Solidification morphology and phase selection in drop-tube processed Ni-Fe-Si intermetallics

摘要

Drop-tube processing was used to rapidly solidify droplets of Ni64.7Fe10Si25.3 and Ni59.7Fe15Si25.3 alloys. In the larger droplets, and therefore at low cooling rates, only two phases, gamma-Ni31Si12 and beta(1)-Ni3Si were observed. Conversely, in the smaller droplets, and therefore at higher cooling rates, the metastable phase Ni25Si9 was also observed. The critical cooling rate for the formation of Ni25Si9 was estimated as 5 x 10(3) K s(-1). SEM and TEM analysis reveals three typical microstructures: (I) a regular structure, comprising single-phase gamma-Ni31Si12 and a eutectic structure between gamma-Ni31Si12 and beta(1)-Ni3Si; (II) a refined lamellar structure with a lamellar spacing <50 nm comprising gamma-Ni31Si12 and beta(1)-Ni3Si; (III) an anomalous structure with a matrix of Ni25Si9 and only a very small proportion of a second, and as yet unidentified, phase. These results indicate that there is an extended stability field for Ni25Si9 in the Ni-rich part of the Ni-Fe-Si ternary system in comparison to the Ni-Si binary system. With an increase of cooling rate, an increasing fraction of small droplets experience high,undercoolings and, therefore, can be undercooled into the Ni25Si9 stability field forming droplets consisting of only the anomalous structure (III). The Fe atoms are found to occupy different substitutional sites in different phase, i.e. Fe substitutes for Ni in the gamma phase and Si in the L1(2) (beta(1)) phase respectively. (C) 2015 Elsevier Ltd. All rights reserved.