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

Turbine blades in gas turbine engines are subjected during operation to triaxial stress fields. For the description of the deformation behaviour of anisotropic single-crystal blades, constitutive equations are required which take account of modifications to the deformation processes caused by evolution of the gamma/gamma' microstructure during service (gamma' rafting). A microstructure-dependent, orthotropic Hills potential, whose anisotropy coefficients are connected to the edge length or gamma' particles, has been applied. The shape of gamma' particles remains cubic below exposures at 700 degreesC. At high temperatures (above 850 degreesC) the gamma' particles coalesce to rafts, and the viscoplastic response of the superalloy is continuously modified. This reduces the creep resistance of [001] orientated specimen. After tensile loading of the [001]-orientated specimens at 1000 degreesC, the rafting of gamma' in the (100) plane was observed as expected, whereas the [111] specimens did not reveal gamma' rafting. Torsionally loaded specimens exhibited rafting only in the near [001]-orientated surface regions of the specimen. The deformation in the [111] tensile and [001] torsion specimens 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 simulated successfully by the microstructure-dependent model.