TITLE: Ahmed Belkarty - Nucleation kinetics of Lead-free solder joints

Project title: Nucleation kinetics of Lead-free solder joints

Investigator: Ahmed Belkarty

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. 

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.

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.

Investigator: Suki Adande

Supervisors: Dr Ben Britton

Collaborators: AVIC-BIAM

Duration: 01/06/2014 - 01/06/2017

Description:The aim of this project is to characterise and understand components of Ni superalloys in coated systems using high fidelity micromechanical tests. The techniques we will employ include; TEM, FIB, EBSD and in-situ testing combined with more standard metallographic techniques such as optical microscopy.

Micromechanical tests can isolate specific regions of interest within an alloy and examine dislocation mechanics at the local scale (micro-nanometres) in an attempt to better understand strengthening mechanisms associated with this scale. Micropillar compression enables small volumes of material to be tested. Compression tests can be subsequently analysed through load/displacement/time data and correlated with EBSD / slip trace analysis to better understand and quantify the material properties / deformation mechanics of the system. Compression tests will be done on different regions and orientations of industrial nickel based superalloys utilising coating chemistries that form a functionally graded alloy. The mechanical properties and intermetallic chemistries of these pillars will be related with chemistry and dislocation structure using SEM based EDS and dislocation contrast in the TEM. This data can be used to improve modelling of functionally graded coatings, and provide experimental input into physically based models.

Investigator: Bartosz Barzdajn

Supervisor: Professor Fionn Dunne and Professor Tony Paxton (King's College London)

Collaborators: Rolls-Royce and King's College London

Duration: 03/01/2014 - 28/02/2017 (PhD Studentship)

Description: Structural integrity in austenitic and ferritic steel will be investigated using fundamental materials modelling to further understanding of nucleation, short crack growth, linear elastic fracture mechanics and effects at ferrite interfaces. One focus will be on austenistic to ferritic boundary effects and later interfaces with carbides will be considered. This project will provide a clear methodology to link between quantum effects at the atomistic scale up length and time scales to influence modelling efforts in crystal plasticity modeling.

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: 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:  Farah Ismail

Supervisors: Dr Barbara Shollock and Professor Trevor Lindley

Duration: 12/11/2012 - 11/11/2016 (PhD Studentship)

Collaborators: Rolls Royce plc

Description: A new class of cobalt based alloys has become possible when the face-center cubic (FCC) structured gamma (γ) Co-solid solution is strengthened with gamma prime (γ’) Co3(Al,W) phase which is an intermetallic compound with an ordered L12 structure. The two phases have similar lattice parameters and therefore a high degree of coherency when γ’ particles are dispersed throughout the γ Cobalt matrix.

The present project is concerned with a further requirement, the development of γ’-strengthened Cobalt based superalloy with sufficient resistance to oxidation at high temperature. Different alloying elements (chromium and nickel) were added to the Co-Al-W ternary alloy in varying amounts and isotopic oxidation experiments at different temperatures were carried out. In isotopic oxidation, a two-stage oxidation experiment is used where 18O^- serves as a tracer element during the second stage oxidation. A larger investigation aims is to characterise the oxidation and oxygen transport mechanisms in Co-based superalloys.

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: Muzi Li

Supervisors:Professor Barbara Shollock and Professor Fionn Dunne

Collaborators: Dr Gaofeng Tian (Aviation Industry Corporation of China)

Duration: 01/01/2014 - 01/05/2017 (PhD Studentship - AVIC)

Description: Gamma prime precipitate (γ’ ) is the most important strengthening phase in powder metallurgy nickel-base superalloy. Its size, morphology and distribution have an important influence on the mechanical properties. Secondary gamma prime phase plays a key role in high temperature properties. This main aim of this project is to explore the relationship among: (i) the heat treatment parameters; (ii) γ/γ’ microstructural characteristics; (iii) mechanical properties (including micro-hardness, micro elastic modulus, mechanical properties of samples subjected to heat treatment), which will provide great help to heat treatment parameters optimization that generates the ideal γ/γ’ microstructure resulting in the best compromise of mechanical properties.

Investigator: Zhenqi Li

Supervisors: Dr Chris Gourlay

Duration: 02/01/2014 - 01/1/2017 

Description: First generation Pb-free solders such as SAC305 and SN100C are alloys of Sn and the transition metals Ag, Cu and/or Ni. These transition metals all form intermetallic compounds with Sn and have near-zero solubility in the beta-Sn phase. Therefore, first generation Pb-free solders can be treated as in-situ composites with a matrix of near-pure Sn and a dispersion of intermetallic phase(s). This project seeks to control the size, volume fraction and locations of the intermetallics in the solder microstructure. The research combines synchrotron X-ray imaging with analytical electron microscopy to develop methods to optimise the Cu6Sn5 size and distribution in hypereutectic solders.

Investigator: Zhaolong Ma

Supervisor: Dr Christopher Gourlay

Duration: 01/08/2013 - 30/09/2016 (CSC-Imperial Scholarship)

Description: Pb-free solder joints usually undergo high and variable undercooling before β-Sn nucleates and then solidify to contain only a few β-Sn grains. In Sn-Ag-Cu solders, this leads to large Ag3Sn blades and highly anisotropic physical and mechanical properties in solder joints. The aim of this project is to study how micro-alloying additions (Co, Mn, Ti, and Al) influence the nucleation undercooling and grain-size of β-Sn, and to develop additions to suppress undercooling and control the β-Sn grain size in SAC305 solder and its joints. The nucleation mechanisms of these additions are being investigated by combining analytical scanning electron microscopy (imaging, EDX and EBSD), with differential scanning calorimetry (DSC).

Investigator: Osamudiamen Omoigiade

Supervisors: Professor Rongshan Qin and Dr Christopher Gourlay

Duration: 04/08/2014 - 08/03/2017 (PhD Studentship - TATA)

Collaborators: Dr Arunasu Haldar (TATA Steel) and Dr Shu Yan Zhang (Rutherford Appleton Laboratory)

Description: The pearlite phase is a ferrite and cementite mixture with high interfacial energy and is a common constituent in low alloy steels as it contributes quite significantly to strength, so much so that fully pearlitic structures find use in high strength applications such as wires for bridge suspension cables. However, its microstructure renders it mechanically anisotropic. Therefore, growing pearlite with a tailored microstructure enables us to probe the strength of the structure more effectively.

My research is therefore centred on manipulating steel microstructures by using an external field to induce transformations in the solid state for which pearlite lends itself to being a good candidate due to its relatively high interfacial energy. Moreover, external fields have been shown to exhibit measurable influence on the thermodynamic properties in steel systems and could accordingly alter the microstructure evolution during processing.

In order to study these effects, neutron diffraction experiments are implemented to reveal the structure and property changes in phase transformations. These changes in microstructure can then be transcribed into changes in physical properties, and finally implemented on larger scales with the aid of our industrial collaborators.

Investigator: Khandaker M Rahman

Supervisors: Dr David Dye

Duration: 05/10/2009 - 30/09/2013 (PhD Studentship)

Collaborators: DSTL, Tata Steel

Description:TWinning Induced Plasticity (TWIP) steels are single phase austenitic alloys that successfully combine the properties of high strength and ductility. TWIP steels are an ideal candidate material for applications where the absorption of energy is required, such as armour or automotive crash safety systems. The TWIP effect arises due to the formation of thin lenticular deformation twins during straining. These twins act as strong barriers to dislocation movement. This results in a dynamic grain refinement process, leading to an increasing work hardening capacity and superior ductility. In this study we aim to better understand the mechanical properties of the material and the twinning process. This is achieved using a range of techniques including in-situ X-ray synchrotron diffraction, high strain rate Hopkinson bar and blast testing and self-consistent modelling. The results will then be augmented using a range of microscopy techniques.

Investigator:Giuseppe G Scatigno

Supervisors: Dr M. R. Wenman, Dr F. Giuliani and Prof. M. P. Ryan

Duration: 2/7/2012 - 3/7/2015 (PhD Studentship - EPSRC)

Description: Stress corrosion cracking of austenitic stainless steels continues to be a major cause of concern in the nuclear industry. This work focuses on the effect of cold work on the propagation of chloride-induced transgranular crack growth in 304L type stainless steel. Several characterisation techniques, such as focused ion beam, secondary ion mass spectroscopy, scanning electron microscopy and electron backscatter diffraction were used to understand key mechanisms.

Investigator: Hao Shang

Supervisor : Dr Chris Gourlay

Duration: 05/03/2013 - 06/03/2016(PhD Studentship)

Description: During Pb-free soldering, the nucleation of beta-Sn usually occurs at high undercooling (10s of K) and is often variable from joint to joint. Zn is one of the most effective additions at suppressing the nucleation undercooling for beta-Sn. The aim of the project is to understand the mechanisms by which Zn additions reduce the nucleation undercooling and to explore ways to develop a reproducible polycrystalline solder microstructure. The project will involve reflow soldering, thermal analysis and optical/electron microscopy.

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.

Investigator: Dr Vassili Vorontsov

Supervisors: Dr David Dye

Collaborators: Dr Jonathan Barnard (University of Cambridge) and Rolls-Royce plc.

Duration: 1/11/2011 - current (EPSRC Doctoral Prize Fellowship)

Description: This project involves research of defects and interfaces in advanced engineering alloys, such as Co-Al-W superalloys, using atomic-resolution electron microscopy techniques, including spherical aberration corrected phase contrast TEM imaging and Z-contrast STEM. In a recent study the effects of alloying on the structure of the gamma/gamma-prime interface in the Co-Al-W based superalloy system were shown to have two types of interface between the coherent γ and γ' phases: the compositional; and the order-disorder (i.e. structural) gradients. While these interface types are closely linked to one another, they have been found to possess differing widths and their effective centres do not always lie on the same line. This is likely to affect the mechanical properties of the alloys, such as creep and fatigue resistance, due to the impact of interface width on the ability of dislocations to glide from one phase to another. Therefore, understanding of such effects will become important when designing new alloys.

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: Dr Zhen Zhang

Supervisors: Professor Fionn Dunne and Dr Ben Britton

Collaborators: Rolls-Royce plc/Timet/Westinghouse/EDF

Duration: 06/01/2014 - 06/01/2016 (PDRA - HexMat)

Description: We will investigate dwell fatigue, shear band formation, and slip transfer at grain boundaries on high performance superalloys applied in areospace engineering. The purpose is to understand the deformation mechanisms from micro-scale levels as well as to interpret experimental observations based on crystal plasticity models.

Investigator: Zebang Zheng

Supervisor : Professor Fionn Dunne and Dr Daniel Balint (Mech Eng)

Duration: 01/10/2012 - 31/12/2016 (MSc + PhD Studentship)

Description: This project will link high fidelity crystal plasticity (CP) modelling with discreet dislocation dynamics (DD) to model micro-deformation by slip in a grain boundary region. This approach will be used to address the stress states in grain boundary regions, with constitutive CP laws derived from DD models, thought to cause problems during cold dwell fatigue in Ti 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.

Title: Design and development of new materials for gas turbines – high strength titanium alloys

Investigator: Dr. Alexander (Sandy) Knowles

Supervisors: Prof. D. Dye

Duration:  Oct 2015–present DARE (www.darealloys.org) PDRA, Oct 2016–17 EPSRC Doctoral Prize Fellow

Description: Design and development of new materials for gas turbines as part of the EPSRC ‘Designing alloys for resource efficiency (DARE) - a manufacturing approach’ partnership across the University of Sheffield, Imperial College London, Kings College London and the University of Cambridge as well as ten industrial partners, including: TIMET, Rolls-Royce, Tata Steel, Siemens and Magnesium Elektron.

One direction of this project focuses on the design of new bcc refractory metal rich beta titanium alloys reinforced with intermetallic bcc superlattice precipitates. Alloys have been produced that comprise remarkable ultra-fine lamellar bulk nano-structures and have demonstrated exceptionally high strengths. Further development of these alloys is being made through my EPSRC Doctoral Prize Fellowship.

A second direction supports the development of new TIMETAL 575 and 407 commercial titanium alloys and the characterisation methods that accelerate the path to commercialisation in collaboration with TIMET. On TIMETAL 575, the mechanisms of Si strengthening additions are being studied using advanced electron microscopy and mechanical testing. While detailed fatigue studies are being performed at multiple lengthscales on TIMETAL 407, so as to further understand its impressive fatigue performance.

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 electron microscopy techniques to gain understanding of fundamental behaviours in such 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-resolution structure and chemistry will be observed using techniques such as HRTEM and ALCHEMI, while dislocation behaviours will be studied using mechanical testing and in-situ TEM experiments.

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: 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.

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.

Investigator: Dr Vivian Tong

Supervisors : Dr Ben Britton (Primary), Fionn Dunne (Secondary)

Duration: 11/10/2016 - 31/03/2018 (HexMat PDRA)

Description:  Hexagonal close-packed (HCP) alloys such as titanium and zirconium are used in the aerospace and nuclear industries respectively. HCP alloys have anisotropic mechanical properties which affect in-service performance. Micromechanical testing of single orientations or grain boundaries provides an opportunity to isolate intrinsic materials properties and test them directly, which is not possible to obtain from macro-scale testing as the response is averaged out over many grain orientations. In particular, the strain rate sensitivity (SRS) of Ti alloys has been shown to be linked to facet nucleation in dwell fatigue. Micropillar compression directly measuring SRS of different slip systems, coupled with crystal plasticity modelling, has informed mechanistic understanding of cold dwell fatigue.

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.