EBSD and DSC.

Investigator: Ning Hou

Supervisors: Dr Chris Gourlay

Duration: 3/10/2014 - 30/9/2017 

Description:Most electronic solders have near-eutectic composition. One aim of my project is to quantify the competition between stable Sn-Ni3Sn4 and metastable Sn-NiSn4 eutectic microstructures during controlled unidirectional growth. The focus is on the influence of interface growth rate and impurities on phase selection. Another aim is to measure and calculate the eutectic coupled zone for eutectics important in soldering such as Sn-Cu6Sn5, Sn-Pb, Sn-NiSn4 / Sn-Ni3Sn4 and Sn-Ag3Sn.

Investigator: Dr Sergey Belyakov

Supervisor: Dr Chris Gourlay

Duration: 30/06/2016 - 01/07/2013 (PDRA)

Collaborators: Nihon Superior Co., Ltd.

Description:Sn-0.7Cu-0.05Ni has been used as a Pb-free solder since 1999 and has become a popular choice for wave-soldering. This project seeks to understand microstructure formation during reflow soldering and microstructure evolution in service when any Sn-Cu-Ni alloy is soldered to Cu or Ni substrates. The research aims to control and predict solder joint reliability in this system and to understand the how joint miniaturisation affects microstructure formation and stability. This project involves controlled solidification experiments, reflow soldering and analytical electron microscopy.

Read More:

• S.A. Belyakov, C.M. Gourlay, NiSn4 formation during solidification of Sn-Ni alloys. Intermetallics. 25 pp. 48-59, 2012
• S.A. Belyakov, C.M. Gourlay, NiSn4 formation in Ni-Sn and ENIG-Sn couples. Journal of Electronic Materials. 41(12) pp. 3331- 3341, 2012
• S.A. Belyakov, C.M. Gourlay, Role of Fe impurities in the nucleation of metastable NiSn4, Intermetallics. 37, pp. 32-41, 2013

Investigator: Simon Wyatt

Supervisors: Dr. T Ben Britton (primary), Prof. Fionn P E Dunne

Funding:  EPSRC STU0126694 (50%), Rolls-Royce Group plc (50%) 

Abstract

Metals are widely used for load-bearing applications in complex environments. Their properties are dependent on the underlying behaviour of the material microstructure, which is naturally anisotropic due to the discreet and crystallographic nature of slip and anisotropic elastic properties. This project focuses on developing efficient methods of modelling the evolution of crystallographic texture in two-phase alloys using efficient crystal plasticity based upon the fast Fourier transform. Working with Rolls-Royce plc, materials will be characterised using HR-EBSD to determine textures which will further stimulate the computational work.

Investigator: Dr. Sudha Joseph

Supervisors: Dr David Dye

Duration: 21/09/2014 - 21/09/2016 (HexMat PDRA)

Description: Dwell fatigue is a deleterious failure mechanism in compressor discs made of titanium alloys used for aero engine applications, which can be of concern. It can give rise to sub-surface initiation and brittle cleavage-like, facetted features. This can be controlled/avoided by the fundamental understanding of the failure mechanisms. This project aims to understand the deformation mechanisms in disc alloy Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti-6242Si) under dwell conditions. In particular, the focus is on crack initiation since life of the component under these circumstances is dominated by nucleation. One of the interests is to investigate the damage mechanisms at a low stress level, where the creep mechanisms start activated and compare it with the damage mechanisms at high stress level, where the dwell effect is dominated by creep mechanisms. It involves extensive mechanical (static, cyclic, dwell and creep) and microstructural characterization (SEM, EBSD and TEM) of the alloy. The main objective is to look at the slip behavior and dislocation interactions in crack initiated facets of the FIB milled samples using advanced TEM techniques. The dislocation studies will be carried out in relation to the crystallography of primary α grains. The role of hydrogen in dwell behavior of the alloy will also be investigated.    

Investigator: Te-Cheng Su

Supervisor : Dr Chris Gourlay

Duration: 06/10/2014 - 31/03/2018 (Imperial College PhD Scholarship)

Collaborators: Tomoya Nagira, Osaka University; Hideyuki Yasuda, Kyoto University; Japan Synchrotron Radiation Research Institute (JASRI)

Description: The investigation of deformation of partially solidified alloys by in-situ X-ray synchrotron radiography has been widely reported in the last five years. It develops from the 2-D direct observation and a series of image analysis techniques on semi-solid alloys in isothermal holding. Further, some breakthroughs have been made such as tomographic reconstruction to observe the three-dimensional granular behaviour of partially solidified alloys during casting. There are several unexplored combinations of semi-solid microstructure, stress and accumulated strain which may affect the mechanical response of partially solidified alloys under loading. Therefore, the project is investigating the effect of grain size, shear strain rate and solid fraction on deformation mechanisms of Al-Cu alloys and steels in the semi-solid state by the application of in-situ X-ray synchrotron radiography and tomography. We are adapting ideas and methods from soil mechanics to track the microstructural evolution including the liquid flow field, strain field and grain motion during loading in order to develop new perspectives for filling, feeding and defect formation in alloy casting.

Title:  EBSD studies of the evolution of microstructure and damage during the thermal cycling of Pb-free solder materials

Investigator: Tianhong Gu

Supervisors: Dr Ben Britton / Dr Christopher Gourlay

Funding: Self-funded

Description: When electronics fail in service the cause is often thermomechanical failure of the solder joints. The major failure is cracking occurred in the bulk near solder / substrate interface, as shown in Figure 1. Pb-free joints usually contain more than 95% tin phase, which has highly anisotropic thermophysical properties. Thus, tin is particularly sensitive to thermomechanical fatigue caused by cycles of heating and cooling. There is a strong industrial demand to understand the mechanisms leading to thermomechanical fatigue failure, to be able to predict the joint microstructure that is most resistant to thermal cycling, and to develop ways of generating the optimum microstructure through alloy design and processing.Therefore, this project will be focused on investigating the role of stress distribution within solder joint generated from thermal expansion mismatch between solder ball and substrate, and microstructure of solder ball on failure mechanisms of solder joints during thermal cycling.

Discrete dislocation dynamics coupled with discrete solute diffusion to model the effect of hydrogen in steel

Supervised by Daniel Balint and T. Ben Britton

Funded by ICO CDT Nuclear Engineering and AWE

Abstract

Plastic deformation in metals is due to the motion of mobile dislocations subject to shear stresses in excess of a critical value. At particular strain rates and temperatures mobile solutes form atmospheres at the base of dislocations pinning dislocation motion. This results in an inverse strain response, jerky plastic flow and decreased ductility. In the worst case, ductility is reduced significantly and brittle fracture may occur. We attempt to understand the interaction and effects of mobile dislocations and solutes in metals using discrete dislocation dynamics coupled with an appropriate discrete solute diffusion model. In the first instance our model will be developed to describe carbon solutes in saturated iron. A generalized model will be developed so as to give insight to the deleterious effects of hydrogen in industrial steels.

Investigator: Dr Claudia Gasparrini

Supervisor: Dr Mark Wenman

Collaborators: Rolls-Royce Plc, Australian Nuclear Science and Technology Organisation, UK Atomic Energy Authority Materials Research Facility, National Nuclear Laboratory, Culham Centre for Fusion Energy, Oxford University, Manchester University, University of New South Wales.

Duration: 01/12/2017 – 01/12/2019 (Postdoctoral Research Associate)

Description: The aim of this project is to investigate the role of microstructure and processing history on the phenomenon of neutron irradiation embrittlement in reactor pressure vessel (RPV) steels. The materials investigated will all be of the same chemical composition of A508 class 3 ferritic steel but manufactured using different methods.  There will be forged (in current use), hot isostatically pressed and electron beam welded materials.  The position involves help oversee the technical and safety aspects of carrying out irradiations in the OPAL reactor and packaging/transport of the materials to be sent back to the UKAEA Materials Research Facility at Culham.  Micro-mechanical tensile testing and microscopy (SEM and TEM) are performed on unirradiated and irradiated steels in collaboration with the UKAEA Materials Research Facility, Manchester University and the Australian Nuclear Science and Technology Organisation.

Dr Daniel King - Atomic scale modelling of nano-solute-vacancy clusters in reactor pressure vessel steels

Supervisor: Dr Mark Wenman

Collaborators: Rolls-Royce Plc, National Nuclear Laboratory, Culham Centre for Fusion Energy, Australian Nuclear Science and Technology Organisation, Oxford University, Manchester University, University of New South Wales.

Duration: 01/02/2017 – 01/08/2019 (Postdoctoral Research Associate)

Description: The aim of this project is to determine the driving forces and behaviour of nano-scale solute-vacancy clusters, a mechanism responsible for hardening, that occurs due to  neutron irradiation of reactor pressure vessel steels.  The expected outcome is to achieve a mechanistic understanding of this process to support safety cases and models for pressurised water reactor life extensions. This project specifically involves the modelling and prediction of the clustering behaviour of Mn, Ni and Si, in bcc Fe, using density functional theory. Results from these models will be used in a multiscale approach to link fundamental solid state physics calculations with advanced manufacturing of steels for future reactors.  Further, results from this project will allow for less conservative predictions of toughness reduction and support on-going operation for reactors beyond 60-80 years.

Microstructural evolution of 316l stainless steel in laser powder bed fusion

Investigator: Filippo Vecchiato

Supervisors: Dr Mark R Wenman and Dr Paul A Hooper

Duration: 03/10/2015 –31/03/2019 (PhD Studentship)

Description: Laser powder bed fusion technology allows for design flexibility, unfeasible with other destructive manufacturing methods. Therefore, it can be used to produce porous structures and lattices with variable mechanical properties. The project has been focused on the control of the microstructural properties of deposited 316L, by controlling the cooling rates involved in the process, changing the laser parameters used during the deposition.

The procedures used variable laser power and laser exposure time, controlling the grain size in the melt pool, producing different cooling rates. The microstructural formation was compared against the investigation of the cooling rates with a multi-channel high speed thermal camera.

Investigator: Dr Yilun Xu

Supervisor / Line Manger: Prof Fionn Dunne

Title: Crystal Plasticity Modelling of Electronic Solder Joint Performance

Duration: 01/07/2018 - 30/06/2021

Description:

This project aims to predict and control of solder joint reliability in electronics using high fidelity crystal plasticity finite element (CPFE) modelling and discrete dislocation dynamics (DD) techniques. The research involves the establishment of mechanistic crystal-level models for slip and failure (in conjunction with micro-mechanical experimentalists) and the development of solder bead micro-structural models. The output of the modelling guides the design of solder bead microstructure and provides the optimal performance for particular loading regimes, e.g. thermo-mechanical and mechanical shock loading. The project also includes working with colleagues utilising the models through to simulating solder arrays under in-service loading.