Institute of Metallurgy of Ural Department of Russian Academy of Sciences, Ekaterinburg, Russia:

L. Yu. Udoeva, Senior Researcher, e-mail: lyuud@yandex.ru
V. M. Chumarev, Chief Researcher
L. I. Leontev, Chief Researcher
N. I. Selmenskikh, Researcher

Influence of rare earth metals on structural-phase state of Nb – Si eutectic alloy was considered on example of yttrium and scandium. On the basis of phase diagrams and thermodynamic analysis of interactions in Nb – Si – Y and Nb – Si – Sc systems, there was carried out a theoretical estimation of dependences of the phase state of eutectic composites on the temperature and doping element content. For the purpose of experimental verification of calculated data, the model of Nb – 18.7 Si alloys (1.0–6.0% (at.) of yttrium and 0.9–2.6% (at.) of scandium) was obtained by vacuum-arc melting. In the researched samples, a phase composition and distribution of doping elements between structural components were determined with the help of X-ray diffraction, optical microscopy and electron probe microanalysis. There was found that phase formation of eutectic Y and Sc doped Nb – Si alloys basically corresponds to equilibrium principles and obtained thermodynamic models, which can be used for forecasting of phase composition of Nb – Si alloys. Scandium concentration from 0.9 to 2.6% (at.) does not affect the Nbss/silicides ratio in Nb – 18.7 Si eutectic alloy, while yttrium in the range of 1.0–6.0% (at.) increases the Nbss solid solution share twice. Yttrium and scandium significantly increase the dispersity of Nb – 18.7 Si alloy structure due to their surfactant properties and refine the Nb – Si alloys from oxygen impurities, forming refractory oxides Y₂O₃ and Sc₂O₃. Furthermore, these elements stabilize the high-temperature Nb₃Si silicide within the range of test concentrations, hindering the eutectoid decomposition of Nb₃Si → Nb_ss + α-Nb₅Si₃ at the temperature of 1770 ^oC.

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