Experimental study of grain boundary melting in aluminum

Experiments aimed at determining whether grain boundaries melt “completely” to structures which closely resemble their undercooled bulk liquid at temperatures appreciably below the bulk melting temperature, T m were performed. A variety of grain boundaries in Al, containing localized grain boundary dislocation (GBD) arrays, was observed directly by hot stage electron microscopy during heating to close to T m . These included boundaries possessing both 2-D and 1-D periodicity. No delocalization of the GBD cores was observed up to T = 0.96 T m indicating an absence of melting. In additional experiments, polycrystalline Al specimens were partially melted in the hot stage and the grain boundaries in the direct vicinity of the liquid/solid interface were searched for possible melting. No signs of melting were detected for temperatures up to 0.999 T m . The results indicate that complete grain boundary melting occurs only essentially at T m . The results are consistent with atomistic simulations in the literature which indicate that grain boundaries become preferentially disordered upon heating. This process continues progressively and culminates rather abruptly very near T m where complete melting finally occurs. During this process, the boundary structure maintains a degree of “crystallinity”, and only at T m does the distinction between the boundary structure and the bulk liquid disappear completely. Other experimental evidence in the literature supporting this picture is cited. Le but des expériences réalisées est de déterminer si les joints de grains fondent complètement pour donner des structures qui ressemblent de près à leur liquide en surfusion à des températures nettement inférieures à la température de fusion, T m . Plusieurs joints de grains dans Al, contenant des réseaux localisés de dislocations de joints, ont été observés directement par microscopic électronique au cours de chauffages jusqu'aux environs de T m ; il y avait des joints possédant une périodicité à deux et à une dimensions. Les coeurs des dislocations de joints n'ont pas bougé jusqu'à T = 0,96 T m , ce qui indique l'absence de fusion. Dans des expériences supplémentaires, des échantillons polycristallins d'Al ont été partiellement fondus dans le porte-objet chauffant et les joints de grains dans le voisinage direct de l'interface liquide/solide ont été recherchés pour étudier une possible fusion. Aucun signe de fusion n'a été détecté jusqu'à des températures de 0,999 T m . Les résulta