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.

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The creep and fatigue properties of the directionally solidified nickel base superalloy CM247LC DS have been investigated. Constant creep tests have been carried out on specimens with different orientations in the temperature range of 700-1OOO'C at different loads to obtain times to rupture up to 35000 h. The comparison of the creep properties with the IN738LC conventionally cast alloy and with the oxide dispersion strengthened MA6000 alloy has shown the better creep performance of the alloy CM247LC DS in the temperature range of interest for application in land based gas turbines. A series of cyclic load creep tests has allowed to study the effect of load variations on the creep. The effect of cycling stress is to increase the strain rate, compared with the constant load creep tests and then to reduce the rupture life of the alloy. The LCF tests, performed at the temperatures of 85O'C and 950°C in longitudinal strain controlled conditions, have evidenced a fairly stable cyclic response. Basquin and Coffin-Manson relationships can adequately predict the fatigue life of the alloy. The CM247LC DS alloy exhibits a better fatigue life than IN738LC and MA6000 alloys.

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