Influence of texture and microstructure on mechanical properties of high-strength wrought magnesium alloy AZ80

ZHANG Ping(张 平)¹, J. LINDEMANN², C. LEYENS²

1. National Engineering Research Center of Light Alloy Net Forming, Shanghai Jiao Tong University,

Shanghai 200030, China;

2. Chair of Physical Metallurgy and Materials Technology, Technical University of Brandenburg at Cottbus, Germany

Received 28 July 2006; accepted 15 September 2006

Abstract: Monotonic (tensile and compressive) and high cycle fatigue properties of the forged magnesium alloy AZ80 were investigated by using specimens with load axis parallel to longitude (L) or transverse (T) direction. A pronounced directional anisotropy in monotonic tests was observed in AZ80, i.e. the yield stress in T-direction is significantly lower than that in L direction. However, the directional anisotropy is absent in fatigue, fatigue strengths in both L- and T-directions are essentially equal. The absence of directional anisotropy in fatigue is possibly associated with the microstructure of AZ80. A homogeneous single phase structure probably alleviates the directional anisotropy of fatigue properties of the wrought magnesium alloy.

Key words: mechanical properties; tensile; compression; high cycle fatigue; wrought magnesium alloys; AZ80; directional anisotropy

1 Introduction

Magnesium alloys, due to their low densities, are particularly attractive for automotive industry for mass saving[1-2]. Currently, most of magnesium components in automobile are produced by die-casting process because of its high productivity. Compared to die-cast, magnesium components fabricated by mechanical forming exhibit higher ductility and strength[3]. However, owing to the hcp crystal structure of magnesium, mechanical forming is apt to produce a preferred crystal orientation of the basal plane in the wrought products. This preferred crystal orientation (texture) causes the tensile-compressive yield strength asymmetry in magnesium wrought alloys, i.e., the wrought alloys exhibit a high yield stress in tensile but lower yield stress in compression[3-5].

Another influence of texture on wrought magnesium alloys is the directional anisotropy of mechanical properties[3, 6-7]. The directional anisotropy in monotonic properties (tensile and compressive) of wrought magnesium alloys has been widely studied[3]. In general, the extrusion or rolling direction is significantly superior to transverse direction, i.e., in extrusion or rolling direction the magnesium alloys exhibit higher yield and tensile strengths. In contrast, the information on the directional anisotropy of fatigue of the wrought magnesium alloys is very limit. Recently, the directional anisotropy of fatigue strength has been found in extruded magnesium alloys AZ31, AZ80[6-7] and AZ61[8-9]. The directional anisotropy of fatigue was also attributed to the effect of texture by the authors[8-9]. Interestingly, this anisotropy was often accompanied by a microstructural anisotropy in the magnesium alloys[6-7]. Unfortunately, the influence of the microstructural anisotropy on fatigue properties was overlooked.

In the present work, the mechanical properties of the high-strength wrought magnesium alloy AZ80, including tensile, compression and high cycle fatigue, were investigated. Especially, the influence of texture and microstructure on the directional anisotropy of mechanical properties was discussed.

2 Experimental

The high-strength magnesium alloy AZ80 (nominal composition in mass fraction: 8Al, 0.5Zn, 0.2Mn, balance: Mg) forged to the shape of rectangular bar stock was received from Otto Fuchs Metallwerke, Meinerzhagen, Germany. The dimension of the forged bar is 300 mm(L)×60 mm(T) ×11 mm, as shown in Fig.1.

Fig.1 Wrought magnesium alloy AZ80 as received

For mechanical testing, specimens were machined with load axis parallel to longitude (L) as well as transverse (T) direction of the rectangular bars. Tensile tests were performed on threaded cylindrical specimens having gage length of 20 mm. Specimens for compre- ssion tests were cylindrical shape with 20 mm in length and 10 mm in diameter. The initial strain rate applied for both tensile and compression was 8.3×10^-4 s^-1.

For fatigue testing, hour-glass shaped round specimens (6 mm gage diameter) were used. After machining, a layer with thickness of about 100 ?m was removed from the surface of the specimens by electrolytical polishing (EP) in order to avoid the influence of machining on the fatigue results. Fatigue tests were conducted under rotating beam loading (R=-1) at a frequency of about 100 Hz in air.

Crystallographic textures determined by X-ray diffraction using Ni-filtered CuK_a radiation are shown as (0002) pole figures. Light optical microscopy (LOM) was employed for microstructural investigation. Speci- mens for LOM were sectioned, cold mounted, ground, polished and then etched in acetic picral (10 mL acetic acid+100 mL 4% picric acid in ethanol+5 mL H₂O). Polarized light was used to reveal orientation contrast. Grain size was measured by the lineal intercept method.

  3 Results and discussion

The microstructure of the wrought magnesium alloy AZ80 is shown in Fig.2. One can see that the forged alloy consists of fine and equiaxed grains with an average grain size of about 30 μm. No precipitation was observed in LOM.

The crystallographic texture in AZ80 is illustrated in Fig.3. It can be seen that for the forged alloy the basal planes are oriented predominantly parallel to the L-T plane.

Fig.2 Microstructure of as forged magnesium alloy AZ80 (L-axis horizontal)

Fig.3 (0002) pole figure of wrought magnesium alloy AZ80

Fig.4 presents the strain—stress curves in tension for the wrought magnesium alloy AZ80. A strong directional anisotropy in yield stress is observed in forged AZ80, the yield strengths for L- and T-directions are 226 and 157 MPa, respectively. The yield strength in L-direction is significantly larger than in T direction, resulting from the texture in forged material. Due to influence of texture, plastic deformation in T-direction is much easier than in L-direction since basal planes experience large shear stresses by loading in T direction, while relatively smaller shear stresses act on basal planes for loading in L-direction. In addition, a strong work hardening effect in T-direction narrows the disparity in tensile strength between L- and T- directions. Furthermore, the tensile-compressive yield strength asymmetry was also observed in both L- and T-directions of the high-strength wrought magnesium alloy AZ80. The compressive yield strengths for L- and T-directions are 171 and 132 MPa, respectively, significantly lower than the tensile yield strengths. The lower compressive yield strengths are associated with the texture in AZ80, since the texture in the as-forged AZ80 is favourable to form  twinning in compression[5]. Tensile and compressive properties of magnesium alloy AZ80 are summarised in Table 1.

Fig.4 Tensile stress—strain curves of wrought magnesium alloy AZ80(load axis is L and T direction, respectively)

Table 1 Monotonic test results on wrought magnesium alloy AZ80

The stress—cycles curves in air for characteristic directions of the magnesium alloy AZ80 are demonstrated in Fig.5. Interestingly, fatigue strengths in L- and T-directions for the forged AZ80 are essentially equal, about 100 MPa. This indicates that the effect of texture seems to be more pronounced on monotonic properties (tensile and compress) than on fatigue properties of AZ80.

The absence of the directional anisotropy in fatigue is inconsistent with the results reported in Ref.s[6-9]. HILPERT and WAGNER[6] observed a strong directional anisotropy in high cycle fatigue of an extruded magnesium alloy AZ80, the fatigue strengths in L- and T-directions obtained by Hilpert were about 100 and 50 MPa, respectively. Interestingly, the extruded magnesium alloy AZ80 used by HILPERT and WAGNER[6-7] possesses the same composition, similar grain size and texture as the forged AZ80 used in the present work. On that account, it is reasonable to assume that the differences in fatigue behaviour are possibly associated with the different microstructures in the two materials. The material used in the present work exhibits a single phase structure, in which aluminum is dissolved in the a-phase as shown in Fig.2. While in the extruded AZ80 microstructural anisotropy is clearly seen, discontinuous b-phase stringers are present along the extrusion direction[6]. As loading direction (T-direction) is perpendicular to second phase stringers, a high stress concentration around the stringers may be induced due to the pile-up of dislocations during fatigue, leading to the nucleation of fatigue crack. Therefore, existence of b-phase stringers may deteriorate fatigue strength in T-direction, and thus causes the directional anisotropy of fatigue properties in AZ80. It is noted that solid solution treatment or reducing the size of intermetallic compound may improve fatigue strength of magnesium alloys[8-10]. Hence, we conclude that the homogeneous single phase structure of AZ80 in the present work possibly improves the fatigue properties of T-direction and alleviates the directional anisotropy of mechanical properties of AZ80, resulting in the absence of the directional anisotropy in fatigue.

Fig.5 Stress—cycles curve of wrought magnesium alloy AZ80

Fracture surface of the wrought magnesium alloy AZ80 is shown in Fig.6. It is found that all fatigue cracks nucleated at the surface, which can be explained by the load distribution in specimens. During rotating beam loading fatigue, the surface experiences the maximum tension stress, thus the specimen surface is favourable for crack nucleation[11]. Additionally, due to the lack of constraint in grains at free surface, the glided dislocations and twinning during deformation may result in a microscopically irregular surface, which consequently makes the surface as a prevailing site for crack initiation.

4 Summary

The monotonic (tensile and compression) and fatigue properties of the high-strength wrought magnesium alloy AZ80 (forged) have been investigated. In monotonic tests, a pronounced directional anisotropy in yield strength is observed, i.e., the yield stress in T-direction is significantly lower than in L direction. In fatigue tests, however, the fatigue strengths in L- and T-direction of the forged AZ80 are essentially equal, the directional anisotropy is absent. It suggests that both texture and microstructure affect mechanical properties of the high-strength wrought AZ80. In addition, texture and microstructural anisotropies may cause the directional anisotropy in mechanical properties. The influence of texture on the monotonic properties seems to be more pronounced than on fatigue properties. A homogeneous single phase structure possibly improves the fatigue properties of T direction and alleviates the directional anisotropy in fatigue of the wrought magnesium alloy AZ80.

Fig.6 SEM fatigue crack nucleation sites of EP specimens: (a) L; (b) T(Arrows indicate crack nucleation sites)

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(Edited by YANG Hua)

Foundation item: Project supported by Shanghai Pujiang Program and German Federal Ministry of Education and Research (BMBF)

Corresponding author: ZHANG Ping; Tel: +86-21-62933139; E-mail: ppzhang@sjtu.edu.cn