Part V – May 1968 – Communications - Discussion of "Diffusion of Palladium, Silver, Cadmium, Indium, and Tin in Aluminum"*

Anand and Agarwala recently determined diffusion coefficients by the residual activity (Gruzin) method for several tracers in large-grained, high-purity aluminum. D values at any particular temperature in the range of 400" to 630°C were about four orders of magnitude lower for cd115, 1n114, sn113, and pdlm than for Ag. Corresponding values for Do and Q in the Arrhenius equation were also quite small. It was pointed out previously that unusually low diffusion coefficients within the region of a specimen near a free surface may be the major cause of such anomalous results for impurity diffusion in silver and in aluminum.44 Anand and Agarwala considered this possibility but rejected it because their penetration profiles were "homogeneous" and "yielded diffusion coefficients obeying the Arrhenius plot". Neither of these two criteria satisfactorily prove or disprove the presence of a near-surface effect. Such can be accomplished only by direct serial sectioning in which the chips ground from the specimen are assayed for tracer content. In fact, such experiments always should be done prior to use of the somewhat indirect Gruzin method where the finer details of the transport process cannot be determined. There have been several other reports of unusually low D, Do, and Q values for impurity diffusion in silver and aluminum.1"4 It is interesting to note that all of these studies made use of the Gruzin method. Also, in all cases where the same systems were examined by use of a direct serial sectioning technique, large near-surface effects were observed. One must conclude therefore that the reported anomalous diffusion parameters for cd, In114, sn113, and pd in aluminum are due to some near-surface effect, while the results for Agl10 indicate that no such effect was encountered for this system.