利用真空冶炼制备了不同(W+Mo)/Cr比(质量比)的铸造镍基高温合金, 采用OM, SEM和TEM观察了合金试样的微观组织, 研究了(W+Mo)/Cr比对合金组织演化和持久性能的影响. 结果表明, (W+Mo)/Cr比对热处理态组织无明显影响, 主要组成相为g基体、g' 相、初生MC和晶界二次碳化物. 长期时效期间, 合金试样的组织演化主要包括g' 相粗化、拓扑密排相 (TCP) 相析出、MC分解和晶界粗化. 随(W+Mo)/Cr比降低, MC的热稳定性明显降低, 晶界粗化程度升高, 晶界碳化物发生了M₆C→ M₆C+M₂₃C₆→ M₂₃C₆的转变. 同时, TCP相的析出量明显减少. 当(W+Mo)/Cr比为0.22时, 无TCP相析出. 另外, (W+Mo)/Cr比由高于0.55降低至0.37时, TCP相的种类由m相转变为了m与s相共存. g' 相和晶界粗化及TCP相的析出是引起合金持久性能降低的主要原因. 综合(W+Mo)/Cr比对合组织演化和持久性能的影响得出, (W+Mo)/Cr比约为0.37时, 合金具有最佳的持久性能.

The Ni-based superalloys are widely used as microstructural components of modern 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 g' 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 ℃, 4.5 h, air cooling+750 ℃, 10.5 h, air cooling), they are thermally exposed at 850 ℃ for different times. The stress-rupture tests are operated under the condition of 800 ℃, 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 g matrix, g' phase, MC carbide and secondary carbides distributing at grain boundaries. During long-term thermal exposure, the microstructure evolutions occur by g' phase coarsening, TCP phases formation, MC degeneration and grain boundary coarsening. The g' 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 m phase to coexist of m and s 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 g' 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.