Crunch time

2 min read

A form of steel that gets stronger as it deforms is being developed by the Max Planck Institute for Iron Research (MPIR) in Germany.

The material, dubbed TWIP (twinning induced plasticity) steel, could revolutionise car safety as it can deform by up to 80 per cent while maintaining the integrity of the vehicle.

Providing an extra layer of safety, the steel absorbs the energy of an impact then disperses it to surrounding parts of the car as it deforms, or stretches.

'During the deformation, the steel is able to absorb a lot of the crash energy,' said Dr Stefan Zaefferer of the MPIR.

'And while you deform this steel it becomes harder and harder. The idea is that you can protect the people by putting more energy into the steel and less into the people.

'I believe it could have a serious effect on safety. Steels currently used for these crash-relevant parts are either high strength but not very deformable, or deformable but not very strong.'

Like all alloys TWIP gains its properties from changing the crystal lattice structure — the spatial arrangement of atoms — of the metal by adding different elements. These elements change the shape of the lattice and so the metal acts differently. TWIP uses 22-25 per cent manganese, similar to Transformation Induced Plasticity (TRIP), a steel which has been on the market for about 10 years and is strong but with only limited ductility.

What gives TWIP its extreme ductility is the twinning effect caused by stacking faults in the crystal lattice. These faults disturb the regular stacking of the atomic planes and result in a folding effect. Twinning can only be achieved at an ignition temperature, but if the stacking fault energy is too high the effect is not achieved.

But the team at Max Planck is trying to unravel the mystery of why TWIP steel acts as it does. 'We do not yet understand why these metals are so ductile,' said Zaefferer. 'We know that it's something to do with twinning but why it leads to such ductile metal is not understood.

'The composition of these steels is very simple. It's mainly manganese and iron, with aluminium and a little bit of carbon or silicon. It doesn't use expensive materials like nickel in high-strength steels.

'We are doing fundamental research to understand how the physical properties of the metal enable it to be so ductile. The properties depend very much on the chemical content. You can tailor the properties by altering the manganese content.'

Another issue facing the MPIR team is that of hydrogen embrittlement, where the steel takes up hydrogen from water and incorporates it into its structure.

'This may lead to very serious problems. If you add a little hydrogen to the atomic links, you may split them up, which can lead to cracks in the steel,' said Zaefferer.

'We have started a new project with public funding to address these problems and in 10 years, we will have a completely new class of steel from scratch.'

Zaefferer said three steel companies — ThyssenKrupp, Arcelor, and Salzgitter Steels — are already on the way to making products from TWIP steel.

Jon King, director of Corus Automotive, which is working with Salzgitter said: 'We think there will be products on the market in a year or two.'

TWIP steel is being developed for use in cars but Zaefferer believes it could have applications in trains and other modes of transport. It may also be used to create complex geometrical shapes for car frames or other parts.