Grain boundary complexions and pseudopartial wetting

The important class of grain boundary (GB) complexions includes the few nanometer thick layers having composition which strongly differs from that of the abutting grains. Such GB complexions are frequently called intergranular films (IGFs) and can be observed close to the lines of wetting, prewetting and premelting complexion transitions in the bulk phase diagrams. In the majority of systems, the direct transition between complete and partial GB wetting takes place (by changing temperature, pressure, etc.). However, in certain conditions the so-called pseudopartial (or pseudoincomplete, or frustrated complete) GB wetting appears in a phase diagram between complete and partial wetting. In case of pseudopartial GB wetting, the thin GB layer of a complexion (IGF or 2-D interfacial phase) can coexist with large droplets (or particles) of the wetting phase with a non-zero dihedral (contact) angle. Thus, such IGFs can be observed in the two-phase (or multiphase) fields of bulk phase diagrams, in the broad intervals of concentrations, temperature and/or pressure. The IGFs driven by the pseudopartial GB wetting can drastically modify the properties of polycrystals. In this review, we discuss this phenomenon for the technologically important Fe–Nd–B-based hard magnetic alloys, WC–Co cemented carbides and Al-based light alloys.