•Phase equilibrium between α- and β-Nb5Si3 in Nb–Mo–Si alloys was established.•The enthalpy change in the α–β transition for Nb5Si3 was evaluated.•The crystallography of Nb precipitates and Nb5Si3 matrices was identified.•Defect mechanism for the off-stoichiometry of β-Nb5Si3 was studied.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-Nb5Si3 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.

Dislocation character and operative slip systems in α-Nb5Si3 were examined by transmission electron microscopy. Two-phase alloys comprised of (Nb) and α-Nb5Si3 were used in this study. Although few dislocations were present in the α-Nb5Si3 phase of a pre-deformed alloy, many developed after 15% of compressive deformation at 1673 K. Two types of the Burgers vectors were identified for α-Nb5Si3: <100] and 1/2 < 111]. The glide planes of dislocations were defined by the cross products between the Burgers vectors and the line vectors of the dislocations, by which the slip systems that operate in α-Nb5Si3 at 1673 K were determined as {011)<111], {100)<010], and {001)<100].