­­­ Grain size and texture changes of magnesium alloy AZ31 during multi-directional forging

YANG Xu-yue(杨续跃)^{1, 2}, SUN Zheng-yan(孙争艳)¹, XING Jie³,

MIURA Hiromi³, SAKAI Taku³

1. School of Materials Science and Engineering, Central South University, Changsha 410083, China;

2. Key Laboratory of Nonferrous Metal Materials Science and Engineering, Ministry of Education,

Central South University, Changsha 410083, China;

3. Department of Mechanical Engineering and Intelligent Systems, UEC Tokyo, Tokyo 182-8585, Japan

Received 12 June 2008; accepted 5 September 2008

Abstract: Grain size and texture changes of magnesium alloy AZ31 were studied in multidirectional forging(MDF) under decreasing temperature conditions. MDF was carried out up to large cumulative strains of 4.8 with changing the loading direction during decrease in temperature from pass to pass. MDF can accelerate the uniform development of fine-grained structures and increase the plastic workability at low temperatures. As a result, the MDFed alloy shows excellent higher strength as well as moderate ductility at room temperature even at the grain size below 1 μm. Superplastic flow takes place at 423 K and depends on the anisotropy of MDFed samples. The mechanisms of strain-induced fine-grained structure development and of the plastic deformation were discussed in detail.

Key words: magnesium alloy; severe plastic deformation; multidirectional forging; texture; continuous dynamic recrystallization

1 Introduction

Recently, developments of ultrafine grained materials processed by severe plastic deformation(SPD) have been studied because of improvement of the several mechanical properties, such as high flow and fatigue strength, moderate ductility. Several SPD processes have been applied to magnesium alloys to modify their mechanical properties[1-4]. XING et al[5-6] showed that ultrafine grained Mg alloy AZ31 can be produced effectively by using multidirectional forging(MDF) under decreasing temperature conditions. It exhibited superplastic elongation of over 300% at 423 K[7-8], and an uniform elongation of over 13% and an ultimate strength of 530 MPa at room temperature[9]. Such obvious improvement of the mechanical properties resulted from the ultrafine grained Mg alloys processed by MDF.

The aims of present work are to study optimum processes for grain refinement and texture control of Mg alloy AZ31, and to improve the mechanical properties at ambient temperature. The grain refinement and texture changes taking place during MDF and the mechanical properties after MDF are investigated in detail. The mechanisms of grain refinement and of plastic deformation of the fine-grained Mg alloy are analyzed and discussed.

2 Experimental

The alloy used in this study was a commercial Mg alloy AZ31 (Al 2.68%, Zn 0.75%, Mn 0.68%, Cu 0.001%, Si 0.003%, Fe 0.003%, and balance Mg, mass fraction). The rectangular samples with a dimension of 31 mm in length, 21 mm in width and 14 mm in thickness (the axis ratio is 2.22?1.49?1) were machined from the rod parallel to the extrusion direction. The samples were annealed at 733 K for 7.2 ks and then furnace cooled, leading to the evolution of almost equiaxed grains with an average grain size of about 22 mm. Compression tests were carried out at constant true strain rates by a testing machine equipped with quenching apparatus, which made it possible to quench the samples within 1.5 s after deformation was ceased. The samples were deformed by multidirectional forging(MDF) at a strain rate of 3×10^-3 s^-1 with decreasing temperature from 623 K to 423 K, followed by water quenching in each pass (Fig.1). The sample dimension did not change when a strain (?e) in each pass was 0.8. Deformed samples were cut along planes parallel to the final compression axis. Microstructures of deformed samples were observed by using optical microscopy(OM) and transmission electron microscopy (TEM) under an acceleration voltage of 200 kV.

Fig.1 Schematic illustration of MDF under decreasing temperature conditions: (a) Loading direction changed in 90? with pass to pass (x→y→z…) (A pass strain ?e 0.8); (b) Temperature decreased from 623 K to 423 K (WQ—Water quenching)

Tensile specimens with a gauge dimension of 10 mm in length, 4 mm in width, and 1 mm in thickness were machined from MDFed samples to various directions. The basal plane {0001} of the Mg alloy approached roughly perpendicular to the final compression axis in a pass strain of 0.8[10]. Specimens of the tensile axis inclined at 0?, 45? and 90? against the basal plane were named the 0?, 45? and 90? sample, respectively (Fig.2). Tensile tests were conducted in vacuum by using an Instron-type testing machine which was equipped with a hydrogen gas quenching apparatus [11]. Tensile tests were carried out at 298 K and 423 K and at various initial strain rates from 5×10^-5 s^-1 to 5×10^-3 s^-1.

3 Results and discussion

3.1 Grain size changes with MDF

Fig.3 shows the optical microstructures developed

Fig.2 Schematic illustration of tensile specimens with inclination angle of 0?, 45? and 90? to final compression axis (CA)

Fig.3 Optical microstructures evolved during MDF under decreasing temperature conditions in AZ31 alloy: (a) T=623 K,ΣΔε=0.8; (b) T=473 K, ΣΔε=3.2

during MDF under decreasing temperature conditions from 623 K to 473 K. It can be clearly seen in Fig.3 that the grain size decreases rapidly with reducing temperature (and increasing cumulative strain), i.e. the initial grain size of 22.3 mm decreases to about 6.7 mm after 1st-pass compression at 623 K (Fig.3(a)) and 0.8 mm after 4th-pass MDF at 473 K (Fig.3(b)). The grain size developed by the 5th-pass compression at 443 K is further decreased, and about 0.5 mm which is measured by using transmission electron microscopy(TEM).

Changes in the average grain size evolved in the Mg alloy AZ31 during isothermal MDF at 623 K and during MDF under decreasing temperature conditions from 623 K to 423 K are shown by a dashed line and a solid line in Fig.4, respectively. The average grain size developed during isothermal MDF is almost constant within the experimental scatters and about 6.7 mm at strains up to S?e=4.8. On the other hand, the dynamic grain size evolved during MDF under decreasing temperature conditions decreases drastically with repeated MDF. It is concluded from Fig.4 that the MDF processing under dropping temperature conditions may be a most effective method for grain refinement of Mg alloys.

Fig.4 Grain size changes in AZ31 alloy during MDF at temperature of 623 K (dashed line) and with decreasing temperature from 623 K to 423 K (solid line)

3.2 Texture development during deformation

It is well known[10,12] that extruded Mg rod has a strong wire texture, in which the basal plane of the HCP lattice lies roughly parallel to the extrusion direction and so the compression axis at the present case. The initial texture of the Mg alloy sample is gradually reoriented by compression. Namely, the basal plane starts to rotate gradually from around 0? with compression and approaches to near 90? to the compression axis in high strain. The final deformation texture is the basal plane perpendicular to the compression axis. It exists stably even after full annealing at high temperatures[13]. Thus, deformation process accompanied with various strain paths, such as MDF, may be useful for grain size control as well as texture modification of Mg alloy.

3.3 Stress—strain curves

Various samples machined from the MDFed Mg alloy (Fig.2) was deformed in tension at 423 K and at strain rates of 5×10^-5 s^-1 and 5×10^-3 s^-1. Effect of the anisotropy of the MDFed Mg alloy on the true stress—nominal strain (s—e_n) curves is represented in Fig.5. It is seen in Fig.5 that the s—e_n curves are clearly affected by the sample anisotropy and also by strain rate. At a high strain rate of 5×10^-3 s^-1, higher flow stress of above 120 MPa appears clearly depending on the sample anisotropy, while relative lower stress of around 60 MPa appearing at 5×10^-5 s^-1 does not change with the anisotropy. On the other hand, the total elongations to fracture are below 100% at 5×10^-3 s^-1 and seem to be not changed so much by the anisotropy, while the elongations to facture of above 200% change clearly with the anisotropy at 5×10^-5 s^-1.

Fig.5 Effect of anisotropy of MDFed AZ31 alloy on σ—ε curves at 423 K

At a higher strain rate of 5×10^-3 s^-1, the average flow stresses decrease in order of the 0?, 90? and 45? samples. Note here that their basal plane for slip deformation inclines roughly at 0?, 90? and 45? against the tensile axis of each sample (Fig.2). As shown in Fig.5, however, the inclination angle of the basal plane in the 90? sample is not 90?, but around 70? at e=0.8. It is concluded, therefore, that tensile deformation of the MDFed Mg alloy at higher strain rate can be controlled mainly by dislocation motion on the basal plane, and so the 0?, 90? and 45? samples have the hardest, moderate and easiest slip systems, resulting in the highest, moderate and lowest flow stress, respectively.

The flow stresses at high strains of 5×10^-5 s^-1, in contrast, are roughly similar irrespective of the sample anisotropy and the total elongations to fracture of above 200% clearly depend on the initial orientation. This suggests that superplastic deformation can take place in the ultrafine grained Mg alloy at 5×10^-5 s^-1. XING at al[7] studied superplastic deformation of the ultrafine grained Mg alloy at temperatures from 393 K to 473 K and at strain rates ranging from 5×10^-6 s^-1 to 5×10^-3 s^-1, and concluded that superplasticity can be controlled mainly by grain boundary sliding.

3.4 Texture changes during tensile deformation

During tensile deformation at 423 K, changes in the grain size as well as in the texture take place in the ultrafine grained Mg alloy. The texture results summarized are shown schematically in Fig.6. The texture of the 0° sample, i.e. the basal plane parallel to the tensile axis, does not change and exists stably during tensile deformation up to e_n=200%. In contrast, the basal plane in the 45? and 90? samples rotates gradually with straining and approaches parallel to the tensile axis in high strain. It is concluded that the basal plane of the Mg alloy becomes finally parallel to the tensile direction irrespective of the initial textures which is the stable orientation in tensile deformation. This suggests that during tensile deformation of both the 45? and 90? samples the basal slip can operate accompanying with a rotation of the basal plane at lower strains. Then, the 0?, 90? and 45? samples show the highest, moderate and lowest yield stress, as seen in Fig.5(b).

Fig.6 Schematic drawing of texture changes with tensile deformation at 423 K of MDFed sample

On the other hand, a rotation of the basal plane during tension should result in an increase of flow stress at moderate to high strains because of texture (orientation) hardening. The flow stresses appearing at  5×10^-5 s^-1 are roughly constant in high strain and almost the same irrespective of the initial orientations. This suggests that deformation in high strain can be controlled by not only slip deformation, but also grain boundary sliding. It is concluded from the discussion above that deformation of the 45? and 90? samples can be controlled mainly by basal slip and subsequently by grain boundary sliding at low to moderate strains, and finally by grain boundary sliding at high strains. In contrast, deformation of the 0? sample can be controlled mainly by grain boundary sliding in low to high strains because of difficulty of the basal slip. This may lead to an orientation dependence of the total elongations to fracture at 5×10^-5 s^-1, that is, the elongations to fracture increase in the order of the 0?, 90? and 45? samples (Fig.5(b)).

It is interested in Figs.5(b) and 6 that the basal plane of the Mg alloy exists stably during superplastic deformation. This result is in contrast with that in several previous papers on superplasticity[11,14-15]. For example, the initial texture of a 7075 aluminum alloy developed by cold rolling is progressively destroyed by deformation and finally replaced to a very diffused and nearly random one just after tensile straining to 160% [11]. However, the basal plane of the MDFed Mg samples is rotated by slip deformation and approaches parallel to the tensile axis at large strains even under superplastic conditions irrespective of the various initial textures.

4 Conclusions

1) MDF under decreasing temperature conditions can accelerate the evolution of fine-grained structures in Mg alloy. The minimal grain size of 0.36 mm is developed at S?e=4.8 and at 423 K.

2) Total elongations to fracture of ultra-fine grained Mg processed by MDF are above 300%, suggesting superplasticity taking place in the Mg alloy.

3) The strong deformation texture, i.e. the basal plane parallel to tensile axis, exists stably during superplastic deformation of the 0? sample. In the 45? and 90? samples, in contrast, lower flow stresses as well as larger elongations appear accompanying with grain rotation and grain boundary sliding at 423 K and at 5×10^-5 s^-1.

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(Edited by YUAN Sai-qian)

Corresponding author: YANG Xu-yue; Tel: +86-731-8830136; E-mail: yangxuyue@mail.csu.edu.cn