Tensile properties of AZ31 sheet/bar and effects of texture

 ZHOU Qing (周 清)¹, G. ITOH², Y. MOTOHASHI³

1. College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics,

Nanjing 210016, China;

2. Department of Mechanical Engineering, College of Engineering, Ibaraki University, Hitachi 316-8511, Japan;

3. Research Center For Superplasticity, Ibaraki University, Hitachi 316-8511, Japan

Received 28 July 2006; accepted 15 September 2006

Abstract: For understanding the deformation mechanism of AZ31 Mg alloy, two kinds of specimen, hot-rolled sheet and extruded bar were tested in tension at room temperature. Relatively small grain sizes ranging from 32 to 8.7 mm are obtained by annealing at several temperatures. And then they were further cold-rolled with rolling angles of 0?, 45? and 90?, which is the angle between the rolling and the longitude direction of as-received specimen, and annealed at a temperature in recrystallization field. The yield stress were found from the stress—strain curve. The grain size dependence of the yield stress is found not in agreement with the Hall-Petch relationship. The texture of (0002) pole figure of each kind of specimen was tested by X-ray diffractometer. The specimen with strong basal texture shows large yield stress and low work-hardening rate, while the specimen with weak basal texture shows low yield stress and rapid work-hardening rate. These results are interpreted by the slip system activated in the different textures. A combined slip system was used to interpret the yield stress variation with the texture and the deformation mechanism was discussed.

Key words: AZ31 magnesium alloy; mechanical property; slip system; deformation mechanism; grain size; texture

1 Introduction

AZ31 magnesium alloy, a typical wrought material, is used for thin sheet in various light mass applications, such as the shell of mobile phone, digital camera and notebook computer. The research of plastic formability is important for wider the application of the sheet product. Enough high strength is also needed to be designed to support the structure. Mechanical properties and related microstructures have been studied for understanding the deformation mechanism of AZ31 Mg alloy. Through grain refinement methods, like cold rolling (CR)[1], equal channel angle pressing (ECAP)[2] and multi-direction forging[3], both the elongation and the yield stress are increased largely. Grain size affects the yield stress through the Hall-Petch relationship based on the dislocation pile-up theory, which is expressed by σ_y=σ₀+kd^-1/2, where σ_y is the yield stress, d is the grain size, k is a constant and σ₀ is a d independent constant. The study on the tensile tests for hot-rolled sheets and square-rods showed good relation of the Hall-Petch relationship between σ_y and d^-1/2 at temperatures of 77, 156, 300, and 420 K[4]. From those results, σ₀ is con- cluded in support of the theory of Armstrong that it is related to the critic resolved shear stress (CRSS) of the basal slip. However, four plottings of σ_y and d^-1/2 do not seem enough for a reasonable regressive line. Furthermore, the texture characteristic of the square-rod is almost the same as that of the sheet, insufficient to obtain a convinced slipping mechanism from the result of comparison of sheet yield behavior with bar’s.

Long elongation has been found in the grain-refined AZ31 alloy[5]. The steady-state behavior in the stress—strain curve obtained from the uniaxial tensile test at the room temperature, showed recovery tendency, which is usually a high temperature superplastic deformation. The twinning and the high-order slip system relax the stress concentration near the grain boundary and a homogeneous deformation are then activated, which are considered as the reason of the recovery.

At the yielding stage, σ_y is only related to prime slipping and no twinning is included because twin strain is small and critic resolved shear stress (CRSS) of twin deformation is low[6], particularly when the grain size is small[7] in the present experiment. However, elongation is related to the high-order slipping or cross-slipping, or in some cases, twinning. Twining takes the part as an additional mechanism when dislocation slipping cannot perform the homogeneous deformation[8].

In this paper, two kinds of specimens with largely different texture characteristics were presented. Tensile tests were performed at room temperature at one constant cross-head speed in order to obtain the mechanical properties. Grains with different diameters were obtained through the method of annealing at several temperatures. Grain refinement method used here was the cold-rolling and subsequent annealing. The dislocation slipping behavior was discussed to find the deformation mechanism of the sheet and the bar.

2 Experimental

Hot-rolled AZ31 alloy sheet with the thickness of 1.35 mm and extruded bar with the cross section of 60 mm×30 mm were commercially obtained. The speci- mens with the dimension of 80 mm×30 mm×1.35 mm and 60 mm×30 mm×4 mm were cut from the hot- rolled sheet and the extruded bar, respectively. They were prepared for further cold-rolling. L, LT and ST were referred to as the direction of the rolling or the extrusion the long transverse and the short transverse direction, respectively. Before cold-rolling, as-received specimens were annealed at the temperature range between 150-400 ℃. Average grain diameters were measured by the method of linear interception. As-received specimens were further cold-rolled with the reduction of 15% and a rolling angle, which was determined as the angle between the rolling direction and the former rolling/extrusion direction, 0?, 45? and 90?, respectively. Then, they were annealed at 300 ℃ for 30 min.

The tensile specimens with the gauge length of 10 mm and gauge width of 2.2 mm were machined with the L direction parallel to the tensile axis. Then, tensile tests were performed on a screw driven machine at a constant cross-head speed of 1×10^-2 s^-1 at room temperature. The textures of both the as-received and the cold-rolled specimens were examined by X-ray diffractometer using Cu K_β radiation operated at 50 kV/20 mA. The Schulz reflection method was used and the scanning angle is in the range of 20?-90? with a step of 5?.

3 Results and discussion

The stress—strain curves are shown in Fig.1, where we find that the curve of the extruded bar shows low yield stress and rapid strain hardening rate, and however, the curve of the sheet shows high stress and low strain hardening rate. It should be explained that the yield stress here corresponds to 0.2% offset flow stress. With the CR treatment, the yield stress of the bar increases and the strain hardening rate is reduced largely; while the two parameters for sheet do not change. The sheet shows higher yield stress and lower strain hardening rate than the bar. The yield stress and elongation, together with the average grain diameter are listed in Table 1. From Table 1, the grain size of the CR specimen is found reduced. It is also found that the reduce is larger in bar than in sheet, which implies that the CR treatment is more effective to the bar than to the sheet. Besides CR effect on the yield stress, the elongation after CR in both specimens are also changed to be increased. The effect of rolling angle on both the grain sizes and the yield stresses is not exhibited.

Fig.1 Stress—strain curves for as-received (a) and cold-rolled sheet and bar (b)

Table 1 Grain size, yield stress and elongation for AZ31 sheet and bar

The plotting between σ_y and d^-1/2, together with the data in the annealing tests, are shown in Fig.2. From Fig.2, it is found that the dependence of the grain size on the yield stress is not in agreement of the Hall-Petch relationship because of the minus values of σ₀ shown in three regressive lines of L-LT, LT-ST bar and sheet. The texture in each kind of specimen is the same, thus no texture factor should be included. Fig.2 indicates that the relationship between the yield stress and the grain size of the sheet and bar may not be in accordance to the Hall-Petch relationship in this paper. Further investigation or other mechanism should be pursued in the future.

Fig.2 σ_y vs d^-1/2 of AZ31 sheet and bar

Difference of yield stresses between the sheet and the bar has been found from Table 1. This difference can be considered caused by the different textures. The textures of the sheet and bar tested by X-ray diffraction technique are shown in Figs.3 and 4, respectively. Comparing the pole figure of the as–received sheet with the bar, which are shown in Fig.3(a) and Fig.4(a) respectively, weaker intensity and larger area basal fiber feature are shown in the bar; however stronger and concentrated intensity in basal texture is exhibited in the sheet. From Fig. 3, it is also found that the center of the pole pattern of the as-received bar is deviated from the circle center with an angle of about 10?; the same can be seen in the CR bar. The pattern of the CR bar shows increased intensity and concentrated area, which indicates stronger texture than that of as-received. Seen from Fig.4, all four pole figures show strong basal texture. With the change of the rolling angle, the pole patterns are elongated along the rolling direction.

The texture effect on the yielding behavior is related to the orientation angle between the normal direction of the activated slip plane and the applied stress direction. As described in the experimental, when the applied stress direction is parallel to extrusion/rolling direction and only one grain is considered, to the pole pattern in Fig.3(a), the basal slip is possible the activated prime slip system for the reason of the random orientation of grain; For the specimen with strong basal texture in Fig.3(b) and Fig.4, basal slip cannot be activated because the slip plane is parallel to the stress axis in basal (0002) plane. Also it is known that CRSS in single crystal is in the plane with an angle of 45? between the normal direction of slip plane and the stress axis. Thus, other prismatic and pyramid slipping system should be possibly activated and act as the prime slip.

Regarding to the properties of polycrystalline, because the activated slip systems in neighbor grains must satisfy the integrated deformation rule, no less than 5 slipping systems in one grain should be activated. Thus, it is complex to pursue the prime slip system which is finally reflected to the critic resolved shear stress (CRSS) of the slip plane or the yield behavior. The tested pure Mg single crystal CRSS for basal slip is as small as 0.5-0.6 MPa[9], whereas for prismatic slip it is near 40 MPa[10] and for pyramidal slip it is near 50 MPa[11]. Because of the solution strengthen; the CRSS of correspondent slip in the AZ31 alloy is slight higher than pure Mg. Furthermore, the number of slip system model in HCP structure is 2, 2, ＞5 for basal, prismatic and pyramidal, respectively. Since the limited slip system and the CRSS value of each slip system, the combined slip system is used to qualitatively interpret the relation of the texture to the yield stress. It is deduced that the combined slip systems of basal+pyramidal is the yielding mechanism of as-received bar resulting to small yield stress, prismatic+pyramidal slips is the yielding mechanism of CR bar resulting to middle yield stress, and the pyramidal slip is the yielding mechanism of both as-received and CR sheet in result of high yield stress as schematically shown in Fig.5.

Fig. 3 (0002) pole figures of as-received (a) and cold-rolled bar (b)

Fig. 4 (0002) pole figures of as-received (a) and cold-rolled sheet with rolling angle 0? (b), 45? (c) and 90? (d)

Fig.5 Schematic diagraphs of textures and combined slipping: (a) Strong basal texture clike in Fig.4; (b) Random texture clike in Fig.3(a); (c) Basal texture with a 10? deviated angle (like in Fig.3(b))

From Table 1, the elongation is found increased with the reduction of the grain size, which is in agreement with the results obtained by KOIKE[5], who has reported the steady-state recovery at the room temperature deformation of fine-grained AZ31 alloy and has given detailed interpretation of the recovery behavior. Moreover, the elongation is affected by the texture. Fig.2(a) shows higher elongation of sheet than bar, which would be reverse if it was usually considered that the specimen with random texture had better plastic deformation ability. From Fig.1, it is found that the strain hardening rate of the bar is faster than the sheet. Faster strain hardening rate causes rapid increment of the internal stress. Stress concentration along grain boundary will lead to production of void and finally to the failure of the specimen. Thus, the high strain hardening rate is related to the low elongation when other conditions of the specimens are the same.

4 Conclusions

1) With the cold-rolling and subsequent annealing treatment, the grain size is reduced, especially to the extruded bar the reduction is larger.

2) The yield stress is failure to relate to the grain size according to the Hall-Petch relationship.

3) A combined prime slipping systems model was established to interpret the relationship between the yield stress and the texture. Basal+pyramidal is the yielding mechanism of as-received bar resulting to small yield stress, prismatic+pyramidal slips is the yielding mechanism of CR bar resulting to middle yield stress, and the pyramidal slip is the yielding mechanism of both as-received and CR sheet in result of high yield stress.

4) The elongation is correlated to the grain size and the texture. Strain hardening rate is a main factor to affect the plastic strain and the failure strain.

References

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(Edited by YANG Hua)

Corresponding author: ZHOU Qing; Tel: +86-25-84896469; E-mail: anzhouqing@nuaa.edu.cn