

EUTECTIG RESEARCH: THE ALLOYS OF LEAD AND TIN. 99 



however, only occur by the slow process of solid diffusion, and therefore in most cases 

 the solid formed will consist of layers of solid solution whose concentration varies 

 more or less continuously, increasing from the centre outwards. If the formation of 

 such layers has taken place, and time has not been allowed for diffusion to obliterate 

 them, the whole of the alloy will not be solid when the point q is reached, since the 

 average composition of the solid formed up to that time will be lower in its content ot 

 B than the alloy as a whole ; the remaining liquid portion will therefore contain more 

 than n per cent, of B, and as the alloy cools further, fresh layers of solid solution will 

 be formed containing more than n per cent, of B. This will continue until the 

 remaining liquid attains the concentration represented by b, but as soon as this 

 concentration is exceeded, the remaining liquid will solidify as eutectic. This will 

 only happen if the rate of cooling is rapid, or if the rate of diffusion is very slow, and 

 the concentration (n) of the alloy not far below the saturation point of the solid 

 solution. Once, however, that a small proportion of eutectic is formed in this way, 

 the meta-stable equilibrium thus established can only be abolished by the process of 

 diffusion through successive solid layers, and it is reasonable to suppose that this 

 diffusion would be still further retarded by the fact that in the formation of the 

 eutectic some of the B constituent has been mechanically separated ; diffusion thus 

 has a larger task to perform in overcoming this separation as well as in obliterating 

 the concentration gradient existing in the mass of the solid solution itself. 



On the basis of this consideration it is possible to account for the divergence 

 between the results of RoBERTS-AuSTEN and those given here. In the experiments 

 of ROBERTS- AUSTEN the rate of cooling was probably such as to allow of the formation 

 of layers of solid solution differing widely in concentration, with the result that alloys 

 lying in reality well within the limits of solid solubility showed the presence of small 

 quantities of eutectic. This view is confirmed by the authors' observations on the 

 effect of different rates of cooling on the indications of the cooling-curves. With 

 moderately rapid cooling the results of ROBERTS- AUSTEN as to the presence of eutectic 

 in alloys very near the lead end of the series have been confirmed. With more 

 moderate rates of cooling, such as that employed in taking the curves of Series A and 

 B, the appearance of the eutectic is postponed to concentrations near 10 per cent, of 

 tin, while in the still more slowly cooled series B and C no eutectic becomes apparent 

 until much higher concentrations are attained. 



In consequence of these observations it appears necessary, in order to ascertain the 

 limit of solubility with accuracy, to expose the alloys to such thermal treatment as 

 will facilitate the attainment of complete equilibrium at a temperature just below the 

 "solidus." Prolonged exposure to a temperature of 175 C. was chosen for this 

 purpose, since this temperature lies safely below the " solidus," even in alloys 

 containing eutectic, while it lies well above the temperature of the transformation 

 already referred to. The exposure to this temperature was carried out in electrically 

 heated ovens whose temperature could be accurately regulated, and in some cases the 



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