Nodular graphite in ductile iron: A scanning Auger microscopy study

Ulrich Hagemann, Interdisciplinary Center for Analytics on the Nanoscale, University of Duisburg-Essen, Duisburg, Deutschland
B. Smaha, Institut für die Technologie der Metalle, University of Duisburg-Essen, Duisburg, Germany
S. Maqbool, Institut für die Technologie der Metalle, University of Duisburg-Essen, Duisburg, Germany
Rüdiger Deike, Institut für die Technologie der Metalle, University of Duisburg-Essen, Duisburg, Germany

The addition of magnesium to molten iron results after inoculation in the formation of nodular graphite during eutectic solidification. Rounded nodules reduce stress in the surrounding area of the graphite and inhibit the creation of cracks and thus lead to the enhanced strength and ductility of spheroidal graphite iron in comparison to lamellar graphite iron.

Although ductile iron has been produced commercially since the 1950s and in large quantities (e.g. roughly 20 million tons in 2015) the precise mechanism behind the nodule formation has not been satisfactorily explained. Many theories have been put forth and discussed, however a consensus on the correct mechanism has not yet been reached.

Sample production
Commercial pig iron is melted in a high frequency furnace and heated to 1520°C. The liquefied iron is poured into a permanent mold containing magnesium grains. With this setup it was possible to freeze down the eutectic solidification of liquid iron creating graphite nodules which are only 2µm or smaller, thus making it possible to investigate the possible nucleus for nodule formation.

Scanning Auger microscopy
Scanning Auger microscopes offer the high spatial resolution of a scanning electron microscope and combine it with Auger spectroscopy which can determine the chemical composition of surfaces on the nm-scale.
The Auger effect describes the emission of an electron from an atom after the filling of an inner-shell vacancy. If a core-hole is filled by an electron from an outer shell the energy difference between the two energy levels is dissipated. Though this happens mostly in the form of a photon, the energy can be transferred to another, which is then emitted from atom. The kinetic energy of this so-called Auger electron (after Pierre Auger, albeit Lise Meitner independently discovered the process roughly a year earlier) is thus dependent both on the element from which it is emitted as well as its chemical environment.
The core-hole needed for the electronic transition can either be created by x-rays or energetic electrons. In Auger spectroscopy electron beams with primary energies of 3kV up to 25kV are used. While the latter offer the highest spatial resolution (ca. 5nm) the former have a larger auger electron yield.

With the use of scanning auger microscopy it was possible to investigate the nature of the roughly 500nm large particles, which are believed to be a possible nuclei for graphite nodule growth. These Mg particles consist of two parts, one which is rich in Sulphur, the other is rich in phosphorous and oxygen.

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