Fe-Al-Nb phase diagram investigation and directional growth of the (Fe, Al)2Nb-(Fe, Al, Nb)ss eutectic system

An attractive alternative method for the manufacture of high temperature structural materials may be achieved from a eutectic transformation found in the Fe-Al-Nb system, which generates the (Fe, Al)2Nb and (Fe, Al, Nb)ss solid phases. Eutectic and off-eutectic samples from the Fe-Al-Nb system were prepared by arc melting under argon atmosphere, which were subsequently directionally solidified using a vertical Bridgman crystal growth unit. The ingots obtained were investigated using optical and electron scanning microscopy with the purpose to evaluate the effect of the growth condition on the microstructure. Primarily, the growth mode of the (Fe, Al)2Nb/(Fe, Al, Nb) eutectic was considered by examining off-eutectic samples. The solidification experiments carried out under different conditions revealed that the microstructure of this system is very sensitive to a growth rate variation. At a low growth rate, the eutectic microstructure was regular, even though several types of defects were found. As the growth rate was increased, a transition from lamellar to fibrous morphology was observed. Some aspects of this transition were also evaluated.