“Wetting” phase transitions by the second solid phase for linear defects (grain boundary triple junctions)

In this work, the wetting phase transition of grain boundaries (GBs) and their triple junctions (GB TJs) by the second solid phase in the magnesium-based alloy EZ33A is studied. The condition for complete wetting for the GB TJ ($\sigma_{\text{GB}}>\sqrt{3}\sigma_{\text{SS}}$) is weaker than that for GBs ($\sigma_{\text{GB}}>2\sigma_{\text{SS}}$). Therefore, if the transition from partial to complete wetting occurs with increasing temperature, then all GB TJs should become completely wetted at a temperature $T_{\text{wTJ}}$, which is lower than the temperature $T_{\text{wGB}}$, at which all GBs become completely wetted. For the first time, it has been found experimentally that GB TJs are completely wetted at $T_{\text{wTJ}}=(380\pm10)\,^\circ$C, which is approximately $70^\circ$C lower than $T_{\text{wGB}}=(450\pm10)\,^\circ$C. The wetting phase at the GBs is the intermetallic compound (Mg,Zn)$_{12}$RE. A similar phenomenon was previously observed for GB TJ wetting with a liquid phase [B.B. Straumal, O. Kogtenkova, and P. Zieba, Acta Mater. 56, 925 (2008)].