The γ/γ'-microstructure of nickel-base superalloys gradually degrades during high temperature loading which deteriorates the mechanical properties. In the work presented the kinetics of microstructural degradation of the superalloy CMSX-4 was investigated metallographically in a wide parameter field (T, σ, t).The effect of microstructural degradation on mechanical properties was determined by mechanical testing of specimens pre-annealed under load. The laboratory results were compared with the microstructure of ex-service blades of CMSX-4.

A detailed analysis of solidification path has been conducted in a typical 3 rd generation Ni-base superalloy, CMSX10K. Using calorimetry and X-ray spectroscopy the solidification sequence, pertinent phase transformation temperatures, evolution of fraction solid with temperature and the accompanying micro-segregation has been determined. Particular emphasis is directed at the latter stages of solidification when non-equilibrium phases form within the inter-dendritic regions. A detailed characterisation of this constituent is carried out to highlight the severity of microsegregation. Relevance and applications of these results in the design of optimum heat treatment cycles to eliminate the as-cast coring and also guidelines from the view of alloy design and its implications to castability are discussed.

Creep behavior of the single crystal nickel-base superalloy René N5 is investigated as a function of material thickness. The results point out that reducing the thickness from 1.0 to 0.2 mm leads to both shorter creep lives and much higher overall creep strain rates of thin specimens. The orientation of the specimen is an important factor too but has a weaker influence on creep behavior under the given test conditions than the specimen thickness. IntroductionSingle crystal nickel-base superalloys are used in blades and vanes of stationary gas turbines and aero engines. Their lifetimes are mainly limited by fatigue, creep and hot corrosion at elevated service temperatures. To increase lifetime a possible strategy is to reduce the material temperature by cooling. In order to optimize both the cooling efficiency and the weight of fast rotating turbine blades a general trend is to reduce the wall thickness of the hollow investment casting parts. The relation between creep properties and section thickness was rarely investigated [1][2][3][4][5][6][7][8][9][10]. For the polycrystalline nickel-base superalloy PWA 1484 Duhl [1] found a five fold reduction of creep rupture life if the specimen thickness is diminishing from 4 to 0.5 mm which mainly depends on grain size and micro structural defects. Doner and Heckler [3,4] observed in uncoated single crystal CMSX-3 a 30% loss in creep rupture life if the wall thickness was reduced from 3.18 to 0.76 mm at 982°C and if the stress level was below 275 MPa. They also found that the time to reach 1% strain was unaffected by wall thickness at a constant stress level. Seetharaman and Cetel [6] reported similar results for single crystal PWA 1484 with wall thicknesses of 1.76 and 0.38 mm respectively and stresses below 275 MPa at 982°C. They concluded that oxidation and the more constrained plastic deformation are the major contributions for early failure of thin specimens. Doner and Heckler [3,4] found that creep rupture properties of aluminized nickel-base superalloys are less influenced by the specimen thickness. By contrast Seetharaman and Cetel [6] mentioned a loss of creep rupture lifetime of 30-40% with a thickness reduction from 1.52 to 0.25 mm for coated Knowledge and understanding of the creep behavior is a fundamental prerequisite for component life-time predictions. Therefore the effect of thickness reduction on the creep properties of uncoated and aluminized single crystal nickel-base superalloy René N5 is investigated in this study. In addition deviations from the [001] orientation are incorporated into this study which is part of a ongoing research carried out for the next years.

The effects of a 2.0 at.% addition of Ta to a model Ni-Al-Cr superalloy aged at 1073 K are assessed using scanning electron microscopy and atom-probe tomography. The addition of Ta results in appreciable strengthening, and the morphology is found to evolve from a bimodal distribution of spheroidal precipitates, to cuboidal precipitates aligned along the elastically soft <001>-type directions. Tantalum is observed to partition preferentially to the γ'-precipitate phase and decreases the mobility of Ni in the γ-matrix sufficiently to cause an accumulation of Ni on the γ-matrix side of the γ'-precipitate/γ-matrix heterophase interface.

Udimet 720@ nickel-base superalloy is in current use as the material for the stage-two power turbine disk for the T800 engine. This alloy was selected because of its attractive level of tensile strength and resistance to low cycle fatigue (LCF). Advancements in novel isothermal forging technology have enabled the production of complex disk forgings called cluster forgings.A single cluster forging produces seven PT2 disks in a single forging operation. Coupled with recent advances in powder metallurgy (P/M) billet production featuring consolidation by HIP followed by extrusion, turbine disk forgings can be produced at a significant cost reduction as compared to single-piece hot die forged disks made from conventional cast/wrought billet. P/M Udimet 720 material displays excellent mechanical properties, competitive with other high strength disk alloys.Turbine disks produced from isothermally forged P/M Udimet 720 passed component test requirements established for the T800 engine. A significant cost reduction resulted from the use of this process.