Experiments were carried out to investigate the influence of third addition elements such as Li, Be, Mg, Ca, Ti, V, Co, Ni, Ge, Mo, Cd, In, Sn, Sb, Pb, and Bi on the formation of G.P.zones in Al-2.5 at%Zn alloys.
The results are as follows:
(1) Ca, V, Mo, Ni, Co and Ti amounting to less than 0.2 at% do not interact with G.P. zones of Zn atoms, but decrease the rate of formation of the G.P.zones. This effect is similar to that of Fe, Cr, Mn or Zr, and it can be reasonably explained in terms of the increase in density of dislocations and in boundary area by the grain refining and the existence of these insoluble compounds which could act as effective sinks to cause the decrease in concentration of quenched-in vacancies.
(2) Sn(≤0.1 at%) and Ge(≤0.2 at%) also do not interact with G.P.zones of Zn atoms, but these solute atoms, especially Sn decrease remarkably the rate of formation of the G.P. zones. This effect is similar to that of Si, and it could be interpreted from the fact that the binding energy between the solute atoms and vacancies is greater than that between Zn atom and vacancy.
(3) Be, Mg, Cd and In decrease the rate of formation of the G.P.zones in an equal degree to or more than Ge. It is clear that Mg and Be atoms in the amount of more than 0.03 at% interact with Zn atoms and participate in the G.P. zones. Moreover, from the results of age-hardening for a long period, it seems that Cd atoms more than 0.03 at% also have interaction with G.P.zones of Zn atoms. Thus, the rate of clustering will be affected by the mobility of solute additions/Zn/vacancy groups as well as the magnitude of the binding energy between the solute atoms and vacancies.
(4) Pb and Li decrease slightly the rate of formation of the G.P. zones. This may be mainly due to a little larger binding energy between Pb or Li atom and vacancy than that between Zn atom and vacancy, although the clustering of Li atoms themselves and the interaction between Li clusters and Zn atoms must also be taken into consideration. From this experiment, it is not clear whether Sb and Bi have influence on the formation of the G.P. zones.
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