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| Bujnošková K., Drápala J., Štěpánek J. |
| SEGREGATION BEHAVIOUR OF NIOBIUM AND ITS INFLUENCE ON STRUCTURAL CHARACTERISTICS OF MOLYBDENUM SINGLE CRYSTALS PREPARED BY ELECTRON BEAM FLOATING ZONE |
Abstract
When preparing quality single crystals of low-alloyed alloys on the base of molybdenum as well as other high melting metals, it is necessary to investigate influence of alloying elements on the single crystal structural, mechanical and physical properties and their behaviour during crystallization under given conditions. The aim of the present work was to evaluate the influence of niobium on structural perfection of single crystals of molybdenum and its behaviour from the view of segregation characteristics. For this purpose single crystals of low-alloyed alloys Mo-1.5 at.% Nb were prepared by the electron beam floating zone melting under various conditions of crystallization (1, 3, 5 mm/min). These single crystals were submitted to metallographical, chemical and X-ray analyses. It was found that alloying of molybdenum single crystals with niobium resulted in disintegration of block structure, i.e. reducing the sub-grain dimensions and increasing the angle of disorientation of their boundaries. Simultaneously, the density of dislocations increased. It can be concluded from X-ray Lauegrams that the single crystals of these alloys were prepared with crystallographical orientation of the growth axis [110]. The performed line chemical analysis of specimens proved creation of niobium micro-segregations, so called growth striations, which showed themselves on concentration profiles as periodically alternating areas with increased and decreased concentrations of niobium. The existence of these chemical inhomogeneities suggests that stationary conditions of the crystal growth were disturbed during the zone melting, i.e. fluctuations of microscopic rate of growth occurred due to convection in the melt. Effective distribution coefficients of niobium in molybdenum for individual conditions of crystallization were determined from the chemical analyses results. They agree relatively well and approach to the calculated theoretical value of the equilibrium distribution coefficient.
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| Keywords: tungsten|molybdenum|single crystal|electron beam floating zone melting|plasma metallurgy| |
| No 4 (2007), p. 481-488 |
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