Phase transitions in wetting films at the surface of Ga-Pb alloys

Surface phase transitions at Ga-rich liquid surfaces have been investigated in Ga-Pb alloys with low lead content. In the region of the liquid-liquid miscibility gap, the Pb-rich liquid phase completely wets the surface of the Ga-rich phase at coexistence. Observations have been made of demixing and solidification of the Pb-rich liquid film. Ga-rich alloys, which are single-phase below the monotectic temperature, can be undercooled below the liquidus, as far as the metastable binodal line where the Pb-rich wetting liquid film forms and solidifies into thin {111} Pb crystals. These films completely redissolve upon reheating to the liquidus temperature. Freezing occurs at surfaces because of complete wetting of the liquid rich in the high melting point component and the hysteretic character of the solidification transformation. Such ''surface'' experiments allow assessment of the stable and metastable liquidus lines of the Ga-Pb phase diagram in the vicinity of the monotectic temperature.