中国空间站难熔Nb-Si和Nb-Si-Zr合金凝固组织形成机理研究

This study reports successful solidification experiments of Nb-Si and Nb-Si-Zr alloys conducted aboard China Space Station, investigating the microstructure formation mechanisms under microgravity with suppressed natural convection. The research unveils the metastable liquid density, solidification surface shrinkage morphology, and microstructural evolution of binary Nb_84.1Si_15.9 hypoeutectic, Nb_82.7Si_17.3 eutectic, and ternary Nb₇₂Si₂₂Zr₆ hypereutectic alloys. The results indicate that shrinkage cavities in both hypoeutectic and eutectic alloys form at junctions of multiple solid regions centered on nucleation sites. However, at junctions of three or more solid regions, the hypoeutectic alloy displays multiple symmetrically distributed independent (Nb) dendrites. The hypereutectic alloy surface shows uniformly distributed micro-shrinkage cavities. The (Nb) dendrites on hypoeutectic and eutectic surfaces display both two-dimensional symmetric growth patterns and three-dimensional oriented growth characteristics, while Nb₃Si/Nb₅Si₃ dendrites on the hypereutectic surface show columnar and flaky characteristics. A distinctive decoupling growth phenomenon of (Nb)/Nb₃Si eutectic was observed in hypoeutectic and eutectic alloys. Regarding eutectic structures: hypoeutectic Nb_84.1Si_15.9 alloys show alternating lamellar and rod-like eutectic morphologies at low undercoolings, transitioning to worm-like, lamellar, and faceted growth patterns at high undercoolings; eutectic Nb_82.7Si_17.3 alloys exhibit radial growth from nucleation sites at low undercoolings; hypereutectic Nb₇₂Si₂₂Zr₆ alloys display faceted and lamellar eutectic structures at high undercoolings. Theoretical calculations indicate that the variation of nucleation rate with undercooling is crucial for competitive phase nucleation, while the higher nucleation rate and ultrafast growth velocity of Nb₃Si/Nb₅Si₃ dendrites under microgravity lead to unique microstructures distinct from terrestrial solidification.