Experimental investigation of pre-melting in metals

Melting of a pure crystalline solid has long been known to be a three-stage process. The first is surface melting, the second is defect melting and the third is bulk melting. The most prominent defects in pure solids are grain-boundaries. Surface melting was first observed in 1985 using atom scattering in Pb at 100K below bulk melting. Grain-boundary melting has been simulated, but there has been no experimental confirmation of it in pure materials. We developed a simple optical method of observing the three stages of the melting and have used it to observe directly the three stages of melting in Sn and other metals.