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Modellierung der Annihilation von Mikroporen in einkristallinen Nickelbasis-Superlegierungen während heiß-isostatischem Verdichten

Fachliche Zuordnung Mechanische Eigenschaften von metallischen Werkstoffen und ihre mikrostrukturellen Ursachen
Förderung Förderung von 2015 bis 2019
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 280132084
 
Erstellungsjahr 2020

Zusammenfassung der Projektergebnisse

The scientific objective of the project was understanding and simulation the HIP process of single crystals of nickel-base superalloys. The problem is quite complex due to very limited information about this process, specific processing conditions (very high HIP temperature) and the microscopic level of the occurring physical processes. Two possible mechanisms of the pore annihilation during HIP were considered: Vacancy mechanism of the pore dissolution by diffusion of vacancies from the pores to the LABs; dislocation mechanism of the pore closure by plastic deformation. It was impossible to identify unambiguously the HIP mechanism from the results of performed SEM and TEM investigations of HIPed specimens, because the evidences for both processes were found. Therefore, it was concluded that the both mechanisms are active and compete. The modeling performed for the standard HIP conditions of superalloy CMSX-4 (1288°C, 103 MPa) supported more the vacancy mechanism than the dislocation mechanism. The vacancy model properly describes the kinetics of the pore annihilation as well as the pressure dependence of the pore annihilation rate. In contrast, the conventional Crystal Plasticity (CP) model predicts dependencies that significantly differ from experimental results. To explain this discrepancy, the non-homogeneity of dislocation sources at the scale of pores must be considered. A model of vacancy diffusion and dislocation transport around pores has been developed. Thereby, it has been assumed that LABs are the only dislocation sources. The Flux Corrected Transport (FCT) method has been implemented in the FEniCS code to solve the transport equations. Instead of the classical displacement-based FE method, the stresses were directly calculated from the dislocation density tensor. First simulations of dislocation motion around a pore suggest that the location of the pore with respect to the dislocation sources plays a crucial role: Since the pore itself is the sole source of shear stresses, there is only little dislocation nucleation when the pore is sufficiently far away from the sources. In accordance, pore shrinking should take place by vacancy diffusion. This could explain the overestimation of pore shrinking rate by conventional CP, since CP implicitly assumes that perfect dislocation sources are homogeneously distributed. It should be noted that the performed modeling was made possible by a large number of supporting experimental studies, including numerous creep tests, diffusion experiments, HIP experiments, synchrotron tomography, SEM and TEM. In conclusion, despite the difficulties encountered during this project valuable practical results were obtained: The performed HIP experiments, characterization of the porosity in HIPed specimens (synchrotron tomography, SEM) and the developed vacancy model can be used for optimization of the parameters of industrial HIP of single-crystal superalloy CMSX-4. The limits of application of classical CP at the microscopic scale were demonstrated. The new dislocation model is expected to be useful to simulate deformation processes by glide and climb at a scale for which the distance between dislocation sources cannot be neglected. Finally, a great amount of the obtained experimental results could be used to support the development of more refined HIP models.

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