High entropy alloys (HEAs), containing five to thirteen metallic elements, with a concentration in the range of 5 to 35 % for each element, exhibit very interesting properties (mechanical, tribological, formability, magnetism...). Their high mixing entropy promotes the formation of simple solid solutions with amorphous or nanocrystallized structure. Bulk pieces of these alloys are known to be stable at relatively high temperature (until 800°C). We study the stability of AlCoCrCuFeNi thin film at temperatures in the range 110 -810 °C. HEA thin films are deposited by magnetron sputtering of mosaic targets. In-situ X-ray diffraction performed during annealing evidences damages of the film above 510°C depending on the initial structure (or chemical composition) of the as-deposited HEA. Energy Dispersive Spectroscopy (EDS) and Scanning Electron Microscopy (SEM) analysis carried out before and after annealing on both studied samples, show that partial evaporation of the thin film, crystalline phase transformation and chemical reaction with the substrate may take place during annealing.
We have studied the deposition of AlCoCrCuFeNi high entropy alloy (HEA) thin films on Si (100) substrates by DC magnetron sputtering process. Three mosaic targets have been used for easily tailoring the film composition. Energy dispersive X-ray spectrometry analysis has shown that chemical composition can be modified around the nominal value by tuning the ratio of the powers applied to the magnetron targets. The deposition rate is directly related to the power sum. Moreover, various surface morphologies have been evidenced by scanning electron microscopy and correlated to the crystalline phases present in the films. Morphology and crystalline structure have been found to depend on the chemical composition. Wetting contact angle has been measured with water droplets, showing that the hydrophobic properties of the thin films depend on their characteristics.
Convection in a cavity with a free surface and heated from the side is studied by a combination of flow visualization and particle image velocimetry. In these experiments, buoyancy and thermocapillarity are of comparable importance in driving the convection. The Prandtl number of the working fluid, the cavity aspect ratio, and the ratio of Rayleigh to Marangoni numbers are all held fixed; the primary experimentally varied parameter is the imposed temperature difference, which varied from 0.3 to 20 °C, resulting in a range of Marangoni numbers between 6×103 and 4.2×105. For low Marangoni numbers, the flow is steady and two dimensional, as expected. The global nature of the flow is in good agreement with available numerical simulations of combined thermocapillary-buoyancy driven convection. At higher Marangoni number, Ma>1.5×105, we observe a transition to steady three-dimensional convection. The nature of this transition is typical of an imperfect bifurcation, and the flow structure is investigated both qualitatively and quantitatively. It is concluded that for the parameter values studied the first unstable mode consists of steady three-dimensional, approximately cubical, vortical structures that are periodic along the axis of the cavity. The three-dimensional flows observed by visualizations are in remarkable agreement with the recent numerical computations of Mundrane and Zebib [Phys. Fluids A 5, 810 (1993)].
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