Mechanical Properties of Nb-18Si-5Mo-5Hf-2C In-Situ Composite Prepared by Arc-Casting Method

Abstract

Niobium-base in-situ composite, Nb–18Si–5Mo–5Hf–2C (in mol%), was fabricated by arc-casting method and heat-treated at 2070 K for 20 h in vacuum. The microstructure is composed of Nb solid solution (Nb_SS) matrix phase and secondary phase of Nb₅Si₃. The Nb₅Si₃ was found to have two kinds of structure: one is the α phase reported in the Nb–Si binary phase diagram and the other is a hexagonal phase with Mn₅Si₃-type structure. To evaluate the mechanical properties of the composite, nano-indentation hardness tests and tensile tests were conducted. The hardness and elastic modulus at ambient temperature were evaluated using nano-indentation hardness tester and the tensile properties were examined by an Instron-type materials testing machine at temperatures of 300, 1470, 1570, 1670 and 1770 K, and at an initial strain rate of 1×10⁻⁴ s⁻¹. The hardness and elastic modulus of the Nb_SS phase are 480 GPa and 170 GPa and those of the silicide are 1480 GPa and 380 GPa. The composite exhibited the excellent ultimate tensile strength of 450 MPa at 1470 K, and the brittle to ductile transition temperature is between 1470 and 1570 K . The fracture behavior of the in-situ composite is complex, however, involving cleavage of the Nb₅Si₃, Nb_SS/Nb₅Si₃ interface separation, ductile rupture of the Nb_SS and correlations of these.