Elsevier

Materials Science and Engineering: A

Volume 594, 31 January 2014, Pages 287-291
Materials Science and Engineering: A

Rapid communication
Annealing hardening in detwinning deformation of Mg–3Al–1Zn alloy

https://doi.org/10.1016/j.msea.2013.11.080Get rights and content

Abstract

The present work reports the effect of annealing treatment on detwinning deformation in Mg alloy AZ31 and pure Mg that have pre-strained twins. It shows that appropriate annealing enhances, rather than reduces, the yield strength of the pre-strained AZ31, but it does not cause any strengthening of the pre-strained pure Mg. STEM–EDS mapping shows that both Al and Zn segregate to twin boundaries in the pre-strained AZ31 after the annealing process. It is proposed that it is the pinning of twin boundary by segregated solute atoms that results in an increased activation stress for detwinning deformation and, hence, annealing hardening.

Introduction

Basal slip and {101¯2}1¯011 twinning constitute the main deformation modes of Mg alloys at room temperature [1], [2], [3], [4]. For hot rolled Mg alloys with a basal texture, {101¯2}1¯011 twinning dominates the initial plastic deformation when the alloys are compressed along the transverse direction (TD) (or the rolling direction (RD)), or are extended along the normal direction (ND) [1], [3]. The compression in the extrusion direction of the extruded Mg alloys with a basal fiber texture is also governed by 1¯011{101¯2} twinning. In general, the {101¯2} twins generated by pre-loading can also detwin during reloading along some specific directions [5], [6], [7]. The detwinning of {101¯2} twins mainly takes place under two types of loading conditions: (1) loading, unloading, followed by reloading along the opposite direction of the initial loading; (2) compressing along TD (or RD) of hot rolled plates, unloading, followed by re-compressing along ND [5], [6]. The twinning–detwinning process is very common in fatigue tests of textured Mg alloys that are subjected to cyclic loading of compression and tension [8], [9], [10], [11], [12], [13], [14]. As the detwinning proceeds by twin boundary migration that does not require the nucleation of new twins, the activation stress for detwinning is quite low. For example, in-situ neutron diffraction measurements demonstrate that the critical resolved shear stresses (CRSS) for twinning and detwinning in the polycrystalline Mg alloy ZK60 (Mg–6 wt%Zn–0.6 wt%Zr) are about 15 MPa and 6 MPa, respectively [7].

Previously, the detwinning deformation has been extensively studied for plastically deformed Mg alloys without any annealing treatments [2], [5], [7], [14]. The influence of annealing treatment on detwinning deformation of Mg alloys is therefore unclear. It is traditionally accepted that annealing treatment generally removes the lattice defects in metals and, thus, causes strength drop. However, in the present paper, we demonstrate that an annealing hardening occurs in the detwinning deformation of Mg alloy AZ31. We further report that such an annealing hardening phenomenon does not occur in detwinning deformation of pure Mg. The corresponding strengthening mechanism is discussed.

Section snippets

Experiments and methods

Hot rolled plates of Mg alloy AZ31 and pure Mg were used in the present study. They both had a typical basal texture. The samples containing {101¯2} twins were prepared by compression along TD of the plate. The detwinning deformation was studied by a further re-compression along ND of the pre-compressed samples at a strain rate of 10−3 s−1. Inhomogeneous deformation regions are commonly known to appear after the compression of block-shaped samples, which greatly affects the deformation

Results

Fig. 1 shows the mechanical properties of pre-strained AZ31 samples under compression along ND. For comparison, the compression strain–stress curve along TD of the hot rolled AZ31 plate, which deforms predominantly by {101¯2} twinning, was also shown (designated TD-Com). The strain–stress curve of the TD-Com sample has a plateau, which is the signature for the predominant occurrence of {101¯2} twinning. Such a plateau also appears in all curves of the pre-strained samples. It is generally

Discussion

AZ31 Mg alloy has a nominal concentration of 2.7 at% Al and 0.4 at% Zn. Al has an equilibrium solid solubility of 11.8 at% at the eutectic temperature and 3.2 at% at 200 °C in the Mg matrix [16]. The solubility of Zn in Mg is about 2.4 at% at the eutectic temperature of 340 °C and about 1.3 at% at 200 °C [16]. Generally, precipitates are absent in AZ31. It is generally accepted that solute atoms in alloys can segregate to high-energy grain boundaries, but not to low-energy twin boundaries. However, a

Conclusion

In summary, we report an annealing hardening phenomenon that occurs during compression tests of pre-strained Mg alloy AZ31, in which detwinning is the major deformation mode. Our experiments further demonstrate that the annealing hardening phenomenon does not take place in the detwinning deformation of pure Mg that is pre-strained and tested under a similar condition. In Mg alloy AZ31, the solute atoms segregate to {101¯2} twin boundaries after annealing, and pin the twin boundaries, which

Prime novelty statement

Detwinning is one of the most important deformation modes of Mg alloys. For pre-strained Mg alloys containing deformation twins, commonly seen in Mg products such as plates, sheets and extrudates, detwinning often dominates the deformation process under certain loading conditions, which greatly influences the deformation behavior and mechanical properties. In this paper, we report an interesting annealing hardening phenomenon in the detwinning process of pre-strained Mg alloy AZ31, which

Acknowledgment

The current study is co-supported by National Natural Science Foundation of China (51371203, 51101175 and 51131009) and National Key Basic Research Program of China (2013CB632204 and 2013CB632205). We are also particularly grateful to Professor Ze Zhang of Zhejiang University for the access to the HAADF-STEM and STEM–EDS work in his lab.

References (17)

  • S.G. Hong et al.

    Acta Mater.

    (2010)
  • X.Y. Lou et al.

    Int. J. Plast.

    (2007)
  • Y.C. Xin et al.

    Scr. Mater.

    (2012)
  • M.R. Barnett

    Scr. Mater.

    (2008)
  • G. Proust et al.

    Int. J. Plast.

    (2009)
  • Y.N. Wang et al.

    Acta Mater.

    (2007)
  • L. Wu et al.

    Acta Mater.

    (2008)
  • C.L. Fan et al.

    Mater. Sci. Eng. A

    (2009)
There are more references available in the full text version of this article.

Cited by (86)

View all citing articles on Scopus
View full text