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The creep behavior of single crystals of the nickel-based superalloy CMSX-4 was investigated at 1288 °C, which is the temperature of the hot isostatic pressing treatment applied to this superalloy in the industry. It was found that at this super-solvus temperature, where no gammaPrime-strengthening occurs, the superalloy is very soft and rapidly deforms under stresses between 4 and 16 MPa. The creep resistance was found to be very anisotropic, e.g., the creep rate of [001] crystals was about 11 times higher than that of a [111] crystal. The specimens of different orientations also showed a very different necking behavior. The reduction of the cross-sectional area psi of [001] crystals reached nearly 100 pct, while for a [111] crystal psi = 62 pct. The EBSD analysis of deformed specimens showed that despite such a large local strain the [001] crystals did not recrystallize, while a less deformed [111] crystal totally recrystallized within the necking zone. The recrystallization degree was found to be correlated with deformation behavior as well as with dwell time at high temperature. From the analysis of the obtained results (creep anisotropy, stress dependence of the creep rate, traces of shear deformation, and TEM observations), it was concluded that the main strain contribution resulted from <01-1>{111} octahedral slip.
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.