On the breakdown of the Nix-Gao model for indentation size effect

ABSTRACT

A new approach accounting for the indentation size effect in metals with the dislocation mediated deformation mechanism is proposed in the paper. The model is based on the assumption that the number of dislocations contained in the ‘effective’ plastic zone under the contact area incrementally scales with the penetration depth with $h^m$, which leads to the modification of the Nix-Gao model, so that the hardness dependence on penetration depth can be expressed as: $H=H_0\sqrt{1+{\displaystyle \frac{h_0}{h}\left(1-e^{-{\displaystyle \frac{h^n}{h_1}}}\right)}}$. The approach is verified on a set of materials including single crystals, pure polycrystalline metals as well as alloys. The effect of materials parameters and the indenter geometry on the validity of the model is discussed.

KEYWORDS:

• Hardness
• indentation size effect
• dislocations
• plasticity of metals

Acknowledgements

Financial support by the ERDF under the project No. CZ.02.1.01/0.0/0.0/17_048/0007373 ‘Damage Prediction of Structural Materials’ and by the Technology Agency of the Czech Republic under the project No. TH02020565 ‘Assurance of Safe and Long Term Operation of Nuclear Reactor Pressure Vessel Internals’ is gratefully acknowledged.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by ERDF [grant number CZ.02.1.01/0.0/0.0/17_048/0007373]; Technology Agency of the Czech Republic [grant number TH02020565].