Impact of pile-up on indentation hardness of neutron-irradiated 15Ch2MFA steel

Background/aim: The mechanical response of metallic materials during instrumented indentation is strongly influenced by the deformation mode beneath the indenter, which may involve sink-in or pile-up. This study aims to evaluate the impact of these modes on the prediction of the modified Meyer hardness of 15Ch2MFA steel and its associated mechanical properties in both non-irradiated and neutron-irradiated states. Methods: Analytical formulations were developed to determine the modified Meyer hardness value from the maximum indentation load, contact stiffness and reduced elastic modulus, while accounting for the deformation mode. The influence of substituting one deformation mode for the other was then assessed for both material states. Results: The results demonstrate that incorrectly identifying the deformation mode can introduce significant errors in hardness estimation. For non-irradiated 15Ch2MFA steel, the difference between the two modes of deformation was approximately 0.5%, corresponding to an overestimation of around 11 MPa. For neutron-irradiated steel, however, the effect was substantially greater: an error of around 8%, with an overestimation of approximately 186 MPa. Conclusion: Accurate identification of the indentation deformation mode is essential for reliable mechanical characterisation. This effect is particularly pronounced in the case of neutron-irradiated 15Ch2MFA steel, which indicates that irradiation-induced hardening increases the sensitivity of indentation-based hardness assessment to the assumed deformation mode.