LAUSR

The effects of high undercooling and a large cooling rate can be achieved by the use of a containerless drop tube technique, which is conducive to rapid solidification and formation of a metastable phase. Here, the rapid solidification of Fe 78 Si 13 B 9 (S 1 ) and Fe 78 Si 9 B 13 (S 2 ) alloys was completed under microgravity condition. Based on theoretical calculations, a maximum undercooling of 433 K (0.29 T L ) and 412 K (0.28 T L ) was obtained, respectively. The microstructure evolution and the formation of an amorphous-nanocrystalline structure for the two alloys were compared and analyzed. The results show that S 2 alloy has better amorphous forming ability and higher hardness. During the solidification of S 1 alloy, the primary phase α-Fe grows by the manner of dendrites, and the secondary dendrite arm spacing decreases exponentially with increased undercooling. An amorphous-nanocrystalline structure is developed when the undercooling is increased up to 388 K; S 2 alloy forms an amorphous-nanocrystalline structure at an undercooling of 275 K and is completely amorphized after exceeding an undercooling of 402 K. In addition, the hardness and elastic modulus are acquired by nanoindentation technology under different degrees of undercooling. The phase constitution, morphology, distribution, and grain refinement of the alloys have important effects on the micromechanical properties of these alloys.