The Structure of Alloys
Meera -Now we meet Professor Peter Lee from the University of Manchester who’s hoping to use Diamond for some clarity about the structure of metal alloys.
Peter – I study the solidification of metals, the processing of metals as well as other materials. In doing that, there is all sorts of kinetic reactions happening and microstructures forming. So the strength of any alloy is not just in its composition, but that composition causes it to form small crystals, different phases, and those phases, very often, are nanostructures. So the strengthening goes from a scale of 10 nanometres where you have small precipitants which are pinned and in fact in alloys, nano-reinforcements has been used for over 103 years now and that then evolves and goes up to a scale of microns and then up to a structure and scale of millimetres. I use Diamond in order to investigate the composition, morphology and interaction of these different phases of these structures.
Meera - And what particular alloys do you look into, focus on?
Peter – One of the main ones I look at is aluminium alloys. These alloys are used in everything from automotives through to air space components. The bulk of these alloys in high-performance applications has been using primary aluminium. The reason they use primary aluminium instead of secondary or recycled aluminium is that when we recycle aluminium we almost as people are slipping in various different stale components. It also just comes through when you break up and recycle an automobile you end up with, very simply, nuts and bolts which are almost always steel. We then re-melt it, the iron concentration in the alloy goes up. Adding that iron concentration, as soon as it goes over 0.5 weight percent, starts causing another phase that when it’s very fine can be reinforcing, but when it’s large can actually be detrimental to the properties. It forms a phase which is highly facetted, it has flat planar edges, so it’s like a diamond, but the phase it forms, instead of being like a diamond and being nice a chunky, is elongated, or needle like. Now imagine structures which are very pointy inside of a material, they act like stress concentrators so they can actually mis-shape, create damage and cause early failure.
Meera - And now knowing this information are you trying to see what’s possible to make recycled aluminium more desirable?
Peter – Exactly. These structures go from nano and when they’re nano they’re beneficial. When you start getting the iron level up, they can actually become as long as 200 microns, almost a millimetre, long and it’s when there are these long sharp shards that they can be detrimental. What we are using Diamond for is in order to determine how we can alter the composition, go through and add a very small percentage of the different alloy compositions or add heterogeneous nuclei that will convert these from a few large structures to many very fine structures and convert something that’s detrimental to something that’s’ actually beneficial within the alloy. If we can do that, we can make the recycled alloys as useful as the primary aluminium alloys.
Meera - And how much of a challenge is that? What do you need to do to go in there and break those long shards and increase the strength around there as well?
Peter – It’s a tremendous challenge. These alloys have 6 to 8 different components. Each of these components are varied in a weight percent from a few parts per million, up to, going to, 1000 parts per million. If you now think of a sort of combinatorial experiment where you are varying from parts per million through to thousands of parts per million with 8 different components, very quickly you can work out that it is actually billions of experiments you can perform in different compositions. What we’re using Diamond for, is to directly observe the kinetics to determine what are the critical phenomena. It’s a real challenge because the few things that we predicted do seem to be probable, aren’t stable in molten aluminium. Molten aluminium has a great ability to dissolve many, many things within it. You also have to be able to make these particles at a very tiny size and get them into this molten melt which means that you need to be able to have them fully leaded by the aluminium. So, there’s lots of challenges we’ve got, lots of great science to carry on doing.
Meera - But what are the benefits of using increasing amounts of recycled aluminium alloys rather than primary sources?
Peter – The huge benefit of using recycled aluminium is that when you make the first original primary aluminium, it comes from digging up bauxite, reducing the bauxite to alumina. Then taking the alumina, which is basically one aluminium atom to 2 oxygen atoms, and reducing that. When you do that, each of those oxygen atoms is converted into a CO2 atom. You are also consuming huge amounts of energy. All that means is that you’re producing somewhere between 7 and 10Kg of CO2 per kilogram of primary aluminium. When you recycle it, you’re really just re-melting and purifying it. That in general, uses less than 0.5 of a kilogram, or 1/20th of the CO2 that’s released. That, if we can go through and convert just 5% of the World’s primary aluminium production to secondary, in new applications which are high-value-added, mean that we can save 15 million metric tonnes of CO2 per year.
Meera - That’s quite a saving, simply by understanding the internal structure and chemistry of metal alloys. That was Professor Peter Lee from the University of Manchester.