Mechanical behavior of nanostructured metals

Nanostructured Fe-10 atomic% Cu alloy powders were synthesized by attrition milling to have an extended solid solution with a grain size of 18 nm. Hot-isostatic-pressing at 600;Mechanical behavior was investigated by Vickers hardness, uniaxial compression, and uniaxial tension testing. The grain-size dependence of strength followed normal Hall-Petch behavior. The Hall-Petch constants were in general agreement with those of bulk iron. A yield strength anisotropy between tension and compression was observed.;Plastic deformation of these alloys occurred by inhomogeneous, localized shear banding from the yield point until fracture or buckling. There were no observations of uniform plasticity. The stress-strain response was nearly perfectly plastic. Transmission electron microscopy revealed that the grain structure within the bands was highly elongated in the shear direction, while the structure outside the bands was identical to the undeformed samples. The perfectly plastic response and shear band deformation were similar to the behavior of metallic glasses and amorphous polymers as discussed herein.;The yield strength anisotropy and angles of the shear band planes suggested that plastic flow in these nanostructured alloys depended on the hydrostatic as well as the deviatoric components of stress. A continuum plasticity model was applied with a pressure-sensitive yield criterion to understand the flow localization, the strength anisotropy, and the shear band orientation with respect to the stress states and the sample geometries. A gradient plasticity theory was found to correlate the shear band widths as a function of grain size quite well.;A new model was developed to explain the deviations from normal Hall-Petch behavior observed in ultra fine grained solids. The new model takes into account the increasing influence of grain boundary material with decreasing grain size. The model compared favorably to data from the literature and to the behavior of the nanostructured alloys studied herein.