Evolution of dislocation structures and deformation behavior of iron at different temperatures: Part II. dislocation density and theoretical analysis

Lan, Ya‐Qian; Klaar, Hans-Joachim; Dahl, Winfried

doi:10.1007/bf02801172

Cited by 37 publications

(18 citation statements)

References 15 publications

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“…(1) holds for the specimens cold-rolled up to 90% in reduction. [3][4][5] The dislocation density estimated by W-H method almost agrees well with the results obtained by neutron diffractometry 6) and TEM observation, 7,8) and thus, the measurement using X-ray diffraction could be regarded to be a valid method for obtaining dislocation density of steels, at least ferritic single-structured iron. However, when the dislocation density of commercial low-carbon steels is measured by W-H method, the effect of pearlite structure should be taken into account.…”

Section: Introductionsupporting

confidence: 72%

Effect of Pearlite Structure on Lattice Strain in Ferrite Estimated by the Williamson-Hall Method

et al. 2015

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The effect of pearlite on the X-ray diffraction peak reflected from ferrite phase in ferrite-pearlite steel was investigated using normalized carbon steels with different volume fraction of pearlite and a hypereutectoid steel with different interlamellar spacing. The lattice strain in ferrite phase, which causes the broadening of X-ray diffraction peak, was increased in proportion to both of the volume fraction of pearlite and the inverse of interlamellar spacing. As a result, the lattice strain in ferrite-pearlite steel can be simply formulated as a function of them. On the other hand, TEM observation reveals that pearlite has low-density dislocation in ferrite phase. This result suggests that the misfit between ferrite and cementite in pearlite generates the significant amount of elastic strain, which leads to the increase in lattice strain. Therefore, the dislocation density must be overestimated in carbon steels with pearlite, if it is estimated from the experimental lattice strain directly.

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Section: Introductionsupporting

confidence: 72%

Effect of Pearlite Structure on Lattice Strain in Ferrite Estimated by the Williamson-Hall Method

et al. 2015

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“…[28,29,30] At this point, we can define a dislocation density in cell walls ( w ) and a dislocation density in cell interiors ( i ). The dislocations in cell walls are accumulated in a very small volume in comparison with those remaining in cell interiors [28][29][30][31] (Figure 10). The mean cell size at these strains is smaller than others previously reported; [14,15] thus, the material has undergone moderate deformation.…”

Section: Dislocation Arrangementmentioning

confidence: 98%

Change of Young’s modulus of cold-deformed pure iron in a tensile test

Benito

Jorba

Manero

et al. 2005

Metall Mater Trans A

115

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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.

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“…It is worth noting that most of the materials tested had been prestrained. The plastic deformation produces an increase in the dislocation density in both steels 24,29,33) and a decrease in the pinning points in the dislocations that are moved during deformation process, as for some time after deformation they are free from interstitial atoms. 34,35) These conditions are suitable for microplastic strain to appear.…”

Section: Inelastic Effects On Loadingmentioning

confidence: 99%

“…However, the rate of increase of free dislocations is high at the beginning but is reduced as plastic strain advances. 18,33,36) Therefore, the fall in the E T values and the increase in the percentage of non-linear or plastic recovery during unloading should stop at high degrees of deformation. These values are approximately 0.06 for TRIP 700 and 0.08 for TRIP 800.…”

Section: Inelastic Effects On Unloadingmentioning

confidence: 99%

Study of the Inelastic Response of TRIP Steels after Plastic Deformation

2005

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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...

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Evolution of dislocation structures and deformation behavior of iron at different temperatures: Part II. dislocation density and theoretical analysis

Cited by 37 publications

References 15 publications

Effect of Pearlite Structure on Lattice Strain in Ferrite Estimated by the Williamson-Hall Method

Effect of Pearlite Structure on Lattice Strain in Ferrite Estimated by the Williamson-Hall Method

Change of Young’s modulus of cold-deformed pure iron in a tensile test

Study of the Inelastic Response of TRIP Steels after Plastic Deformation

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