Thermal stability and mechanical behavior of nanocrystalline iron

The purpose of this study was to investigate the mechanical properties of nanocrystalline Fe. Since mechanical properties are best measured on bulk samples, the mechanically attrited Fe powders have to be compacted by temperature assisted compression. The knowledge of the thermal stability of nanocrystalline Fe is therefore crucial for the compaction step.; Isothermal grain growth data are analyzed using two different models of grain growth, one of which takes pinning forces on the grain boundaries into account. A temperature and time dependent change in the grain growth behavior was observed. At higher temperatures and longer times, the model taking the pinning forces into account model gives an activation energy of 248 kJ / mol. The change in the grain growth behavior is believed to be due to the onset of the solute drag effect for higher temperatures and longer times.; The nc Fe was between 2 and 8 times stronger than coarse grained Fe. Within the margin of error, the elastic modulus of the nc Fe was the same as in coarse grained Fe. The Vickers hardness as a function of the grain size was described with a Hall-Petch slope which was smaller than that in coarse grained iron but similar to that of highly strained coarse grained Fe. A change of the morphology around the indentations was observed around 20 nm. In tension the material failed in a macroscopically brittle manner, while local ductility in the form of shear bands in very concentrated areas was observed for samples with grain sizes {dollar}{lcub}>{rcub}20{dollar} nm. The elastic failure in tension for these samples is believed to be flaw controlled. The failure mechanism in samples with grain sizes {dollar}{lcub}<{rcub}20{dollar} nm was particle debonding. The variation of the flaw size are a result of the processing conditions. The compressive characteristics of the nc Fe were similar to those of an elastic-perfectly plastic material with low strain hardening coefficients and a low room temperature strain rate sensitivity.