Modelling of the mechanical behaviour of the single-crystal turbinealloy CMSX-4 during thermomechanical loading

Abstract

Turbine blades in gas turbine engines are subjected during operation totriaxial stress fields. For the description of the deformation behaviour ofanisotropic single-crystal blades, constitutive equations are required whichtake account of modifications to the deformation processes caused by evolutionof the γ/γ' microstructure during service (γ' rafting). Amicrostructure-dependent, orthotropic Hills potential, whose anisotropycoefficients are connected to the edge length of γ' particles, has beenapplied. The shape of γ' particles remains cubic below exposures at700 °C. At high temperatures (above 850 °C) the γ'particles coalesce to rafts, and the viscoplastic response of thesuperalloy is continuously modified. This reduces the creep resistance of⟨001⟩ orientated specimen. After tensile loading of the⟨001⟩-orientated specimens at 1000 °C, the rafting ofγ' in the (100) plane was observed as expected, whereas the⟨111⟩ specimens did not reveal γ' rafting. Torsionallyloaded specimens exhibited rafting only in the near⟨100⟩-orientated surface regions of the specimen. Thedeformation in the ⟨111⟩ tensile and ⟨001⟩ torsionspecimens occurred by octahedral slip of dislocations and not by cubic slip,as expected from theoretical considerations. Rafting did not occur in the⟨111⟩-orientated specimens. This anisotropy change is simulatedsuccessfully by the microstructure-dependent model.