Abstract
Hardness measurements and electron microscopic observations were made to study factors affecting the aging of Mg–Zn alloy and Mg–Zn–(Ag) alloys. In regard to the homogeneous and heterogeneous nucleation in these alloys, direct aging, two-step aging and strain aging were carried out, with the results summarized as follows:
(1) The aging sequences are distinguished by the intersection temperature in the C-curve diagram. Below about 100°C, the nucleation rate largely depends on the quenching rate, the concentration of zinc and the small amount of silver (0.04wt%).
(2) In the C-curves which were obtained by plotting the incubation time against the reciprocal of the absolute aging temperature, the nose or the lower part of it gives an optimum condition for the pre-aging. As regards the direct aging and the two-step aging, silver atoms positively act on the nucleation of the pre-β′ or the β′ phase.
(3) The rates of precipitation of the β′ and β phases are accelerated by cold working before aging. Electron microscopic observations show that these phases preferentially precipitate on dislocations and fine structures such as twin boundary and sub-boundary.
