In order to investigate the strength and the deformation properties of metals in nearly perfect crystal, nanoindentation test is performed on aluminum samples with the various purities: 99.9999%(6N), 99.99%(4N) and 99%(2N). It is widely known that the strength decreases with increasing the purity of metals. In nanoscale, however, the strength has been revealed to close the ideal shear strength, because the area in nanoscale is expected to behave as a perfect crystal.
In this study, the nanoindentation system that is able to obtain indentation load vs. penetration depth curves is employed. The experiment is performed on ultrahigh-purity aluminum (99.9999%), and on high-purity aluminum (99.99%) and purity aluminum (99%) so as to discuss the relationship between the purity level and the mechanical properties at room temperature. It is revealed that penetration depth decreases with increasing purity, and the critical shear stress estimated from the experimental results indicates the value close to the ideal shear strength. These results suggest that a perfect crystal is harder than an imperfect crystal. Furthermore, the indentation load vs. penetration depth curves indicate the discontinuous deformation of the metals. It is considered that these discontinuities are caused by the existence of impurities or the initiation and the multiplication of dislocations in these samples.
