A Study of the Coalescence Process Inside the Miscibility Gap in Zn-Bi Alloys
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A STUDY OF THE COALESCENCE PROCESS INSIDE THE MISCIBILITY GAP IN ZN-BI ALLOYS .. BERGMAN and H. FREDRIKSSON The Royal Institute of Technology,
Stockholm,
Sweden
ABSTRACT The coalescence process has been studied in four different hyper-monotectic Zn-Bi alloys with 4-10wt % Bi . The alloys have been studied by isothermal treatments at a temperature just above the monotectic temperature and at different holding times. The alloys have been studied both at microgravity and at normal gravity. The change of the particle distribution as a function of composition and holding time has been investigated. The changing of the particle distribution has been compared with the theory of particle collision. The validity of the collision theory is investigated with respect to the theory of liquid phase sintering. INTRODUCTION It is expected that liquid-phase immiscible alloys solidified in the low-gravity environment of space should exhibit a fine homogeneous structure consisting of a dispersion of the minority phase in the matrix. Ang and Lacy (1) as well as Gelles and Markworth (2) and Ahlborn and Lbhberg (3) made some experiments of this type in rocket flights. In all the experiments a strong separation of the phases and a coarse microstructure was found. These results were unexpected and most of the proposed explanations include Marangoni convection or other surface tension phenomena. Carlberg and Fredriksson (4) used Zn-Bi alloys in a series of experiment in the Swedish part of the TEXUS II rocket programme. Some of these samples showed a more homogenous structure but not as fine as would be expected from diffusional growth alone. They suggested that some kind of collisi~n effect must have played a decisive part during the precipitation. In order to investigate whether the coarsening of the structure could be explained with the theory of particle collision, a series of experiment have been performed in the Swedish part of the TEXUS III rocket programme. Quenched samples were treated isotermally at a temperature slightly above the monotectic temperature. The results will be shown and discussed in this report. EXPERIMENTAL TECHNIQUES The phase diagram ZO-Bi is shown in Fig. 1 according to Hultgren et al. (5). The figure shows that there is a miscibility gap with a maximum point at a temperature of 373K and at a molfraction of 0.17 (38wt pct Bi). There is monotectic point at a temperature of 639K and at a mrolfraction of 0.005 (1.9 pct Bi). All samples investigated had a composition between the maximum point and the mo-
564 notectic point. In space six samples with a molfraction of Bi in the range 0.013 to 0.034 (4-10 wt pct Bi) were processed in mvirror furnaces. in reference 6. Some elThe furnaces are principally described the control ectronical changes were however done to facilitate of eonstanttemperature. The samples had a cylindrical shape with a diameter of 6 mm and a length of 12 mm. A type K termocouple with O.D.0.5 mm was placed in the center of each sample. The samples were melted and heated to a temperature j
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