The Al–B–Nb–Ti system. VI. Experimental studies and thermodynamic modeling of the constituent Al–B–Nb system

• Experimental analysis of 20 distinct Al–Nb–B alloys by XRD, EDS, WDS and DTA techniques.
• Thermodynamic modeling of the complete Al–Nb–B system by the CALPHAD approach.
• Thermodynamic calculations of phase equilibria in comparison to experimental data.

Phase equilibria and phase transformations in the ternary Al–B–Nb system were investigated by XRD, SEM/WDS and SEM/EDS techniques, DTA, and optic pyrometry. No ternary compound, including the ternary phases reported in literature, was found. It was experimentally measured that the Al–Nb phases ε (NbAl3), σ (prototype σCrFe) and δ (prototype Cr3Si) have evident ternary extensions with maximal solubility of boron amount to 1.2, 10.4 and 3.2 at.%, respectively. The Al solubility in niobium borides is also significant.The obtained results were used to elaborate the thermodynamic description of this ternary system based on recently published thermodynamic models of the constituent binary subsystems using the CALPHAD approach (Thermo-Calc/PARROT). The Al–B–Nb reaction scheme, projections of the liquidus and solidus surfaces as well as a number of isothermal sections and isopleths were calculated using the proposed thermodynamic description and compared with the experimental results. The calculations were shown to reproduce adequate the experimental data. The main features of the phase equilibria in the range of melting/solidification are the existence of two quasibinary sections, NbAl3–NbB and NbB2–AlB12, eutectic LE ↔ ε + σ + (NbB) at 1850 K and cascades of invariant U-type reactions.