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SHEAR BEHAVIOR AND RELATED MECHANISMS IN MATERIALS PLASTICITY

Texture and Mechanical Behavior of Magnesium During Free-End Torsion

[+] Author and Article Information
Benoît Beausir

Laboratoire de Physique et Mécanique des Matériaux, Université de Metz, Ile du Saulcy, 57045 Metz, FranceFaculté de Génie, Université de Sherbrooke, Sherbrooke, QC, J1K 2R1, Canada

László S. Tóth

Laboratoire de Physique et Mécanique des Matériaux, Université de Metz, Ile du Saulcy, 57045 Metz, France

Fathallah Qods

Department of Material Science and Engineering, School of Engineering, Semnan University, 3519645399 Semnan, Iran

Kenneth W. Neale

Faculté de Génie, Université de Sherbrooke, Sherbrooke, QC, J1K 2R1, Canada

J. Eng. Mater. Technol 131(1), 011108 (Dec 22, 2008) (15 pages) doi:10.1115/1.3030973 History: Received January 07, 2008; Revised October 10, 2008; Published December 22, 2008

Torsion experiments were carried out on pure magnesium (99.9%) and the magnesium alloy AZ71 under free-end conditions of testing. The alloy had an axisymmetric initial texture, while the pure Mg samples were prepared from a rolled plate with a nonaxisymmetric initial texture. The torque as a function of the twist angle was measured at different temperatures (room temperature, 150°C, and 250°C). During twisting, systematic shortening of the samples was observed (Swift effect). The evolution of the crystallographic texture was analyzed by electron backscattering diffraction measurements. The occurrence of dynamic recrystallization (DRX) was detected in pure Mg at 250°C. The Swift effect in the axisymmetric samples was simulated with the “equilibrium equation” approach using polycrystal modeling. In the nonaxisymmetric samples, the texture was simulated at different angular positions with the help of the viscoplastic self-consistent model. The changes in the textures due to DRX were explained in terms of the Taylor factor. Finally, the texture evolution was interpreted with the help of the behavior of ideal orientations and persistence characteristics of hexagonal crystals in simple shear.

Copyright © 2009 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Sample dimensions

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Figure 2

Shape of pure Mg sample deformed at 250°C with a strain rate of 7.7×10−4 s−1. (a) Photo of the surface of the bar after torsion displaying the helices. (b) Longitudinal cross section of the deformed specimen with black spots showing the positions where textures were measured. (c) Cross section of the deformed specimen showing its elliptical shape. (d) Positions of the measurement places before deformation as indicated by the black full circles. The angles are the rotations that took place to reach the final positions.

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Figure 3

Shear strain–axial strain curves for (a) pure magnesium and (b) AZ71

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Figure 4

Stress-strain curve for AZ71 obtained in free-end torsion at a shear rate of 2×10−3 s−1

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Figure 5

EBSD maps after deformation of (a) 0.9 shear in AZ71 and (b) 1.6 shear in pure magnesium. The color code (c) corresponds to the direction of the r axis of the sample within the unit triangle of the hcp stereographic projection.

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Figure 6

Measured and simulated texture evolution during torsion of magnesium AZ71

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Figure 7

Initial texture of the pure Mg sample, measured in the undeformed part, in the head of the sample. Note that this texture is strongly nonaxisymmetric. The initial plate normal direction is along the horizontal axis.

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Figure 8

Texture evolution in the four measured positions: Face1, Ear1, Face2, and Ear2 in (10.0) and (00.2) pole figures

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Figure 9

Ideal orientations of magnesium under simple shear as they appear in the (a) (10.0) and (b) (00.2) pole figures (Beausir (17)). The fiber names are B, basal; P, prismatic; Y, pyramidal; C1, pyramidal-I; and C2, pyramidal-II.

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Figure 10

Slip system families in hexagonal structures; basal, prismatic, pyramidal ⟨a⟩, pyramidal ⟨c+a⟩/I, and pyramidal ⟨c+a⟩/II

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Figure 11

Texture evolution for the torsion of AZ71 in the φ2=0 ° ODF section. (a) Initial texture, (b) deformation texture, (c) simulated texture by simple shear at γ=0.9, and (d) simulation by simple shear at γ=4.0.

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Figure 12

(a) Comparison of experimental and simulated strain hardening in AZ71. (b) Simulated strain hardening at the four measured positions in the pure Mg bar.

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Figure 13

Texture evolution for torsion of pure Mg in the ⟨⟨Face-1⟩⟩ position in the φ=90 ° ODF section. (a) Initial texture, (b) deformation texture, (c) simulated texture, and (d) map of the Taylor factor in the same section of the orientation space.

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Figure 14

Texture evolution for pure Mg in the ⟨⟨Face-2⟩⟩ position displayed in the φ2=30 ° ODF section. (a) Initial texture, (b) deformation texture, and (c) simulation.

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Figure 15

(a) Measured texture in the pure Mg sample in the ⟨⟨Face-2⟩⟩ position displayed in the φ=40 ° ODF section. (b) Map of the Taylor factor in the same section of the orientation space. Thick lines indicate the minimum Taylor factor position at the fiber position.

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