Diffusion and interfacial segregation phenomena occurring in the Si amorphous-crystal system under MeV Ar ion irradiation are reviewed. Implanted Au, Ag and Cu in amorphous Si undergo radiation enhanced diffusion in the temperature range 300-700 K with Arrhenius-like behavior with activation energies of 0.37, 0.39 and 0.17 eV respectively. Segregation and trapping of Au at the amorphous-crystal interface occurs when recrystallization is induced with 2.5 MeV Ar ion irradiation. The Au segregation is analogous to behavior at liquid-solid interfaces. The Au is trapped in crystalline Si at concentrations some ten orders of magnitude in excess of equilibrium concentration.