Stability of the Nb₅Si₃ phase in the Nb-Mo-Si system

Phase equilibrium between the α-Nb₅Si₃ and β-Nb₅Si₃ phases has been studied in the Nb-Mo-Si ternary system. The high temperature β-Nb₅Si₃ phase is stabilized by Mo additions to yield a relatively narrow α/β two-phase field in the Nb-Mo-Si ternary system. The enthalpy of the α-β phase transformation has been evaluated from the experimentally determined two-phase field boundaries lines in Nb-xMo-37.5Si (x = 0-10) alloys as 21 (±3) kJ/mol-atom. The β-Nb₅Si₃ phase exhibits a temperature dependent solubility, which results in the formation of Nb_ss precipitates both in the α- and β-Nb ₅Si₃ matrices. The orientation relationship between Nb_ss and Nb₅Si₃ has been identified by transmission electron microscopy. Lattice parameter variation of the β-Nb₅Si₃ phase suggests that the departure of the β-Nb₅Si₃ phase from stoichiometry toward (Nb + Mo) compositions is derived from the anti-site substitution of Nb and Mo atoms for Si sites.