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Casas C.,Polytechnic University of Catalonia | Tejedor R.,Polytechnic University of Catalonia | Rodriguez-baracaldo R.,National University of Colombia | Benito J.A.,Polytechnic University of Catalonia | And 3 more authors.
Materials Science and Engineering A | Year: 2015

The strength and ductility of bulk nanostructured and ultrafine-grained iron containing 0.39% oxygen by weight was determined by tensile tests. Samples were obtained by consolidation of milled iron powder at 500. °C. Heat treatments were designed to cover a wide range of grain sizes spanning from 100 to 2000. nm with different percentages of coarse and nanostructured grain areas, which was defined as a 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 analysed following two approaches. The first one was based on the strong effect of oxides and involved the use of a mixed particle-grain boundary strengthening model, and the second one was based on simple grain boundary strengthening. The mixed model underestimated the strength of nanostructured samples, whereas the simple grain boundary model worked better. However, for specimens with a bimodal grain size, the fitting of the mixed model was better. In this case, the more effective particle strengthening was 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 containing oxides promoted strain hardening due to the increase in dislocation activity. © 2014 Elsevier B.V. Source

Benito J.A.,Polytechnic University of Catalonia | Benito J.A.,Fundacio Ctm Center Tecnologic Of Manresa | Casas C.,Polytechnic University of Catalonia | Tejedor R.,Fundacio Ctm Center Tecnologic Of Manresa | And 2 more authors.
Proceedings of the Euro International Powder Metallurgy Congress and Exhibition, Euro PM 2011 | Year: 2011

In this work consolidated specimens of iron have been obtained by a warm static pressing method of a nanostructured iron powders produced by mechanical milling. After consolidation and suitable heat treatments, tensile test specimens have been obtained with different grain sizes within the nanostructured and ultrafine ranges. Mechanical milling has been modified in order to obtain different oxygen contamination in the nanostructured powders, so three kind of consolidated specimens with different oxygen content have been obtained. Tensile tests of samples with similar microstructure but with different oxygen content have been compared and the role of oxygen in the microstructure studied. In general, as the oxygen content increases, the strength increases but ductility of consolidated specimens is seriously affected. Source

Benito J.A.,Fundacio Ctm Center Tecnologic Of Manresa | Benito J.A.,Polytechnic University of Catalonia | Cobo R.,Fundacio Ctm Center Tecnologic Of Manresa | Lei W.,Polytechnic University of Catalonia | And 4 more authors.
Materials Science and Engineering A | Year: 2016

The stress-strain response of a Fe-17.5Mn-0.7C-2Al TWIP steel during cyclic loading has been investigated by means of tension-compression tests within the strain limits of ±2%, ±5% and ±10%. In addition, the microstructural evolution during the ±5% cyclic test has also been studied. The difference between the forward and reverse stress for each pre-strain has been analyzed at 0.2% offset strain and at the strains in which forward and reverse curves were parallel in order to study the Bauschinger effect (BE) and permanent softening, respectively. The evolution of the BE with pre-strain for this steel is similar to other FeMnC TWIP steels, that is, increasing values of BE are obtained as the pre-strain increases. However, its absolute values are half those reported in the literature on other FeMnC steels. This diminution of the BE is related to the lower activity of mechanical twinning in FeMnCAl TWIP steels at the pre-strains herein investigated, which promotes less polarized stresses in the matrix due to the lower dislocation storage capacity.Regarding permanent softening, the evolution is similar to that of the BE and the same analysis can be applied. During reverse compression, a slight increase of twin thickness and twin spacing with respect to the first tensile stage took place. This fact might be linked to the lower flow stress observed in the permanent softening period during reverse straining. © 2016. Source

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