Abstract
The effect of different initial microstructures of single crystal nickel base superalloy, frequently used as blade material for gas turbines, defined by gamma prime morphology, has been investigated under the creep and creep-fatigue conditions at 900 °C and 500 MPa. For cyclic loading the wave form of stress as a function of time was of trapezoidal shape with a hold time of 10 s at the upper stress level. The fracture life-time and cycles to fracture were criteria being considered to evaluate deformation behaviour of alloy defined by different structure states under complex creep-fatigue loading. The TEM study of thin foils cut from fractured specimens was used to analyze the deformation mechanism in strengthened matrix in dependence on initial alloy structure. As the results showed, the higher creep strength was achieved in case of structures with cuboidal gamma prime morphology. However, the introduction of the cyclic stress component onto creep stress resulted in the lifetime reduction in comparison with the simple creep deformation and in modification of the fracture mode morphology.

Key words
nickel superalloy, directional solidification, heat treatment, microstructure, gamma prime phase, creep, cyclic creep, deformation mode, dynamic recovery, fracture mechanism, lifetime

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