The effect of oxide particles on the strength and ductility of bulk iron with a bimodal grain size distribution

Abstract

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.

Prediction of Generation of High- and Low-Angle Grain Boundaries (HAGB and LAGB) During Severe Plastic Deformation

“…On the other hand, other authors have reported even smaller grain sizes for iron processed by different SPD techniques, e.g. Tejedor et al [26] and Casas et al [27] obtained grain size values of ~200 nm. In those cases, the greater grain size reduction can be attributed to the deformation techniques used by the authors (High Pressure Torsion (HPT) and mechanical milling respectively), which are well known by their capacity to generate more dislocations at high deformations.…”

Improvements in nanoscale zero-valent iron production by milling through the addition of alumina

“…Consequently, the reactivity of M48h-Al O particles must then be larger than in the cases where no alumina is used during milling (Köber et al 2014 ;Li et al 2009 ) or where the particles are not milled, as is the case of N25P (Crane and Scott 2012 ). Here it should be added that during milling there is a breakdown of the initial oxide layer of former particles which causes a number of the oxides to become finely dispersed throughout the new welded particles, giving rise to new surfaces with less oxygen content (Casas et al 2015 ;Li et al 2009 ).…”