Indentation creep: New machine and test methodology development at various length scales, high temperatures and low deformation rates

• Applicant: Professor Dr.-Ing. Karsten Durst
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins ; Metallurgical, Thermal and Thermomechanical Treatment of Materials
• Term: since 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 326946902

In the first funding period, a new high temperature and high load nanoindenter, which is operated inside a scanning electron microscope (SEM) under vacuum condition has been developed and successfully installed at TU Darmstadt. The operational temperature of the new system ranges from RT up to 1100°C and dynamic indentation experiments with max. loads of up to 1 N are possible. During the first period, the system was successfully applied to glass, Mo and Ni and material properties like modulus of elasticity, hardness as well as rate sensitivity and activation volume have been successfully determined in the whole temperature range. Furthermore, new test protocols, like the constant contact pressure method have been developed for determination of creep properties and the brittle to ductile transition has been successfully analyzed using dynamic indentation testing. High temperature nanoindentation experiments on alloys for high temperature application remains however challenging, mainly due to the high hardness of the alloys at elevated temperature and the strong chemical interaction of the tip material with the samples. This leads to very high tip wear and potentially unreliable data acquisition. Nevertheless, we were able to show that by applying test procedures using large indentations depths and/or short contact times, with a new step load creep method, even critical materials such as Ni can be successfully tested at ultra-high temperatures. During the second funding period it is planned to address the issues mentioned above by developing a user toolbox for testing relevant high temperature materials like Ni-based alloys at or close to their application temperature. This toolbox will be designed as an application guide to select the appropriate test procedure, tip geometry as well as a suitable combination of sample and tip material for the respective purpose.The focus here will be on the development of new loading protocols for non-pyramidal indenter geometries for characterizing material parameters on different length and time scales, in particular, for testing relevant Ni-based HT alloys. Therefore, different type of indentation experiments will be performed on reference materials (Ni-solid solutions, NiAl, IN718, ERBO1A) with well-known mechanical properties at elevated temperatures, whereas the data evaluation of new tip geometries will be supported by Finite Element modelling. Furthermore, new and more stable tip materials will be sought and tested within the project. Therefore, a diffusion couple approach is used to analyze the chemical interaction between critical sample and potential new tip materials. Chemically inert materials are then used as new tip materials, studying their performance during indentation testing at elevated temperature.

• DFG Programme: Research Grants