Evolution of type II residual strains in dependence on the microstructure of nickel base superalloys

• Applicants: Dr. Michael Hofmann; Dr.-Ing. Christian Krempaszky; Professor Dr. Guido Schmitz; Di Wang, Ph.D.
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: from 2016 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 280883331

Macroscopic residual stresses may influence the mechanical properties of industrial components strongly. Thus, the reliable analysis of residual stresses is crucial.By diffraction methods, the macroscopic residual stresses are usually determined relative to a stress-free reference sample, which is cut from the component of interest. The macroscopic relaxation of the stress state during cutting is assumed to leave the type II residual stresses (intergranular and interphase stresses) unaffected. However, particularly in high performance alloys with multiple crystallographic phases inside and which undergo complex thermo-mechanical treatments during their production process, a change of type II residual stresses during macroscopic relaxation is conceivable. In a diffractometric residual stress analysis this may cause high spurious stresses and strains, which may lead to high costs due to overestimated residual stresses and thus due to excessively designs. Our preliminary work points to the fact, that the evolution of intergranular and interphase micro strains may be governed by different microstructural mechanisms in IN718 and Haynes282. The goal of this research project is the fundamental understanding of the sources of different micro mechanical behavior of these two nickel based superalloys.The influence of different microstructural parameters (phase fractions, grain sizes, type of phases) will be studied on samples of IN718 and Haynes282 differing in microstructure due to specially designed heat treatment sequences. The characterization of the microstructure will be preformed by conventional methods as optical micrographs and scanning electron microscopy combined with different high end methods as atom probe tomography, transmission electron microscopy and neutron scattering methods. This combination provides a characterization of the microstructure over different length scales and gauge volumes. The evolution of intergranular and interphase micro strains will be done by in-situ neutron diffraction. The comparison of the microstructure before and after plastic deformation combined with the results of the in-situ tests, will help to enlighten the detailed microstructural mechanisms during plastic deformation. The influence of different microstructural parameters on the micro mechanical material behavior will be identified by this procedure. Supplementary simulations will help to describe, analyses and interpret the experimental findings.

• DFG Programme: Research Grants