The structure of Ti -Nb -B alloys that are cast and annealed at subsolidus temperatures and at 1400°C is experimentally analyzed (x-ray diffraction, metallography, and electron probe microanalysis), and so are temperatures of their phase transformations (differential thermal analysis and pyrometry). No ternary phases are found in the alloys. Projections of solidus and liquidus surfaces, an isothermal section at 1400°C, and a vertical section at 7.5 at.% B are constructed. A reaction scheme is proposed for alloy crystallization. INTRODUCTIONNiobium and boron are regarded as alloying additions for titanium-based materials. Niobium enhances the hightemperature oxidation resistance of conventional titanium solid-solution-hardened alloys [1]. Boron is used to refine grains and clear grain boundaries in conventional titanium alloys [2], new prospective TiAl (TiAl + Ti 3 Al) alloys doped with niobium [3][4][5][6][7], and materials based on ternary orthorhombic Ti 2 NbAl [8-10]. Titanium-matrix composites reinforced by borides and alloyed by niobium [11] and other metals have been under intensive research for the last 15 years. As noted in [12], binary eutectic alloys (Ti) + TiB contain only 7.7 vol.% of the reinforcing boride phase (9 vol.% by our estimates); therefore, alloys reinforced by boride in addition to the titanium solid-solution-hardened matrix should be considered prospective.However, there is a lack of published data on phase equilibria in the ternary Ti -Nb -B system. Neither is there a scientific basis for developing and improving such materials. The only exception is the paper [13], which reported an isothermal section at 1400°C. There are reports on studies of the sections Nb -TiB 2 [14-16] and NbB 2 -TiB 2 [17,18].The objective of this paper is to construct a phase diagram for the ternary Ti -Nb -B system in the Ti -TiB 2 -NbB 2 -Nb region at high temperatures and to examine the melting (crystallization) of alloys. Table 1 provides crystal structure data for Ti -Nb -B phases. Alloys of 43 different compositions are analyzed in [13] by x-ray diffraction (powder technique, V-K α radiation). The specimens were produced from titanium (99.6 wt. % Ti), niobium (99.5 wt.% Nb), and boron (99.3 wt.% B) powders. Pre-sintered compacts were melted in an arc furnace and annealed in vacuum at 1400°C for 50 h. Ternary compounds were not found in the system.A continuous series of solid solutions are confirmed to exist between TiB 2 and NbB 2 isostructural borides, as earlier reported in [17,18]. The solubility of titanium in Nb 3 B 4 (5 at.%), NbB (30 at.%), and Nb 3 B 2 (to 5 at.%) and of niobium in TiB (~5 at.%) is determined. We estimated the composition of the metal phase using a figure from [13]: this * The parentheses are used here and below to denote a solid solution.

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