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Gas turbines are widely used for a variety of purposes including power generation, compression or as jet engines in aircrafts. The critical components of a gas turbine are the high-pressure turbine blades which operate under severe conditions. These include thermo-mechanical loadings over temperatures ranging from room temperature up to 1100°C.
While a large number of constitutive models for single crystals have been proposed, most applications are restricted to special loading scenarios, temperature range and deformation mechanisms. In particular, a number of models are focused on pure creep. Only a few papers consider application of both creep and fatigue. Applications of the constitutive models to long-term stress relaxation are even scarcer. The new model assumes deformation-induced softening and can properly reproduce the viscous behavior at different time scales.
The model has been calibrated with the uniaxial tests at 800°C and 950°C in [001], [011] and [111] specimens of a nickel-basis superalloy. The predicted creep, short- and long-term relaxation and cyclic tests are in reasonable agreement with the experimental observations.
Data about the creep of metals and their alloys at temperatures close to the melting point are very limited. The reason is that most engineering alloys are used at temperatures below 0.6-0.8 of their melting point, so, investigation of creep at higher temperatures has usually no practical relevance. For some special applications however it is important, in our case hot isostatic pressing (HIP) of single-crystal turbine blades cast from nickel-base superalloys. In order to remove porosity the blades are HIPed at temperatures above GammaP-solvus where superalloy has no strengthening GammaP-phase and therefore is very soft. E.g., the company Howmet Castings hips the superalloy CMSX-4 at 1288°C, which corresponds to a homologous temperature of about 0.97=1561 K/1612 K (solidus temperature). Knowledge about the creep of CMSX-4 at this temperature and understanding of the creep mechanisms are necessary to model the kinetics of pore closure during HIP.
CMSX-4 single-crystals of [001] orientation and few single-crystals of different orientations, [011], [123] and [111], were tested under creep conditions at 1288°C in the stress range between 4 and 16 MPa. At this temperature which is above the GammaP-solvus (for CMSX-4 1280°C) the superalloy has single phase structure representing the Gamma-solid solution of nickel strengthened by solute atoms. On creep curves of CMSX-4 single-crystals of different orientations measured at 1288°C/10 MPa it is seen that despite such a high homological temperature, 0.97, CMSX-4 shows very high anisotropy of creep rate. The average creep rate of [001] single-crystal in the range 0-30% strain is about 11.5 time faster than that for [111], a ratio, which is even higher than at the practically relevant temperatures 750-1100°, see e.g.. Approximation the strain rate – stress dependence by the Norton power law gave a stress exponent n of about 6 which is an indication of dislocation creep. The specimen shape after testing, analysis of traces of plastic deformation by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) investigations indicate on dislocation slip on the octahedral system <011>{11-1}.This result however does not support the current doctrine that at high temperatures creep of metals and alloys are climb controlled. It is concluded from the obtained results that even at such a high homologous temperature, 0.97, dislocation movement by slip is more preferable than by climb if only relatively week obstacles are present like solute atoms and low angle boundaries (LABs).
It is remarkable that under used testing conditions the necking and recrystallization behavior of differently oriented single-crystals is very different. E.g., the [001] single-crystal showed very large local strain during necking, Phi=99.8%, and no recrystallization (see Fig. 3a), while the [111] single-crystal small necking, Phi=62%, accompanied by recrystallization. Such a specific deformation and recrystallization has to be undestood.
The obtained creep data of CMSX-4 was introduced in a finite element model in order to simulate pore closure during commercial HIP at a temperature of 1288°C.
Gas turbines are widely used for a variety of purposes including power generation, compression or as jet engines in aircrafts. The critical components of a gas turbine are the high-pressure turbine blades which operate under severe conditions. These include thermo-mechanical loadings over temperatures ranging from room temperature up to 1100°C.
While a large number of constitutive models for single crystals have been proposed, most applications are restricted to special loading scenarios, temperature range and deformation mechanisms. In particular, a number of models are focused on pure creep. Only a few papers consider application of both creep and fatigue. Applications of the constitutive models to long-term stress relaxation are even scarcer. The new model assumes deformation-induced softening and can properly reproduce the viscous behavior at different time scales.
The model has been calibrated with the uniaxial tests at 800°C and 950°C in [001], [011] and [111] specimens of a nickel-basis superalloy. The predicted creep, short- and long-term relaxation and cyclic tests are in reasonable agreement with the experimental observations.