Research project

It is well known that, under high-temperature service conditions, nickel-base superalloys exhibit the phenomenon of directional coarsening, so-called “rafting”, of the gamma/gamma prime microstructure which, in its initial state, consists of cuboidal gamma prime particles in a gamma matrix. For typical alloys with a negative lattice mismatch, rafting leads to a plate-like microstructure aligned perpendicular (parallel) to the tensile (compressive) stress axis. In most cases, the tensile creep resistance is reduced by rafting. Based on theoretical considerations, it was proposed that a raft structure aligned parallel to the stress axis should enhance the creep resistance. Hence, experiments were designed in which monocrystalline SRR 99 specimens were first subjected to a small compressive creep strain (ca. 0.4%) in order to induce a raft structure parallel to the stress axis. Then, these specimens were tested in tensile creep tests at 900°C, 950°C and 1000°C till strains of about 3% (local strains larger than about 2% are not permitted to occur in a component). In all cases, it was found that the initial creep behaviour up to about 2% strain was superior to that of specimens with the initial microstructure. Subsequently, the raft structure, being the “wrong” one, became unstable. In conclusion, it can be stated that prior introduction of a raft structure aligned parallel to the stress axis enhances the tensile creep resistance appreciably for all practical purposes and, as shown earlier, leads also to a significant improvement of the isothermal high-temperature fatigue behaviour.