Several experimental superalloy compositions based on the alloy IN792 were investigated. Creep rupture properties were measured and microstructures in the as-heat-treated condition and after creep rupture testing were evaluated. Two types of topologically closed packed (TCP) phases have been observed. Type 1 occurs in alloys with increased additions of the ␥Ј-formers Al and Ta and shows detrimental effects on creep rupture life. Type 2 precipitates in Re-containing alloys are much smaller in size and volume fraction than type 1 and are found not to influence the creep rupture properties. The different effect of type 1 and 2 TCP phases can be rationalized in terms of the amount of precipitate and the resulting matrix depletion of solid solution strengtheners.
The effect of withdrawal rate on hot tearing or solidification cracking during directional solidification (DS) was explored. High withdrawal rates were found to reduce the hot tearing tendency. There is a general refinement of the microstructure when the withdrawal rate is increased. First, the dendrite arm spacing appears smaller. Second, the columnar grain size is reduced. Third, the remaining liquid is finely dispersed and increasingly discontinuous. Finally, a more uniform distribution of strain is obtained because of the presence of more grain boundaries, and grain boundary cohesion becomes stronger due to the elimination of continuous liquid films. The reduced hot tearing susceptibility is attributed to the uniform distribution of strain and stronger grain boundary cohesion.
A nickel-base superalloy for directional solidification (DS) of industrial gas turbine blades was developed based on alloy IN792 with approximately 12 mass % Cr. From preliminary test results a temperature gain in stress-rupture strength of 30 K can be expected compared to Re-free aero engine alloys with columnar grains. This significant strength improvement can be achieved by adding 2 to 3 mass % Re in the group of 12 mass % Cr alloys. With balanced Cr-, Al-, and Ti-contents they also exhibit superior hot corrosion resistance compared to typical high Al / low Cr / low Ti aero engine alloys. The castability for large blades was proved.
The effect of grain boundary (GB) misorientation on hot tearing susceptibility of directionally solidified (DS) nickel-based superalloys was explored. We found that the castability of second generation nickel-based superalloy CMSX-4 is inferior to DS superalloy IN792, an alloy well known for bad castability. The castability of CMSX-4 is somewhat improved at a higher solidification rate. The hot tearing tendency increases with increasing GB misorientation angle. As feeding tendency becomes greater with increasing misorientation, this points to the importance of GB cohesion for solidification cracking in the alloy. Microstructure investigation reveals that hot tearing is associated with formation of continuous gamma and gamma prime eutectic films at the GB in CMSX-4. We assume that the gamma and gamma prime eutectic, which reflects the remaining liquid at the end of solidification, prevents the impinging dendrite arms from touching and in this way decreases cohesion.
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