Influence of Cooling Rate on Microstructural Formation of Melt-Spun Fe-Al-Nb Ternary Alloy

Fe-Al-Nb ternary alloys as a sort of high-temperature structure materials are paid more attention in recent years. The pseudobinary eutectic composed of Nb(Fe,Al)₂ and α-Fe phases in Fe-Al-Nb alloy transformed from lamellar shape to fiber with the increase of growth rate in directional solidification. Heat treatment techniques were applied to investigate the strengthening mechanism related to microstructural formation. However, influence of cooling rate on microstructure especially pseudobinary eutectic is not clear yet. In this work, rapid solidification and the microstructural formation of Fe_67.5Al_22.8Nb_9.7 ternary alloy were investigated by melt spinning technique to reveal the rapid solidification mechanism of the alloy. As the wheel rate increases from 10 m/s to 40 m/s, the thickness of alloy ribbon decrease by one order of magnitude, i.e. from 67.70 mm to 4.69 mm, the cooling rate increases by seven times, i.e. from 1.24×10⁶ K/s to 9.53×10⁶ K/s. Consequently, the sample shape transforms from regular ribbon to regular ribbon, fishbone-like ribbon and droplets. The microstructure consists of Nb(Fe,Al)₂ and α-Fe phases. The rise of wheel rate leaded to the microstructural transition and refinement, as well as the refinement in terms of eutectic interlamellar spacing and grain size (i.e. grain diameter) measured using Image-Pro Plus software. On condition that the wheel rate is less than 40 m/s, characteristics are divided into two regions, i.e. primary a-Fe phase plus lamellar pseudobinary eutectic near free surface region and anomalous pseudobinary eutectic near roller surface region. As the wheel rate increases from 10 m/s to 30 m/s, lamellar eutectic becomes fragmented and the amount of anomalous pseudobinary eutectic enlarges. Once the wheel rate is up to 40 m/s, anomalous pseudobinary eutectic is the only microstructure of the fishbone-like ribbon. Meanwhile, the alloy droplets with the diameter size ranging from 90 mm to 1500 mm were achieved at the wheel rate of 40 m/s. Owing to the relative low cooling rate, the microstructure of the alloy droplet consist of primary α-Fe phase and lamellar pseudobinary eutectic. As the droplet diameter decreases, the primary α-Fe phase transforms from dendrite to equiaxed grain and the pseudobinary lamellar eutectic is refined.