Research Projects: Metallurgy

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.

The project will initially examine the adaptation of the replication process for higher melting point metals and the identification of suitable processing routes. As part of this work there will be a need to adapt existing equipment, and potentially construct new machines for processing. The resulting foams will be characterised using a range of experimental techniques for mechanical and transport properties, and their suitability for a number of potential applications assessed.

Metal Injection Moulding (MIM) is a method used for the rapid and low cost production of net-shape metal parts. In this process metal powder is mixed with a polymer carrier and injected into a mould, before being sintered to form a dense part. The method is particularly suitable for the processing of very high melting point reactive metals, such as titanium. Low cost titanium containing large amounts of porosity could find weight-saving structural applications in, for example, transport.

A number of techniques for creating controlled porosity in MIM components will be examined, with the aim of producing as wide a range of structures as possible. The structure of these foams will be characterised, and their mechanical properties assessed. Further work will look at applying the same techniques to similar metals and alloys, including stainless steel and Ni-Ti alloy, which displays shape memory properties.

The progress of engineered structures towards ever finer scales has resulted in metallic parts reaching sizes where the mechanical behaviour is significantly influenced by the interaction dislocations with the sample free surfaces.

Recent compression experiments on micropillars machined from various metals using focussed ion beam (FIB) techniques indicate a dependence of yield strength on sample dimensions (a size effect) once the size of the structure drops below about 10 μm. However, questions have been raised concerning changes to the material and the dislocation structure during the FIB process.

The project will be in collaboration with researchers at EPFL in Lausanne, Switzerland (http://lmm.epfl.ch/), who have developed an alternative method to cast single crystal aluminium of small dimensions and complex shapes without influencing the dislocation structure. Electron microscopy will be used to characterise the samples both before and after testing. Nanoindentation facilities in the department will be used to perform compression tests on cast micropillars, and more complex mechanical tests on cantilevered beams and other sample geometries with dimensions on the order of 1-10 μm. Analysis of the resulting data will aim to verify the existence of a size effect and to understand its origin.

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.

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, such as hot and cold isostatic pressing, Conform (continuous extrusion) and direct particulate rolling, for titanium alloy powder feedstock, with a view to providing downstream processing routes for powder from emerging reduction technologies, such as the FFC Cambridge process. 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.

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 such as Ti-5553 and Ti55531 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.

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.

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.

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).

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 thewse approaches and will make extensive use of the department’s thermomechanical simulation equipment and modelling expertise, and liaison with steel industry personnel.

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.

Plate is traditionally rolled from cold-charged thick slabs in a low utilisation reversing process, usually nowadays augmented with powerful post-rolling cooling systems. These methods of production are inefficient in energy and asset use terms, but they give maximum flexibility in terms of temperature strain path. Work will centre on the following areas

i) feasibility of hot charging of slabs (dependent on the solution kinetics of microalloyed steels)
ii) overall strain and strain path constraints on achievable properties
iii) characterisation of strain temperature paths for mini-mill rolling and cooling configurations.

Steel for high integrity plate applications is traditionally made through the basic oxygen route with low and tightly controlled scrap use, intensive secondary steelmaking and low speed continuous casting to thick sections. For thick walled pipe, ingot casting is seeing increasing use. These methods lead to maximised steel cleanliness and assure sufficiency of mechanical working in microstructural development, but they are extremely energy intensive. Work will centre on

i) EAF versus BOF route cleanliness and energy use
ii) Secondary steelmaking influences on properties and product consistency
iii) Influence of microalloying practice on castability

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;
i) ZrO₂ affects the break temperature
ii) ZrO₂ acts as a nucleation site
iii) ZrO₂ significantly affects the flux viscosity
iv) ZrO₂ affects the mould flux crystallinity, and therefore the heat transfer characteristics
A variety of techniques including viscosity measurements, hot stage microscopy, and numerical modelling will be used.

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.

The proposed project aims to address issues related to the relationship between local solidification conditions and operator defined process parameters. This will form the basis for PhD research within the IMMPETUS and IMPC research groups and supervised by Dr Iain Todd. The project will involve research in both experimental and a modelling and will concentrate on Nickel alloys 625 and other possibly Hastalloy X or Waspaloy. The aim is to understand the ALM process from a microstructural perspective and, hence, to facilitate the use of these processes in the production of structural parts.

Periodic trusses are attractive structures for applications where a combination of high strength and light weight are desirable – e.g. in aerospace applications. The current problem facing their use is the lack of a suitable approach for the prediction of their properties. The aim of this project is therefore to further develop a modelling approach pioneered by Professor Askes and to test this by experiment through the design, manufacture and mechanical testing of truss structures via Electron Beam and Laser Direct write technologies.

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.

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.

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.

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.

Iron oxide based materials are well known as industrial catalysts. Ferrous catalysts are produced through combination of iron oxide with varying levels of promoter species to give the pre-catalyst, which can then be reduced to generate the relevant active iron material. Understanding the properties of the final materials both during and after production is vital to improving its properties. Although catalysts of this type have been studied in the past from a chemical angle, it is hoped that investigation of them and their manufacture from a modern materials perspective may lead to new understanding of their properties and potential applications.

Initial feasibility studies at Sheffield have shown that small-scale production of these materials is relatively simple. This PhD project will allow Johnson Matthey and the Dept of Engineering Materials in Sheffield to continue the preliminary research and include fundamental studies of the materials themselves. The project will involve the synthesis and characterisation of new and existing iron oxide catalysts using the new clean melting facility at Sheffield. The materials produced will be investigated and analysed at Sheffield using our extensive range of materials analysis techniques. In addition, Johnson Matthey’s research site in the UK in collaboration with chemists and engineers in the catalysis research team. The project covers both the production and materials aspects of the catalyst as well as its catalytic performance and will therefore be co-supervised by both a technical product team and the manufacturing science group within the Johnson Matthey Technology Centre.

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.

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.

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.

While much work has been done on the friction stir welding (FSW) of aluminium, the technology frontiers of friction welding are rapidly evolving and there is great interest in extending the technology to welding high performance and high melting temperature metals. For example, the advantages of FSW high performance titanium alloys has been recognised for some time, but the problems are severe: the poor thermal conductivity leads to severe temperature gradients, which is a major issue given that Ti can only be worked at such high strain rates over a narrow temperature range. Recent innovative changes in the FSW process have made the FSW of titanium alloys feasible. Other friction based welding techniques include linear friction welding (LFW), which is now an important process in the development of the integrally bladed rotor (Blisk). IMMPETUS, through collaboration with TWI, has recently developed experience in the microstructure and mechanical property analysis of friction welded materials with particular emphasis on FSW of high temperature alloys. FSW and LFW are highly non linear processes, involving severe changes in strain direction, abrupt changes in strain rate and steep thermal gradients, making prediction of microstructure and understanding of the process extremely difficult. We have a novel and unrivalled approach to investigate FSW and LFW for alloys where there is currently little knowledge, namely steels and Ti alloys using our world class arbitrary strain path machine (ASP). Our collaboration with TWI, which gives us direct access to instrumented friction welding machines, will give us a direct method for validating the output from the ASP. In addition to the mechanical testing, the project will involve extensive microstructural characterisation using high resolution electron backscatter diffraction (EBSD) for example.

Full funding is available for UK/EU for an exciting PhD project on the Processing and Characterisation of Iron Oxide Materials for use in catalysis, supported by Johnson Matthey. The research will be carried out in the Department of Engineering Materials of the University of Sheffield, in collaboration with Johnson Matthey Catalysts and the Johnson Matthey Technology Centre.

Iron oxide based materials are well known as industrial catalysts. Ferrous catalysts are produced through combination of iron oxide with varying levels of promoter species to give the pre-catalyst, which can then be reduced to generate the relevant active iron material. Understanding the properties of the final material both during and after production is vital to improving its properties. Although catalysts of this type have been studied in the past from a chemical angle, it is hoped that investigation of them and their manufacture from a modern materials perspective may lead to new understanding of their properties and potential applications.

Initial feasibility studies at Sheffield have shown that small-scale production of these materials is relatively simple. This PhD project will allow Johnson Matthey and the Department of Engineering Materials in Sheffield to continue the preliminary research and include fundamental studies of the materials themselves. The project will involve the synthesis and characterisation of new and existing iron oxide catalysts using the new clean melting facility at Sheffield. The materials produced will be investigated and analysed at Sheffield using our extensive range of materials analysis techniques. In addition, Johnson Matthey will provide further analytical support where required as well as the opportunity to test the catalytic performance of the materials. The project will include the opportunity to spend some time working at Johnson Matthey’s research site in the UK in collaboration with chemists and engineers in the catalysis research team.

The project covers both the production and materials aspects of the catalyst as well as its catalytic performance and will therefore be co-supervised by both a technical product team and the manufacturing science group within the Johnson Matthey Technology Centre. The academic supervisors at Sheffield will be Professor Panos Tsakiropoulos and Professor Tony West.

Those interested in this project should contact asap Professor P Tsakiropoulos at p.tsakiropoulos@sheffield.ac.uk or Dr Matthew Lunn at Matthew.Lunn@matthey.com for further information.