Enhancement of the High-Temperature Tensile Creep Strength of Monocrystalline Nickel-Base Superalloys by Pre-rafting in Compression

Abstract

In recent years, the development of Y.-hardened monocrystalline nickel-base superalloys focused on different aspects of alloy composition and microstructural refinement. In particular, the formation of so-called y/y'-raft structures, lying typically, in the case of a negative y/y'-lattice misfit 6, perpendicular to the stress axis during high-temperature tensile creep had attracted much attention. Unfortunately, this microstructural transformation leads in most cases to deteriorated creep properties. Earlier attempts to prevent rafting by suitable prior annealing treatments had been in so far unsuccessful, that while the applied annealing treatment retarded raft formation, it inevitably lead to microstructural coarsening accompanied by creep acceleration. The goal of the present extensive study is to show that suitable pre-rafting in compression, leading to a y/y'-raft structure parallel to the stress axis in the case of a negative lattice misfit, enhances not only the isothermal hightemperature fatigue strength, as shown earlier, but also the hightemperature creep properties. The observed macroscopic creep behaviour will be discussed with reference of microstructural observations.

The Effects of Different Alloying Elements on the Thermal Expansion Coefficients, Lattice Constants and Misfit of Nickel-Based Superalloys Investigated by X-Ray Diffraction

“…in that region of a blade in which the combined action of thermal and mechanical loads is most severe (stress above ca. 150 MPa, temperature ≤ 950 C, compare [18]), the development of the raft structure leads to a deterioration of the mechanical properties of the alloy [9,19]. In this respect, it is interesting to investigate the effects of different alloying elements on the development of the γ/γ microstructure.…”

On Thermomechanical Fatigue of Single-Crystal Superalloys

“…Regarding the TMF properties, Neuner et al [65] have shown that the pre-rafting of the microstructure seems to influence the fatigue life differently, depending on which TMF cycle is used. Further, Tetzlaff et al [66] showed that pre-rafting improves high-temperature creep properties.…”

“…Creep deformation at high temperature leads to rafting of the γ/γ -microstructure and generally, the resistance to creep is improved due to this phenomenon because dislocations are prevented from climbing around the γ -particles and instead have to cut through the γ -particles [18]. More specifically, it has been shown that rafts parallel to the loading axis, obtained from pre-rafting, are beneficial to creep properties [66]. Rafting occurs rapidly; for example at 1150°C/100 MPa the rafting has been shown to be completed already after 10 h [51].…”

Effect of the Prior Microstructure Degradation on the High Temperature/Low Stress Non-isothermal Creep Behavior of CMSX-4® Ni-based Single Crystal Superalloy

“…According to Tetzlaff and Mughrabi,[24], and under the present low stress conditions, the P-type structure should be the most creep resistant due to its larger areas of vertical /' interfaces where the dislocation climb process is impeded most strongly. This lower creep strain rate compared to a cuboidal /' microstructure and to a N-type ' rafted one is maintained as long as fragmentation of the vertical plates is avoided.…”