Observations of interface premelting at grain-boundary precipitates of Pb in Al

This work reports direct observations showing the effect of size and interface structure on premelting behaviour of nanoscale inclusions. Using ill-situ transmission electron microscopy it was possible to observe premelting of individual Pb inclusions in Al, each bounded by two distinctly different topotaxial interfaces. Such particles were generated by precipitating single-crystal Pb inclusions a few tens of nanometres in size at grain boundaries in Al. At equilibrium these particles adopt compound shapes, made from two segments whose shape and interface structure is characteristic of their misorientation With the matrix crystal. Only one of these interfaces premelts. In close agreement with a simple model, the width of the liquid layer depends reversibly on undercooling and interface curvature, and hence on particle size. The observed behaviour confirms previous reports on interface-dependent melting. By observing the selective melting of different interfaces for the first time on individual particles, it was possible to rule out experimental uncertainties and to show unambiguously that inclusion melting depends strongly on interface structure.