Welcome to Interfaces, the newsletter from the Department of Materials Science and Engineering at the University of Sheffield. Every issue, we’ll bring you news from the world of Materials, from us and elsewhere, and how discoveries made through the years affect our lives today.
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Working in collaboration with colleagues from Imperial College London, Professor Iain Todd from the Department of Materials Science and Engineering at the University of Sheffield has been taking a novel approach to the development of engineering components produced using additive manufacturing.
Additive manufacturing (AM), also known as 3D printing, is often used to produce engineering components. By utilising lattice structures (such as that shown below) to replace solid materials, these components are much lighter that their solid counterparts, and can be engineered in such a way that they also exhibit property combinations that are inaccessible to conventional solids. These structures are known as architected materials.
These lattice structures typically have a uniform layout with nodes all conforming to a regular array with the struts between the nodes all following common planes: and herein lies the problem.
The work, detailed in Nature magazine on 17 January 2019, explains how these uniform lattices replicate the structure of a metallic single crystal: the nodes in the AM lattice are equivalent to the atoms in the single crystal and the struts are equivalent to the atomic bonds. In each of these structures, the atomic planes, or nodes are all perfectly aligned.
While in some applications, such as the high temperature end of a jet engine, single crystal materials are ideal because of their ability to withstand deformation at extreme temperatures, they have limitations relating to their mechanical performance. This limitation is also observed in AM parts with a uniform lattice structure.
When the structure is put into compression, once the force is sufficient to cause permanent deformation, the lattice shears along one or more of the planes of nodes. With nothing to inhibit this shearing, the collapse becomes catastrophic.
In polycrystalline materials – those with many crystals – the alignment of the atomic planes is random, so when a shear force is in a particular direction, a crack will slow down or stop when it meets a crystal where the atoms are aligned differently from the crystal in which the crack initiated. Moreover, it is possible to introduce different materials in the form of phases, precipitates or inclusions used to strengthen the materials; these materials also help to inhibit crack propagation.
It is this fundamental metallurgical understanding that inspired scientists at Imperial College London and the University of Sheffield to mimic polycrystalline microstructures in AM lattices with the aim of developing robust, damage-tolerant architected materials.
Through the computer modelling of atomic structures, scaling them up and creating meso-structures based on polycrystalline materials, engineers are transforming the way that materials are designed, for which the name ‘meta-crystals’ has been coined.
Experimental testing of components made from these meta-crystals has demonstrated that they are highly energy absorbant, with the polycrystal-like material able to withstand almost seven times the energy before failure than the materials that mimic the single-crystal structure.
While the basic metallurgical concepts are being used to inspire the development of architected materials, researchers are using the creation of architected materials as an alternative approach to study complex metallurgical phenomena.
Prof Iain Todd of The University of Sheffield: “This approach to materials development has potentially far-reaching implications for the additive manufacturing sector. The fusion of physical metallurgy with architected meta-materials will allow engineers to create damage-tolerant architected materials with desired strength and toughness, while also improving the performance of architected materials in response to external loads.
“And while these materials can be used as standalone structures, they can also be infiltrated with other materials in order to create composites for a wide variety of applications.”
Dr Minh-Son Pham of Imperial College London: “This meta-crystal approach could be combined with recent advances in multi-material 3D printing to open up a new frontier of research in developing new advanced materials that are lightweight and mechanically robust, with the potential to advance future low carbon technologies.”
Further details of the research can be found in Nature, Vol 565, Issue 7739, 17 January 2019 (DOI: 10.1038/s41586-018-0850-3).
Gravy isn’t just for Christmas. Make enough and there’ll be plenty of slices left for many roast dinners to come.
But if you prefer your gravy with a more pourable and smooth consistency, Dr Julian Dean is the man to talk to. Featured in The Times just before Christmas, Julian was able to educate frustrated cooks around the country with the science behind lovely lump-free gravy.
We all know good gravy when we taste it. it’s about flavour, smell and consistency. You want it to tantalise your olfactory receptors, complement the accompanying food and to coat it generously. You don’t want it to get stuck in your teeth.
Using the combined knowledge of Dr Joanne Buckley from the Department of Chemistry and our own Dr Dean, The Times were able to reveal the best method for making gravy.
The most common method is the apply heat to a combination of fat from your roast mead, stock and flour. The flour provides starch, a carbohydrate. When hot, individual starch molecules can absorb liquid and expand. During cooking, as a gravy is warmed, water molecules are given energy and start to move more. “They bump into each other, off the sides and bottom of the pan and when they have enough oomph and hit a starch molecule, the structure is compromised and the starch absorbs water,” Dr Buckley explains.
The starch matrix swells to form a framework that provides a thickening base for the delicious meaty juices from the roast. The process must, however, be managed carefully. A cook must separate the starch molecules so that the particles can expand uniformly.
We suspected that poor gravy is caused, principally, by incorrect materials processing. To test this, a team led by Julian Dean made gravy in three different ways. The viscosity of each batch was tested – good gravy needs to be thick enough to coat a vegetable but viscous enough to pour. Lumpiness was measured by passing them through a sieve with 2mm holes.
Our experiments suggest that the surest route to bad gravy is to add your stock too early. In our first experiment Dr Dean immediately combined 50g of fat (we used butter) with 350ml of water. Then 50g of plain flour was added. The mixture was heated to 70C. About 30 per cent of the resulting slop took the form of lumps, some over 30mm in diameter: not very nice. The liquid portion was too runny to coat a parsnip.
To avoid this a cook must make a “roux”, which involves first mixing the fat and flour. The fat coats the starch molecules and separates them, allowing for the formation of a matrix that will absorb the stock when you add it.
But do not add your stock too quickly. For our second trial we mixed our flour and melted fat to make a roux. Then we added 350ml of water in one go. This created a material that was three times thicker than a batch made properly.
Nearly 20 per cent of the gravy, by weight, was formed of small particles, between about 2mm and 3mm across. They were suspended in the fluid – not quite lumpy, but certainly not right.
The missing ingredient was patience. For our third experiment a roux was mixed from the fat and flour and then 350ml of water was added slowly. This mixture was heated to 70C, to give our water molecules some oomph. This yielded a smooth, lump-free sauce.
Finally, a warning: do not swap plain flour for the same amount of corn flour. When we did this we ended up with a sauce ten times thicker. Avoid this outcome unless you want to ask: “Anyone want gravy – one slice or two?”
There’s no question that our students are keen to learn, and it’s fascinating to hear how they combine the knowledge that they gain on their course with topics that interest them outside lectures.
Throughout their course, they produce pieces of work to demonstrate their understanding, and these can be presented in a number of ways.
In his first podcast, third year MEng student, James Nohl, finds out about the importance of silver in the heritage of Sheffield.
Listen to James’s podcast here.
Professor Dan Allwood recently took the opportunity to have a chat with newly-promoted Professor Nicola Morley about her research and how she has got to where she is today. Here’s what they talked about.
Dan: Welcome Nicola it’s good to talk to you
Nicola: Lovely to speak to you too Dan
D: How did you end up in a Materials department? What was your story?
N: I’m the oldest of five girls. My other sisters are also as high flying as I am. My sister Rosalind is a Consultant Anaesthetist at the Manchester Children’s Hospital; my sister Caroline is the overall photo editor of Farmers Weekly down in London; my sister Debbie’s a physiotherapist in Birmingham; and my sister Kelly is a Maths teacher in Shropshire at a very high ranking school. And I say that because we all have a very sciency background. My parents both have science degrees; my dad has a degree in Chemistry and my mum Microbiology. So as a family we’ve always been interested in science and doing science.
D: So you were destined to be a scientist, then.
N: I had realised by the age of ten or so that writing wasn’t my speciality. I really wasn’t fussed by reading and writing. I love reading murder mysteries but that’s it! I was really interested in the workings of the world and science. I took Maths, Physics and Chemistry at A-level and my dad did his degree at Oxford so I thought I might as well give that a shot, and I got at place there to study Physics at Merton College, which I didn’t realise at the time was one of the top colleges to do Physics in.
D: Tell us a little bit about your time in Oxford.
N: I did four really very enjoyable years at Oxford. I really loved it. I worked very hard and I also played very hard! I rowed for the college, I played netball for the college, I played cricket for the college, I played badminton for the college. I was also very involved with the CU and St Aldate’s. But I also worked hard and I learned within my first year how to time manage to get what I needed done.
I never had an essay crisis in my student days. I went to the library between lectures to get the work I need to do done, do my revision and then I could play outside of those times. That’s how I learned how to do life, if I’m honest, and how to get what needs doing done, and get on and enjoy the rest of it.
From there I did two summer jobs at the National Physical Laboratory. We were encouraged to go and get training and I applied and got two eight-week placements. One was working in the magnetics group and one was working in the electronic kilogram group. So I have seen the electronic kilogram! They have recently announced how to use fundamental constants to define the kilogram instead of a standard mass of platinum and I got to work on that twenty years ago with them, working out how to do it and it was really fascinating.
I never thought I’d do a PhD, I always thought I’d go into industry and then one of the guys there turned round to me and said to go and do a PhD, go and do four years of research and then carry on. By then I’d done a couple of projects and it was probably just at the tip of the wave of superconductivity, before it was about to crash. I found it interesting and had done a couple of projects on it so I went up to Durham to work with Damian Hampshire for my PhD.
D: Is that when you moved over from Physics to Materials Science?
N: It was in the Physics Department in Durham. Durham doesn’t have a Materials department. I did four years there and enjoyed it. I enjoyed doing the research and if anything it was closer to Materials Science and Engineering than Physics; I spent most of the time making samples which is very typically what we do in Materials, changing compositions, and I built a piece that went onto the bottom of a cryostat that could measure superconductivity at 300 milliKelvin (that’s temperature), 15 Tesla (that’s magnetic field!) and it was a new technique, so it was more of an Engineering project than pure Physics.
D: A cryostat is a system that takes things down to very low temperatures, right?
N: Yes, so if you use liquid helium you can reach 4.2 Kelvin. This was a liquid helium-3 system, so that means it can go down to 300 milliKelvin, which is 0.3 degrees above absolute zero. There are issues that come with that and I learned how to do a lot of work. A PhD takes a lot of patience as well. From there, I applied to a handful of postdoc positions and one of them was with Professor Mike Gibbs in Sheffield, which was a two-year position.
When Professor Gibbs moved over to the Materials Department, I moved with him and took up a lectureship in the Department.
D: So, you have experience of Physics departments, with your time working in Durham and Sheffield Physics and then at NPL in Physics groups, and then you’re in a Materials department. Do you notice any differences in the two?
N: Physics is a beautiful science. Physicists come up with theories that they want to understand, but don’t really have an influence on everyday life. I do work on the muon source and I sit on panels there and people are pinging atoms and looking at structures at say, 10 milliKelvin, because they want to understand what happens to atoms at that temperature. What I like about the Materials Department is that we still do science and try to understand what’s going on but we try to make it so that we can tune it or understand what’s going on so we can make it useful. And that’s one of the things I do love about doing Materials Science – I can do things that are useful for the world, not just ‘look I’m pinging two atoms at 10 milliKelvin at high magnetic field and they do some random oscillation’.
It’s a different mind-set I think. In Materials you still do some of the interesting science but you are often looking at where it can work, whereas physicists often purely want to ask ‘what happens if…?’ ‘Why does this happen?’ and don’t always look at the end point or make it consumer-useful. I think that partly comes from my mum, who wants to understand why my science is going to be any use to her!
D: Now you’re working in Materials Science and Engineering, what are you currently researching?
N: I have a grant as part of a very large consortium led by Imperial College’s Aerospace, which is looking at ‘Structural Health Monitoring of Composites’, so at the moment a lot more planes are being made of carbon fibre, up to 70% of a plane, because it’s lighter, has the same strength properties as metals but as it’s lighter it uses less fuel. The problem with carbon fibre is that it damages quite easily and, unlike metals where there are very tried and tested techniques to monitor the damage, it’s much more difficult to monitor the damage and it has much more complex damage mechanisms. And the problem is that if you don’t detect what is known as ‘barely visible damage’ early, it can cause cracks and delaminations that can essentially cause catastrophic failure if it’s not caught.
D: (Apologies to anyone reading this in a plane!)
N: So, what we’re trying to do now is we’re trying to come up with techniques to detect the damage early. So it’s called ‘Structural Health Monitoring’ and it’s early detection. You can use pizeoelectrics, so you put a piezoelectric on the surface and they send waves into the material.
D: What are piezoelectrics?
N: Piezoelectrics, essentially, if you strain them they change their charge, or you can use them where you excite them and they produce what’s down as a Lamb wave through a materials, and that can be picked up and any damage will change their frequency. Fibre optics are very similar and you can use the two together or separately. Again, under strain, you send an optical signal through the fibre optics, which has a certain signature. If it’s strained or there’s any damage (damage in the composite causes a strain) it can pick it up.
D: So it affects how the light transmits through the fibre?
N: And you can use these and pinpoint the damage. Something I’ve worked on for the past 15 years is magnetostrictive materials. These are very similar to piezoelectrics but these materials, if you strain them you change their magnetisation. So what we’re looking at we’re trying to create a strain sensor which can go onto the surface of the composites such that if there is damage we can detect where the damage is early and work it out.
D: So, these sensors, under strain, the magnetic properties change and then you detect the magnetic properties.
N: Yes. So we’ve been looking at ribbons on the surface with a handheld part to look over the surface of it and that has worked but there’s obviously weight issues if you’re going to add more ribbon. The other way we’re working at the moment is we’re using magnetic nanoparticles in the epoxy that’s already put on the surface to then see if that gives us a big enough magnetic change.
D: Excellent – so you’re functionalising the structural material. That’s really interesting. I know that you’re heavily involved in other research, but I’d like to save that for another day. Nicola, thanks very much for talking to us.
In the next issue, Dan will be talking to Nicola about her work on High Entropy Alloys.
Powering the world’s computers requires an astonishing amount of energy; information and communication technologies account for around 7% of the world’s energy consumption, and the demand is only expected to increase into the next decade.
One reason that current complementary metal-oxide semiconductor (CMOS) computer technology is so power hungry is that it requires a constant supply of energy to operate; as soon as power is lost, a computer “forgets” what it was doing and will have to start over again once the power is restored.
A solution to this would be to make logic gates, the fundamental building blocks of a computer, from materials that show non-volatile properties—i.e., those that allow information to be retained without using any energy at all.
In a recent article in Advanced Functional Materials, the research team, led by members of the Department of Materials Science and Engineering at the University of Sheffield has demonstrated the feasibility of one such technology. Their work shows how swirling nanoscale “tornados” of magnetism, known as vortex domain walls, can be flowed through complex networks of nanowires composed of nickel and iron in a manner that reproduces the behavior of common logic gates.
In their approach, binary data (i.e, 0s and 1s) are encoded directly within the winding of the vortices—if the magnetism rotates clockwise, the vortex represents binary 1, while if it rotates anticlockwise, the vortex represents binary 0. Importantly, the vortices are non-volatile and retain their winding even when no power is supplied to the devices.
In addition to being a novel way of creating non-volatile logic, the approach shows advantages over previous attempts to create logic devices from magnetic nanowire networks. Specifically, in the approach proposed, sets of data injected simultaneously could move through the device in a synchronized fashion, something that wouldn’t be true for previously proposed devices.
The authors believe that the next steps in developing this technology will be to demonstrate robust methods of reading data from, and writing data to, the devices, as well as developing new magnetic materials that will allow the logic gates to operate consistently and without errors.
So, you think you know your materials? Can you identify the picture above? Post your comments below, or Tweet us @msesheffield with the hashtag #WhatsTheMatter.
The flower shape oxide crystal of Hematite crystal developed on surface of a high
silicon steel specimen at 850°C for 180 minutes. It was taken in 2014 by Daryoush Ahmadi.
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