Abstract: With irons stretched at various limit strains, the dependence of pre-plastic strain on the size effect of nanoindentation hardness are studied by a nanoindenter. It is found that both the hardness HM0 (corresponding to statistically stored dislocation density) and the true stress of iron show the same variation with the increase in the pre-plastic strain. When the pre-plastic strain increases from 0 to 10%, the dislocation density in iron increases sharply, resulting in a quick increase in the tensile stress (from 150 Mpa to 320 Mpa) and HM0 (from 1.13 Gpa to 2.05 Gpa). However, with pre-plastic strain further increasing from 10% to 23%, the dislocation configuration in iron is trans-lated from equal-axis dislocation cell into thin stripy dislocation cell. At the same time, both the tensile stress and HM0 show a slow increase with strain. This study will help to explore the origin of the indentation size effect.

Key words: Dislocation, Indentation size effect, Nanoindentation hardness, Plastic strain