Structural materials: metal-silicon-Boron. The Nb-rich corner of the Nb-Si-B system

Phase equilibria in the Nb-Nb5Si3-NbB region were studied in the melting (crystallization) range by means of light microscopy, XRD, SEM and EMPA on alloys after arc-melting and annealing at 1800degreesC and at subsolidus temperatures. Phase transition and melting temperatures were determined by DTA and pyrometric Pirani-Alterthum technique resulting in a solidus projection and two isopleths, Nb77Si23-Nb77B23 and Nb99Si1-Nb5Si2B. The T-2-phase Nb5Si3-xBx (0 less than or equal to x less than or equal to 2, Cr5B3-type) was found to form equilibria with (Nb), NbB, Nb3Si, and with the T-1-phase (Mn5Si3 derivative type). The T2-phase melts incongruently (Nb5Si1.8B1.2 at 2245degreesC) and forms a quasibinary eutectic with the niobium solid solution on a minimum tie-line at similar to1880degreesC. (C) 2003 Elsevier Inc. All rights reserved.