Research Projects: Advanced Metallic Materials

4 PROCESSING AND PROPERTIES OF HIGH MELTING POINT METAL FOAMS
Supervisor: Dr R Goodall

Metal foams produced from higher melting point metals (such as copper, nickel, titanium, superalloys and shape memory alloys) would be of interest in a wide range of applications, including biomedical implants and the absorption of sound in aero engines. The replication process, where metal is cast into the spaces in an assembly of grains of a refractory but water soluble material, which is then dissolved away to leave a sponge-like structure of metal, has been shown to provide good control of the foam pore size, shape and structure, but has only been extensively explored for aluminium.

Work in this area includes the development of processing techniques to give controlled foam structures, and the characterisation of the materials produced in order to understand the links between foam structure and metal microstructure and the material properties, such as mechanical behaviour or thermal and electrical conductivity. Once these links have been made they can be used to design optimum foam structures for applications.

5 PROPERTIES OF POROUS STRUCTURES WITH INTERMEDIATE LEVELS OF ORDER
Supervisor: Dr R Goodall

are either stochastic (i.e. random) metal foams or sponges, or highly ordered lattice structures with regular unit cells. The former are often produced by the injection of gas or removable solid phase into a melt or powder mixture, and as such the properties are frequently close to being isotropic, while the latter may be created by layer manufacturing techniques, which can be time consuming for large production runs of identical components.

In this project, processing techniques to create structures combining elements of both of these classes will be investigated, and the mechanical, electrical and thermal properties of the resulting materials will be investigated. This will include methods for combining regions of random pores with lattice structured regions, to obtain anisotropic properties or specific behaviour such as negative Poisson’s ratio (auxetic) materials. Structural modelling will be used to suggest which structural combinations could show promise for certain applications, and these predictions will be validated experimentally.

6 EFFECT OF HIGH STRAIN RATE DEFORMATION PROCESSES ON THE DAMAGE TOLERANCE AND SERVICE PERFORMANCE OF AEROSPACE TITANIUM ALLOYS
Supervisors: Dr M Jackson

The majority of aerospace titanium alloy components will be either high-speed machined, peened using laser shock or mild steel shot and/or burnished. Such techniques impart a high degree of surface deformation at very high strain rates. Peening, for example, is an established technique designed to impart compressive residual stresses that provide enhanced damage tolerance. However, recent work at Sheffield has shown it to be deleterious to the surface microstructure of the component, providing enhanced oxygen diffusion kinetics and leading to a loss of creep strength. The aim of this project is to determine the effect of important high deformation processes, such as shot peening, laser shock peening and high speed machining on the surface microstructure and subsequent damage tolerance for a range of titanium alloys. The deformation mechanism with regard to alpha-beta morphology and texture will investigated using both scanning and transmission electron microscopy, including electron backscatter diffraction. The effect of thermal cycling during service will also be investigated to determine the alpha case formation and oxygen diffusion kinetics measured using secondary ion mass spectrometry. The subsequent change in mechanical properties will be measured via standard tensile and fatigue testing.

7 DEVELOPMENT OF SOLID STATE DOWNSTREAM PROCESSING ROUTES FOR THE PRODUCTION OF LOW COST TITANIUM ALLOYS FOR NON-AEROSPACE MARKETS
Supervisors: Dr M Jackson

Over the last decade, a number of low cost titanium reduction technologies have received a lot of attention and sponsorship. These processes have the potential to compete with the Kroll process, providing an alternative source of material, particularly for the non-aerospace market sectors. An alternative, cheaper source of titanium powder and significant developments in processing cost reduction are essential if target markets such as the automotive industry are to be penetrated to any extent at all. Although many of the emerging extraction processes produce a powder/granular final product, there has been little emphasis on downstream processing of such feedstock into a useable form. If there is to be a step change in the cost of titanium (similar to that achieved in aluminium in the late nineteenth century) then disruptive technologies that consolidate the titanium particulate through affordable non-melt routes directly into non-aerospace grade useable forms is essential. The proposed programme of work aims to develop cheap non-melt consolidation methods for titanium alloy powder feedstock, with a view to providing downstream processing routes for powder from emerging reduction technologies. A major outcome of the work will determine whether it is economically and commercially viable for the production of titanium alloy bar/rod and sheet via such non-melt routes.

8 PREDICTION OF MICRO-TEXTURAL AND PROPERTY EVOLUTION OF TITANIUM ALLOYS USED FOR HIGH STRENGTH AIRFRAME AND LANDING GEAR FORGINGS
Supervisors: Dr M Jackson and Dr B Wynne

Isothermal forging (IF) is essential for the fabrication of titanium aerospace components, such as high strength landing gear forgings. To reduce the high processing costs it is essential for the industry to accurately predict microstructural and microtextural evolution and thus, property evolution with respect to IF variables. A testing methodology for evaluating and predicting the microstructural evolution of titanium alloys during subtransus IF has been developed at Sheffield. The project will exploit the unique World leading thermomechanical compression facility within the department to forge high strength beta titanium alloys at near beta transus temperatures to obtain microstructural and textural information for a range of strains within a single specimen. The rheological behaviour of beta titanium alloys, which generally exhibits flow softening, will be fitted using a recently developed constitutive approach that incorporates an internal microstructural variable. The aim will be to use a finite element modelling package to produce strain and lambda profiles, which correspond to the equivalent microtextural profiles of the test specimens, providing a database of texture and subsequent mechanical properties for defined IF conditions.

9 EFFECT OF FRICTION AND LUBRICATION ON MULTIPASS HOT DEFORMATION
Supervisor: Dr E J Palmiere

In the conventional production of iron and aluminium alloys, a large variety of rolling conditions occur by which a complex forming history is imposed on the material. The effects of the multipass deformation on the final material properties may be manifold and furthermore, the interdependence of process parameters can significantly affect the eventual microstructure and subsequently its properties according to the type of rolling schedule and equipment. The issue is further complicated with the incorporation of the imminent effects of friction and lubrication of the slab. Past researches on Plane Strain Compression (PSC) testing and Finite Element (FE) modelling have shown that strains within the PSC testing samples are not uniform and thus eventually the variation in microstructure. This research was setup to study the effects of lubrication and friction on the tests; incorporating with it developing a practice guide in the positional sampling of microstructures in PSC test pieces. The research would include the use of the new Servotest machine in the department and the JEOL 6400 SEM particularly the EBSD technique. Quantification of the microstructure would be performed by quantitative metallurgy methods developed.

10 THERMOMECHANICAL PROCESSING OF PLATE STEELS UNDER NON-EQUILIBRIUM CONDITIONS
Supervisor: Dr E J Palmiere

Plate steels represent an important class of steels to the construction, energy and shipbuilding industries This project will focus on the influence of processing history (e.g., under conditions of dynamic changes in deformation temperature, strain, strain rate, cooling rate) in both single phase austenite and ferrite+austenite regions, together with alloy chemistry on the mechanical properties. As such, this project will involve the use of physical deformation simulations of flat rolling processes coupled with quantitative microscopy (optical and electron microscopy, including EBSD) and mechanical testing.

11 THERMOMECHANICAL PROCESSING FOR ULTRA-FIN FERRITE GRAINS IN DUAL PHASE STEELS
Dr E J Palmiere

Dual phase steels represent an important class of steels to the automotive industry. This project will focus on the influence of processing history (e.g., deformation temperature, strain, strain rate, cooling rate) and alloy chemistry on the final sheet formability. The overall aim will be to determine processing windows which result in ultra-fine ferrite grains, and the impact that such a microstructure will have on mechanical properties. As such, this project will involve the use of physical deformation simulations of flat rolling processes coupled with quantitative microscopy (optical and electron microscopy, including EBSD).

12 INTENSE WATER-COOLING DURING OR AFTER HOT ROLLING TO IMPROVE STEEL PRODUCTION
Supervisors: Dr E J Palmiere and Visiting Professor A A Howe, in association with Tata Steel

Intense water-cooling after the final hot rolling pass is a well-established technology to improve product properties, but has certain limitations, e.g. grain refinement and property improvement through thicker products, or maintenance of the required shape and flatness. Various adjustments can be made to the rolling schedule, e.g. rolling overall at lower temperatures, or particularly with a period of intense cooling during a hold in the rolling schedule, that could reduce the need for accelerated cooling after rolling, and/or allow further improved properties than can be accomplished by established approaches. This project will investigate these approaches and will make extensive use of the department’s thermomechanical simulation equipment and modelling expertise, and liaison with steel industry personnel.

13 DEVELOPMENT OF CoCrMo ALLOYS FOR HIP JOINT PROSTHETIC APPLICATIONS
Supervisor: Prof WM Rainforth

In recent years, metal-on-metal (MoM) hip replacements based on CoCrMo alloys have increasingly become the preferred choice for younger and/or more active patients due to their superior wear resistance, longer service duration and reduced inflammatory osteolysis resulting from such devices. CoCrMo is generally regarded as a corrosion resistant alloy, due to the tenacious oxide film (1–4nm thick) formed on the metallic surface containing mainly Cr2O3. Nevertheless, CoCrMo is reported to be susceptible to corrosion when implanted into the human body, especially within a tribological contact such as a knee or hip. Implant components fabricated from CoCr based alloys have been reported to produce elevated Co, Cr (and Ni) ion concentrations in body fluids. It is important, therefore, to understand the mechanisms, such as wear, that lead to the liberation of the metal ions. There have been a number of investigations into the wear mechanisms that occur in the artificial hip joints. Abrasive wear has been seen on some of the retrieved CoCrMo hip joints and is one of the possible wear mechanisms on such joints. Interestingly, there has also been much interest in the role of a so-called nanocrystalline layer that forms on the surface of CoCrMo alloys, with some suggestions that it enhances wear resistance, but other evidence suggest it enhances the rate of corrosion. In all cases, the surface degradation mechanisms are related to the composition and microstructure of the alloy, although there remains significant controversy on which is the optimum microstructure. This project will seek to manufacture a range of CoCrMo alloys with varying composition and microstructure. Detailed analysis of the wear resistance will be undertaken in simulated body fluids. Detailed electron microscopy will be used to understand the deformation mechanisms that occur at the worn surfaces. Based on this information, further alloy development will be undertaken.

14 DEVELOPMENT OF NEW SHAPE MEMORY ALLOYS
Supervisors: Prof WM Rainforth, Prof I Todd

Shape memory alloys (SMAs) are well established commercial materials that have stimulated huge academic interest, the most common by some margin being Nitinol (50.8%Ni 49.2%Ti). The shape memory effect is associated with a phase transformation from the equilibrium crystal structure to a metastable martensite under the influence of applied stress or a temperature change. The transformation is reversible, simply by heating. The behaviour of SMAs clearly depends strongly on the martensite start (Ms) and finish (Mf) temperatures. A wide range of temperatures can be obtained through a wide range of alloy compositions, from the binary (Ti-V, Ti-Mo, and Ti-Nb) through the ternary (Ti-Mo-Ge, Ti-Mo-Sn, Ti-Nb-Sn, Ti-Nb-Al etc) and quaternary (Ti-25Pd-24Ni-1W). Recently, it has been realised that SMAs have interesting wear properties since substantial elastic deflections at contacting asperities can significantly reduce the local contact stress and dissipate the contact stresses through a larger volume of material. An additional issue with the use of Nitinol in biomedical applications is the potential for biotoxicity from the presence of nickel. Therefore, there is extensive international research aimed at developing Ti based shape memory alloys that do not contain nickel, using elements that are known to be bioinert. This project will manufacture and test some of these promising binary and ternary shape memory alloys (such as the Ti–30Ta–1Al and Ti–30Ta–1Sn noted above). The microstructure will be investigated in detail. The wear properties will also be explored.

15 PROCESSING AND STRUCTURE OF HIGH STRENGTH STEELS
Supervisors: Professor W M Rainforth and Dr E J Palmiere

Many key value added alloys require intercritical processing to give the required properties: high strength steels for the automotive sector for high strength:weight ratio (short term target of 1000MPa strength and 25% ductility); high strength plate steels for large diameter trans-continental gas pipelines. Regardless of whether intercritical processing involves deformation or heat treatment, product properties depend on the extent to which chemical partitioning between phases approaches equilibrium and therefore on the properties of the individual phases. In recent years there have been significant strength gains by using dual phase (DP), transformation induced plasticity (TRIP) and multiphase steels but it is anticipated that further gains will require process and composition changes. We have developed laboratory simulations which accurately replicate the whole commercial process route from hot rolling through complex intercritical anneal (including the rapid heating and cooling cycles) right the way through to the bake hardening cycle. We are therefore in a unique position to study the effect of changes to the chemical composition, particularly in microalloying, and to determine the microstructural evolution throughout the process. In high strength plate steels the sections are substantially thicker, resulting in significant microstructural gradients through the material, while the additional thermal mass of the material makes microstructural refinement more difficult. This project will make use of our world leading thermomechanical simulation equipment to develop the microstructure/property relationships as a function of process route for the latest high strength steels.

122 NEW CASTING TECHNOLOGIES
Supervisor : Dr RP Thackray

The development of technologies such as thin slab casting, direct strip casting and twin roll casting have resulted in a number of engineering challenges such as the design of materials suitable for metal delivery, design of mould and SEN systems, understanding of heat transfer and solidification, microstructural and property characterisation and the influence of roll properties on product quality. With the increasing use of new steel grades, (HSLA, TRIP, TWIP, DP) there may be a need for an understanding of the behaviour of these steels or assessment of the feasibility of these steels for use in these casting processes. This can be achieved by understanding of the heat transfer and microstructure development during strip casting use and implementation of PROCAST and CALCOSOFT continuous casting programmes to give us the capability of modelling traditional continuous casting processes but also twin roll and strip casting. Previous work has found that twin roll or direct cast material suffers from consistency issues (particularly low C steel), so additional work could concentrate on investigation of alloy chemistry on properties of strip cast material and development of thermal treatments for strip cast materials.

16 TUNDISH MODELLING AND VALIDATION
Supervisor : Dr R P Thackray (with Sheffield Forgemasters)

In the continuous casting of steel a tundish is a vessel situated between the ladle and the mould, and is primarily designed to ensure the smooth transfer of steel into the mould. But the tundish also plays a very important role in the production of reproducibly high quality steel. Rather than being seen as simply a distribution vessel or holding tank, various operations such as inclusion removal and modification, alloy addition, and temperature and compositional homogenisation, can be carried out in the tundish, and so the tundish is a key factor in the production of, for example clean steel.

Many studies have been carried out in recent years to investigate fluid flow phenomena in tundish systems, using combinations of physical and mathematical modeling techniques such as CFD, particle image velocimetry (PIV), water modeling, and pulse tracer addition to simulate the complex flow in tundishes with arrangements of dams and weirs, and to provide estimates of parameters such as residence time distribution (RTD).

However, there have been comparatively few studies which have taken a through process approach to look at the effect of tundish design on the quality of the final product. This proposal aims to do that using a combination of modeling and experimentation to study the critical issues in steelmaking and thermomechanical processing in the production of turbine rotors.

17 EFFECT OF ZrO2 ADDITIONS ON MOULD FLUX PERFORMANCE
Supervisors: Dr R P Thackray

One of the most common and damaging accidents occurring during continuous casting is the so called sticker breakout, where the solid outer shell of steel collapses, and the molten core pours through, causing operations to be shut down for long periods of time. Causes of these accidents are thought to be poor lubrication, and examination of material from a sticker breakout reveals a build up of ZrO2 at the affected site.

This project will examine the effect of ZrO2 on the properties of selected mould fluxes to ascertain whether;

1. ZrO2 affects the break temperature
2. ZrO2 acts as a nucleation site
3. ZrO2 significantly affects the flux viscosity
4. ZrO2 affects the mould flux crystallinity, and therefore the heat transfer characteristics
5. A variety of techniques including viscosity measurements, hot stage microscopy, and numerical modelling will be used.

18 PERFORMANCE ON DEMAND: CONTROL AND PREDICTION OF AEROSPACE ALLOY MICROSTRUCTURES
Supervisor: Professor I Todd

Additive manufacturing technologies, such as Direct and Shaped Metal Deposition (DMD/SMD), as well as finding use in the rapid prototyping of engineering components are looking increasingly promising as rapid manufacturing methods. Until now the development of the technology has focussed on producing the required component form on-demand and for the production of one-off or non-structural components this has proved highly effective: this could be referred to as Form On-Demand (FOD). Moving this technology on from one associated with rapid prototyping to one which can be used to form structurally critical components will also require that we can produce components which have both the desired form and suitable metal microstructures: we can term this combination Performance on Demand (POD). There has been significant research effort in the field of FOD and much of the published work in the open literature on direct metal deposition technologies deals with this aspect. In contrast, there are very few widely published papers dealing with the control of solidification microstructures – the majority of the published work being concerned with Ti alloys and functionally graded materials.

Building an understanding of the solidification microstructure selection process and a predictive modelling capacity would clearly be advantageous when designing a deposition strategy, from the point of view of obtaining a component microstructure that is fit for purpose. Knowing the local solidification conditions and being able to influence control over them is also advantageous from the perspective of controlling the size and morphology of carbides, nitrides and intermetallic phases in Nickel based superalloys.

19 HIGH ENTROPY ALLOYS – A GATEWAY TO NOVEL METALLIC MATERIALS?
Supervisor: Professor I Todd

High entropy alloys are equiatomic multicomponent (usually greater than 5 elements) alloys which have been recently reported as possessing remarkable strengths (>2GPa) and deformation to failure (>20%). This combination of properties is clearly of interest but there is no clear understanding of whether a particular collection of metallic elements will lead to the formation of a suitable microstructure. These alloys – in spite of their being multicomponent – usually consist of only 2 phases bcc and fcc and the behaviour of the alloys seems to be strongly dependent on the spatial distribution and volume fractions of the two phases. This project will take an approach based on recent work related to the development of novel Ti alloys and bulk metallic glasses as a departure point and will seek to clarify the underlying mechanisms behind the formation and properties of this new class of alloys.

20 DEFORMATION MECHANISMS IN NANOSTRUCTURED HEXAGONAL CLOSE PACKED METALS
Supervisors: Professor I Todd, Dr B Wynne and Professor M Rainforth

Research in ultrafine grained (ufg) and nanocrystalline (nc) metals has concentrated, generally speaking, on fcc materials where huge increases in yield strength have been observed in pure metals and structures combining these high strengths with improved ductility have also been reported. There is less work on cph metals and the aim of this research would be to develop a deeper understanding of the mechanisms in cph metals when the grain size falls below 100nm. The influence of texture and other microstructural features such as twin density on mechanical behaviour will also form an intrinsic part of the research.

21 DEVELOPMENT OF Ti BASE ALLOYS FOR BIOMEDICAL APPLICATIONS
Supervisor: Professor P Tsakiropoulos

Titanium alloys offer several benefits for biomedical applications, including lower elastic modulus, excellent corrosion resistance and enhanced biocompatibility. Commercial purity and alpha-beta Ti alloys are the primary Ti alloys currently used for biomedical application; metastable beta Ti alloys also offer great opportunities for biomedical applications. The potential to have low modulus of elasticity is particularly important for hard tissue replacement where stress shielding, a phenomenon where re-absorption of natural bone and implant loosening arises because of the difference in elastic modulus between natural bone and hard tissue implant, is one of the primary causes requiring revision surgery. This project will investigate the design and development of Ti base alloys suitable for biomedical applications with reduced elastic modulus and close to that of the bone.

22 DESIGN AND DEVELOPMENT OF OXIDATION RESISTANT COATINGS FOR REFRACTORY METAL ALLOYS
Supervisor: Professor P Tsakiropoulos

The temperatures experienced by turbine engine airfoils with TBC coatings can approach 1150 °C. This is essentially the limit for nickel-based superalloys. In order to achieve service temperatures higher than those of nickel-base superalloys materials with significantly higher melting points are required. Currently, Nb and Mo silicide base alloys are considered as likely candidates. However, even the most oxidation resistant of these alloys would require oxidation protection with coatings. Furthermore, both Mo and Nb suffer from pest oxidation at T<800 °C. In this project single or multiphase oxidation resistant coatings/bond coats will be designed to provide oxidation protection in both the pest oxidation (<800C) and high temperature (>1100 °C) regimes. The project is suitable for PhD candidates that are interested in alloy design, phase equilibria, and microstructural characterisation using x ray diffraction and electron probe microanalysis.

23 PHASE TRANSFORMATION IN Nb SILICIDE BASE ALLOYS
Supervisor: Professor P Tsakiropoulos

The temperatures experienced by modern gas turbine engine airfoils with TBC coatings can approach 1150 °C. This is essentially the limit for nickel-based superalloys because most advanced superalloys melt at 1350 °C, chemical segregation in the superalloy can lead to incipient melting at 1270 °C and the interaction zone between the bond coat of the TBC and the airfoil can melt at temperatures less than 1250 °C. For the high-temperature applications required for the next generation of jet engines, refractory metal alloys, in particular Nb- and Mo-silicide base alloys with melting temperatures exceeding 1750 °C are considered to be the most likely candidates. In this project solid state phase transformations in selected Nb silicide base alloys that are currently under development in the Department will be studied. The project is suitable for PhD candidates that are interested in alloy design, phase equilibria, physical metallurgy and microstructural characterisation using x ray diffraction and electron microscopy and microanalysis.

24 THERMOMECHANICAL PROCESSING AND MICROSTRUCTURE ANALYSIS OF HIGH TEMPERATURE AEROSPACE ALLOYS
Supervisor: Dr B P Wynne

Aluminium and magnesium alloys are popular choice materials for introducing high strength to weight ratios in applications such as automobiles. However, in most industrial hot working operations the deforming material experiences a large range of varying deformation conditions. One such variation is the strain path which the material experiences which can vary significantly in both space and time. Therefore, knowledge of such effects on flow behaviour and microstructure evolution is important for developing accurate models of the industrial process. This project intends to investigate this by using a state of the art strain path test machine which has seamless control on strain path at strain rates close to those experienced by the material in the industrial process. The test material can be decided at the start of the project but it must be deformed at hot working temperatures. Data such as flow stress, recrystallised grain size and crystallographic texture will then be used to quantify the effects of a non-linear strain path history.

25 ELECTRON BACKSCATTERED DIFFRACTION ANALYSIS (EBSD) ANALYSIS OF THERMOMECHANICALLY PROCESSING METALS AND ALLOYS
Supervisor: Dr B P Wynne

This project will use a recently acquired field emission gun (FEG) SEM equipped with EBSD analysis equipment. Recent advances in the EBSD technique, that have significantly improved angular resolution, in conjunction with a FEGSEM have made it a feasible alternative to the TEM as a tool for quantifying deformation microstructure. Parameters such as sub-grain orientation and size, misorientation distribution, misorientation gradients, and orientations and growth rates of recrystallising grains can now be routinely quantified over a statistically significant sample size. Such variables form the basis of the newly developing physically-based equations that describe microstructure evolution in hot-worked metals. Therefore this project will examine the applicability of the EBSD technique for a wide range of thermomechanical processed alloys including aluminium, IF steel and Ti-based alloys.