Enhanced age hardening response and precipitation evolution of elastic stress aged Mg–Zn alloys

Abstract

How to improve the age hardening response of Mg alloys in economical and effective ways is an important and interesting issue. The age hardening response and precipitation evolution of Mg–Zn alloys with and without external elastic stress are investigated. Microstructural observations of stress-free aged samples show that the low hardness is mainly due to the low number density and relatively small aspect ratio of β1′ precipitates, while a large number of nano-scale strengthening β1′ precipitates appear in the α-Mg matrix at the early stage of stress aging. In addition, the aspect ratio of β1′ precipitates in the stress aged samples is evidently higher than that of the stress-free aged samples at the same peak aged stage. Abundant dislocations introduced by the applied stress act as nucleation sites of precipitates and rapid diffusion path of solutes, which are mainly responsible for the remarkable increment in hardness and promoted kinetics. Precipitate-free zones ( PFZs) are observed in both samples with and without stress. In the stress aged samples, the formation of PFZs is mainly controlled by the motion of dislocations, which is different from the solute-vacancy depletion mechanism in the stress-free samples. Results imply that the external stress plays a similar role to that of commonly used methods in reducing the size, increasing the density, and promoting the precipitation kinetics, but has a negligible stress-orienting effect on the precipitate evolution.