- Email: [email protected]
Effect of equal channel angular pressing on the deformation behaviour of magnesium alloy AZ31 under uniaxial compression
Journal of Materials Processing Technology 184 (2007) 294–299
Effect of equal channel angular pressing on the deformation behaviour of magnesium alloy AZ31 under uniaxial compression Z. Z´uberov´a a,∗ , Y. Estrin a , T.T. Lamark a , M. Janeˇcek b , R.J. Hellmig a , M. Krieger a a
Clausthal University of Technology, Institute of Material Science and Engineering, Agricolastr. 6, 38678 Clausthal-Zellerfeld, Germany b Charles University, Faculty of Mathematics and Physics, Ke Karlovu 5, 121 16 Prague 2, Czech Republic Received 19 September 2006; received in revised form 16 November 2006; accepted 22 November 2006
Abstract Deformation behaviour of magnesium alloy AZ31 under uniaxial compression was investigated. AZ31 samples produced by squeeze casting, hot rolling and hot rolling with subsequent equal channel angular pressing (ECAP) were tested. It was shown that ECAP processing produces a fine-grained microstructure that gives rise to improved mechanical properties. © 2006 Elsevier B.V. All rights reserved. Keywords: ECAP; AZ31; Compression behaviour
1. Introduction Magnesium alloys are among the lightest metallic materials used for structural applications. Their high strength/weight ratio predetermines these materials for a broad range of technical applications, particularly in the automotive and aircraft industries as well as in manufacturing of electronic devices. Mg alloys are commonly produced as cast material because of their good castability, acceptable quality, dimensional control and a relatively high strength. Difficulties with plastic forming of the alloys are a consequence of their hexagonal close-packed structure. However, the fabrication of wrought magnesium alloys also has considerable potential because of their advantageous mechanical properties (higher ductility and strength) as compared to cast ones [1,2]. Plastic forming of Mg alloys is enhanced at elevated temperatures, because non-basal slip can be activated in addition to basal slip. Superplasticity is also observed in finegrained magnesium alloys [3]. A possible way for improving mechanical properties of magnesium and its alloys is by processing them by equal channel angular pressing (ECAP). In order to optimize the plastic forming process, it is important to understand the effect of temperature and strain rate on the flow stress. Recently, the high temperature deformation behaviour of AZ31 alloy has been examined extensively by Ishikawa et al. [2]. AZ31
∗
Corresponding author. Tel.: +49 5323 722753; fax: +49 5323 723148. E-mail address: [email protected] (Z. Z´uberov´a).
0924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.11.098
is a medium strength alloy with low notch fatigue sensitivity [4] and good damping behaviour. However, this alloy has a low corrosion resistance [5] and low creep resistance above 130 ◦ C. ECAP is a relatively new method for manufacturing bulk materials with fine to ultra-fine microstructure by severe plastic deformation [6]. High tensile and compressive strength of materials prepared by ECAP can be expected according to the Hall–Petch relation [7,8]. It is further desirable to find processing conditions for which the strength enhancement is not accompanied by a loss of ductility. The ECAP die is a block with two intersecting channels of identical cross-sections. The billet to be processed has the same cross-section as the channel and has to be well lubricated. It is placed into the first channel and then pressed into the exit channel by a plunger. Ideally, severe deformation by simple shear occurs in a zone where the two channels meet. As the billet is pushed through this zone, new regions of material successively experience this shear strain providing a uniform shear deformation of the billet—apart from its end regions. Strain accumulation can be produced by repeated pressing of the sample. Between the subsequent passes the sample can be rotated. The most commonly used route is BC which involves rotating the specimen 90◦ around its axis between successive passes. This has been shown to result in an equiaxed grain structure with enhanced mechanical and other properties [9]. ECAP of AZ31 was reported in Refs. [10–12]. Various ECAP conditions were investigated. Grain refinement and deformation twins after ECAP processing in the temperature range of 200–250 ◦ C were observed [10]. Furthermore, recrystallisation
Z. Z´uberov´a et al. / Journal of Materials Processing Technology 184 (2007) 294–299
295
was found to occur above 200 ◦ C by Kamado et al. [10]. Average grain size of 1 m after eight passes of ECAP by route BC at 200 ◦ C was reported in [11]. Agnew et al. [12] conducted ECAP of AZ31 at 300 ◦ C because they experienced cracking of samples at a low temperature of 200 ◦ C. They neither found any significant grain refinement nor a change in the tensile properties at 300 ◦ C. The objective of the present work was to study the deformation behaviour under uniaxial compressive loading of magnesium alloy AZ31 prepared by three processing techniques: squeeze casting (SC), hot rolling (HR) and hot rolling followed by four passes of equal channel angular pressing via route BC (HR-ECAP4). 2. Experimental procedures Fig. 1. Microstructure of AZ31SC. Conventional magnesium alloy AZ31 (nominal composition 3 wt% Al and 1 wt% Zn) was used as test material. It was prepared by three different processing techniques. The first state was squeeze cast (SC) and the second one hot rolled (HR). The third state was produced by four ECAP passes (route BC ) from hot-rolled material (HR-ECAP4). Squeeze casting was realized by the two-step loading procedure (pressure of 80 MPa for 15 s followed by 140 MPa for 90 s). The temperature of the casting die was 200 ◦ C. Hot rolling was conducted at 400 ◦ C. The materials were tested in the as-processed states. Specimens for ECAP pressing having the dimensions of 10 mm × 10 mm × 60 mm were machined from the hot-rolled material. The angle between the two channels in the ECAP die was Φ = 90◦ . The cross-sectional dimensions of both channels were 10 mm × 10 mm. The material of the die was a tool steel X38CrMoV51. The die was placed in a servohydraulic testing machine Instron 8502 with a maximum load capacity of 200 kN. ECAP was carried out at 200 ◦ C at a speed of 5 mm/min. For easier pressing, temperature resistant lubricant MoS2 was used. No cracking of specimens occurred under these conditions, unlike in the case presented in [12], when higher pressing speeds were used. It should be stressed that in the present work, the main target was not the processing route leading to these three different states, but rather the effect of the ensuing microstructure on the post-processing behaviour under compression. Microstructures of specimens in as-processed conditions (SC, HR) as well as after compression tests were observed with an optical microscope ZEISS Axioplan 2. Specimens from SC were cut with no specific orientation, while those from HR were cut parallel to the rolling direction. The specimens were mechanically polished and subsequently etched by picric acid. The microstructure of specimens after ECAP and after the following compression tests were investigated by transmission electron microscopy (TEM) using a Philips CM 200 electron microscope operated at 200 kV. TEM specimens were cut from middle sections of the pressed billets perpendicular to the pressing direction. Thin foils for TEM were first mechanically polished and finally electropolished in a Tenupol 5 double jet polishing unit in a LiCl + Mg perchlorate + methanol + buthyloxyethanol solution at −45 ◦ C. Cylindrical specimens having a diameter of 10 mm and a height of 15 mm were used for compression tests. As in microstructure investigations, specimens of squeeze cast material were cut from a billet at a random orientation. Specimens prepared from hot-rolled material were taken parallel to the rolling axis. Specimens from the ECAP material were machined from the middle portions of the billets. Uniaxial compression tests were conducted in air using an electromechanical Instron 4507 testing machine at four different temperatures: 20, 100, 200 and 300 ◦ C at a constant cross-head speed corresponding to an initial strain rate of about 10−3 s−1 .
Fig. 2. After four ECAP passes the hot-rolled material (AZ31HR–ECAP4) displayed a grain size of 1–2 m as illustrated in Fig. 3. The grains were equiaxed with sharp grain boundaries. Many dislocations were observed in the ECAP material as shown in Fig. 4.
3.2. Compressive behaviour Figs. 5–7 show the true stress versus true strain curves for squeeze cast material (AZ31SC) and the hot-rolled material prior to ECAP (AZ31HR) and after four ECAP passes (AZ31HR–ECAP4) for four temperatures: 20, 100, 200 and 300 ◦ C. Standard equations were used for calculating the true stressstrain curves from the load versus cross-head displacement curves obtained in compression tests in the strain range where no sizeable barrelling effect was present. A comparison of the curves for AZ31SC tested at 20 and 100 ◦ C shows that both the maximum strength and the ductility increase with increasing temperature in this temperature range, cf. Fig. 5. For 200 and 300 ◦ C, the flow stress drops below that found at 100 ◦ C. Deformation at 20 and 100 ◦ C terminated with the fracture of the specimens. The maximum compressive strength reached by the specimens deformed at 20 and 100 ◦ C was 248 and 329 MPa, respectively. Deformation at 200 and 300 ◦ C was stopped at a strain of about 0.4 because for higher degrees of deformation the requirement of uniaxial loading was no longer met. The yield stress (determined as the flow stress at a strain offset of 0.01) decreases with increasing deformation temperature, Fig. 8, as does the strain-hardening rate. AZ31HR exhibits a different deformation behaviour, see Fig. 6. The yield stress for this material is higher than that for SC material for the strain rate of
3. Experimental results 3.1. Microstructure The microstructure of AZ31SC is depicted in Fig. 1. It consists of large grains with an average size of 450 m. The SC material contains dendrites. The microstructure of AZ31HR was homogeneous with a grain size of about 20 m,
Fig. 2. Microstructure of AZ31HR.
296
Z. Z´uberov´a et al. / Journal of Materials Processing Technology 184 (2007) 294–299
Fig. 5. True stress vs. true strain curves for AZ31SC.
Fig. 3. Microstructure of AZ31HR–ECAP4.
10−3 s−1 used in the tests, cf. Fig. 8. At 20 and 100 ◦ C, a sharp yield point is visible. With increasing temperature, the yield stress decreases, but its temperature variation is now more pronounced than for the SC state. The experiments at 200 and 300 ◦ C were again stopped at a strain of about 0.4. The maximum compressive strength of the HR material at 20 and 100 ◦ C (473 and 455 MPa, respectively) is higher than in AZ31SC and is reached at lower strains, Fig. 9. Fig. 6. True stress vs. true strain curves for AZ31HR. The yield stress of AZ31HR–ECAP4 (Fig. 7) at 20 ◦ C is very close to that of hot-rolled specimens not processed by ECAP. The difference in the yield stress values between AZ31HR–ECAP4 and the hot-rolled material that did not undergo ECAP increases with the test temperature. The ECAP-treated material exhibits the highest value of the yield stress at all temperatures investigated. Strain to failure at 20 ◦ C is higher for SC material than for HR material. On the
Fig. 4. Dislocations in grains of AZ31HR–ECAP4.
Fig. 7. True stress vs. true strain curves for AZ31HR–ECAP4.
Z. Z´uberov´a et al. / Journal of Materials Processing Technology 184 (2007) 294–299
Fig. 8. Temperature dependence of the true yield stress of AZ31 in three different conditions studied.
Fig. 9. Maximum compressive stress (MCS) vs. the strain at which it was attained. other hand, strain to failure at 100 ◦ C is the highest for HR-ECAP4, followed by that for SC, and is the lowest for HR material that did not undergo ECAP pressing. This indicates the enhanced ductility of the fine-grained material. The maximum strength (for AZ31HR–ECAP4) of about 355 MPa at 20 ◦ C lies between those of AZ31SC and AZ31HR. The effect of ECAP on the flow stress of the hot-rolled material practically disappears at 200 ◦ C. Microstructures of AZ31SC, AZ31HR and AZ31HR–ECAP4 after compression tests at 200 ◦ C are presented in Figs. 10–13. In Fig. 10, a deformed dendritic microstructure is seen. The microstructure of deformed AZ31HR presented in Fig. 11 contains small (about 1 m) grains co-existing with coarser grains about 20 m in size. Fig. 12 shows the microstructure of AZ31HR–ECAP4 as observed by transmission electron microscopy. As could be expected, compression following ECAP did not lead to further changes in the grain size. However, a higher dislocation density resulting from compression test is obvious, cf. Figs. 4 and 13.
Fig. 10. Microstructure of AZ31SC after compression test at 200 ◦ C.
et al. in [11]. They observed a grain size of 1 m after eight passes of ECAP by route BC at 200 ◦ C. These differences in grain size were found to influence the compressive properties significantly. By contrast, Agnew et al. [12] did not find substantial changes in grain size and mechanical properties, which probably resulted from the high pressing temperature they used. The experimental results from the compression tests show that at and below 200 ◦ C the highest yield stress is associated with the smallest grain size (AZ31HR–ECAP4), followed by that of AZ31HR. This trend is in agreement with the grain size dependence of the yield stress according to the Hall–Petch relation [7,8]. This grain size dependence of mechanical properties was also reported for a magnesium alloy with low aluminium content (0.9 wt%) after two-pass-ECAP at 200 ◦ C [13]. On the other hand, at room temperature, strain to failure was found to drop with decreasing grain size. In contrast, Yamashita et al. [13] reported increasing elongation to failure, albeit for the case of tensile deformation. The largest strain to failure at 100 ◦ C was found for AZ31HR–ECAP4. With increasing temperature the mechanical behaviour is changed. It is interesting to note that the yield stress of AZ31 prepared by SC has the lowest level, but it decreases with temperature fairly slowly. This relative thermal
4. Discussion AZ31 specimens in three different conditions associated with the processing history of the alloys were tested in compression at 20, 100, 200 and 300 ◦ C. The average grain size depends on the specimen condition. The squeeze cast material had a coarse grained microstructure. Hot-rolled material had medium-sized grains that were substantially reduced (down to 1–2 m) after four ECAP passes. This behaviour was also reported by Mukai
297
Fig. 11. Microstructure of AZ31HR after compression test at 200 ◦ C.
298
Z. Z´uberov´a et al. / Journal of Materials Processing Technology 184 (2007) 294–299
Fig. 12. TEM micrograph of AZ31HR–ECAP4 after compression test at 200 ◦ C.
stability is distinct from the temperature variations of the yield stress of hot-rolled AZ31. While the material in this condition exhibits a higher compressive yield stress at 20 ◦ C, the yield stress drops off with temperature in a much more precipitous way. Thus, the value of the yield stress of AZ31SC at 300 ◦ C is
the highest one, although it is obviously too low for the alloy to be applied at this temperature. Strain hardening was observed at all temperatures and it was significant, especially at 20 and 100 ◦ C. When AZ31 (HR, HR-ECAP4) was deformed at 200 and 300 ◦ C, a quasi-steady state flow region was observed over a broad strain range. This suggests the occurrence of dynamic recovery. The flow stress decreased with increasing temperature monotonically. This behaviour agrees with the results reported by Ishikawa et al. [2] who studied high temperature compressive properties of AZ31 over a strain rate range from 10−3 to 10−1 s−1 . They suggested that the predominant deformation process at the low strain rate was dislocation creep controlled by pipe diffusion at low temperatures and by lattice diffusion at high temperatures. It was further suggested that a low dislocation density in magnesium alloys is a result of the scarcity of slip systems in hcp structure [2]. The high dislocation density detected by TEM in the present work, along with a significant strain hardening observed at room temperature, suggests that deformation is carried by a dislocation mechanism. It should be noted that we did not observe twinning for deformation at 200 ◦ C, which is in agreement with the work of Agnew and Duygulu [14] who reported very limited twinning in fine-grained rolled Mg alloys and claimed the non-basal slip of a or slip of a + c dislocations together with dynamic recrystallisation to be possible deformation mechanisms in fine-grained hexagonal materials at elevated temperatures. 5. Conclusions The compressive properties of magnesium alloy AZ31 in three different conditions were examined at the nominal strain rate of 10−3 s−1 in a temperature range from 20 to 300 ◦ C. The values of the yield stress and maximum strength were shown to be sensitive to the microstructures produced by the three preparation techniques used. At 20 ◦ C, the highest yield stress was found for hot-rolled material after substantial grain refinement by equal channel angular pressing (AZ31HR–ECAP4), while the squeeze cast material AZ31SC exhibited the lowest value of the yield stress. Due to different rates of decrease of the yield stress with temperature, at 300 ◦ C the yield stress for the squeeze cast material turned out to be the highest. ECAP was demonstrated to provide a slight increase of the compressive yield strength of the hot-rolled material at all temperatures, the effect being more pronounced at elevated temperatures. Acknowledgements The work was funded by DFG through the grant Es 74/9-1 and by GACR through the grant 106/05/2347 (M.J.). References
Fig. 13. TEM evidence of high dislocation density of AZ31HR–ECAP4 after ¯ B = [0 1¯ 1 1]). compression test at 200 ◦ C (dark field, g = (1¯ 0 1 1),
[1] H.-K. Kim, Y.-I. Lee, Ch.-S. Chung, Fatigue properties of a fine-grained magnesium alloy produced by equal channel angular pressing, Scripta Mater. 52 (2005) 473–477.
Z. Z´uberov´a et al. / Journal of Materials Processing Technology 184 (2007) 294–299 [2] K. Ishikawa, H. Watanabe, T. Mukai, High temperature compressive properties over a wide range of strain rates in an AZ31 magnesium alloy, J. Mater. Sci. 40 (2005) 1577–1582. [3] R. Lapovok, R. Cottam, P.F. Thomson, Y. Estrin, Extraordinary superplastic ductility of magnesium alloy ZK60, J. Mater. Res. 20 (6) (2005) 1375–1378. [4] L. Kunz, P. Lukaˇs, Y. Estrin, Z. Z´uberov´a, Notch sensitivity of cast AZ31 magnesium alloy, Mater. Eng. (Materi´alov´e inˇzinierstvo) 12 (3) (2005) 88–91. ˇ ady, Cathodic anticorro[5] B. Hadzima, P. Palˇcek, M. Chalupov´a, R. Can´ sive protection of magnesium alloy AZ91, Kovove Mater. 41 (4) (2003) 257–269. [6] V.M. Segal, Materials processing by simple shear, Mater. Sci. Eng. A 197 (1995) 157–164. [7] E.O. Hall, The deformation and ageing of mild steel, 3 discussion of results, Proc. Phys. Soc. London 64B (381) (1951) 747–753. [8] N.J. Petch, The cleavage strength of polycrystals, J. Iron Steel Inst. 174 (1953) 25–28. [9] K.T. Park, H.J. Lee, Ch.S. Lee, B.D. Ahn, H.S. Cho, D.H. Shin, Effect of ECAP strain on deformation behaviour at low temperature superplastic
[10]
[11]
[12]
[13]
[14]
299
regime of ultrafine grained 5083Al alloy fabricated by ECAP, Mater. Trans. 45 (3) (2004) 958–963. S. Kamado, T. Ashie, H. Yamada, K. Sanbun, Y. Kojima, Improvement of tensile properties of wrought magnesium alloys by grain refining, Mater. Sci. Forum 350–351 (2000) 65–72. T. Mukai, M. Yamanoi, H. Watanabe, K. Higashi, Ductility enhancement in AZ31 magnesium alloy by controlling its grain structure, Scripta Mater. 45 (2001) 89–94. S.R. Agnew, T.M. Lillo, J. Macheret, G.M. Stoica, L. Chen, Y. Lu, D. Fielden, P.K. Liaw, Assessment of equal channel angular extrusion processing of magnesium alloys, in: Proceedings of the Magnesium Technology Symposium (2001), New Orleans, Louisiana, February 11–15, 2001, pp. 243–247. A. Yamashita, Z. Horita, T.G. Langdon, Improving the mechanical properties of magnesium and a magnesium alloy through severe plastic deformation, Mater. Sci. Eng. A 300 (2001) 142–147. S.R. Agnew, O. Duygulu, Plastic anisotropy and the role of non-basal slip in magnesium alloy AZ31B, Int. J. Plasticity 21 (2005) 1161– 1193.
Effect of equal channel angular pressing on the deformation behaviour of magnesium alloy AZ31 under uniaxial compression Z. Z´uberov´a a,∗ , Y. Estrin a , T.T. Lamark a , M. Janeˇcek b , R.J. Hellmig a , M. Krieger a a
Clausthal University of Technology, Institute of Material Science and Engineering, Agricolastr. 6, 38678 Clausthal-Zellerfeld, Germany b Charles University, Faculty of Mathematics and Physics, Ke Karlovu 5, 121 16 Prague 2, Czech Republic Received 19 September 2006; received in revised form 16 November 2006; accepted 22 November 2006
Abstract Deformation behaviour of magnesium alloy AZ31 under uniaxial compression was investigated. AZ31 samples produced by squeeze casting, hot rolling and hot rolling with subsequent equal channel angular pressing (ECAP) were tested. It was shown that ECAP processing produces a fine-grained microstructure that gives rise to improved mechanical properties. © 2006 Elsevier B.V. All rights reserved. Keywords: ECAP; AZ31; Compression behaviour
1. Introduction Magnesium alloys are among the lightest metallic materials used for structural applications. Their high strength/weight ratio predetermines these materials for a broad range of technical applications, particularly in the automotive and aircraft industries as well as in manufacturing of electronic devices. Mg alloys are commonly produced as cast material because of their good castability, acceptable quality, dimensional control and a relatively high strength. Difficulties with plastic forming of the alloys are a consequence of their hexagonal close-packed structure. However, the fabrication of wrought magnesium alloys also has considerable potential because of their advantageous mechanical properties (higher ductility and strength) as compared to cast ones [1,2]. Plastic forming of Mg alloys is enhanced at elevated temperatures, because non-basal slip can be activated in addition to basal slip. Superplasticity is also observed in finegrained magnesium alloys [3]. A possible way for improving mechanical properties of magnesium and its alloys is by processing them by equal channel angular pressing (ECAP). In order to optimize the plastic forming process, it is important to understand the effect of temperature and strain rate on the flow stress. Recently, the high temperature deformation behaviour of AZ31 alloy has been examined extensively by Ishikawa et al. [2]. AZ31
∗
Corresponding author. Tel.: +49 5323 722753; fax: +49 5323 723148. E-mail address: [email protected] (Z. Z´uberov´a).
0924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.11.098
is a medium strength alloy with low notch fatigue sensitivity [4] and good damping behaviour. However, this alloy has a low corrosion resistance [5] and low creep resistance above 130 ◦ C. ECAP is a relatively new method for manufacturing bulk materials with fine to ultra-fine microstructure by severe plastic deformation [6]. High tensile and compressive strength of materials prepared by ECAP can be expected according to the Hall–Petch relation [7,8]. It is further desirable to find processing conditions for which the strength enhancement is not accompanied by a loss of ductility. The ECAP die is a block with two intersecting channels of identical cross-sections. The billet to be processed has the same cross-section as the channel and has to be well lubricated. It is placed into the first channel and then pressed into the exit channel by a plunger. Ideally, severe deformation by simple shear occurs in a zone where the two channels meet. As the billet is pushed through this zone, new regions of material successively experience this shear strain providing a uniform shear deformation of the billet—apart from its end regions. Strain accumulation can be produced by repeated pressing of the sample. Between the subsequent passes the sample can be rotated. The most commonly used route is BC which involves rotating the specimen 90◦ around its axis between successive passes. This has been shown to result in an equiaxed grain structure with enhanced mechanical and other properties [9]. ECAP of AZ31 was reported in Refs. [10–12]. Various ECAP conditions were investigated. Grain refinement and deformation twins after ECAP processing in the temperature range of 200–250 ◦ C were observed [10]. Furthermore, recrystallisation
Z. Z´uberov´a et al. / Journal of Materials Processing Technology 184 (2007) 294–299
295
was found to occur above 200 ◦ C by Kamado et al. [10]. Average grain size of 1 m after eight passes of ECAP by route BC at 200 ◦ C was reported in [11]. Agnew et al. [12] conducted ECAP of AZ31 at 300 ◦ C because they experienced cracking of samples at a low temperature of 200 ◦ C. They neither found any significant grain refinement nor a change in the tensile properties at 300 ◦ C. The objective of the present work was to study the deformation behaviour under uniaxial compressive loading of magnesium alloy AZ31 prepared by three processing techniques: squeeze casting (SC), hot rolling (HR) and hot rolling followed by four passes of equal channel angular pressing via route BC (HR-ECAP4). 2. Experimental procedures Fig. 1. Microstructure of AZ31SC. Conventional magnesium alloy AZ31 (nominal composition 3 wt% Al and 1 wt% Zn) was used as test material. It was prepared by three different processing techniques. The first state was squeeze cast (SC) and the second one hot rolled (HR). The third state was produced by four ECAP passes (route BC ) from hot-rolled material (HR-ECAP4). Squeeze casting was realized by the two-step loading procedure (pressure of 80 MPa for 15 s followed by 140 MPa for 90 s). The temperature of the casting die was 200 ◦ C. Hot rolling was conducted at 400 ◦ C. The materials were tested in the as-processed states. Specimens for ECAP pressing having the dimensions of 10 mm × 10 mm × 60 mm were machined from the hot-rolled material. The angle between the two channels in the ECAP die was Φ = 90◦ . The cross-sectional dimensions of both channels were 10 mm × 10 mm. The material of the die was a tool steel X38CrMoV51. The die was placed in a servohydraulic testing machine Instron 8502 with a maximum load capacity of 200 kN. ECAP was carried out at 200 ◦ C at a speed of 5 mm/min. For easier pressing, temperature resistant lubricant MoS2 was used. No cracking of specimens occurred under these conditions, unlike in the case presented in [12], when higher pressing speeds were used. It should be stressed that in the present work, the main target was not the processing route leading to these three different states, but rather the effect of the ensuing microstructure on the post-processing behaviour under compression. Microstructures of specimens in as-processed conditions (SC, HR) as well as after compression tests were observed with an optical microscope ZEISS Axioplan 2. Specimens from SC were cut with no specific orientation, while those from HR were cut parallel to the rolling direction. The specimens were mechanically polished and subsequently etched by picric acid. The microstructure of specimens after ECAP and after the following compression tests were investigated by transmission electron microscopy (TEM) using a Philips CM 200 electron microscope operated at 200 kV. TEM specimens were cut from middle sections of the pressed billets perpendicular to the pressing direction. Thin foils for TEM were first mechanically polished and finally electropolished in a Tenupol 5 double jet polishing unit in a LiCl + Mg perchlorate + methanol + buthyloxyethanol solution at −45 ◦ C. Cylindrical specimens having a diameter of 10 mm and a height of 15 mm were used for compression tests. As in microstructure investigations, specimens of squeeze cast material were cut from a billet at a random orientation. Specimens prepared from hot-rolled material were taken parallel to the rolling axis. Specimens from the ECAP material were machined from the middle portions of the billets. Uniaxial compression tests were conducted in air using an electromechanical Instron 4507 testing machine at four different temperatures: 20, 100, 200 and 300 ◦ C at a constant cross-head speed corresponding to an initial strain rate of about 10−3 s−1 .
Fig. 2. After four ECAP passes the hot-rolled material (AZ31HR–ECAP4) displayed a grain size of 1–2 m as illustrated in Fig. 3. The grains were equiaxed with sharp grain boundaries. Many dislocations were observed in the ECAP material as shown in Fig. 4.
3.2. Compressive behaviour Figs. 5–7 show the true stress versus true strain curves for squeeze cast material (AZ31SC) and the hot-rolled material prior to ECAP (AZ31HR) and after four ECAP passes (AZ31HR–ECAP4) for four temperatures: 20, 100, 200 and 300 ◦ C. Standard equations were used for calculating the true stressstrain curves from the load versus cross-head displacement curves obtained in compression tests in the strain range where no sizeable barrelling effect was present. A comparison of the curves for AZ31SC tested at 20 and 100 ◦ C shows that both the maximum strength and the ductility increase with increasing temperature in this temperature range, cf. Fig. 5. For 200 and 300 ◦ C, the flow stress drops below that found at 100 ◦ C. Deformation at 20 and 100 ◦ C terminated with the fracture of the specimens. The maximum compressive strength reached by the specimens deformed at 20 and 100 ◦ C was 248 and 329 MPa, respectively. Deformation at 200 and 300 ◦ C was stopped at a strain of about 0.4 because for higher degrees of deformation the requirement of uniaxial loading was no longer met. The yield stress (determined as the flow stress at a strain offset of 0.01) decreases with increasing deformation temperature, Fig. 8, as does the strain-hardening rate. AZ31HR exhibits a different deformation behaviour, see Fig. 6. The yield stress for this material is higher than that for SC material for the strain rate of
3. Experimental results 3.1. Microstructure The microstructure of AZ31SC is depicted in Fig. 1. It consists of large grains with an average size of 450 m. The SC material contains dendrites. The microstructure of AZ31HR was homogeneous with a grain size of about 20 m,
Fig. 2. Microstructure of AZ31HR.
296
Z. Z´uberov´a et al. / Journal of Materials Processing Technology 184 (2007) 294–299
Fig. 5. True stress vs. true strain curves for AZ31SC.
Fig. 3. Microstructure of AZ31HR–ECAP4.
10−3 s−1 used in the tests, cf. Fig. 8. At 20 and 100 ◦ C, a sharp yield point is visible. With increasing temperature, the yield stress decreases, but its temperature variation is now more pronounced than for the SC state. The experiments at 200 and 300 ◦ C were again stopped at a strain of about 0.4. The maximum compressive strength of the HR material at 20 and 100 ◦ C (473 and 455 MPa, respectively) is higher than in AZ31SC and is reached at lower strains, Fig. 9. Fig. 6. True stress vs. true strain curves for AZ31HR. The yield stress of AZ31HR–ECAP4 (Fig. 7) at 20 ◦ C is very close to that of hot-rolled specimens not processed by ECAP. The difference in the yield stress values between AZ31HR–ECAP4 and the hot-rolled material that did not undergo ECAP increases with the test temperature. The ECAP-treated material exhibits the highest value of the yield stress at all temperatures investigated. Strain to failure at 20 ◦ C is higher for SC material than for HR material. On the
Fig. 4. Dislocations in grains of AZ31HR–ECAP4.
Fig. 7. True stress vs. true strain curves for AZ31HR–ECAP4.
Z. Z´uberov´a et al. / Journal of Materials Processing Technology 184 (2007) 294–299
Fig. 8. Temperature dependence of the true yield stress of AZ31 in three different conditions studied.
Fig. 9. Maximum compressive stress (MCS) vs. the strain at which it was attained. other hand, strain to failure at 100 ◦ C is the highest for HR-ECAP4, followed by that for SC, and is the lowest for HR material that did not undergo ECAP pressing. This indicates the enhanced ductility of the fine-grained material. The maximum strength (for AZ31HR–ECAP4) of about 355 MPa at 20 ◦ C lies between those of AZ31SC and AZ31HR. The effect of ECAP on the flow stress of the hot-rolled material practically disappears at 200 ◦ C. Microstructures of AZ31SC, AZ31HR and AZ31HR–ECAP4 after compression tests at 200 ◦ C are presented in Figs. 10–13. In Fig. 10, a deformed dendritic microstructure is seen. The microstructure of deformed AZ31HR presented in Fig. 11 contains small (about 1 m) grains co-existing with coarser grains about 20 m in size. Fig. 12 shows the microstructure of AZ31HR–ECAP4 as observed by transmission electron microscopy. As could be expected, compression following ECAP did not lead to further changes in the grain size. However, a higher dislocation density resulting from compression test is obvious, cf. Figs. 4 and 13.
Fig. 10. Microstructure of AZ31SC after compression test at 200 ◦ C.
et al. in [11]. They observed a grain size of 1 m after eight passes of ECAP by route BC at 200 ◦ C. These differences in grain size were found to influence the compressive properties significantly. By contrast, Agnew et al. [12] did not find substantial changes in grain size and mechanical properties, which probably resulted from the high pressing temperature they used. The experimental results from the compression tests show that at and below 200 ◦ C the highest yield stress is associated with the smallest grain size (AZ31HR–ECAP4), followed by that of AZ31HR. This trend is in agreement with the grain size dependence of the yield stress according to the Hall–Petch relation [7,8]. This grain size dependence of mechanical properties was also reported for a magnesium alloy with low aluminium content (0.9 wt%) after two-pass-ECAP at 200 ◦ C [13]. On the other hand, at room temperature, strain to failure was found to drop with decreasing grain size. In contrast, Yamashita et al. [13] reported increasing elongation to failure, albeit for the case of tensile deformation. The largest strain to failure at 100 ◦ C was found for AZ31HR–ECAP4. With increasing temperature the mechanical behaviour is changed. It is interesting to note that the yield stress of AZ31 prepared by SC has the lowest level, but it decreases with temperature fairly slowly. This relative thermal
4. Discussion AZ31 specimens in three different conditions associated with the processing history of the alloys were tested in compression at 20, 100, 200 and 300 ◦ C. The average grain size depends on the specimen condition. The squeeze cast material had a coarse grained microstructure. Hot-rolled material had medium-sized grains that were substantially reduced (down to 1–2 m) after four ECAP passes. This behaviour was also reported by Mukai
297
Fig. 11. Microstructure of AZ31HR after compression test at 200 ◦ C.
298
Z. Z´uberov´a et al. / Journal of Materials Processing Technology 184 (2007) 294–299
Fig. 12. TEM micrograph of AZ31HR–ECAP4 after compression test at 200 ◦ C.
stability is distinct from the temperature variations of the yield stress of hot-rolled AZ31. While the material in this condition exhibits a higher compressive yield stress at 20 ◦ C, the yield stress drops off with temperature in a much more precipitous way. Thus, the value of the yield stress of AZ31SC at 300 ◦ C is
the highest one, although it is obviously too low for the alloy to be applied at this temperature. Strain hardening was observed at all temperatures and it was significant, especially at 20 and 100 ◦ C. When AZ31 (HR, HR-ECAP4) was deformed at 200 and 300 ◦ C, a quasi-steady state flow region was observed over a broad strain range. This suggests the occurrence of dynamic recovery. The flow stress decreased with increasing temperature monotonically. This behaviour agrees with the results reported by Ishikawa et al. [2] who studied high temperature compressive properties of AZ31 over a strain rate range from 10−3 to 10−1 s−1 . They suggested that the predominant deformation process at the low strain rate was dislocation creep controlled by pipe diffusion at low temperatures and by lattice diffusion at high temperatures. It was further suggested that a low dislocation density in magnesium alloys is a result of the scarcity of slip systems in hcp structure [2]. The high dislocation density detected by TEM in the present work, along with a significant strain hardening observed at room temperature, suggests that deformation is carried by a dislocation mechanism. It should be noted that we did not observe twinning for deformation at 200 ◦ C, which is in agreement with the work of Agnew and Duygulu [14] who reported very limited twinning in fine-grained rolled Mg alloys and claimed the non-basal slip of a or slip of a + c dislocations together with dynamic recrystallisation to be possible deformation mechanisms in fine-grained hexagonal materials at elevated temperatures. 5. Conclusions The compressive properties of magnesium alloy AZ31 in three different conditions were examined at the nominal strain rate of 10−3 s−1 in a temperature range from 20 to 300 ◦ C. The values of the yield stress and maximum strength were shown to be sensitive to the microstructures produced by the three preparation techniques used. At 20 ◦ C, the highest yield stress was found for hot-rolled material after substantial grain refinement by equal channel angular pressing (AZ31HR–ECAP4), while the squeeze cast material AZ31SC exhibited the lowest value of the yield stress. Due to different rates of decrease of the yield stress with temperature, at 300 ◦ C the yield stress for the squeeze cast material turned out to be the highest. ECAP was demonstrated to provide a slight increase of the compressive yield strength of the hot-rolled material at all temperatures, the effect being more pronounced at elevated temperatures. Acknowledgements The work was funded by DFG through the grant Es 74/9-1 and by GACR through the grant 106/05/2347 (M.J.). References
Fig. 13. TEM evidence of high dislocation density of AZ31HR–ECAP4 after ¯ B = [0 1¯ 1 1]). compression test at 200 ◦ C (dark field, g = (1¯ 0 1 1),
[1] H.-K. Kim, Y.-I. Lee, Ch.-S. Chung, Fatigue properties of a fine-grained magnesium alloy produced by equal channel angular pressing, Scripta Mater. 52 (2005) 473–477.
Z. Z´uberov´a et al. / Journal of Materials Processing Technology 184 (2007) 294–299 [2] K. Ishikawa, H. Watanabe, T. Mukai, High temperature compressive properties over a wide range of strain rates in an AZ31 magnesium alloy, J. Mater. Sci. 40 (2005) 1577–1582. [3] R. Lapovok, R. Cottam, P.F. Thomson, Y. Estrin, Extraordinary superplastic ductility of magnesium alloy ZK60, J. Mater. Res. 20 (6) (2005) 1375–1378. [4] L. Kunz, P. Lukaˇs, Y. Estrin, Z. Z´uberov´a, Notch sensitivity of cast AZ31 magnesium alloy, Mater. Eng. (Materi´alov´e inˇzinierstvo) 12 (3) (2005) 88–91. ˇ ady, Cathodic anticorro[5] B. Hadzima, P. Palˇcek, M. Chalupov´a, R. Can´ sive protection of magnesium alloy AZ91, Kovove Mater. 41 (4) (2003) 257–269. [6] V.M. Segal, Materials processing by simple shear, Mater. Sci. Eng. A 197 (1995) 157–164. [7] E.O. Hall, The deformation and ageing of mild steel, 3 discussion of results, Proc. Phys. Soc. London 64B (381) (1951) 747–753. [8] N.J. Petch, The cleavage strength of polycrystals, J. Iron Steel Inst. 174 (1953) 25–28. [9] K.T. Park, H.J. Lee, Ch.S. Lee, B.D. Ahn, H.S. Cho, D.H. Shin, Effect of ECAP strain on deformation behaviour at low temperature superplastic
[10]
[11]
[12]
[13]
[14]
299
regime of ultrafine grained 5083Al alloy fabricated by ECAP, Mater. Trans. 45 (3) (2004) 958–963. S. Kamado, T. Ashie, H. Yamada, K. Sanbun, Y. Kojima, Improvement of tensile properties of wrought magnesium alloys by grain refining, Mater. Sci. Forum 350–351 (2000) 65–72. T. Mukai, M. Yamanoi, H. Watanabe, K. Higashi, Ductility enhancement in AZ31 magnesium alloy by controlling its grain structure, Scripta Mater. 45 (2001) 89–94. S.R. Agnew, T.M. Lillo, J. Macheret, G.M. Stoica, L. Chen, Y. Lu, D. Fielden, P.K. Liaw, Assessment of equal channel angular extrusion processing of magnesium alloys, in: Proceedings of the Magnesium Technology Symposium (2001), New Orleans, Louisiana, February 11–15, 2001, pp. 243–247. A. Yamashita, Z. Horita, T.G. Langdon, Improving the mechanical properties of magnesium and a magnesium alloy through severe plastic deformation, Mater. Sci. Eng. A 300 (2001) 142–147. S.R. Agnew, O. Duygulu, Plastic anisotropy and the role of non-basal slip in magnesium alloy AZ31B, Int. J. Plasticity 21 (2005) 1161– 1193.