Formation and reversion of Guinier-Preston zones in Al-5.3 at.% Zn

Abstract

Formation, reversion, and reformation of GP zones in Al-5.3 at.% Zn were studied through measurement of electrical resistance. Young's modulus, critical resolved shear stress, and small-angle scattering of X-rays. A definite zone radius of approximately 9 Å, a critical size for scattering of conduction electrons, is associated with the resistance maximum or plateau which is observed during aging. This size is independent of aging temperature or whether the zones form on aging or reaging after reversion. After a long aging treatment, about 1 4 of the Zn atoms present are associated with zones; after a long reaging treatment, only about 1 4 are associated with zones. The intensity maximum in the small angle X-ray scattering curve early in the aging process is apparently due to a shell depleted in Zn about each zone. It is due to interference between zones late in the process. Early in the process, the zones are more uniform in size than later on; coarsening occurs after an initial depletion of the matrix. There is no incubation time for aging or reaging. The kinetics of formation of zones on aging and reaging depend upon vacancy concentration and supersaturation. These vary with aging temperature, time, and previous treatment. In the absence of a large excess in vacancies (this is the situation during reaging or late in the direct aging process) the relation of the composition of the solid solution about the zone to the spinodal composition is particularly important and the rate of zone formation decreases with an increase in aging temperature. In the initial stages, when there are large numbers of vacancies, the rate increases with an increase in aging temperature.