Interdiffusion of Sn and Pb in Liquid Pb-Sn Alloys

摘要

Coefficients for the interdiffusion of Sn in Pb-richalloys and Pb in Sn-rich alloys were established using 1.5-mm-diameter capillaries and the semi-infinite rod technique.Interdiffusion coefficients are presented for the entireconcentration range from pure Pb to pure Sn, for temperaturesfrom 668 to 1031 K. The concentration dependence of theinterdiffusion coefficients was determined by establishing theconcentration along the length of the capillaries and calculatingthe coefficients using a finite-difference technique. Theinterdiffusion of Sn in Pb, extrapolated to 0 at. pct Sn, is given byD valence 8.8 X 10~(-8) exp - (22, 600/RT) m~2/sand that for Pb in Sn, extrapolated to 0 at. pct Pb, byD valence 2.4 X l0~(-8) exp - (19, 300/RT) m~2/sThe "average" value for the interdiffusion of Sn in Pb, for theconcentration range from 0 to 74 at. pct Sn, is given byD valence 1.1 X 10~ (-7) exp - (25, 200/RT) m~2/sand the average value for the interdiffusion of Pb in Sn, for theconcentration range from 0 to 26 at. pct Pb, is given byD valence 1.3 X 10~ (-8)exp - (22, 600/RT) m~2/sThe values obtained for the coefficients agree reasonably wellwith previous results for the diffusion of Sn in Pb-rich alloysand are consistent with solvent self-diffusion coefficients forpure Pb and pure Sn. However, while the diffusion coefficientsobtained from these Arrhenius equations are likely of the rightorder of magnitude, it is concluded that the results are affectedby fluid flow in the capillaries, resulting in higher than actualactivation energies. It is suggested that, for the capillary-reservoir technique, convective flow in the reservoir across theopen end of the capillaries induces "lid-driven" flow in theupper portions of the capillaries, resulting in higher than actualdiffusion coefficients, particularly for the Sn-rich alloys, sincethe Sn-rich end of the capillaries was open to the reservoir.Because of fluid motion induced in the capillaries, all of theresults for solute and self-diffusion in Pb, both present andprevious, are likely erroneous because they were obtained usingthe capillary-reservoir technique.Some previous results for solvent self-diffusion in liquid Snwere obtained using either the thin disk or the semi-infinite rodtechnique and, since these results agree with results obtained inmicrogravity, it is concluded that the nonreservoir methods mayprovide a means of obtaining more accurate liquid diffusion data.