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

作者：Zhiwei Du^*; Yonggang Peng^*; Hang Teng; Zhong Cao; Kui Zhang; Minglong Ma; Xiaolei Han; Ting Li; Cong Che; Yongjun Li

来源：Journal of Magnesium and Alloys, 2023, 11(07): 2326-2339.

Crystal structures, growth characteristics, and transformation of the precipitates in a Mg-7Gd-5Y-1Nd-2Zn-0.5Zr(wt.%) alloy aged at200 ℃ 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 smallerβ precipitates dissolve to form joints. β1precipitates essentially evolve from pre-β1precipitates, with four-point diamond structures formed by RE/Zn atomic substitution and atomic migration based on the original α-Mg structure.