Activation parameters from nanoindentation repeated creep experiments: Spherical indenter vs cylindrical flat punch

This study explores a repeated creep indentation approach to investigate the underlying deformation mechanisms in metals by extracting key deformation activation parameters. Using a thermally stable in-situ nanoindenter, repeated indentation creep tests were performed on nanocrystalline (nc) nickel (Ni), and the results were systematically compared with corresponding micropillar compression experiments. To explore the influence of tip geometry and material response, two distinct non-self-similar indenter geometries, spherical and cylindrical flat punch indenters, were used. The spherical indenter exhibited a linear decay in stress relaxation with increasing depth, while the flat punch showed minimal stress relaxation (∼10 MPa) on nc Ni. This behavior is reflected in the extracted activation parameters: for the spherical indenter, the activation volume increased linearly with representative indentation strain from 4.25 to 13.7 b3, whereas for the flat punch, it remained stable at 11.8 ± 1.5 b3. Due to obtaining consistent activation parameters with the cylindrical flat punch, it was further used to perform repeated indentation creep tests on single-crystal chromium (sx Cr) and single-crystal nickel (sx Ni). These tests demonstrated the reliability of the cylindrical flat punch geometry in capturing activation parameters across different material systems.