Nov 08, 2023

The metal that brought you cheap flights

It made the age of cheap foreign holidays possible, and for years it was what made margarine spreadable. Nickel may not be the flashiest metal but modern life would be very different without it.

Deep in the bowels of University College London lies a machine workshop, where metals are cut, lathed and shaped into instruments and equipment for the various science departments.

Chemistry professor Andrea Sella stands before me holding a thick, two-metre-long pipe made of Monel, a nickel-copper alloy. Then he lets it fall to the ground with a deafening clang.

"That really speaks to the hardness and stiffness of this metal," he explains, picking up the undamaged pipe.

But another reason Monel is a "fantastic alloy", he says, is that it resists corrosion. Chemists need ways of handling highly reactive materials - powerful acids perhaps, or gases like fluorine and chlorine - so they need something that won't itself react with them.

Gold, silver or platinum might do, but imagine the price of a 2m-long pipe made of gold. Nickel, by contrast, is cheap and abundant, so it crops up everywhere where corrosion is a concern - from chemists' spatulas to the protective coating on bicycle sprockets.

But nickel can produce other alloys far quirkier than Monel, Sella is eager to explain.

Take Invar, an alloy of nickel and iron. Uniquely, it hardly expands or contracts with changes in the temperature - a property that comes in very handy in precision instruments and clocks, whose workings can be interfered with by the "thermal expansion" of other lowlier metals.

Then there is Nitinol.

Sella produces a wire in the shape of a paperclip - but it is far too easy to twist out of shape to be of use holding sheets of paper together. He mangles it in his fingers, then dips it into a cup of boiling water. It immediately writhes about… and turns back into a perfect paperclip.

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Nickel demonstration

Nitinol has a special memory for the shape in which it is first formed. And its composition can be tuned, so that at a particular temperature it will always return to that original shape. This means, for example, that a rolled-up Nitinol stent can be inserted into a blood vessel. As it warms to body temperature, the stent opens itself out, allowing blood to flow through it.

But all these alloys pale in significance compared to a special class of alloys - so special they are called "superalloys". These are the alloys that made the jet age possible.

The first jet engines were developed simultaneously in the 1930s and 40s, by Frank Whittle in the UK and by Hans von Ohain in Germany, both on opposing sides of an accelerating arms race.

Those engines, made of steel, had serious shortcomings.

"They didn't have the temperature capability to go above about 500C," explains Mike Hicks, head of materials at Rolls-Royce, the UK's biggest manufacturer of jet turbines. "Its strength falls off quite quickly and its corrosion resistance isn't good."

In response, the Rolls-Royce team that took up Whittle's work in the 1940s went back to the drawing board - one with the periodic table pinned on to it.

Tungsten was too heavy. Copper melted at too low a temperature. But nickel - with a bit of chromium mixed in - was the Goldilocks recipe. It tolerated high temperatures, it was strong, corrosion-resistant, cheap and light.

Today, the descendants of these early superalloys still provide most of the back end of turbines - both those used on jet planes, and those used in power generation.

"The turbine blades have to operate in the hottest part of the engine, and it's spinning at a very high speed," says Hicks's colleague Neil Glover, head of materials technology research at Rolls-Royce.

"Each one of these blades extracts the same power as a Formula 1 racing car engine, and there are 68 of these in the core of the modern gas turbine engine."

The gas stream that these stubby blades sit in is 1700C - some 200C above the melting temperature of the blade itself.

This feat is achieved thanks to a heat-resistant ceramic coating, as well as "cooling" air - itself some 650C - which is drawn from further up the engine into the hollow blade and then bled out over the surface of the blade via tiny holes.

The ability of superalloys to operate at such extreme temperatures is what makes your holiday to the Algarve or Florida affordable.

"The hotter the turbine can operate, the more efficient the engine as a whole, and the less fuel it uses," Neil explains.

But the blades must deal with more than just extreme temperatures. They rotate so quickly that the centrifugal load on them is equivalent to several tons.

This, combined with regular heating and cooling, can lead to a problem known as "creep" - the blade slowly elongates until it begins to bite into the turbine casing.

Most metals are made up of myriad tiny crystals, called grains, which are fused together. But the grain boundaries are a source of weakness, allowing the crystals to slip and the material to deform.

So Rolls-Royce deals with this problem by creating the blade as a single crystal - growing it from a melt using directional solidification, akin to the copper sulphate crystals you might have grown in a school chemistry experiment as a child.

In effect, the blade is like a gemstone, with a single atomic lattice all the way through.

The alloys have also been improved by adding other elements - 10 or more in total - enabling the turbine designer to tune the material properties of each engine component.

And it is because of these additional alloying ingredients that the story of the jet engine also turns out to be the story of another chemical element - one far more enigmatic than nickel.

That element is rhenium. Adding it to the superalloy helps further resist creep.

But rhenium also happens to be one of the scarcest substances on earth. It forms only one part per billion of the earth's crust. The entire annual worldwide production of rhenium is a mere 40 tonnes, and more than three-quarters of it goes into superalloys.

So next time you are taxiing down a runway, you can thank nickel - but spare a thought for its obscure cousin, rhenium, too.

But at the beginning I mentioned margarine, and at this point you may be wondering what it has in common with superalloys and jet engines.

The answer is, not much. Margarines are made primarily from vegetable oils and fats. The thing is that most of these are too liquid to be spread on toast - and if so, nickel can be used to make them more viscous and "buttery".

This is done via a chemical reaction called hydrogenation, in which hydrogen is pumped into the oils, along with a tiny smidgen of nickel, which acts as a catalyst. The nickel doesn't react with the oils, instead it acts as a molecular machine, enabling the oils to react with the hydrogen. The resulting more hydrogen-rich fats are thicker - and spreadable.

So should we thank nickel for margarine too? Maybe, or maybe not.

The hydrogenation process can produce two types of fat molecule - the kinked cis-fats, and the less welcome and straighter trans-fats. These trans-fats do not commonly occur in nature, and have been associated with high cholesterol levels and associated heart disease and strokes. This has spawned a move towards using palm oils instead, which are naturally thicker and more spreadable, especially when combined with emulsifiers. Yet this has opened up a whole new can of worms - the destruction of rainforests to make way for palm oil plantations.

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