Reversible “Wetting” of grain boundaries by the second solid phase in the Cu–In system

The reversible wetting of grain boundaries by the second solid phase in the copper-indium system has been observed. With an increase in the temperature, the contact angle $\theta$ between the (Cu)/(Cu) grain boundary in a Cu-based solid solution based and particles of the $\delta$-phase (Cu$_{70}$In$_{30}$)decreases gradually. Above $T_{\text{W}}=370\,^\circ$C, the first (Cu)/(Cu) grain boundaries completely “wetted” by the $\delta$ phase appear in Cu-In polycrystals. In other words, the $\delta$ phase forms continuous layers along grain boundaries and $\theta=0$. At $440\,^\circ$C, the fraction of completely wetted grain boundaries reaches a maximum ($93\%$), whereas the average contact angle reaches a minimum $(\theta=2^\circ)$. With a further increase in the temperature, the fraction of completely wetted grain boundaries decreases and vanishes again at $T_{\text{DW}}=520\,^\circ$C. This phenomenon can be explained by an anomalous shape of the solubility limit curve of indium in a solid solution (Cu).