Department of Materials Science & Metallurgy: Research papers of the month

Department of Materials Science & Metallurgy

Research papers of the month

September 2016

 

Difficult diffusion in multicomponent steels

It could be argued quite reasonably that the interpretation of growth rate data on the formation of ferrite in steels, is in a state of crisis. This is because of a failure to account for the limitations of experimental techniques and genuine difficulties with the theory of diffusion-controlled growth in multicomponent steels. One might be forgiven in deducing from the published literature that whenever there is a gap between theory and experiment, it is explained by appealing to  free energy dissipations that have little in the way of supporting evidence.

It is demonstrated here that the current kinetic theory that relies on the formation of sharp concentration gradients is unphysical. There is a substantial  penalty associated with the creation of such sharp changes in composition. Therefore, the abrupt concentration spikes would never occur in practice.

The actual distribution of solute would be over distances orders of magnitude larger than currently calculated, leading to slower growth rates than are predicted currently. The consequences of this conclusion place doubt both on the transition from local to paraequilibrium, and whether the latter state exists at all for reconstructive transformations.

Figure: Growth at high supersaturations with negligible partitioning of manganese during transformation. The bulk alloy composition is designated by the symbol .

H.K.D.H. Bhadeshia, "Some difficulties in the theory of diffusion-controlled growth in substitutionally alloyed steels", Current Opinion in Solid State and Materials Science, available online 3 August 2016, in Press.

http://dx.doi.org/10.1016/j.cossms.2016.07.004

 

A new AFM technique to study tribofilm-microstructure correlation

In order to prevent wear, the performance of bearing oils is augmented by special chemicals referred to as anti-wear (AW) additives. Their function is to develop a thin sacrificial protective layer known as tribofilm, a product of chemical reactions triggered by tribological conditions. At the microscale, tribofilms developed by some AW additives are well known to cover the surface of the material with an uneven layer of patch-like morphology. This is detrimental to performance, as certain regions of the material are often left bare without any protective film. This also leads to increased surface roughness and, as a result, higher friction, which is undesired.

To investigate the structure of the tribofilm and its relation with the microstructure, a new atomic force microscopy (AFM) technique was developed. AFM topography micrographs of the same region were obtained before applying EDTA (ethylenediaminetetraacetic acid) to remove the tribofilm, after applying EDTA, and subsequently after etching; this unveils the microstructure of the underlying steel. By overlaying the three micrographs the new technique has allowed, for the first time, to establish the correspondence between the microstructure of bearing steels, the morphology of the tribofilm and bearing performance.

J. J. Rydel, R. H. Vegter, and P.E.J. Rivera-Diaz-del-Castillo, "Tribochemistry of bearing steels: A new AFM method to study the material–tribofilm correlation", Tribology International, 98 (2016) pp. 74–81

http://dx.doi.org/10.1016/j.triboint.2016.01.055