Study of the Microstructure and Phase Composition of Crude Niobium Electron-Beam Remelting Sublimates

Abstract

crude niobium. The phase composition of sublimates formed during the first remelting of crude niobium is studied by scanning electron microscopy and X-ray microanalysis. Sublimates typically form structures of two types, i.e., dendritic and lamellar that is connected with niobium refining process conditions and the effect of various factors on sublimate crystallization. Niobium electron beam remelting sublimates are inconsistent with respect to both chemical and phase compositions. A layered structure is typical for the porous part of sublimates that are the basis of the material. The microstructure of another type is dendritic, typical for lamellar and rounded particles with a metallic sheen located within pores or dispersed in the layered part of sublimates. Alternation of phases within each layer is noted for a layered structure. The composition of the phases containing niobium is close to NbAl3 and a solid solution based on niobium (more than 90% Nb). Within the zone of phases consisting of aluminum, there are areas accumulating iron and silicon of variable composition that may be attributed to solid solutions based on Al5FeSi and Al3Fe. The structure of the metallized part of the sublimates consists of light rounded dendrites of niobium aluminide NbAl3, between which there is a phase close in composition to Al8Fe2Si. It is noted that single particles of pure niobium metal are found within the structure of the metallized part of sublimates. It is noted that as expected the main phase is NbAl3 intermetallic. Niobium is also present in the form of silicide Nb5Si3. Aluminum is found in sublimates in elemental form, in solid solution with iron and silicon, or in the form of oxide. Impurity elements are distributed between an oxide phase based on α –Al2O3 (Fe, Si, Ni) and Al–Fe–Si (Cr, Mn, Ni) solid solution.