Influence of the Grain Boundary Phase Transitions on the Diffusion-Related Properties

摘要

Grain boundary (GB) phase transitions can change drastically the properties of polycrystals. 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 equilibrium. 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 equilibrium contact with the liquid phase. The liquid phase has to substitute the GB and to separate both grains. The experimental data on GB wetting phase transitions in the systems Al-Sn, Al-Ga, Al-Sn-Ga, Cu-In, Cu-Bi, Fe-Si-Zn, Mo-Ni, W-Ni, Zn-Sn and Zn-In are analysed. 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. The experimental data on GB segregation, energy, mobility and diffusivity in the systems Cu-Bi, Al-Ga, Al-Pb and Fe-Si-Zn obtained both in polycrystals and bicrystals are analysed. In case if two solid phase are in equilibrium, the GB "solid state wetting" (or covering) can occur. In this cas the layer of the solid phase 2 has to substitute GBs in the solid phase 1. Such covering GB phase trasition occurs if the energy of two interphase boundaries between phase 1 and 2 is lower than the GB energy in the phase 1. The data on the "solid state wetting" GB phase transitions in systems Al-Zn, Zr-Nb, Fe-C, W-Ni, Mo-Ni and W-Cu are analysed. The tie-lines of the covering phase transitions can also have the continuation in the one-phase areas of the bulk phase diagrams. In that case one can speak about GB solvus lines and formation of equilibrium GB layers of the phases unstable in the bulk. The GB phase transitions lead to the drastic change of the GB properties, particularly their diffusion permeability. As a result, the GB phase transition influence such diffusion-controlled properties as liquid-phase and activated sintering, superplasticity, soldering, processing of semi-solid materials etc.