Effect of Heat Treatment on the Microstructure and Mechanical Properties of Nb-base Superalloys

The effect of heat treatment on the microstructure and mechanical properties of Nbss/Nb5Si3 in situ composites prepared by conventional high-rate-solidification Bridgeman process (HRS) was investigated. The microstructure evolution of samples during heat treatment process were analyzed by SEM, XRD and EPMA. Results show that the as-cast alloy consists of Nbss, gamma-(Nb, Ti)(5)Si-3, beta-(Nb, Ti)(5)Si-3 and primary carbide phase; after heat treatment, the beta-(Nb,Ti)(5)Si-3 phase is transformed to alpha-(Nb,Ti)(5)Si-3 phase and the secondary carbide is precipitated. Meanwhile, the silicide phase gradually dissolves or breaks into small pieces, with the morphology changing from the needle into granules. The transformation (Nb,Ti)(5)Si-3 -> (Ti,Nb)(5)Si-3 occurs when aging at 1150 degrees C. The compressive strength and room temperature tensile strength of heat-treatment specimens are improved, while the tensile strength of as-cast and heat-treated samples at 1000 degrees C is at the same level. The temperature has a great influence on the fracture mode, which is cleavage fracture at low temperature and ductile fracture at high temperature.