Samples of nanostructured and ultrafine grained steels with carbon content ranging from
0.05 to 0.55%wt. have been obtained by a warm consolidation process from mechanically milled
powders and subsequent heat treatments. In general, homogeneous grain size distributions were
obtained except for the low-carbon steel in which a bimodal grain size distribution was observed
when it was heat treated at high temperatures. The stress-strain response has been studied by means
of compression tests. Nanostructured materials showed high strength but poor results in terms of
ductility. In the low-ultrafine range (mean grain size between 100-500 nm) the three materials
showed an increase in the ductility with strain softening. Finally, when the average grain size was
close to 1 µm samples showed larger ductility and strain hardening.
This paper reviews the ductility of nanostructured and ultrafine iron obtained using a variety of methods. Mechanical milling of powder and subsequent hot consolidation, one of the most popular methods offer high mechanical strength but poor ductility. Improvements made in the consolidation processes and the introduction of final heat treatments, in addition to new approaches such as spark plasma sintering and high pressure torsion, have increased the total plastic strain of nanostructured iron. The development of bimodal structures enables the existence of strain hardening and more uniform deformation. The paper also includes a steel study, which finds that the hardness of milled powder and the role of carbon atoms inside ferrite grains make it more difficult to improve the ductility of nanostructured samples.
Abstract.The strength and ductility of bulk nanostructured and ultrafine grained iron with 0.39% oxygen in weight was determined by tensile tests. The samples were obtained by consolidation at 500 ºC of milled iron powder. Heat treatments were designed to cover a wide range of grain sizes from 100 to 2000 nm with different percentages of coarse and nanostructured grain areas, which is defined as bimodal grain size distribution.Transmission electron microscopy was used to determine the diameter, volume fraction and location of oxides in the microstructure. The strength was analyzed following two approaches, the first one based on a high influence of oxides, a mixed particle-grain boundary strengthening model, and the second one based on simple grain boundary strengthening. The mixed model underestimated the strength of nanostructured samples whereas simple grain boundary model worked better. However, for specimens with bimodal grain size the fitting of the mixed model was better. In this case, the more effective particle strengthening is related to the dispersion of oxides inside the large ferrite grains. In addition, the bimodal samples showed an acceptable combination of strength and ductility. Again, the ferrite grains with oxides inside promoted strain hardening due to the increase in dislocation activity.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.