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Abigail Ackerman - Secondary Alpha in Ti-6Al-2Sn-4Zr-6Mo

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.

Dr Ioannis Bantounas – Effect of Microstructure & Micro-Mechanisms Responsible for High Temperature Mechanical Properties of Co-Based Superalloys

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.

Ioannis Bantounas
Backscattered electron micrograph showing secondary and tertiary gamma prime precipitates in a Co-based superalloy.

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

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.

Felicity Dear - Fundamental mechanisms in titanium alloys

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.

 

Dr Sudha Joseph - Dwell fatigue crack initiation in compressor disc alloy Ti-6262Si

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.    

Thomas Kwok - Alloy and process development for high strength medium

Title: Alloy and process development for high strength medium Mn steels

Investigator: Thomas Kwok

Supervisor: David Dye

Funding: A*STAR

Duration: 09/01/2018 – 09/01/2022

 

High Mn Twinning Induced Plasticity (TWIP) steels were previously the subject of intense research. Their remarkable ductility (>30%) and high rate of strain hardening allowed for high energy absorption during deformation. These properties made TWIP steels very attractive for energy absorbing applications such as automotive crash bumpers and armour plate. However, numerous problems were encountered during the industrialisation of this class of steels. These problems include edge cracking during hot rolling, Mn segregation and excessive Mn vaporisation, and can be largely attributed to the large Mn content (16 – 30 wt%). As a result, a new class of steels termed ‘medium Mn steel’ was created with significantly less Mn (6 – 12 wt%) while attempting to preserve the properties of high Mn TWIP steels.

 

The aim of this project is two-fold. The first aim is to understand the new deformation methods in medium Mn steels such as the recently identified TWIP+TRIP mechanism. The second aim is to use this understanding to develop a novel medium Mn steel in rolled format that is industrially competitive in terms of cost and processing.

Hannah Lord - Microstructure Effects on Fatigue in (New) Titanium Alloys

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.

Tom McAuliffe - Deformation mechanisms in Co/Ni-base superalloys

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.

Tom McAuliffe
Figure 1: Secondary electron image of the ɣ’ distribution in a Co/Ni-base superalloy (Knop et al, “A new polycrystalline Co-Ni superalloy” JOM, vol. 66, no. 12, pp. 2495-2501, 2014)

Lucy R Reynolds - Cobalt based Superalloy development

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.

Samuel Rogers - Alloy Conceptual Design and the Fundamental Mechanisms of Galling & Wear in the PWR Environment

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.  

Yitong Shi - Environmental Effects on Cracking in Titanium Alloys

Investigator: Yitong Shi

Project TitleEnvironmental 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.

Claire Trant - Fatigue in Gamma Titanium Aluminide

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.

Dr. Xin Xu - Designing Alloys for Resource Efficiency

Dr. Xin Xu - Designing Alloys for Resource Efficiency

Investigator: Dr Xin Xu

PI: Prof. David Dye

Collaborators: Rolls-Royce

Funding: EPSRC

Duration: 01/05/2018– 28/09/2019

Description:

Improving the energy efficiency of transportation tools is an effective way to reduce the greenhouse gas emission. Therefore, industries have a strong demand for high strength, high ductility alloys to make transportation tools lighter and safer, meanwhile, to reduce the production cost. Twinning induced plasticity (TWIP) and transformation induced plasticity (TRIP) have been demonstrated to enhance the strength and ductility simultaneously. The aim of this project is developing high strength, high ductility steels and titanium alloys taking the advantage of TWIP and TRIP effects in addition to other strengthening mechanisms. Key methods include thermodynamic modelling (Thermo-Calc), processing route design, mechanical testing and microstructure characterisation using scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy, synchrotron and neutron diffraction and imaging.

Dr Xin Xu
Fig. 1 Tensile properties and microstructure of a TWIP Ti alloy