Grain boundary wetting phenomena in the Al-Cu system

The complete and incomplete wetting of (Al) / (Al) grain boundaries by the melt L is observed in the (Al)+L two-phase region of the Al-Cu phase diagram. Already slightly above the eutectic transformation temperature, approximately 20 % of the (Al) / (Al) GBs in the polycrystal are completely wetted by the melt. Their fraction increases with increasing temperature and reaches 100 % at a temperature of 600°C. Above this temperature, all (Al) / (Al) GBs remain completely wetted by the melt L. In the (Al)+Al2Cu two-phase region of the Al-Cu phase diagram the particles of the second solid phase Al2Cu (θ-phase) are observed in the (Al) / (Al) GBs and in the bulk. The Al2Cu precipitates form a nonzero (and rather large) contact angle with (Al) / (Al) GBs. Thus, the complete wetting of the (Al) / (Al) GBs by the second solid phase Al2Cu is not observed in the Al-Cu system. From this point of view, the Al-Cu alloys differ from the previously studied Al-Mg and Al-Zn alloys where the (Al) / (Al) GBs can be completely wetted by the second solid phase (Al3Mg2 and (Zn), respectively).