| Nanocrystalline copper samples were produced and analyzed with the aim of elucidating how very small grain sizes affect mechanical properties of crystalline materials. The mechanical properties and microstructure of the material were investigated. Important mechanical properties explored include yield strength, hardness, and ductility. Such microstructural parameters as grain size, grain morphology, grain size distribution, and bulk density were also evaluated. Grain size and strength in conventional metals are related by the Hall-Petch relation, which is expected to lose relevance at low grain sizes. At average grain sizes of 10–100 nm, previous researchers often noted deviations from Hall-Petch predictions of strength. In contrast, samples in the same grain size regime tested in compression were found to adhere to the relation for both yield strength and hardness in this study. At the lowest grain sizes, a mild reversal in the trend of the Hall-Petch relation may be visible, suggesting a possible shift in the dominant deformation mechanism. In addition, Transmission Electron Microscopy (TEM) was used to observe deformation of nanocrystalline copper in situ. Grains of 40 nm and perhaps smaller sizes appear to deform by dislocation motion, though other mechanisms may also be active. Extensive examinations of loose powder were also performed via TEM. Wide grain size distributions were likely to have arisen during consolidation, because uncompacted powders have a more uniform and smaller distribution. Processing of samples was controlled to achieve a variety of grain morphologies for property evaluation. |
