Influence of {10-12} extension twinning on the flow behavior of AZ31 Mg alloy

doi:10.1016/j.msea.2006.09.069

Materials Science and Engineering: A

Volumes 445–446, 15 February 2007, Pages 302-309

https://doi.org/10.1016/j.msea.2006.09.069 Get rights and content

Abstract

Uniaxial compression tests were performed on samples cut along the extrusion direction from AZ31 Mg alloy tubes. A stage of increasing work hardening rate was observed on representative true σ–ɛ curves. Specimens compressed to various strain levels were examined by optical microscopy and electron backscattered diffraction (EBSD) techniques. The results indicate that the widespread formation of intersecting {10-12} extension twins is responsible for the increased strain hardening rate.

Introduction

Two types of deformation modes, slip and twinning, generally take place in Mg and its alloys during plastic deformation. Previous research has shown that the flow curve generated when extension is applied along the c-axes of most of the grains differs considerably from that generated when contraction is applied along the relevant c-axes [1], [2], [3], [4], [5], [6]. It is generally agreed that the pronounced yield asymmetry is associated with the activation of {10-12} twinning during extension (but not contraction) along the c-axis. Thus the asymmetry results from the combination of a sharp initial texture and the polarity of deformation twinning.

Twinning plays two important roles in Mg and its alloys. It contributes to texture evolution by reorienting the twinned areas of the grains. Furthermore, the strain hardening behavior of Mg alloys can also be influenced by twinning [6], [7], [8]. Currently, there are opposing views regarding the consequences of deformation twinning on the subsequent strain hardening behavior. Rohatgi et al. [9] have suggested that the reason why twinning retards the decrease in strain hardening rate is because of the effective grain refinement that results from twinning. By contrast, based on his study of samples aligned for c-axis extension, Barnett [4] has pointed out that reorientation of the c-axes by almost 90° has a more significant effect on strain hardening than the grain refinement produced by twinning.

The phenomenon of enhanced strain hardening occurring in association with twinning has also been observed in titanium by Salem et al. [10]. They suggested that the basic mechanism of strain hardening by deformation twinning is the inhibition of non-coplanar slip by dislocation pile-up and storage at the twin-matrix boundary. This is also a kind of “grain refinement” explanation. From the above studies, it is quite clear that deformation twinning plays an important role in the strain hardening response of hexagonal metals, although consensus has not been reached about the causes of this effect.

By using neutron diffraction-based strain measurements, Brown et al. indicated that the texture evolves quickly and quite sharply when {10-12} extension twinning occurs during the deformation of AZ31B [8]. However, they did not carry out a systematic evaluation of microstructure evolution during plastic deformation and so a suitable correlation between the microstructural events and the different stages of the flow curve is lacking. Because of the importance of an accurate characterization of the evolution of microstructure and texture during deformation, an investigation was conducted on textured polycrystalline Mg samples deformed in uniaxial compression. The aims of the current study were to determine the twinning modes that operate during the deformation of extruded Mg alloy tubes and to clarify the influence of twinning on the strain hardening behavior.

Section snippets

Experimental

The AZ31 tubes were extruded using porthole dies by Timminco Metals in Aurora, Ontario, Canada; they had nominal outer diameters of 70 mm and wall thicknesses of 4 mm. Cylindrical specimens 3.3 mm in diameter and 4.75 mm long were machined from the AZ31 tubes with their axes aligned along the extrusion direction. The chemical composition of this batch of material is presented in Table 1.

Uniaxial compression tests were conducted on a servo-controlled MTS machine. The samples were first heated to the

Flow behavior

Examples of the true stress–true strain curves obtained at various temperatures are presented in Fig. 2. The flow curves exhibit abrupt yielding at around 60–80 MPa followed by a short range of fairly conventional work hardening behavior. After a plastic strain of about −0.08, a point of inflection appears in the flow curves and the rate of strain hardening begins to increase with increasing strain. Finally, at strains of −0.14 to −0.18, the flow stress reaches its peak and begins to decrease.

Discussion

The microstructures in Fig. 4, Fig. 5 are in agreement with those observed by Barnett et al. [7]. It seems that the low apparent fraction of twins in the sample arises because many of the grains have been entirely consumed and reoriented by twinning. This indicates that the potential for twinning is essentially exhausted by the end of stage III. The changes in microstructure illustrated in Fig. 4 show that {10-12} extension twins develop quite quickly. At 200 °C and 0.1 s⁻¹, it only takes 1.1 s

Conclusions

The evolution of {10-12} extension twinning has been studied on specimens cut from extruded AZ31 Mg alloy tubes. It has been shown that certain twinning events coincide with specific stages of the flow curve. These can be described as follows: stage I, the onset of primary {10-12} extension twinning accompanied by macroscopic yielding; stage II, primary twin formation spreads, the flow stress begins to increase, but the work hardening rate remains low; stage III, the formation of dense secondary

Acknowledgments

This research was sponsored by General Motors of Canada and the Natural Sciences and Engineering Research Council of Canada. Discussions with Dr. Matthew Barnett of Deakin University in Australia are gratefully acknowledged. SG thanks to the FNRS of Belgium for financial support.

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Influence of {10-12} extension twinning on the flow behavior of AZ31 Mg alloy

Abstract

Introduction

Section snippets

Experimental

Flow behavior

Discussion

Conclusions

Acknowledgments

Acta Metall.

Scr. Metall. Mater.

J. Light Met.

Mater. Sci. Eng. A

Acta Mater.

Mater. Sci. Eng. A