An X-ray computed tomography study of three-dimensional microstructure evolution for undercooled SiNi three-phase peritectic-type alloys

Abstract

Most of previous investigations on microstructure evolutions of undercooled peritectic alloys mainly performed on two-dimensional sections, and three-dimensional (3D) microstructure explorations of undercooled peritectic alloys contribute to understanding the crystal growth characteristics and spatial distribution features of constituted phases in 3D space geometry. In this work, 3D microstructure evolutions of undercooled SiNi peritectic-type alloys were investigated based on glass fluxing experiments assisted by the characterization of X-ray computed tomography. In experimental undercooling range, undercooled SiNi alloys appeared as primary Si phase surrounded by peritectic NiSi₂ phase plus the eutectic (NiSi+NiSi₂) phase inside the interdendritic gap. Primary Si and peritectic NiSi₂ phases with the major characteristics of plate-like structures became more with a rise in the undercooling. Meanwhile, plate-like Si phase shrank in the spatial direction of its maximum size, whereas peritectic phase thickened in three dimensions. With increasing the undercooling beyond a threshold value, lamellar eutectic evolved into anomalous eutectic. The volume-fraction reduction and microstructure refinement of eutectic phase happened under a higher undercooling. Owing to the upward movement of Si phase with a smaller density, the farther away from the sample bottom the more the primary Si phase and peritectic NiSi₂ phase, and the less the eutectic phase. Because of the relatively higher undercooling in the outside shell region of the sample, plate-like Si phase within the outside shell region was smaller and primary and peritectic phases were more than those inside the internal spherical regions of the sample, whereas the volume fraction of eutectic phase in the center of the sample was relatively higher.