Effects of Won Microstructural Stability of the Third Generation Ni-Based Single Crystal Superalloys

Ni-based single crystal superalloys are widely used in the manufacture of aero engine turbine blades because of the excellent mechanical properties at high temperature. With the development of single crystal superalloys, the content of refractory elements is constantly increased (especially Re) to improve the high temperature capability, which in turn leads to the decrease in microstructural stability of alloys, such as the TCP phase precipitation. It is important to find one element which not only can maintain high temperature performance but also does not evidently promote TCP phase precipitation and is very cheap in price to replace Re partially. W is one of the most important solution strengthening elements in superalloys, its diffusion rate in Ni matrix is close to Re and far below the other alloying elements, meanwhile, the advantage of low price make it to be the most suitable substitute of Re. However, there is little work about the effect of W on microstructural stability in Re contained third generation superalloys. In this work, the effects of W on the elemental segregation, elemental partitioning ratio of γ/γ', microstructure evolution and TCP phase precipitation during thermal exposure at 950, 1000 and 1050℃ have been investigated in a third generation Ni-based single crystal superalloys with varied contents of W (6%~8%, mass fraction). The results show that the addition of W has no obvious effect on segregation of the alloying elements of as-cast alloys as well as the morphology, size and volume fraction of γ' phase after heat treatment. During the thermal exposure at 950℃, the connection and deformation of γ' phase are accelerated, but its coarsening rate is decreased with increasing W content. The TCP phases precipitated in three alloys during thermal exposure are mainly μ phase and σ phase. The area fraction of TCP phases is increased slightly with the W addition during thermal exposure, which is the largest at 1000℃, less at 950℃ and the least at 1050℃.