Kirkendall Diffusion Research Articles

Activity-Coefficient and Vacancy Flow Effects on Tracer Diffusion Coefficients in Silver-Gold Alloys

In work reported previously, measurements of silver and gold tracer activities were made in silver-gold-alloy Kirkendall diffusion couples in which the tracers were originally placed in the interface. Semilogarithmic plots of these data were straight lines, but the coefficients measured were not self-diffusion coefficients because of the effects of the composition gradient. These straight-line graphs are now explained, and an approximate equation for the diffusion coefficients is derived from the theory reported earlier. Some of the measured values disagree with the calculation and the disagreement is outside of the estimated errors.

Structural changes in single crystal copper-alpha brass diffusion couples

Structural changes associated with Kirkendall diffusion in single crystal copper-alpha-brass couples have been studied. Vapour—solid couples in which zinc was diffused into copper from the vapour were investigated using metallographic and X-ray techniques. The following effects were found under certain conditions in the diffusion zone: 1. (1) dislocation formation; 2. (2) arrangement of dislocations into subboundaries; 3. (3) recrystallization and formation of new grains; 4. (4) twin formation. The dislocation density and fineness of substructure were greatest at the lowest diffusion temperatures. Re-crystallization was found at low diffusion temperatures, and twin formation was always associated with recrystallization. An explanation of these phenomena is given in terms of the production and subsequent redistribution of dislocations by climb and slip mechanisms during diffusion.

Growth of voids in metals during diffusion and creep

A study has been made of the thermodynamic conditions under which voids may grow from suitable nuclei in a metal by a mechanism of vacancy aggregation under the influence of stress and vacancy supersaturation. The results are applied to the intergranular void formation observed during creep experiments and also void formation during Kirkendall diffusion. In the case of creep it is postulated that nuclei may grow into voids by receiving stress-motivated vacancy currents from nearby grain boundary sources. In the case of Kirkendall diffusion it is first shown that the development of stress in the diffusion zone depends directly upon the existence of a vacancy supersaturation. The roles of vacancy supersaturation and stress in producing voids are then discussed individually. It is concluded that there is sufficient vacancy supersaturation present to produce voids within grains if nuclei of reasonable size are present. The effect of stress in the absence of large stress concentrations is probably to increase the equilibrium vacancy concentrations at sinks within the grains thereby diverting extra vacancies into voids. Void formation at grain boundaries, where the vacancies are maintained near equilibrium, is explained as a result of stress.