UIFFLSIU-N l-\ .METALS ] 2"^ 



Ihc eonceniration considorcd in the dilfusiou process^. 'Jlic silver-lead 

 and gold-lead systems provide examples of the behaviour just referred to. 

 ►Starting with a coneentrated silver-lead alloy, in which the silver is as 

 finely divided as possible, then the silver which has migrated by diffusion 

 will constantly be replaced by dissolution from the grains of silver. The 

 system therefore consists of a constantly saturated solution of silver in 

 lead from which silver diffuses into pure lead. As will be seen later, ihe 

 solubility of silver in lead can l)e determined 1)y means of this behaviour. 



In only a few cases have wv us(hI an alternative to the spectroscopic; 

 method. The silver content of lead alloys has been determined, for 

 example, by Ihe usual method of volatilizing the l(>ud and assaying Ihe 

 residual lead of silver. It was also necessary to employ radioactive 

 methods in order to determine the self-diffusion rate. At higher temper- 

 atures, where the selfdiffusion is already somewhat larger, the diffusion 

 constant could be determined as follows : A radioactive lead isotope is 

 condensed on the surface of inactive lead and the ionization due to 1he 

 a-])articles emitted by the radioactive lead is measured. The system is 

 then heated to the experimental temperature and the ionization, which 

 now has a lower value owing to the diffusion which has taken place, is 

 measured again. The self-diffusion constant can be determined from this 

 decrease due to diffusion. At a lower temperature this method, which 

 indeed is very sensitive, proved not sensitive enough and had to be 

 replaced by another. The recoil yield before and after diffusion was 

 measured and the diffusion constant was calculated from the decrease 

 by means of a formula developed by R. Furth-. While the first method 

 enables the diffusion constant to be determined down to 10~'^ cm^/day. 

 the use of the second method permits diffusion constants of 10-^^ cm^ day 

 or less to be obtained. 



The solubility of a metal in a solid phase can be determined from th(> 

 analysis of diffusion since only those particles of the metal which are 

 distributed as atoms are involved in the diffusion process. The dis- 

 fussion is best illustrated by an example. 



(a) Starting with a concentrated alloy the diffusion is allowed to 

 proceed until all parts of the originally pure layer of lead are saturated 

 with the diffusing metal. Since an increase in the diffusion time is no 

 longer accompanied by an increase in concentration there now exists a 

 saturated solution the analysis of which yields the solubility directly. 

 At 288 °C', for example, it is impossible to produce by diffusion a silver- 

 lead alloy containing more than 0.13 atomic per cent of silver. 



1 ff. G. Grube (Z. Metallk. 19, 438 [1927]), who has (Iclcrminod a scmmp.s of 

 diffusion velocities in high-melting metals. With regard to the problem of diffusion 

 in alloys refer also to the many puV)lieations of G. Tammann and his scliool. 



2 R. FtJRTH, Hamlbuch dcr pliy.sikal. and techn. Mechanik 7, <)S7 (1930). 



