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 Nbss precipitates both in the α- and β-Nb 5Si3 matrices. The orientation relationship between Nbss 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.
- A. Silicides, various
- B. Crystallography
- B. Phase transformation
- B. Thermodynamic and thermochemical properties
- D. Electron microscopy, transmission