Displacement-controlled nanoindentation stress relaxation tests for reliable determination of activation volumes

Abstract

This study presents the development of nanoindentation stress relaxation testing as a robust method for extracting deformation activation parameters in materials. Using an in-situ , displacement-controlled nanoindenter, stress relaxation tests were conducted on nanocrystalline (nc) nickel (Ni) with Berkovich, cube-corner, and spherical tips. Results showed good agreement with uniaxial micropillar compression relaxation tests performed on the same sample, demonstrating the reliability of the testing protocol and data analysis methodology. The apparent and effective activation volumes were comparable, indicating minimal changes in deformation substructure during indentation relaxation. Effective activation volumes averaged 8.11 ± 3.3 b3 for indentation and 5.91 ± 0.8 b3 for micropillar compression, with corresponding strain rate sensitivity exponents ( m ) of 0.028 ± 0.002 and 0.028 ± 0.001, respectively. These consistent values underscore the reliability of the indentation-based approach for probing transient plasticity. Additional tests on single-crystal (sx) Ni and chromium (Cr) revealed strong and weak strain dependence of activation volume, respectively. Overall, this work establishes nanoindentation stress relaxation as a promising technique for investigating size effects and transient plasticity mechanisms in metals.