Mechanical properties of glassy nanopillars
A comparative, computational study of size effects in nanoglasses and homogeneous bulk glasses
by K. Albe
We have investigated the influence of structure size on the mechanical properties of NG and HG nanopillars with 7 nm grain size and diameters ranging from 4.5 up to 54 nm by means of MD simulations. Simulations were done for two different glasses, namely, Cu 64 Zr 36 and Pd 80 Si 20 , as representatives of metal–metal and metal–metalloid systems, respectively. Different from previous studies, the NGs were produced by consolidation rather than Voronoi tessellation and thus have a more realistic microstructure. Our results show a clear difference in the deformation mode between NG and HG for the 36 and 54 nm nanopillars, independent of the glass type. While HG nanopillars exhibit a stress drop and strain localization developing in a shear band, NG nanopillars show ductile deformation behavior with softening at larger engineering strains and deformation by necking. The tensile ductility of about 13–15% found in our simulations is in agreement with 15% plastic strain observed for a 400 nm Sc 75 Fe 25 NG nanopillar using in situ tensile tests in a transmission electron microscope (Wang et al., 2015). HG and NG nanopillars with D = 4.5 nm, where the pillar diameter is smaller than the average grain size d = 7 nm of the NG, deform by necking since the nucleation of STZs on the surface is dominating. In the HG nanopillars with D = 9 and 18 nm, shear banding is more obvious in Cu 64 Zr 36 than in Pd 80 Si 20 . When reducing the NG nanopillar diameter to near or double the average grain size, strain softening appears at the larger engineering strain (>20%). Moreover, structural relaxation after a cyclic loading leads to local recovery, and the stress increases upon reloading. We determined Young’s modulus and yield strengths from stress–strain curves of tensile deformations. We find that both properties are smaller in the NG nanopillars as compared with their homogeneous counterparts in both glasses. From Young’s modulus values and the shear band energy, the critical stress for shear band formation is estimated. We find that the predicted critical stress values are quite consistent with the observed deformation modes.