Rapid solidification mechanism of Fe-Al-Nb alloy droplet and its influence on microhardness under microgravity condition

High temperature Fe-Al-Nb alloys will be prospectively applied to the industrial field, i.e., aviation, gas turbine, etc. In this paper, rapid solidification of Fe67.5Al22.8Nb9.7 ternary alloy under microgravity condition is realized by using drop tube containerless processing technique. Our purpose is to investigate the microstructural transition pattern and relevant micromechanical properties, and then to reveal the influence of rapid eutectic growth on application performance. The sample of 2 g is placed in a quartz tube with an orifice at the bottom, and the quartz tube is then placed at the top of 3 m drop tube. The sample is inductively melted and further superheated to a certain temperature with the protecting mixture gas composed of argon and helium. The alloy melt is ejected through the orifice by an argon gas flow and dispersed into fine droplets. The droplets are undercooled and finally rapidly solidified during their free fall in the drop tube. The alloy droplets with the diameter sizes ranging from 40 to 1000 m are achieved. The liquidus temperature of the alloy is 1663 K. The microstructure of the alloy consists of Nb(Fe, Al)2 and (Fe) phases. In the master alloy prepared by arc melting, the segregation along the gravity direction takes place because of the difference in cooling rate inside the master alloy. By comparison, the microstructures of the alloy droplets are homogeneous. The variations of thermodynamical parameters with droplet size are analyzed. As droplet diameter decreases, its Nusselt and Reynolds numbers rise from 3 to 8 and from 5 to 137, respectively, its undercooling and cooling rate increase from 50 to 216 K and from 1.23103 to 5.53105 K s-1 respectively. This causes the corresponding microstructural transition. A small amount of primary Nb(Fe, Al)2 phase transforms from dendrite to equiaxed grain, the lamellar eutectic is replaced by the fragmented eutectic. The relationship between eutectic interlamellar spacing and undercooling satisfies an exponential equation, indicating that the eutectic is refined by three times. Consequently, mainly owing to the eutectic refinement, the microhardness of the alloy increases by 10% with the increase of undercooling according to the Hall-Petch behavior in terms of both eutectic grain size and interlamellar spacing. Compared with the microstructure of the alloy undercooled to the same level under electromagnetic levitation in our recent work, the microstructure in drop tube is more refined due to the larger cooling rate, contributing to the microhardness of the alloy increasing by 2%-6%.