Vacancy-solute Complexes Research Articles

Effect of titanium additions on the aging characteristics of an al-zn-mg alloy

A minute titanium addition (0.04 pct) was added to an Al-Zn-Mg alloy in order to determine the effect of titanium on microstructure and aging characteristics of this alloy. Titanium was found to retard the kinetics of precipitation and to prevent solute segregation to grain boundaries on air quenching. Below G.P. solvus temperature, hardening of the alloy was due both to dislocation loop formation and to G.P. zone formation. Above G.P. solvus temperature, hardening was due only to formation of precipitate. These results have been explained in terms of titanium reducing the likelihood of the formation of solute-vacancy complexes, causing a reduction in zinc and magnesium mobility.

A model for the growth of Guinier-Preston zones-the vacancy pump

Vacancies highly in excess of equilibrium are thought to be responsible for the high rates of formation of Guinier-Preston zones (G.P. zones) after the quench from high temperatures. A kinetic model is developed for the growth of spherical G.P. zones based on the diffusion of solute-vacancy complexes to the zones. Under certain favorable physical conditions, when the complex arrives at the zone-matrix interface, the solute and vacancy are no longer bound; and the vacancy can now contribute to enhancing a vacancy concentration gradient directed radially outward from the zone. The vacancies are thus driven back into the matrix to again collide with solute atoms, once more becoming complexes. In this way the zone acts as a vacancy pump, maintaining a high vacancy concentration for long periods of time. Mathematical equations are derived based on this model, and kinetic data for Al-rich Zn is considered. A rather good comparison with experiment is found for early in the zone-forming process.

Self-diffusion of lead, thallium and bismuth in the solid lead-thallium system

Results are reported for the self-diffusion of both lead and thallium over the whole range of solid solution for these elements (0–87 atomic per cent T1) for the temperature interval, 206−323°C. Data for the trace impurity diffusion of bismuth over the same concentration range are also given for a more limited temperature range. All three elements show the same qualitative dependence of their diffusion coefficient upon alloy composition: namely, a slight maximum at about 35 atomic per cent thallium and a deep minimum at about 75 atomic per cent thallium at all temperatures. The results are compared with the predictions of the corresponding states principle and with theories based upon the Johnson vacancy-solute complex mechanism. A correction applied to the corresponding tates principle, based upon lattice parameter deviations fromVegard's Law, leads to agreement with observation. Some evidence for the existence of a superlattice at alloy compositions in the neighborhood of PbTl 7 is inferred from trends in the ratios D Tl / D Pb and D Bi / D Pb with thallium concentration.