Formation and growth of precipitates in a Mg-7Gd-5Y-1Nd-2Zn-0.5Zr alloy aged at 200 °C

Abstract

Crystal structures, growth characteristics, and transformation of the precipitates in a Mg-7Gd-5Y-1Nd-2Zn-0.5Zr (wt.%) alloy aged at 200 °C for various durations were investigated using transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). A detailed Mg-Gd type precipitation sequence for Mg-Gd-Y-Nd-Zn alloys was proposed as follows: supersaturated solid solution → solute clusters → zigzag GP zones + β′′(I) → β′ → β′+ protrusions/joints → pre-β1 → β1 → β. Solute clusters formed in the early stage of aging consisted of one or more rare-earth (RE)/Zn-rich atomic columns with different configurations. RE/Zn-rich solute clusters grew into zigzag GP zones and β′′(I) as aging time extending. The paired-zigzag GP zones might grow up to be β′ precipitates directly. In the peak- and plat-aging stages, the number of solute clusters in the matrix decreased until they disappeared, and most existed as zigzag arrays and super hexagons. Protrusions formed at the end of β′ at an angle of 120°, then grew into joints when two different β′ variants encountered together. Protrusions/joints comprise zigzag arrays, super-hexagons, β′F, β′′(II), βT, and hybrid structures rich in solute atoms, and act as catalysts for the growth of the β′ variants. Larger β′ grow by joints consumption while smaller β′ precipitates dissolve to form joints. β1 precipitates essentially evolve from pre-β1 precipitates, with four-point diamond structures formed by RE/Zn atomic substitution and atomic migration based on the original α-Mg structure.