Mechanical response of nanocrystalline steel obtained by mechanical attrition

The mechanical properties of bulk specimens of nanocrystalline 0.55% C steel with a grain size of 30 nm and a relative density higher than 97% have been determined. Samples were obtained by cold compaction and warm sintering at 425 °C of nanocrystalline powders obtained by mechanical attrition in a planetary ball mill. In both processes an Ar protective atmosphere was used in order to avoid oxygen contamination. X-ray diffraction (XRD) and Transmission electron microscopy (TEM) analysis indicated that a volume-averaged grain size of 30 nm is maintained after the warm consolidation processes. TEM studies also showed equiaxed ferrite with no dislocations inside the grains. However, the grain size distribution was no homogeneous as large grains of 100 nm were observed. An average hardness of 8.5 GPa was obtained, in good agreement with other bulk specimens of nanocrystalline Fe or eutectoid carbon steel prepared by other authors. Compression tests of bulk specimens at a strain rate of 10⁻⁴ s⁻¹ showed a compression strength near 2,500 MPa with an absolute lack of ductility. Nanoindentation measurements at room temperature provided a strain rate sensitivity parameter of 0.012, indicating that the deformation mechanism is somehow governed by diffusion mechanisms.