1. Introduction 2. The physical metallurgy of nickel and its alloys 3. Single crystal superalloys for blade applications 4. Superalloys for turbine disc applications 5. Environmental degradation: the role of coatings 6. Summary and future trends.

The Superalloys: Fundamentals and Applications

The evolution during heat treatment of the as-forged microstructure of the high strength superalloy UDIMET 720Li has been studied. Particular emphasis has been placed on the characterisation of γ ′ precipitation kinetics using optical and transmission electron microscopies (TEM) and subsequent image analysis. The observations are interpreted using thermodynamic, phase transformation and precipitate hardening theories. The results have implications for the gas turbine manufacturers. Through a better understanding of the evolution of the microstructure during ageing, a heat treatment of 24 h at 700°C is proposed, which is believed to be optimal. This allows full advantage to be taken of the properties of the alloy, whilst reducing the costs and time associated with the heat treatment schedules. Moreover, the data presented allows the variation in properties across a U720Li forging to be estimated.

Heat treatment of UDIMET 720Li: the effect of microstructure on properties

Precipitation hardening of superalloys by ordered γ′-particles

The mechanical properties of Ni3AI (γ’) are first reviewed, with particular emphasis on the flow-stress and creep properties. It is shown that the flow stress of Ni3AI has a very unusual temperature dependence: it increases with increasing temperature. This behaviour is shown to be related to the nature of the dislocation dissociation in Ni3AI, the increase in flow stress being the result of thermally activated cross-slip from {111} planes to {010} planes of ½(110) screw dislocations. This model is shown to be in agreement with the observed changes in the flow stress with changes in test temperature, sample orientation, and sense of the applied stress. The effects of compositional changes on the flow stress are also discussed. Stage II work hardening, strain-rate effects, and the creep deformation of γ’ are reviewed and shown to be only poorly understood. The flow strength of γ/y’ alloys is then discussed, and it is shown that many of the anomalies in the behaviour of pure γ’ are also found in γ/γ...

Mechanical properties of Ni3AI and nickel-base alloys with high volume fraction of γ'

On the prediction of the yield stress of unimodal and multimodal γ′ Nickel-base superalloys

Some new aspects concerning particle hardening mechanisms in γ' precipitating Ni-base alloys—I. Theoretical concept

Some new aspects concerning particle hardening mechanisms in γ' precipitating nickel-base alloys—II. Experiments

The isothermal aging behavior of the cuboidal ordered γ′ precipitates of the cast nickel-based superalloy IN 100 was investigated at temperatures between 800 and 1000°C. Despite the complication connected with morphological changes of the γ′ particle microstructure due to coarsening and raft formation a classical Ostwald ripening law was confirmed. Comparing the calculated critical resolved shear stress (CRSS) increase with the experimental data exhibits the functional dependencies on the mean γ′ size as predicted by theory [B. Reppich, in: R.W. Cahn et al. (Eds.), Materials Science and Technology, vol. 6, in: H. Mughrabi (Ed.), Plastic Deformation and Fracture of Materials, Wiley-VCH, Weinheim, Germany, 1993, pp. 311–357]: (a) a parabolic increase of the CRSS for cutting of fine under-aged γ′ particles (weak pair-coupling); (b) a transition to cutting by strongly pair-coupled dislocations for increasing particle sizes; and (c) a deviation from the hyperbolic CRSS decrease due to strong pair-coupled cutting towards Orowan by-passing for non-coupled single dislocations at over-aged particles.

Order strengthening in the cast nickel-based superalloy IN 100 at room temperature

The high-temperature creep behavior of the oxide-dispersion-strengthened (ODS) nickel-base superalloys MA 754 and MA 6000 has been investigated at temperatures up to 1273 K and lifetimes of approximately 4000 hours using monotonic creep tests at constant true stressσ, as well as true constant extension rate tests (CERTs) at\(\dot \varepsilon \). The derivation of creep rupture-lifetime diagrams is usually performed with conventional engineering parametric methods, according to Sherby and Dorn or Larson and Miller. In contrast, an alternative method is presented that is based on a more microstructural approach. In order to describe creep, the effective stress model takes into account the hardening contributionσp caused by the presence of second-phase particles, as well as the classical Taylor back-stressσp caused by dislocations. The modeled strain rate-stress dependence can be transferred directly into creep-rupture stress-lifetime diagrams using a modified Monkman-Grant (MG) relationship, which adequately describes the interrelation between\(\dot \varepsilon \) representing dislocation motion, and lifetimetf representing creep failure. The comparison with measured creep-rupture data proves the validity of the proposed micromechanical concept.

Creep lifetime prediction of oxide-dispersion-strengthened nickel-base superalloys: A micromechanically based approach

Experimental results of extensive transmission and scanning electron microscopy investigations on a range of commercial two-phase superalloys (Nimonic PE 16, Nimonic 80 A, Nimonic 90, Nimonic 105 and IN-100) are reported. The growth of the unimodal size-distributed ordered γ′ (Ni3Al, Ti) precipitates during one-stage isothermal aging obeys strictly the common t13 law independent of the degree of misfit, volume fraction and particle shape (spheres, cuboids and cuboidal arrays). Deviations from the t13 behaviour are observed only in the final overaging stage because of drastic changes in the γ′ morphology γ′ volume fractions increase monotonically with increasing aging time, and the particle number densities decrease monotonically with increasing particle size, suggesting that the morphological development is governed by the combined decomposition and Ostwald ripening.

B. Reppich

Papers

Some new aspects concerning particle hardening mechanisms in γ' precipitating Ni-base alloys—I. Theoretical concept

Some new aspects concerning particle hardening mechanisms in γ' precipitating nickel-base alloys—II. Experiments

Order strengthening in the cast nickel-based superalloy IN 100 at room temperature

Creep lifetime prediction of oxide-dispersion-strengthened nickel-base superalloys: A micromechanically based approach

Electron microscopy of γ′ particles in nickel-based superalloys