Grain size dependence of mechanical properties in nanocrystalline selenium

Porosity-free nanocrystalline element selenium (nc-Se) samples with the mean grain sizes ranging from 8 to 70 nm were: synthesized by complete crystallization of the melt-quenched amorphous Se solid. Mechanical properties including microhardness (H-nu) and elastic modulus (E) of the nc-Se samples were measured by means of nanoindentation tests and microhardness tests, respectively. With a reduction of grain size, the nc-Se samples were found to be substantially hardened. But the grain size dependence of H-nu does not follow a simple Hall-Fetch relation over the whole grain size range, exhibiting three distinct stages corresponding to three different Hall-Fetch slopes. The maximum Hall-Fetch slope was found to be in the grain size range of 15-20 nm, corresponding to large values of the: elastic modulus. This behavior can be explained in terms of the lattice distortion in the nc-Se samples that was experimentally determined by using quantitative x-ray diffraction measurements. A conclusion is drawn that the lattice structure of the nm-sized crystallites may play an important role in mechanical properties of nanocrystalline materials.