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Changes in Young's modulus E (determined according to ASTM E-111) of polycrystalline pure iron deformed by a tensile test at room temperature are determined. From its original mean value, 210 GPa, E decreased with deformation to a mean value of 196 GPa at ϭ 0.060. Thereafter, slight recovery occurred and E stabilized to 198 GPa until the appearance of neck ( ϭ 0.100). With the aim to examine the causes of this behavior, residual stresses and textures were measured and the dislocation structure was observed by transmission electron microscopy (TEM). Longitudinal residual stresses increased from the first step of deformation ( ϭ 0.015) and remained constant until the samples fractured. There was no significant difference in texture throughout the deformation process, during which the increment of ␣ fiber was smooth. Thus, the decrease of E cannot be attributed to residual stresses or textures. A relationship between dislocation arrangement and decrease of E is proposed. Following the model established by Mott, dislocations can bow out in their glide planes, giving extra elastic strain and thus a decrease of E. The increase of the dislocation density during the first steps of deformation lowers the E values, since the extra elastic strain increases. At higher strains, when the cellular dislocation structure has formed (between ϭ 0.060 and 0.080), the dislocations in cell interiors are capable of giving an extra elastic strain, whereas the dislocations trapped in the cell walls are not. However, the dislocation density in cell interiors is lower than the dislocation density in the early stages of deformation in which the cell structure has not been developed. This produces the slight recovery of E measured at these strains. From ϭ 0.080, the values of E stabilized since no changes in dislocation density in cell interiors are observed.

A study of the elastic response before and after tensile plastic strain was undertaken for two commercial low-alloyed TRIP steels. These steels, TRIP 700 (C-Mn-Al alloy) and TRIP 800 (C-Mn-Si) are commercial alloys used in sheet metal stamping. The behaviour of the instantaneous tangent modulus (E T ) versus stress during loading and unloading was measured for each degree of prestrain. Loading curves show a decrease in the E T of the deformed samples as compared with the undeformed state. Though at low stresses a highly linear response was measured for both steels, a decrease was obtained for TRIP 700 as strain increased, whereas TRIP 800 remained unchanged. During unloading, a progressive decrease in E T was obtained in all deformed states, with lower chord modulus values as the tensile plastic prestrain increased. The inelastic response observed is attributed mainly to microplastic strain caused by the displacement of mobile dislocations. Thus, the differences between the two TRIP steels studied are related to the microstructure and the different dislocation structures observed in them. A notable consequence of this study is a better accuracy in the prediction of springback passes due to a better understanding of these inelastic effects that stems from going beyond mere use of traditional Young's modulus values.KEY WORDS: TRIP steels; springback; Young's modulus; dislocation structure. This microplastic deformation results firstly from the short range of motion of mobile dislocations, 19) and secondly from the bowing of the dislocation line between pinning points following models proposed by Mott and Friedel,20,21) and by Granato and Lücke. 22) This extra deformation, which occurs below the internal strength level of the material, is recoverable, and is affected by the dislocation density and the total length of dislocation line that is able to move or bow out. Thus, when e mp grows, a decrease in E is expected. The following Eq. (2), proposed by Granato and Lücke,22) summarizes this idea:where b is the magnitude of the Burgers vector, r is the density of the mobile dislocations and x is the average displacement of a dislocation line of length l.As springback appears after tools have been removed, it is the behaviour of Young's modulus 'during unloading' that should be investigated to obtain a more accurate prediction of the recovery from strain after forming. Ghosh et al. 13,23) used uniaxial tensile tests to study strain recovery after plastic prestrain for a type of steel used in sheet metal stamping, and demonstrated the nonlinearity of the unloading part of the stress-strain curve. This "inelastic behaviour" results in an extra deformation that is not predicted by the usual values of Young's modulus, and again is resumed as microplastic strain. This deformation must be accounted for in FEM simulations in order to better predict the final shape of the pieces stamped. The nature of microplastic strain during unloading is mainly associated with the backward movement of the dislocations that pile up during...

Nanoscale Zero Valent Iron (nZVI) represents a promising material for subsurface water remediation technology. However, dry, bare nZVI particles are highly reactive, being pyrophoric when they are in contact with air. The current trends of nZVI manufacturing lead to the surface passivation of dry nZVI particles with a thin oxide layer, which entails a decrease in their reactivity. In this work an activation procedure to recover the reactivity of air-stable nZVI particles is presented. The method consists of exposing nZVI to water for 36 hours just before the reaction with the pollutants. To assess the increase in nZVI reactivity based on the activation procedure, three types of nZVI particles with different oxide shell thicknesses have been tested for Cr(VI) removal. The two types of air-stable nZVI particles with an oxide shell thickness of around 3.4 and 6.5 nm increased their reactivity by a factor of 4.7 and 3.4 after activation, respectively. However, the pyrophoric nZVI particles displayed no significant improvement in reactivity. The improvement in reactivity is related mainly to the degradation of the oxide shell, which enhances electron transfer and leads secondarily to an increase in the specific surface area of the nZVI after the activation process. In order to validate the activation process, additional tests with selected chlorinated compounds demonstrated an increase in the degradation rate by activated nZVI particles.