Rapid solidification of Ni-Si-Fe intermetallics in drop tube

The rapid solidification of the Ni74.7-xFexSi25.3 (x = 0, 10 and 15 at.%) alloys was studied by the drop tube technique aiming to investigate the formation of intermetallic compounds and microstructural evolution at high cooling rates. The particles of different diameters were obtained, with 75-850 μm particles for the Ni-25 at.% Si alloy and 53-850 μm particles for the Ni74.7-xFexSi25.3 (x = 10 and 15 at.%) alloys. Several characteristic techniques were used to analyze the as-solidified samples, including optical microscopy, SEM, TEM, DTA and XRD.
For the Ni-25.3 at.% Si alloys, the metastable phase Ni25Si9 formed as the dominant phase in all size ranges of the particles, with γ-Ni31Si12 and β1-Ni3Si also being present. Three typical microstructures were observed: (1) regular lamellar and (2) anomalous eutectic structures comprising the Ni25Si9 and β1-Ni3Si phases; (3) heteroclite structure with the matrix of Ni25Si9. The formation of the eutectic structures indicates that there is a possible eutectic reaction for the Ni25Si9 and β1-Ni3Si phases. With increasing cooling rate, an increasing fraction of the droplets formed the entire heteroclite structure.
The solidified phases and microstructures of the Ni-Fe-Si alloys were deeply influenced by the cooling rate. At low cooling rates, only γ-Ni31Si12 and a compound with the same structure as β1 were obtained, while the additional metastable phase Ni25Si9 formed in the small particles (53-212 μm). Three typical microstructures were observed with increasing cooling rate: (1) regular structure comprising the single-phase γ-Ni31Si12 and eutectic structure of γ-Ni31Si12 and β1-Ni3Si; (2) refined lamellar structure with the wide band of γ-Ni31Si12; (3) anomalous structure with the matrix of Ni25Si9. It is suggested that there is an extended Ni25Si9 stability field in the Ni-rich part of Ni-Fe-Si ternary system. With increasing cooling rate, an increasing fraction of small droplets experience high undercoolings and, therefore, can be undercooled into the Ni25Si9 stability field forming the entire anomalous structure.