Steel yourself: Crystal structure and properties

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  • Published: Sep 15, 2015
  • Author: David Bradley
  • Channels: X-ray Spectrometry
thumbnail image: Steel yourself: Crystal structure and properties

Steel yourself

 Structural change in steel: Scientists at the Max-Planck-Institut für Eisenforschung use images from a transmission electron microscope (grey) to make linear defects in an alloy of iron (Fe) and manganese (Mn) visible. Atom probe tomography shows them the distribution of the iron (blue) and manganese atoms (green). They have put green iso-surfaces into the image where the concentration of the manganese atoms is 12.5 percent. In the superimposed images, the researchers can see that the manganese atoms accumulate along the linear defects; the crystal structure which forms there is different to the surrounding material. [less] © M. Kuzmina/MPI für Eisenforschung

The crystal structure of a metal can change at linear defects and so alter its properties according to new research from scientists in Germany.

As every good chemistry textbook will tell you, metals have a number of useful properties such as strength and ductility. Alloying metals has allowed humanity to adjust these and other properties for millennia, but with some limitations. Modifying the number and type of structural defects in a metal alloy to more precisely control the material is a target of materials science and metallurgical research, Now, a team at the Max-Planck-Institut für Eisenforschung in Düsseldorf, Germany, has introduced a novel type of defect, which they refer to as a linear complexion, that can be seen in manganese-rich steel. The team describes complexions as chemically and structurally distinct regions inside dislocation cores in the form of a linear defect.

Among the most well known of modified metals is steel, which was first used almost 4000 years ago, with evidence of its production in ironware from Anatolia (specifically Kaman-Kalehöyük is a multi-period archaeological site in Kirsehir Province, Turkey, around 100 km south east of Ankara 6 km east of the town centre of Kaman) dating back to 1800 BCE. Today, we use countless variations on the theme for a wide range of engineering and technological applications.

Favoured structures

The German researchers, Margarita Kuzmina, Michael Herbig, Dirk Ponge, Stefanie Sandlöbes and Dierk Raabe, investigating manganese steel have discovered that this alloy exhibits a crystal structure at linear defects that is different from that seen typically in this material. Astoundingly, the combined length of linear defects in a cubic metre of steel can amount to a span of up to one light year. Being able to tune this would allow materials scientists to forge steels with different levels of malleability, rigidity and ductility.

"We don’t yet know what effect the spatially confined chemical and structural states in the material have on its properties," explains Raabe, who led the study. "We stumbled across the states more by chance," he adds. The original aim of the research was to look at the micro- and nano-structure of a particularly rigid and ductile manganese steel which has been strengthened with nanoparticles and is used in the landing gear of large aircraft or as tool steel. The team analysed this material using atom probe tomography and revealed that chains of manganese-rich nano-beads are present in the steel.

However, they also noticed that the concentration of the manganese increased along specific lines after they had heat treated the steel. The fine tubes in which the manganese collects are only two nanometres across, but the accumulation does not happen over the whole length rather in chains of manganese-rich nano-beads. The crystal structure has to change to accommodate this increased concentration of manganese atoms at the local level. Whereas iron and manganese usually adopt a body-centred cubic, or martensite, structure. The chain of nano-beads corresponds to the face-centred cubic, or austenite, structure.

Material scientists already knew that such deviations could occur at the two-dimensional level. But, this new discovery takes it up a dimension, with "filigree" austenite structures being seen in the interior of individual martensite crystal grains. "When we saw that the manganese accumulated in thin tubes, we had the idea there could be spatially confined chemical and structural states along linear defects," says Dirk Ponge. The shift in crystal structure at the defects representing a lower energy structure.

The team also used transmission electron microscopy (TEM) to look closely at the linear defects and then mapped the distribution of the atoms in the sample again with the aid of atom probe tomography. The superimposed images obtained with each different technique confirmed how the manganese-rich nano-beads arrange themselves precisely along the linear defects.

Ponge adds that, "The stress is particularly high at the dislocations but the material can apparently reduce stress and thus assume an energetically more favourable state by forming a crystal structure there which would be energetically less favourable otherwise." Using these results, the team has now been able to extend a key formula used by material scientists for working out which structure a material might favours under given conditions.

A new road to Damascus steel

Raabe points out that these states might occur not only in the cylinders of an engine, the blades of a turbine or other materials that are permanently subjected to great heat but also in steel car panels and other engineering components. The team will next investigate what effect the local structural changes have on properties. "Our findings may help to explain an already known behaviour of metals - the fact, for instance, that metals become brittle when they corrode and absorb hydrogen," adds Raabe. They also hope to find way to induce these spatially confined states deliberately to develop new types of steel with specific properties. This kind of control might open up entirely new approaches to steel manufacture for specialist applications.

Related Links

Science 2015, 349, 1080-1083: "Linear complexions: Confined chemical and structural states at dislocations"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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