Evolution and Growth Kinetics of θ Precipitates in Naturally Aged MgLiAlZn Alloy Studied by In Situ Small-Angle X-ray Scattering

Abstract

The MgLiAlZn (LAZ1110) magnesium alloy was studied by in situ small-angle X-ray scattering (SAXS) to reveal the evolution of MgLi2Al θ precipitates (ppts) in the early aging stage at room temperature (RT). DSC and XRD measurements reveal θ ppts in naturally aged alloy. During 0 to 22 hours of early aging at RT, the growth and coarsening of plate-like θ nano-ppts are quantitatively resolved using SAXS in terms of the temporal evolution of the radius, thickness, and relative volume fraction. In this early aging period, the radius of θ ppts grows gradually from 3.1 to 6.9 nm with a nearly constant thickness of 3.7 nm. The relative volume fraction of θ ppts increases rapidly in the early aging stage and then more slowly in the peak aging stage at ~ 17 hours. The growth behavior of θ ppts after the peak aging stage follows the coarsening kinetics and causes hardness softening. It is supposed that the corresponding increase and decrease in hardness can be attributed to the coherency strengthening and coherency loss of the θ ppts, respectively.