科研之友
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摘要
The magnesium-gadolinium (Mg-Gd) system provides an ideal model alloy for precipitation hardening. Evolution of precipitates in a Mg-16.2Gd-0.4Zr (wt.%) alloy during isothermal ageing from 150 degrees C to 300 degrees C was systemically studied using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The widely accepted four-stage precipitation sequence of the Mg-Gd(-Zr) alloy system has been accordingly updated to: super-saturated solid solution (S.S.S.S.) -> ordered solute clusters -> GP zones -> beta' -> beta(F)' + tail-like hybrid structures -> beta(1) -> beta. In addition to carefully revealing an updated precipitation sequence, new insights into the relationships amongst different precipitate phases were obtained and rationalised. (1) Solute clusters and GP zones form during the early stages of ageing and share the same structural unit; (2) Formation of beta' phase is accomplished via ordered stacking of zigzag GP zones along < 10 (1) over bar0 >(x): (3) Coarsening of beta' phase promotes the formation of beta(F)' phase and tail-like hybrid structures; (4) beta(F)' phase facilitates the nucleation of beta(1) phase; and (5) beta(1) and beta phases maintain a low-energy Sigma 9 interface and beta(1) phase finally transforms to equilibrium beta phase in situ. The precipitate evolution at the grain boundaries during ageing were also investigated and grain boundary precipitate free zones were observed in Mg-16.2Gd-0.4Zr alloy aged at 250 degrees C or above.
