Nano-scale precipitation and phase growth in Mg-Gd binary alloy: An atomic-scale investigation using HAADF-STEM

Abstract

In the present study, nanoscale precipitation and phase growth in Mg-15Gd(wt%) binary alloy are comprehensively studied by atomic-resolution high angle annular dark field - scanning transmission electron microscopy (HAADF-STEM). Various nanoscale precipitate structures, including βH, β′, βT′, βM and β′′-like structure, assembled by the “basic unit” as building block, are observed, reconstructed and discussed in detail. In the early stage of precipitation, the precipitate growth basically follows y (nm)=1.53x−2.68 (nm) and protrusions start to grow when the size of a precipitate reaches a threshold value. Long-range diffusion of Gd solute preferentially proceeds along <101¯0>Mg directions and protrusions are formed. A large number of fine β′ precipitates exist and act as the key strengthening structures. The formation of precipitate-free zones in the over-age condition is attributed to the intertwining of precipitates.