Containerless solidification of Ag-Al and Ag-Cu eutectic alloys in a drop tube

Drop tube technique has been used to containerlessly solidify Ag-28 wt.%Al and Ag-28 wt.%Cu eutectic alloys. Inductively melted sample was forced by Ar pressure of 2 bar through the small bore at the lower end of a quartz tube and a thin liquid jet is formed that disperses into small droplets of different size owing to the Rayleigh instability. The droplets cool, undercool and solidify during free fall in time of 1 s under the conditions of reduced gravity in He atmosphere. The cooling rates at the solidification temperature are computed as a function of droplet size by heat balance calculations. They are ranging between 0.5 × 103 and 3.0 × 104 K/s for droplets in size range of 100-900 μm diameters. The cooling rate at the solidification temperature decreases with the increase of droplet size. Nusselt, Reynolds and Prandtl numbers of the droplets were calculated. Microstructures of the alloys were analyzed and eutectic spacings are determined. Microhardness of the droplets with different sizes was measured revealing a dependence on the cooling rate with which the droplets are solidified.