Microstructure characterization of the directionally solidified Nb-Mo-Si eutectic alloys

The Nb-Mo-Si eutectic alloys were prepared using Optical Floating Zone (OFZ) directional solidification method. The effects of rod rotation speed and withdrawal rate on the eutectic morphology were examined. Results show that the rotation speed of seed rod has an obvious effect on structure morphology, while the rotation speed of starting sample rod has unobvious effect. When the withdrawal rate is 5 mm/h, and the seed rod doesn't rotate, a regular Nb _ss-β(Nb, Mo) ₅Si ₃ lamellar structure can be obtained. When the seed rod rotates, some spherical particles and regular lamellar structure firstly appear in the center area of the sample, and then expand outwards along radius direction. Afterwards, the regular lamellar structure is gradually transferred into irregular and broken lamellar structure. When the withdrawal rate is between 3 and 5 mm/h, it is regular lamellar structure. At 10 mm/h, columnar and cellular structures co-exist. At 1 mm/h, the lamellar structure is degenerated and Nb _ss tends to form a net-work structure. The variety of microstructure was discussed in terms of liquid/solid interface characterization and undercooling.