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

Highlights: • Advanced aberration-corrected TEM has been applied to investigate the precipitation in Mg-Gd binary alloy. • The growth of β^′ precipitates at the early stage follows y (nm) = 1.53x − 2.68 (nm). • The long-range diffusion of Gd solutes prefers to occur along $10\overline{1}0$>_Mg directions and protrusions are formed. 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 β_H, β^′, β_T′, β_M 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 $10\overline{1}0$>_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.