Pseudopartial wetting of grain boundaries

The developments of last decade show that properties of fine-grained and nanograined materials are critically controlled by the behaviour of grain boundaries (GBs) and triple junctions (TJs). Moreover, the most advanced experimental methods like high-resolution electron microscopy (HREM) and atom probe microscopy allowed observing that GBs and TJs are frequently not atomically thin and smooth but contain the few nm thick layers or so-called complexions. These layers can appear in equilibrium, non-equilibrium (transient) or steady-state structures. Most interesting, from our point of view, is the phenomenon of the so-called pseudopartial (or pseudoincomplete) GB wetting. It is intermediate between complete and partial GB wetting.

In this work we observed pseudopartial GB wetting in three systems:

In Al–Zn alloys after severe plastic deformation by the high pressure torsion pseudopartially wetted Al/Al GBs with thin Zn-rich layer with uniform thickness of 2-4 nm were found. The presence of such thin Zn-enriched layer can explain the unusually high ductility of Al–Zn alloys after high pressure torsion.

In WC–Co the phenomenon of pseudopartially GB wetting can explain that the minority of WC/WC GBs are completely wetted by Co melt during the liquid phase sintering. Namely the WC/WC GBs have the high contact angle with cobalt binder and, nevertheless, contain the 2-3 nm thin uniform Co-rich layer.

And in the NdFeB-based alloys the few nanometers thin layers of the Nd-rich phase appear between Nd2Fe14B grains due to the pseudopartial GB wetting. By isolating the grains of hard magnetic Nd2Fe14B phase with the 2-4 nm thin uniform Nd-rich layers the pseudopartial GB wetting delivers the optimum magnetic properties to the system.