Superbainite

Structure of superbainite. Inset is a same-scale image of a carbon nanotube. [1]

Structure of superbainite. Inset is a same-scale image of a carbon nanotube. [1]

According to archaeologists, the Iron Age began in 1300 BC and lasted for around two millennia. Today, steels (alloys of iron and carbon) comprise 95% of global metal consumption and this trend shows no sign of declining.

Glancing at the media, however, one would be forgiven for assuming that steel is now a has-been. We are bombarded with stories of novel materials: carbon nanotubes, metallic glasses, graphene, carbon fibre, nickel superalloys. . . all of which are “stronger than steel”.

“Now we can construct space elevators!” claim the articles. “Let’s build a climbing frame to the moon! We’ll use this stuff to make everything!”

The observant among us, however, will note that most cars, trains and buildings still don’t feature superalloys, metallic glass or magic nanotubes. Neither are they invisible; nor do they fly; nor do they do any of the other things that journalists tend to ‘predict’.

Instead, steels somehow remain the best — and cheapest — materials for the job. Also, they are stronger than steel. This is because ‘steel’ is a vague construct used by sensationalists, with an unspecified strength guaranteed to be less than that of a novel material. Metallurgists rarely refer to ‘steel’, just as the Inuit have fifty words for snow, not one of which is ‘snow’.

Steels, on the other hand, are alloys with a colourful variety of properties: strength, toughness, hardness, ductility and wear resistance. They are everywhere: railways, pipelines, bearings, jet engines, armour, nuclear reactors. . . and more. Many of these technologies are cutting-edge in their design, and require high-performance steels to match.

The benefits of steels lie first and foremost in their versatility. Iron atoms can ‘stack’ themselves in various distinct configurations (just like a greengrocer stacking oranges — there are different ways to do it). Each configuration is known as a ‘phase’. As well as the two main iron phases (austenite and ferrite) there is a carbide phase — a compound of iron and carbon (Fe3C) — called cementite.

These phases often exist in combinations and have different properties. For example: cementite is strong and brittle, whereas ferrite is softer and tougher. If we combine them into a structure called pearlite (consisting of alternating layers of ferrite and cementite), we produce a steel that is reasonably strong and tough.

The ability to harness the properties of different steel phases has been around for centuries. The temperature and composition of the steel determines which phases can form. By heating and cooling different areas of a sword in different ways, ancient Japanese swordsmiths could produce a steel blade that was soft and tough in the interior, with an incredibly hard cutting edge.

Today, heat treatment is still used to produce phases or combinations of phases with desirable properties. Alloying elements can also modify the properties: manganese and chromium increase strength; molybdenum prevents embrittlement. It is the metallurgist’s role to explore new methods of heat treatment and alloying in order to produce novel steels.

‘Superbainite’ is one such alloy. It is designed for high-strength applications and is currently one of the strongest metals in existence. Its strength comes from an incredibly fine microstructure consisting of two phases: ferrite and austenite, and it is the morphology of the phases that is important. The ferrite is in the form of plates, separated by thin films of austenite.

In order to deform a metal, it is necessary to force its atoms to change position relative to each other. However, an interface (such as the boundary between two phases, marking a change in the atomic arrangement) acts as a barrier to this atomic ‘rearrangement’, and it becomes harder to deform the material. By refining the microstructure, we can create more interfaces and increase the strength of a metal. The structure of superbainite is unusually fine: the ferrite plates are as thin as 20 nanometres.

Surface relief caused by shear transformation of bainite

The unusually fine structure of superBainite is a result of the displacive transformation, this also result in a characteristic surface relief.

This scale is typically difficult to achieve because it requires very low transformation temperatures (200°C compared to a conventional 400°C). When transforming from one phase to another, steels can release heat, limiting how much they can be cooled. The problem was overcome by harnessing a different type of transformation — the bainite transformation — that re-absorbs the heat that is released. The high density of interfaces in superbainite is the origin of its strength.

But strength, for all its virtues, is not the only important property of a metal. Ductility (how much the metal can be deformed before it fractures) and toughness (resistance to crack propagation) are also key. Yet if a steel is very strong, it may have low toughness or ductility. The real challenge is to obtain an optimum combination of several properties.

Some steels make use of an unusual property of phase transformations. It is possible to induce a change from one phase to another not by heating but by deforming the metal. Deformation involves atomic movement, so it is conceivable that the iron atoms can change configuration, producing a different phase. This can enhance the ductility of steels while retaining their strength, a phenomenon known as transformation-induced plasticity (TRIP).

Returning to superbainite, we find that the two-phase microstructure is a double-edged sword. It is suspected that the TRIP effect is converting the softer austenite into a much harder, more brittle form of iron, resulting in low toughness. If we are to design superbainitic alloys for applications that require high degrees of impact resistance (such as armour), we must identify and remedy the causes of these poor properties, without losing the strength for which it has become famous.

We must also try to accelerate the transformation. While it is desirable to transform superbainite at very low temperatures, it can take 10 days or longer to complete the treatment. A sample of the alloy designed to transform at room temperature currently sits in the Science Museum in London. At the start of its 100-year transformation time, it gives us a glimpse of what we may be able to achieve in the future. There are many exciting advances still waiting to be made. Why pin our hopes on expensive, rare and exotic alternatives, when we are living in the Steel Age?

1. C. Garcia-Mateo, F. G. Caballero and H. K. D. H. Bhadeshia, Development of Hard Bainite, ISIJ International (2003), 43, 1238-1243

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2 Responses

  1. Interesting , I t would be wonderful if you could post Howard clarks HT
    formula without too many graphs for bainite L-6 swords ….. (smile) James ….. JSSUS

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