Internal interfaces like crystallite, grain or phase boundaries are very important microstructure features, as they influences significantly the physical properties of solids. In nanostructured materials, the density of internal interfaces is extremely high, thus the interfaces and interface phenomena become a dominant factor that affects the properties of nanocrystalline materials and nanocomposites. This generally well-known and frequently utilized relationship between the microstructure and properties is also the basic idea behind the “defect engineering”, which employs the internal interfaces as a kind of microstructure defects to tailor the materials properties.
In this talk, local heteroepitaxy and specific orientation relationships between neighbouring nanocrystallites in compact nanocomposites will be discussed as examples of the interface phenomena, which facilitate the formation of lattice strains at internal interfaces, and are therefore often used to increase the materials hardness and/or to stabilize metastable phases at high temperatures. The increase of the hardness will be illustrated on the transition metal nitrides containing aluminium and on the boron nitride nanocomposites. The prospects of the stabilization of metastable phases will be illustrated on the ternary systems (TM,Al)N and (Cr,Zr)2O3-x. The effect of further microstructure defects, e.g., stacking faults, on the phase transformations and decomposition of metastable phases will be discussed. Additionally, this talk will show the complementarity of the diffraction, microscopic and spectroscopic methods when they are applied to study nanostructured materials.