Study of the Microstructure and Phase Composition of Crude Niobium Electron-Beam Remelting Sublimates
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STUDY OF THE MICROSTRUCTURE AND PHASE COMPOSITION OF CRUDE NIOBIUM ELECTRON-BEAM REMELTING SUBLIMATES V. M. Chumarev,1,2 A. G. Upolovnikova,3 and N. I. Sel’menskikh4
UDC 669.14
Results are given for a study of the structure sublimates of formed during electron beam remelting of 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 NbAl 3 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 Al 5 FeSi and Al 3Fe . The structure of the metallized part of the sublimates consists of light rounded dendrites of niobium aluminide NbAl 3 , between which there is a phase close in composition to Al 8 Fe 2Si . 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 NbAl 3 intermetallic. Niobium is also present in the form of silicide Nb 5Si 3 . 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 – Al 2O 3 (Fe, Si, Ni) and Al–Fe–Si (Cr, Mn, Ni) solid solution. Keywords: niobium, microstructure, sublimates, aluminum, electron-beam remelting, phase composition.
As is well known, in order to prepare niobium ingots of high purity electron beam melting of crude niobium is used making it possible to implement purification from impurities due to evaporation and removal from the melted workpiece surface [1–4]. The evaporating impurities and partly niobium are condensed forming sublimates that are a rich raw material. The sublimate niobium content from electron beam remelting (EBR) of crude niobium depends on original composition and the number of niobium remelting stages. As a rule in plants EBR sublimates of niobium depending on composition are either stored or returned to the production cycle niobium manufacture. Introduction of sublimates into a charge for melting crude niobium with a high Nb content (80–90%) does not markedly reduce the quality of refined metal and makes it poss
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