The diffusion coefficient of Bi in dilute liquid alloys of Bi in Sn
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E. D. GIBSON,
AND
In analyzing solidification experiments on Sn-Bi alloys it became necessary to know the diffusion coefficient of Bi in liquid Sn-Bi alloys at the liquidus temperature. A literature search revealed two studies 1'~on dilute liquid alloys of Bi in Sn. The work of Niwa et al " extended from 450 ~ to 600~ and of Buell and Shuckz from 300 ~ to 400~ Unfortunately, when extrapolated to the liquidus temperatures of interest (~ 230~ the two studies gave diffusion coefficients which were different by over a factor of two. Consequently, the present experiments were carried out to provide diffusion coefficient data for this system at the temperature of interest. The conventional capillary-reservoir technique 3 was used. The capillaries were Pyrex tubes ranging in length from 57 to 67 mm with an inside diam of 1 mm and a wall thickness of 0.125 ram. The liquid metal baths were contained in two graphite crucibles of length 10 cm and inside diam 2.2 cm. One bath was pure tin and the other tin plus 10 ppm of Bia~ This bismuth isotope is a y-emitter with a 30 yr half life. Each run involved a total of six capillaries, three to measure the activity of the radioactive bath and three to measure the diffusion process. Both sets of three capillaries were held vertically with the open ends down. The baths were f i r s t outgassed under vacuum at 700"C and hydrogen admitted to remove oxide from the bath surfaces. The baths were then outgassed and the temperature was lowered. With the system under vacuum one set of capillaries was lowered into each bath and then filled by introducing one atmosphere hydrogen. The set of capillaries in the isotope bath was now removed to the upper cool part of the furnace and the system was held for one half hour to insure thermal equilibrium. The set of capillaries in the pure tin bath was then carefully transferred to the radioactive bath and diffusion of Bi20' into these capillaries was allowed to occur for around 24 h. The temperature of the radioactive bath was monitored continuously on a stripchart recorder using a tantalum sheathed ChromelA1umel thermocouple inserted into the bath. The baths were heated with a three zone furnace which had been adjusted to insure a small positive temperature gradient in the vertical direction to eliminate thermal convection. The control system held the temperature to within • I~C. J. D. VERHOEVEN and E. D. GIBSON are Senior Metallurgist and Assistant Metallurgist, respectively, Ames Laboratory-ERDA and M. B. BEARDSLEY is Student, all at Iowa State University, Ames, Iowa 50010. Manuscript submitted December 4, ! 974.
METALLURGICAL TRANSACTIONS B
Fig. 1--Plot of experimental data as natural log D vs reciprocal temperature. Data of Refs. 1 and 2 included as shown. Solid line is least squares fit to authors' data. After removing the glass capillaries the samples were weighed and then counted in a well-type scintill a t i o n c o u n t e r b y m e a s u r i n g t h e t i m e to a c h i e v e 100,000 c o u n t s . T h e b a t h a c t i v i t y w a s t a
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