Elsevier

Acta Materialia

Volume 55, Issue 11, June 2007, Pages 3899-3910
Acta Materialia

Twinning and texture development in two Mg alloys subjected to loading along three different strain paths

https://doi.org/10.1016/j.actamat.2007.03.006Get rights and content

Abstract

The evolution of twinning and texture in two Mg-based (+Al, Mn, Zn) alloys was investigated using uniaxial tension, uniaxial compression and ring hoop tension testing at temperatures from ambient to 250 °C and a strain rate of 0.1 s−1. The results indicate that the initial extrusion texture plays an important role in the formation of different types of twins and that the twinning behavior also depends on the strain path. Contraction and double twinning are the dominant twinning mechanisms in uniaxial tension, while extension twinning prevails in uniaxial compression and ring hoop tension testing. Schmid factor analysis indicates that only components that are favorably oriented (i.e., with the highest SF values) can undergo rapid and complete twinning. The different twinning behaviors are shown to be responsible for the sharply contrasting strain hardening characteristics of the experimental flow curves and dramatic texture changes.

Introduction

In magnesium crystals, strain along the c-axis can only be accommodated by 〈c + a〉 slip or twinning. At low temperatures (<200 °C), Mg exhibits a strong propensity for mechanical twinning since the latter has a lower threshold stress than 〈c + a〉 slip [1], [2], [3], [4]. Two types of twins are frequently reported in magnesium alloys: {10–12}〈10–1–1〉 extension and {10–11}〈10–1–2〉 contraction twins. Extension twins are formed when there is an extension strain component parallel to the c-axis, while contraction twins are activated when there is a contraction strain component parallel to this axis. In addition to primary twinning, secondary twinning can take place within the reoriented primary twins. This is known as double twinning. Generally, contraction twins form first, after which {10–12} extension twins are propagated within the original contraction twins. Double twins lead to a net contraction along the c-axis [2].

Kocks and Mecking [5] observed that strain hardening rates are different for different deformation modes. They attributed this behavior to differences in the texture evolution behavior. In Mg alloys, the variations in the strain hardening behavior can also be ascribed to the influence of the initial texture on slip and twinning. Previous research has indicated that the mechanical response as well as the evolution of texture during straining are significantly affected by the interaction between the strain path and the initial texture. Furthermore, the directionality of the twinning systems leads to strongly anisotropic mechanical behavior in textured magnesium [6], [7], [8], [9], [10], [11], [12], [13]. This opens up the possibility of controlling the mechanical response by designing crystallographic textures appropriate to particular strain paths.

The influence of mechanical twinning on the deformation of Mg has been demonstrated in many studies [14], [15], [16], [17]. On the one hand, the twin boundaries that have formed can act as barriers to dislocation motion, as do grain boundaries, leading to an increase in the work-hardening rate. In addition, they transform glissile dislocations into sessile dislocations within the twin interiors and, hence, contribute to strengthening via the Basinski mechanism (i.e., the trapping of sessile dislocations inside twins) [18]. On the other hand, they accommodate strain along the c-axis, which can give rise to a decrease in the work-hardening rate. Furthermore, the lattice rotation introduced by twinning can enhance or reduce the rate of work hardening, depending on the type of twin formed. As in the case of 〈c + a〉 slip, {10–11} twinning has the effect of rotating the basal planes (by 56°) towards a more favorable orientation for basal glide. Double twinning (which reorients the basal planes by 38°) has the same effect as {10–11} twinning. {10–12} twinning reorients the basal planes by 86°. In the latter case, grains that are oriented unfavorably for slip remain in unfavorable orientations.

Experimental results show that twinning plays an important role at low temperatures and when the grain size is large or the strain rate is high [19], [20]. Although twinning cannot dominate plasticity at large strains, since the shear strains are modest even at large twinned volume fractions, deformation twinning can give rise to a radical reorientation of the volume fraction of the crystal that has twinned. This leads to substantial texture modification [21], [22], [23]. Subsequent plasticity by slip in a twin-modified texture then progresses quite differently due to the anisotropic nature of the deformation mechanisms. It is therefore useful to know how twinning can contribute to the overall deformation of the material.

The aim of the current study was to further the fundamental understanding of the mechanisms that govern the plastic behavior of Mg alloys. A principal objective was to determine the strain dependence of the volume fractions of the two main kinds of twins as well as the temperature and strain rate dependences of these phenomena. This type of information is of considerable importance in crystal plasticity calculations and simulations pertaining to intermediate temperatures, a range in which twinning plays such an influential role.

Section snippets

Experimental method

The samples were taken from tubes extruded using porthole dies by Timminco Metals in Aurora, ON, Canada; they had nominal outer diameters of 70 mm and wall thicknesses of 4 mm. The chemical compositions of the two materials are presented in Table 1. In order to avoid the influence of the seam lines formed during tube extrusion, before machining, each tube was etched for 5 s to reveal the four seam lines. For this purpose, a solution of 12 g CrO3, 15 ml nitric acid and 85 ml H2O was used.

Uniaxial

Initial texture

The textures of the as-received AM30 and AZ31 tubes are presented in Fig. 2. Independent measurements made using X-rays and the EBSD technique were in good agreement. The initial grain orientations can be divided into two groups: one with their c-axes approximately parallel to the radial direction (called the RD component) and the other with their c-axes approximately parallel to the tangential direction (called the TD component). The positions of these two components are illustrated

Effect of initial texture and strain path on twinning

During extrusion, the basal planes of randomly oriented grains are rotated into alignment with the extrusion direction. Thickness reduction, i.e., compression along the RD direction, produces the RD component, while circumferential reduction produces the TD component. In this type of texture, basal slip is suppressed under both tensile and compressive loading conditions since there is little or no resolved shear stress on the basal planes. Instead, twinning and non-basal slip are generally

Conclusions

  • 1.

    During uniaxial tension tests, the applied force favors the formation of contraction and double twins. Although these twins reorient the basal poles, they only make a small contribution to texture change due to limited volume fraction of twinned material.

  • 2.

    Grain size, temperature, strain and strain rate all have significant effects on the volume fraction of contraction and double twins. The latter decreases linearly with temperature and increases linearly with strain, attaining a saturation value

Acknowledgement

This research was sponsored by General Motors of Canada and the Natural Sciences and Engineering Research Council of Canada. The authors are grateful to Dr K. Boyle and R. Eagle of CANMET in Ottawa for carrying out the RHTT tests and to R. Kubic (GM R&D Center, Warren, MI, USA) for technical assistance with respect to scanning electron microscopy. S.G. thanks the Belgian FNRS for financial support.

References (32)

  • T. Obara et al.

    Acta Metall

    (1973)
  • S.R. Agnew et al.

    Acta Mater

    (2001)
  • U.F. Kocks et al.

    Prog Mater Sci

    (2003)
  • S.E. Ion et al.

    Acta Metall

    (1982)
  • M.R. Barnett

    J Light Metals

    (2001)
  • S. Kleiner et al.

    Mater Sci Eng

    (2004)
  • R. Gehrmann et al.

    Mater Sci Eng A

    (2005)
  • S.R. Agnew

    Duygulu Ö Int J Plast

    (2005)
  • M.R. Barnett et al.

    Acta Mater

    (2004)
  • J.W. Christian et al.

    Prog Mater Sci

    (1995)
  • D.W. Brown et al.

    Mater Sci Eng A

    (2005)
  • L. Jiang et al.

    Scripta Mater

    (2006)
  • M.D. Nave et al.

    Scripta Mater

    (2004)
  • M.R. Barnett et al.

    Acta Mater

    (2004)
  • S. Godet et al.

    Script Mater

    (2006)
  • S.R. Agnew et al.

    Acta Mater

    (2006)
  • Cited by (454)

    View all citing articles on Scopus
    View full text