Thermocapillary model of formation of nanostructures on the surface irradiated by low-energy high-current electron beams

Abstract

The paper examines nano and microstructures formed on the surface of hypoeutectic Al–Si alloy (silumin) irradiated by low-energy high-current electron beams. A combination of thermocapillary instability and vapor pressure is assumed to be a principal mechanism initiating formation of these structures. Taking into consideration these phenomena, a dispersion equation is stated and analyzed for low hydrodynamic disturbances on the melt surface. The authors came to a conclusion vapor pressure in low-frequency approximation results in a dual mode correlation between growth rate and wavelength. The first maximum initiated by thermocapillary forces is associated with a wavelength of ∼1 μm; whereas the second one resulted from space-time modulation of vapor pressure is in a range of ∼10 μm. The paper highlights conditions for initiation of thermocapillary instability, taking into account evaporation in a nano-dimensional range of wavelengths. In general, a dispersion equation is demonstrated to have two instable solutions irrespectively of vapor pressure modulation. Therefore, two correlations of growth rate and wavelength are identified. The first one has its maximum in the range 200 to 800 nm, being similar to dimensions of crystallization cells. Apparently, the second correlation furthers formation of periodical structures on the surface.