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: 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.

Modelling Delayed Hydride Cracking and Crack Growth in Zr Cladding

Delayed hydride cracking (DHC) is a phenomenon which can occur in metallic alloys subjected to thermal cycling. At elevated temperatures, hydrogen diffuses preferentially to crack tips and precipitates at high concentrations, forming hydride phases. These hydride precipitates give rise to stress concentration at crack tips and can influence crack growth-rates significantly. DHC is of particular interest in zirconium alloys, which are used as cladding material in nuclear reactors. This project aims to develop novel crystal plasticity models, coupled with hydrogen diffusion models (accounting for hydride formation and dissolution), building upon a computational modelling framework developed by Dr David Wilson, which enabled mechanistic understanding of fatigue crack growth in zirconium alloys. The influence of hydrogen content on crack growth rate and direction in zirconium alloys under cyclic loading will be investigated, and hence, a more mechanistic understanding of cyclic crack growth characteristics in real materials will be gained. More generally, this work aims to provide an understanding of the influence of microstructural characteristics on crack growth-rate, acceleration, deceleration, and tortuosity etc. Another key challenge of this work is to expand the current 2D modelling capabilities and develop a 3D crack-growth modelling framework. The crystallographic plane along which a crack grows can influence the growth-rate by up to 4 to 5 times. This will enable more accurate prediction of crack-growth characteristics in zirconium alloys; however, as existing methodologies are already very computationally expensive, this is a key obstacle to be overcome.

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.

Studies of deformation and twinning in magnesium alloys.

Chaitanya Paramatmuni, Prof. Fionn Dunne

The plastic behaviour of a material during forming is influenced by the extent of its anisotropy. Magnesium (Mg) alloys show high plastic anisotropy caused by the activation of different deformation modes depending on the loading direction. One of the main contributors to this anisotropy is deformation twinning. It is found to be detrimental to the formability of Mg alloys as it abruptly reorients the texture, preferentially activates hard slip systems and also acts as hot spots in the microstructure. While these hot spots aid in the generation of desirable weak texture during heat treatment, in performance they act as high stress concentration spots for crack nucleation and facilitate crack growth.

This project utilises integrated experimental and numerical investigation of twin nucleation and growth on the free-surface of the sample and in the 3D microstructure.  Twin-assisted and slip-assisted twin nucleation in wrought Mg alloy AZ31 and rare-earth Mg alloy WE43 respectively are investigated in detail on the free surfaces of samples. The spatial variation of field variables inside and at the vicinity of twins is explicitly measured using high resolution electron backscatter diffraction (HR-EBSD) to study the twin growth in Mg alloy AZ31. The experimental observations are then integrated with crystal plasticity strain-gradient modelling to investigate twin growth in Mg alloys. Finally, these free-surface investigations are extended to three-dimensions by conducting diffraction contrast tomography (DCT) and 3D-EBSD experiments to obtain 3D grain and twin morphologies, which are further analysed using mesoscale modelling techniques.

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: 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.

Project title: Modelling methods for optimal functionally graded materials by novel processing.

Investigator: Thomas J Porter

Supervisors: Prof. Fionn Dunne and Dr David Stewart

Duration: 01/10/2019 – 30/09/2022

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

 

Project Description: Powder Hot Isostatic Pressing (HIP) is a metallurgy process for consolidating powder particles. High temperature and pressure are applied simultaneously to form a component of complete theoretical density. The application of pressure allows for elimination of porosity and mitigation of grain growth. A near net shape component is produced, with better chemical homogeneity in comparison to traditional casting methods.

The aim of the project is to develop functionally graded components through the HIP process. So far, components are in general made from a single type of material. However, the possibility of hot isostatically pressing two different types of material together would address the need of varying performance along a component. An example is in graded structures coupling pressure vessels and piping in a nuclear power plant.

This project addresses the establishment of computational material modelling methods to facilitate optimally designed HIPped materials and structures, and will likely be partnered with an experimental programme, in collaboration with Rolls-Royce.

Project title: Fatigue crack nucleation around annealing twin boundary in nickel superalloy

 

Investigator: Xiaoxian Zhang

 

Supervisors: Prof. Fionn Dunne

 

Collaborators: Dr Jean Charles Stinville, University of California Santa Barbara

 

Duration: 01/10/2019 – 01/10/2020

 

Funded by CSC and RAEng

 

Description: For polycrystal nickel superalloy, fatigue crack nucleation (FCN) is well known around annealing twin boundary except for non-metallic inclusions. The mechanism about this nucleation is associated with the special crystal orientation and microstructure/morphology of twin. Even the slip behavior (parallel and inclined) happened around twin has been investigated systemically, an intrinsic scientific question remains elusive, that is why fatigue crack preferentially nucleate around twin boundary rather than other grain boundary and grain interior.

This project employ a crystal plasticity finite element analysis coupled a HR-DIC characterization, to investigate the micro-behavior appended around twin boundary during a cyclic loading, providing an understanding about the FCN around twin from an energy approach associated with microstructure.

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.

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Investigator: Dr Yang Liu

Supervisor / Line Manger: Prof Fionn Dunne

Title: Towards validated predictive capability for microstructure-sensitive fatigue and delayed hydride cracking in Zr

Duration: 31/03/2020 - 14/09/2021

Description:

The aim of this work is to establish microstructure-level dislocation-based crystal plasticity (CP) modelling coupled with atomistic hydrogen diffusion for spatially resolved analysis of hydride precipitation and dissolution, hydride cracking and interaction with fatigue-driven crack growth in (proton) irradiated Zr-alloy. The CP modelling method facilitates explicit representation of microstructure (grain and second-phase morphology, crystallography, slip, grain boundary interactions). The work will rely on property extraction and quantitative characterisation (HR-DIC, HR-EBSD) at Manchester and Oxford, discrete dislocation modelling at Imperial and Oxford. New experimental work at Imperial could be conducted to investigate hydride precipitation and dissolution under representative in-service thermo-mechanical loading in (proton) irradiated samples in order to validate the new CP modelling framework developed.

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.