Grain boundary wetting by a solid phase; microstructural development in a Zn–5 wt% Al alloy

A systematic study of grain boundary wetting by a solid phase was carried out for the first time. The microstructure of Zn–5 wt% Al polycrystals was studied in the temperature range 250–375 °C. The Al-rich phase formed either chains of separated lens-like precipitates or continuous layers at the Zn-rich phase/Zn-rich phase grain boundaries upon annealing at different temperatures. The contact angle at the intersection between the Al-rich phase/Zn-rich phase interphase boundaries and the Zn-rich phase/Zn-rich phase grain boundary decreased with increasing temperature. It became zero at a certain temperature, and remained zero above this solid-state wetting temperature, i.e., a continuous Al-rich phase layer covered the Zn-rich phase/Zn-rich phase grain boundaries. The fraction of wetted grain boundaries increased with increasing temperature and was independent of annealing time. The growth of the Al-rich phase at the grain boundaries is controlled by volume diffusion in the matrix phase.