Melting of metals: role of concentration and migration of vacancies at surfaces

The mechanism of melting is investigated by considering the role of surfaces with regard to the concentration and migration of vacancies. For many metals, it is found that while the vacancy concentration in the bulk is similar to 0.37% at the equilibrium melting point (T-0), the vacancy concentration at the free surface is as high as 10%, i.e. similar to that in the bulk at the superheating limit. Melting is believed to be associated with a lattice instability induced at a critical vacancy concentration of similar to 10%, both at the surface and within the crystal lattice. The abrupt increase in vacancy concentration from 0.37 to 10% on melting at T-0 can be explained as a result of melting of the surface. The surface pre-melting behaviour of metals is quantitatively interpreted by considering the vacancy migration there.