Experimental study of grain boundary melting in aluminum

Abstract

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.