“WETTING BY SOLID STATE ” GRAIN BOUNDARY PHASE TRANSITION IN Zn–Al ALLOYS

Abstract. The microstructure of Zn–5wt.%Al polycrystals has been studied between 250 and 375 °C. The evolution of continuous (Al) layers at individual Zn grain boundaries (GBs) has been studiet at 230 and 290 °C. The (Al) phase forms either chains of separated lens-like precipitates or continuous uniform layers at (Zn) GBs. If the GB particles are observed, the contact angle q>0? at the intersection between (Al)/(Zn) interphase boundaries (IBs) and (Zn)/(Zn) GB. With increasing temperature q becomes zero at certain Tws and remains zero above Tws. Above Tws a (Zn) GB is covered by continuous (Al) layer. Tws inversely correlates with the GB energy. The fraction of (Zn) GBs covered by (Al) layers increases with increasing temperature. Therefore, the GB phase transition ‘wetting by solid phase ’ proceeds in Zn–Al alloys. It is thermodynamically similar to the GB wetting phase transition by liquid phase. The tie-line of a GB phase transition is constructed in the conventional bulk Zn–Al phase diagram. Such GB tie-lines are especially important for the nanocrystalline materials. 1