Segregation and surface transport of impurities: New mechanisms affecting the surface morphology of laser treated metals

Melts were produced in as-grown and cleaned gold films by focused nanosecond laser pulses, with fluences too low to cause a measurable ablation. Their dynamics and solidification were followed with time-resolving transmission electron microscopy and analyzed by computer simulations. The liquids perform an oscillating flow on the time scale of 100 ns and solidify with an impurity-dependent large-scale distribution of material superimposed by nonresonant ripples. This behavior could be reproduced by simulations if impurity-dependent thermocapillarity and a new mechanism of distribution of surface active impurities were incorporated into the usual equations of energy, momentum, and mass conservation,i.e., periodically varying segregation at the crystallization front and transport by a mobile surface layer. The liquid is inferred to consist effectively of a fast-moving (less than or equal to 60 m/s) surface layer that accumulates most surface active atoms and slow (less than or equal to 1 m/s) bulk material. (C) 1997 American Institute of Physics.