Grain boundary wetting-related phase transformations in Al and Cu-based alloys

The phase transformations can proceed not only in the bulk phases but also on free surfaces and in grain-and interphase boundaries. In this review we consider the grain boundary phase transformations in Cu- and Al-based alloys. In particular, among those transformations are the transitions between compete and incomplete grain boundary wetting. The wetting phase can be either liquid or solid. If the wetting phase is solid, the fraction of wetted grain boundaries can increase also with decreasing temperature. The transition itself can be discontinuous (of the first order) or continuous (of the second order). The thin layers of grain boundary phases (called also the grain boundary complexions) can occur in the conditions (temperature, pressure and concentration) where only one volume phase is thermodynamically stable. The phenomenon of the pseudo-incomplete (or pseudo-partial) grain boundary wetting is also discussed. In this case the wetting phase characterized by non-zero grain boundary contact angle coexists with a thin continuous layer of grain boundary phase. The new lines of respective grain boundary phase transformations appear in the conventional phase diagrams for the bulk phases. The grain boundary phase transitions can strongly influence the properties of grain boundaries themselves and those of polycrystals as a whole. For example, the presence of grain boundary layers can increase the plasticity (if the phase is ductile) or decrease it (if the grain boundary phase is brittle). The effect of grain boundary phase transitions on properties of polycrystalline material increases with decreasing grain size and becomes critical in nanograined materials.