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.

This paper presents results from a research initiative aimed at investigating high temperature creep deformation mechanisms in Ni-base superalloys through a combination of creep experiments, TEM deformation mechanism characterization, and state of the art modeling techniques. The effect of microstructure on dictating creep rate controlling deformation mechanisms was revealed for specimens with a bimodal γ′ size distribution that possessed different secondary γ′ size, tertiary γ′ volume fraction, and γ channel width spacing. It was found that the less creep resistant microstructure was the one with a greater secondary γ′ size, wider γ channel width, and higher volume fraction of tertiary γ′. Deformation in this microstructure commences by way of a/2<110> dislocations concentrated in the γ matrix at lower strains, which then transition to a SISF precipitate shearing mode at larger strains. The more creep resistant microstructure possessed a finer γ channel width spacing, which promoted a/2<110> dislocation dissociation into a/6<112> Shockley partials at lower strains and microtwinning at higher strains. Dislocation precipitate interaction was further explored using microscopic phase field modeling, which was able to capture key microstructural aspects that can favor dislocation dissociation and decorrelation since this appears to be a precursor to the microtwinning deformation mode. New viable diffusion pathways associated with the reordering processes in microtwinning have been explored at the atomistic level. All of the above activities have shed light onto the complex nature of creep deformation mechanisms at higher temperatures.

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.

A study was conducted on the influence of grain boundary elements (GBE) on several P/M nickel base superalloys. Evaluations included microstructural response to chemistry and processing, tensile, creep, rupture, and fatigue crack growth testing (FCG). Boron was found to be the most influential element in the study, with increased boron leading to increased grain boundary triple point void formation and tendency for Thermally Induced Porosity (TIP). Boron reduced creep life but increased both rupture life and rupture ductility and slightly reduced hold time crack growth rates. Carbon also influenced microstructure by slightly refining grain size, and slightly reduced creep and hold time crack growth rates. Hafnium had slight impacts on yield strength and hold time crack growth rates. Magnesium overall had negligible effects. Tantalum had several significant effects, improving creep life and reducing crack growth rates. Overall, the results confirm that careful control and optimization of grain boundary chemistries and grain boundary microstructure are necessary to achieve maximum performance of P/M superalloys.