微重力下Fe-Al-Nb合金液滴的快速凝固机理及其对显微硬度的影响

采用落管无容器处理技术实现了Fe_(67.5)Al_(22.8)Nb_(9.7)三元合金在微重力条件下的快速凝固,获得了直径为40—1000μm的合金液滴.实验中合金液滴的过冷度范围为50—216 K,冷却速率随着液滴直径的减小由1.23×10~3K·s~(-1)增大到5.53×10~5K·s~(-1).研究发现,Fe_(67.5)Al_(22.8)Nb_(9.7)合金液滴的凝固组织均由Nb(Fe,Al)_2相和(αFe)相组成,且随着液滴直径的减小,初生Nb(Fe,Al)_2相由树枝晶转变为等轴晶,共晶组织发生了约3倍的细化且生长特征由层片共晶向碎断共晶转变;硬质初生Nb(Fe,Al)_2相的析出有效提高了合金的显微硬度.与电磁悬浮条件下同过冷合金的凝固组织对比发现,落管条件下的合金液滴凝固组织更细化,使得合金显微硬度提高了2%—6%.

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 Fe_67.5Al_22.8Nb_9.7 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 μm are achieved. The liquidus temperature of the alloy is 1663 K. The microstructure of the alloy consists of Nb(Fe, Al)₂ and (αFe) 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 1.23×10³ to 5.53×10⁵ 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%.