EFFECTS OF (W plus Mo)/Cr RATIO ON MICROSTRUCTURAL EVOLUTIONS AND MECHANICAL PROPERTIES OF CAST Ni-BASED SUPERALLOYS DURING LONG-TERM THERMAL EXPOSURE

The Ni-based superalloys are widely used as microstructural components of modem turbine engines due to its good high temperature strength, good fatigue and creep property and excellent hot-corrosion resistance. In order to increase their high temperature strength, more and more refractory elements, such as W and Mo, are added into these alloys while Cr content gradually decreases. During long-term aging, these alloys generally experience various microstructural changes, including coarsening of gamma' phase coarsening, formation of a continuous grain boundary (GB) carbide network, precipitation topologically close-packed (TCP) phase, and degeneration of MC carbide. However, there is limited available data about the effect of (W+Mo)/Cr ratios on the microstructural evolution of Ni-based superalloys. In this work, the cast Ni-based superalloys with different (W+Mo)/Cr ratios (mass ratios) are fabricated by vacuum induction furnace. After standard heat treated (1110 degrees C, 4.5 h, air cooling+ 750 degrees C, 10.5 h, air cooling), they are thermally exposed at 850 C for different times. The stress-rupture tests are operated under the condition of 800 degrees C, 294 MPa. Effects of (W+Mo)/Cr ratios on the microstructure evolutions and mechanical properties are investigated by the combination of OM, SEM, TEM and stress-rupture tests. The experiment results show that the (W+Mo)/Cr ratio has no obvious influence on the standard heat treated microstructure, which is mainly composed of gamma matrix, gamma' phase, MC carbide and secondary carbides distributing at grain boundaries. During long-term thermal exposure, the microstructure evolutions occur by gamma' phase coarsening, TCP phases formation, MC degeneration and grain boundary coarsening. The gamma' phase coarsening behavior is not affected obviously by the (W+Mo)/Cr ratio. However, the amount of TCP phases decreases significantly with decreasing of (W+Mo)/Cr ratio and the type of TCP phases transforms from p phase to coexist of p and a phases when (W+Mo)/Cr ratio decreases from 0.55 to 0.37. There are no TCP phases observed in the sample with (W+Mo)/Cr ratio of 0.22. The thermal stability of MC carbide is reduced obviously and the grain boundaries coarsen more severely by the decrease of (W+Mo)/Cr ratio. The degradation of stress-rupture property is attributed to the coarsening of gamma' phase and grain boundaries and the formation of TCP phases. Combined with the effect of (W+Mo)/Cr ratio on the solid solution strengthening, microstructure evolution and stress-rupture property, it can be concluded that the optimum stress-rupture property can be obtained when the (W+Mo)/Cr ratio is about 0.37.