Grain Size Dependence of Mechanical Properties in Nanocrystalline Selenium

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Abstract
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 (Hv) 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 Hv does not follow a simple Hall–Petch relation over the whole grain size range, exhibiting three distinct stages corresponding to three different Hall–Petch slopes. The maximum Hall–Petch 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.