The Influence of the Grain Boundary Phase Transitions on the Properties of Nanostructured Materials

Abstract

Grain boundary (GB) phase transitions can change drastically the properties of nanograined polycrystals, leading to enhanced plasticity or brittleness, increasing diffusion permeability. They influence also liquid-phase and activated sintering, soldering, processing of semi-solid materials. The GB wetting phase transition can occur in the two-phase area of the bulk phase diagram where the liquid (L) and solid (S) phases are in equlibrium. The GB wetting tie line appears in the L+S area. Above the temperature of the GB wetting phase transition a GB cannot exist in equlibrium contact with the liquid phase. The liquid phase has to substitute the GB and to separate both grains. The GB wetting tie-line can continue in the one-phase area of the bulk phase diagram as a GB solidus line. This line represents the GB premelting or prewetting phase transitions. The GB properties change drastically when GB solidus line is crossed by a change in the temperature or concentration. In case if two solid phase are in equilibrium, the GB “solid state wetting” (or covering) can occur. In this case the layer of the solid phase 2 has to substitute GBs in the solid phase 1. Such covering GB phase transition occurs if the energy of two interphase boundaries between phase 1 and 2 is lower than the GB energy in the phase 1.