The influence of microgravity on the solidification of Zn-Bi immiscible alloys
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1. I N T R O D U C T I O N C O M P O S I T E materials are often applied in different situations where unique properties are required. Composite materials consist of two or more materials of different types in a state of physical mixture. One important factor in determining the properties of the material is the distribution and size of the second phase in the matrix. The distribution of the second phase should be as homogenous as possible. In a molten matrix this is normally hindered due to sedimentation, flotation or segregation. It will be expected that a fine homogenous dispersion can be formed under microgravity from a system which shows a miscibility gap and there are several systems which demonstrate a miscibility gap. Amongst the metallic systems are such alloys as A1-Pb, Zn-Pb and Zn-Bi. A few experiments have already been performed under microgravity in systems with a miscibility gap. Ang and Lacy I made a few experiments with Zn-Pb alloys in the STP-flight. Gelles and Markworth 2 as well as Ahlborn and L6hberg 3 have made some experiments in the SPAR rocket program. All the experiments show a strong separation of the two liquid phases and also a coarse microstructure. These results were unexpected and one has tried to explain the observations as being due to a large fluid flow caused by Marangoni convection or due to other surface tension phenomena and there seems to be a need for a lot of more experiments in order to understand and to explain the observations. In the Swedish part of the T E X U S II program a series of experiments with different compositions and different cooling rates have been performed in Zn-Bi alloys. The results will be shown and discussed in this report.
T. CARLBERG, now with M.I.T., Cambridge, MA, and H. FREDRIKSSON, Professor, are with the Department of Casting of Metals, Royal Institute of Technology, Stockholm, Sweden. Manuscript submitted September 27, 1979.
2. E X P E R I M E N T A L T E C H N I Q U E S The phase diagram Zn-Bi is shown in Fig. 1 according to Hultgren et al. 4 The figure shows that there is a miscibility gap with a maximum point at a temperature of 873 K and at a molfraction of 0.17 (38 wt pct BI). There is a monotectic point at a temperature of 689 K and at a molfraction of 0.005 (1.9 pct Bi). All samples investigated had a composition between the maximum point and the monotectic point. In space three alloys with a molfraction of roughly 0.02, 0.09, and 0.17 pct were examined. Two experiments with different cooling rates were performed with the 8 pct alloy and one experiment with each one of the other two alloys. The experiments were performed in four mirror furnaces. The furnaces are described in Ref. 5. The samples had a cylindrical shape with a diameter of 6 m m and a length of 5 ram. A thermocouple of the Chromel-Alumel type was placed in the center of each sample. The samples were melted and heated to a temperature above the miscibility gap. For all the three alloys the lamps only were switched off, and the samples cooled natural. Figure 2 shows the ti
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