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Microstructure and mechanical properties of AZ91D magnesium alloy prepared by compound extrusion
Materials Science and Engineering A 528 (2011) 3930–3934
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Rapid Communication
Microstructure and mechanical properties of AZ91D magnesium alloy prepared by compound extrusion Qiang Chen ∗ , Zhixiang Zhao, Dayu Shu, Zude Zhao Southwest Technique and Engineering Institute, Chongqing 400039, PR China
a r t i c l e
i n f o
Article history: Received 20 July 2010 Received in revised form 12 January 2011 Accepted 12 January 2011 Available online 19 January 2011 Keywords: AZ91D magnesium alloys Severe plastic deformation Microstructure Mechanical properties Compound extrusion
a b s t r a c t Based on conventional extrusion and equal channel angular extrusion (ECAE), a new severe plastic deformation (SPD) method called compound extrusion is developed to fabricate fine-grained AZ91D magnesium alloys. The fine grain size of 6 m is obtained as the accumulated strain increased to 9.146. The AZ91D alloy treated by compound extrusion exhibits good mechanical properties, with a yield strength of 202.2 MPa, a tensile strength of 323.1 MPa and an elongation to fracture of 14.8%. The good mechanical properties of AZ91D alloy treated by compound extrusion are due to grain refinement and to the homogeneous distribution of intermetallic particles. The success in development of compound extrusion proves that compound extrusion can offer a good opportunity for the development of good mechanical properties of as-cast magnesium alloys. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Over the last few years, magnesium alloys have become promising candidates for the replacement of some conventional engineering metals because of the demand of lighter structural materials [1]. However, due to their hexagonal close-packed (HCP) structure and low stacking fault energy, they generally present limited strength and ductility at ambient temperature [2,3]. An effective way to minimize these disadvantages is to develop magnesium alloys with fine-grained microstructures [4]. Equal channel angular extrusion (ECAE) is an effective method, which introduces severe strain repeatedly to a material. At the present, some research [5–10] has been performed on the application of ECAE to magnesium and its alloys. The results of Zheng et al. [5] suggested that after eight-pass ECAE, the microstructure of the Mg–Zn–Y–Zr alloy consisted of grains of about 3.5 m having a low dislocation density. Kim et al. [6] reported that after eight-pass ECAE following route Bc, the mean grain size of as-extruded AZ61 alloy was refined from 24.4 m to 8.4 m. Ding et al. [7] developed a multi-temperature ECAE process, which could produce an ultrafine-grained AZ31 alloy having a grain size 0.37 m. Figueiredo and Langdon [8] reported that the ECAE formed ZK60 alloy was measured as 0.8 m after pressing through
∗ Corresponding author. Tel.: +86 023 68792286. E-mail address: [email protected] (Q. Chen). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.01.028
eight passes. Kulyasova et al. [9] reported the microstructure of the ultrafine-grained AM60 alloy processed by ECAE. They suggested that for the lowest processing temperature, the mean grain size of 10-pass ECAE formed alloy was small as 1 m. Máthis et al. [10] suggested that the initial grain size of 40 m decreased to 1.2 m after eight ECAE passes for AZ91 alloy. Although ECAE was an effective method to refine grain sizes of magnesium alloys, ECAE has been used only in the laboratory because of its low productivity. For mass production, a new severe plastic deformation (SPD) called compound extrusion was proposed based on conventional extrusion and ECAE. The aim of the present study is to investigate the microstructure and mechanical properties of AZ91D alloy treated by compound extrusion.
2. Experimental procedure The material used in the present study was a commercial Mg–Al–Zn alloy, AZ91D, and was received as a commercially as-cast bar. Its nominal composition was 9.0 wt.% Al, 1.0 wt.% Zn, 0.20 wt.% Mn, and balanced magnesium. For compound extrusion, the ascast bar was cut into samples with a diameter of 39 mm and a length of 60 mm. The compound extrusion process was conducted using a split die, fabricated from tool steel. The schematic representation of compound extrusion process was shown in Fig. 1. It consisted of conventional extrusion and modified ECAE. In the section of conventional extrusion die, the semi-angle of conical die was /4 and the extrusion ratio was 9:1. In the section of modi-
Q. Chen et al. / Materials Science and Engineering A 528 (2011) 3930–3934
3931
Fig. 2. Principle of ECAE with inside transient arc radius r and lateral transient arc radius R.
3. Results and discussion 3.1. Estimation of the strain in compound extrusion
Fig. 1. A schematic drawing of compound extrusion.
Iwahashi et al. [11] have estimated the strain in ECAE. They suggested that the same strain was accumulated in each passage through the die, the strain after N cycles, εN was therefore given by
fied ECAE die, it consisted of the conventional ECAE process and the “C” shape ECAE process. Before compound extrusion, an ascast bar and the compound extrusion die were held for 20 min at 120 ◦ C, and then were coated with a lubricant of colloidal graphite mixed with water. Then, they were heated to 300 ◦ C and held for 30 min. During heating, heating jackets were put on the die and the extrusion temperature was controlled within ±5 ◦ C of the setting temperature. During compound extrusion, the billet was inserted into the heated conventional extrusion die and stayed in the die for approximately 5 min, to allow the billet to reach the targeting temperature. Extrusion speed was about 1 mm/s. After compound extrusion, the billet was quenched in cold water. Samples for microscopic examination were cut parallel to the extrusion direction of the billet. After mechanical polishing, samples were etched for 3 s using a solution of 100 ml ethanol, 6 g picric acid, 5 ml acetic acid and 10 ml water. Grain sizes were measured using a mean linear intercept method. For each sample, measurements were taken from the whole sectioned area with 800 intercepts counted per sample. Five tensile samples of dogbone geometry with the 5 mm gauge length, 4 mm width, 2 mm thickness, and 2 mm shoulder radius with the gauge length parallel to the extrusion direction were extracted from the centre portion of the compound extrusion billets using linear cutting machining. Tensile test was performed on Instron-5569 tensile machine, with initial strain rate of 2 mm/min at room temperature. Note that for microstructure examination of different parts labelled points (Fig. 1), the bar was taken from the die different deformation stages whereas the mechanical properties of compound extruded bar were obtained after complete process. The fracture was examined by scanning electron microscopy (SEM). Measurements of Vickers microhardness were made on the vertical extrusion direction of ascast and compound extruded alloys using a Matsuzawa MXT-␣7e microhardness tester. For each measurement, a load of 200 g F was applied for 15 s. Seven measurements were taken on each sample at randomly selected points and then averaged to give the Vickers microhardness.
N εN = √ 2 cot 3
1 2
˚+
1 2
+ csc
1 2
˚+
1 2
(1)
where N was the number of ECAE passes, ˚ was the include angle and was the corner angle. Although Iwahashi et al. considered that N, ˚ and exerted influences on εN , they neglected the effect of inside transient arc radius r and lateral transient arc radius R on εN . In the present research, compound extrusion consisted of two sections: conventional extrusion and modified ECAE. In the section of conventional, accumulated strain ε was depended on extrusion ratio and was given by
ε = ln l/l0 = ln r02 /r12 = ln
(2)
where l was the length of billet after extrusion, l0 was the original length of billet before extrusion, r0 was the initial radius of billet before extrusion and r1 was the radius of billet after extrusion. In the section of modified ECAE, only single pass processing was considered for simplicity. The principle of modified ECAE is illustrated schematically in Fig. 2, where ˚ was the include angle, was the corner angle, r was inside transient arc radius and R was lateral transient arc radius. Note that problems with friction at the die walls may be avoided by use of appropriate lubricants. Therefore, the present analysis assumes the die is fabricated as illustrated in Fig. 1 and the billet is lubricated so that frictional effects can be neglected. In Fig. 2, the shear strain is = a u/d u, where a u = a t + tu = rc + as and d u = ad = L. On the one hand, a u can be obtained from the relationship ab = dc , as + sb = dr + rc , rc = as + sb − dr, rc = as + (R − r) , so that a u = rc + as = 2as + (R − r) . On the other hand, as = adcot( /2 + ˚/2) and a u = 2adcot( /2 + ˚/2) + (R − r) . Therefore, the shear strain for single pass ECAE is given by = 2 cot
2
+
˚ 2
+ (R − r)
L
(3)
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Q. Chen et al. / Materials Science and Engineering A 528 (2011) 3930–3934
Fig. 3. Optical microstructures of AZ91D magnesium alloy formed by compound extrusion with different accumulated strains, (a) 0, point A; (b) 2.197, point B; (c) 2.801, point C; (d) 4.614, point D; (e) 6.427, point E; (f) 8.240, point F and (g) 9.146, point G. The location of point was labelled in Fig. 1.
The equivalent strain, εeq , is represented by
⎡
⎢ εeq = ⎣
2 ε2x + ε2y + ε2z + 3
2 + 2 + 2 xy yz zx
2
⎤1/2 ⎥ ⎦
(4)
In the present research, εx , εy , εz , yz and zx are 0. Therefore, the equivalent strain, εeq , can be expressed as
1 εeq = √ 2 cot /2 + ˚/2 + (R − r)/L 3
(5)
According to Eq. (5), in the section of modified ECAE, the equivalent strain of single pass depends on not only the include angle ˚ and the corner angle , but also inside transient arc radius r, lateral transient arc radius R and the diameter of billet L. When the billet√passes die “C”, the imposed strain of the billet increases by / 3. In the present research, the inside transient arc radius r, the lateral transient arc radius R and the diameter of billet L for compound extrusion are 13 mm, 26 mm and 13 mm, respectively. According to Eqs. (2) and (5), the accumulated strains of √ point A, B, C, D, E, F√and G are 0, 2.197 (ln 9), 2.801 (ln 9 + 3/9), 4.614 (ln 9 + 4 3/9), 6.427 (ln 9 +
Q. Chen et al. / Materials Science and Engineering A 528 (2011) 3930–3934
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√ √ √ 7 3/9), 8.240 (ln 9 + 10 3/9) and 9.146 (ln 9 + 23 3/18), respectively. 3.2. Optical microstructure Fig. 3(a) shows the microstructure of as-cast AZ91D magnesium alloy. As shown in Fig. 3(a), the as-cast AZ91D alloy consisted of ␣Mg matrix and eutectic (␣-Mg + -Mg17 Al12 ). In the as-cast AZ91 alloy, eutectic was located into interdendritic spaces. The presence of eutectic in the microstructure was attributed to non-equilibrium solidification caused by the fast cooling rate of conventional casting process. Although many grains already significantly refined after conventional extrusion, the grain structure was not homogeneous with very fine grains of 2–4 m as well as coarse grains of greater than 30 m (Fig. 3(b)). This so-called “bimodal” microstructure was also observed by Lin et al. [12]. Careful examination of Fig. 3(b) indicated that many fine grains were formed along the grain boundaries of those coarse grains. When the accumulated strain reached 2.801, recrystallisation was almost extended completely and most of old grains were replaced by fine and recrystallised grains (Fig. 3(c)). As shown in Fig. 3(d), the whole matrix of sample was taken up by recrystallised grains and average grain size was about 19 m. With increase in accumulated strain, recrystallised grains were refined and became more equiaxed (Fig. 3(e) and (f)). The fine grain of 6 m had been obtained when the accumulated strain approached 9.146 (Fig. 3(g)). The comparison of the microstructures in Fig. 3(b)–(g) revealed that the mean grain size tended to decrease as the accumulated strain increased from 0 to 8.240. However, when the accumulated strain exceeded 8.240, it was difficult to get more grain refinement further and the average grain size of compound extruded alloy reached to a final-equilibrium fine-grained size. With the increase of accumulated strain, the temperature at which the compound extrusion took place also had a prominent role to play. Since more heat caused by friction was generated during compound extrusion to a high accumulated strain than a low accumulated strain, the coarsening of recrystallised grains would be greater. This might counteract the effect alloy increased accumulated strain, resulting in the decrease of grain refinement ability during compound extrusion. During compound extrusion, once dynamic recrystallisation was completed, the further increase of accumulated strain provided no effect on grain refinement. 3.3. Hardness The mean HV values with the accumulated strain are shown in Fig. 4. It can be seen from Fig. 4 that the value of hardness dra-
Fig. 4. Influence of the accumulated strain on hardness of AZ91D alloy.
Table 1 Mechanical properties of as-cast and compound extruded AZ91D magnesium alloys. Conditions
YS (MPa)
UTS (MPa)
Elongation (%)
As-cast Compound extrusion
63.6 (2.7) 202.2 (4.5)
118.3 (8.3) 323.1 (7.6)
1.6 (0.4) 14.8 (1.6)
matically increased with increasing the accumulated strain from 0 to 4.614. However, when the accumulated strain approached 8.240, the hardness reached a saturated value and it was little influenced by increasing the accumulated strain on certain conditions. This result was in agreement with the microstructure evolution with increasing accumulated strain. The grain refinement and the increased grain boundary were attributed to increase in the value of hardness. 3.4. Mechanical properties and fractography Table 1 shows the tensile test results of the AZ91D alloy in both as-cast and compound extruded states. As shown in Table 1, the values of yield strength (YS) ultimate tensile strength (UTS) and elongation to fracture of the as-cast alloy were 63.6 MPa, 118.3 MPa and 1.6% respectively, and the values of YS, UTS and elongation to fracture of the compound extruded alloy were 202.2 MPa, 323.1 MPa and 14.8%, respectively. Fig. 5 shows typical fracture surfaces of as-cast and compound extruded AZ91D alloys. The SEM image of as-cast AZ91D in Fig. 5(a) shows a transcrystalline fracture, in which a few cracks could be observed. Moreover, the as-cast
Fig. 5. Fractography of as-cast and compound extruded AZ91D magnesium alloys: (a) as-cast alloy and (b) compound extruded alloy.
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AZ91 alloy contained coarse brittle intermetallic particles, which in-homogeneously distributed in the microstructure. Therefore, AZ91D alloy exhibited a little plastic deformation before fracture. It can be seen from Fig. 5(b), tiny and uniform dimples can be obviously observed on the fracture surface of the compound extruded alloy, corresponding to refined and equiaxed grains after compound extrusion. Comparison of Fig. 3(b)–(g) also shows that the higher accumulated strain gives a finer recrystallised grain size. The higher the accumulated strain, the greater the overall grain boundary and sub-grain boundary area. Since grain boundaries were effective obstacles to dislocation motion, and since fine-grained alloy would have a higher density of grain boundaries per unit volume, the strength of AZ91D alloy remarkably increased with decreasing grain size. The remarkably improved ductility was obtained in the compound extruded alloy because more fine grains contributed to the macroscopic deformation and the stress concentrations were accordingly reduced and spread over a wider area. Moreover, the compound extrusion process not only refined the grain size, but also changed the morphology and amount of intermetallic particles. As shown in Fig. 5(b), the breakage of intermetallic particles into smaller parts which facilitated the dislocation motion. Therefore, good ductility was obtained for compound extruded AZ91D alloy. 4. Conclusions By means of the new SPD method called compound extrusion, as-cast AZ91D magnesium alloy has been successfully applied to SPD up to the accumulated strain of 9.146 without the introduction of any significant damage. The fine grain of 6 m has been obtained after compound extrusion. Both the strength and the duc-
tility of the alloy are improved significantly after the processing due to grain refinement and to the homogeneous distribution of intermetallic particles. Furthermore, the value of hardness of AZ91D alloy increases obviously with increasing the accumulated strain from 0 to 8.240. However, the value of hardness increases a little with further increasing accumulated strain. Acknowledgements The authors are grateful for the support from the Natural Science Foundation of China (NSFC) under Grant No. 51005217. Dr. Chen is grateful for the support from China Postdoctoral Science Foundation Grant No. 20100480677. References ´ [1] K.N. Braszczynska-Malik, J. Alloys Compd. 477 (2009) 870–876. [2] S.M. Fatemi-Varzaneh, A. Zarei-Hanzaki, M. Haghshenas, J. Alloys Compd. 475 (2009) 126–130. [3] G. Ben Hamu, D. Eliezer, L. Wagner, J. Alloys Compd. 468 (2009) 222–229. [4] X.S. Huang, K. Suzuki, A. Watazu, I. Shigematsu, N. Saito, J. Alloys Compd. 470 (2009) 263–268. [5] M.Y. Zheng, S.W. Xu, K. Wu, S. Kamado, Y. Kojima, Mater. Lett. 61 (2007) 4406–4408. [6] W.J. Kim, C.W. An, Y.S. Kim, S.I. Hong, Acta Mater. 51 (2003) 3293–3307. [7] S.X. Ding, C.P. Chang, P.W. Kao, Metall. Mater. Trans. A 40A (2009) 415–424. [8] R.B. Figueiredo, T.G. Langdon, Mater. Sci. Eng. A 430 (2006) 151–156. [9] O. Kulyasova, R. Islamgaliev, B. Mingler, M. Zehetbauer, Mater. Sci. Eng. A 503 (2009) 176–180. [10] K. Máthis, J. Gubicza, N.H. Nam, J. Alloys Compd. 394 (2005) 194–199. [11] Y. Iwahashi, J. Wang, Z. Horita, M. Nemoto, T.G. Langdon, Scripta Mater. 35 (1996) 143–146. [12] J. Lin, Q. Wang, L. Peng, H.J. Rovenc, J. Alloys Compd. 476 (2009) 441–445.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Rapid Communication
Microstructure and mechanical properties of AZ91D magnesium alloy prepared by compound extrusion Qiang Chen ∗ , Zhixiang Zhao, Dayu Shu, Zude Zhao Southwest Technique and Engineering Institute, Chongqing 400039, PR China
a r t i c l e
i n f o
Article history: Received 20 July 2010 Received in revised form 12 January 2011 Accepted 12 January 2011 Available online 19 January 2011 Keywords: AZ91D magnesium alloys Severe plastic deformation Microstructure Mechanical properties Compound extrusion
a b s t r a c t Based on conventional extrusion and equal channel angular extrusion (ECAE), a new severe plastic deformation (SPD) method called compound extrusion is developed to fabricate fine-grained AZ91D magnesium alloys. The fine grain size of 6 m is obtained as the accumulated strain increased to 9.146. The AZ91D alloy treated by compound extrusion exhibits good mechanical properties, with a yield strength of 202.2 MPa, a tensile strength of 323.1 MPa and an elongation to fracture of 14.8%. The good mechanical properties of AZ91D alloy treated by compound extrusion are due to grain refinement and to the homogeneous distribution of intermetallic particles. The success in development of compound extrusion proves that compound extrusion can offer a good opportunity for the development of good mechanical properties of as-cast magnesium alloys. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Over the last few years, magnesium alloys have become promising candidates for the replacement of some conventional engineering metals because of the demand of lighter structural materials [1]. However, due to their hexagonal close-packed (HCP) structure and low stacking fault energy, they generally present limited strength and ductility at ambient temperature [2,3]. An effective way to minimize these disadvantages is to develop magnesium alloys with fine-grained microstructures [4]. Equal channel angular extrusion (ECAE) is an effective method, which introduces severe strain repeatedly to a material. At the present, some research [5–10] has been performed on the application of ECAE to magnesium and its alloys. The results of Zheng et al. [5] suggested that after eight-pass ECAE, the microstructure of the Mg–Zn–Y–Zr alloy consisted of grains of about 3.5 m having a low dislocation density. Kim et al. [6] reported that after eight-pass ECAE following route Bc, the mean grain size of as-extruded AZ61 alloy was refined from 24.4 m to 8.4 m. Ding et al. [7] developed a multi-temperature ECAE process, which could produce an ultrafine-grained AZ31 alloy having a grain size 0.37 m. Figueiredo and Langdon [8] reported that the ECAE formed ZK60 alloy was measured as 0.8 m after pressing through
∗ Corresponding author. Tel.: +86 023 68792286. E-mail address: [email protected] (Q. Chen). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.01.028
eight passes. Kulyasova et al. [9] reported the microstructure of the ultrafine-grained AM60 alloy processed by ECAE. They suggested that for the lowest processing temperature, the mean grain size of 10-pass ECAE formed alloy was small as 1 m. Máthis et al. [10] suggested that the initial grain size of 40 m decreased to 1.2 m after eight ECAE passes for AZ91 alloy. Although ECAE was an effective method to refine grain sizes of magnesium alloys, ECAE has been used only in the laboratory because of its low productivity. For mass production, a new severe plastic deformation (SPD) called compound extrusion was proposed based on conventional extrusion and ECAE. The aim of the present study is to investigate the microstructure and mechanical properties of AZ91D alloy treated by compound extrusion.
2. Experimental procedure The material used in the present study was a commercial Mg–Al–Zn alloy, AZ91D, and was received as a commercially as-cast bar. Its nominal composition was 9.0 wt.% Al, 1.0 wt.% Zn, 0.20 wt.% Mn, and balanced magnesium. For compound extrusion, the ascast bar was cut into samples with a diameter of 39 mm and a length of 60 mm. The compound extrusion process was conducted using a split die, fabricated from tool steel. The schematic representation of compound extrusion process was shown in Fig. 1. It consisted of conventional extrusion and modified ECAE. In the section of conventional extrusion die, the semi-angle of conical die was /4 and the extrusion ratio was 9:1. In the section of modi-
Q. Chen et al. / Materials Science and Engineering A 528 (2011) 3930–3934
3931
Fig. 2. Principle of ECAE with inside transient arc radius r and lateral transient arc radius R.
3. Results and discussion 3.1. Estimation of the strain in compound extrusion
Fig. 1. A schematic drawing of compound extrusion.
Iwahashi et al. [11] have estimated the strain in ECAE. They suggested that the same strain was accumulated in each passage through the die, the strain after N cycles, εN was therefore given by
fied ECAE die, it consisted of the conventional ECAE process and the “C” shape ECAE process. Before compound extrusion, an ascast bar and the compound extrusion die were held for 20 min at 120 ◦ C, and then were coated with a lubricant of colloidal graphite mixed with water. Then, they were heated to 300 ◦ C and held for 30 min. During heating, heating jackets were put on the die and the extrusion temperature was controlled within ±5 ◦ C of the setting temperature. During compound extrusion, the billet was inserted into the heated conventional extrusion die and stayed in the die for approximately 5 min, to allow the billet to reach the targeting temperature. Extrusion speed was about 1 mm/s. After compound extrusion, the billet was quenched in cold water. Samples for microscopic examination were cut parallel to the extrusion direction of the billet. After mechanical polishing, samples were etched for 3 s using a solution of 100 ml ethanol, 6 g picric acid, 5 ml acetic acid and 10 ml water. Grain sizes were measured using a mean linear intercept method. For each sample, measurements were taken from the whole sectioned area with 800 intercepts counted per sample. Five tensile samples of dogbone geometry with the 5 mm gauge length, 4 mm width, 2 mm thickness, and 2 mm shoulder radius with the gauge length parallel to the extrusion direction were extracted from the centre portion of the compound extrusion billets using linear cutting machining. Tensile test was performed on Instron-5569 tensile machine, with initial strain rate of 2 mm/min at room temperature. Note that for microstructure examination of different parts labelled points (Fig. 1), the bar was taken from the die different deformation stages whereas the mechanical properties of compound extruded bar were obtained after complete process. The fracture was examined by scanning electron microscopy (SEM). Measurements of Vickers microhardness were made on the vertical extrusion direction of ascast and compound extruded alloys using a Matsuzawa MXT-␣7e microhardness tester. For each measurement, a load of 200 g F was applied for 15 s. Seven measurements were taken on each sample at randomly selected points and then averaged to give the Vickers microhardness.
N εN = √ 2 cot 3
1 2
˚+
1 2
+ csc
1 2
˚+
1 2
(1)
where N was the number of ECAE passes, ˚ was the include angle and was the corner angle. Although Iwahashi et al. considered that N, ˚ and exerted influences on εN , they neglected the effect of inside transient arc radius r and lateral transient arc radius R on εN . In the present research, compound extrusion consisted of two sections: conventional extrusion and modified ECAE. In the section of conventional, accumulated strain ε was depended on extrusion ratio and was given by
ε = ln l/l0 = ln r02 /r12 = ln
(2)
where l was the length of billet after extrusion, l0 was the original length of billet before extrusion, r0 was the initial radius of billet before extrusion and r1 was the radius of billet after extrusion. In the section of modified ECAE, only single pass processing was considered for simplicity. The principle of modified ECAE is illustrated schematically in Fig. 2, where ˚ was the include angle, was the corner angle, r was inside transient arc radius and R was lateral transient arc radius. Note that problems with friction at the die walls may be avoided by use of appropriate lubricants. Therefore, the present analysis assumes the die is fabricated as illustrated in Fig. 1 and the billet is lubricated so that frictional effects can be neglected. In Fig. 2, the shear strain is = a u/d u, where a u = a t + tu = rc + as and d u = ad = L. On the one hand, a u can be obtained from the relationship ab = dc , as + sb = dr + rc , rc = as + sb − dr, rc = as + (R − r) , so that a u = rc + as = 2as + (R − r) . On the other hand, as = adcot( /2 + ˚/2) and a u = 2adcot( /2 + ˚/2) + (R − r) . Therefore, the shear strain for single pass ECAE is given by = 2 cot
2
+
˚ 2
+ (R − r)
L
(3)
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Q. Chen et al. / Materials Science and Engineering A 528 (2011) 3930–3934
Fig. 3. Optical microstructures of AZ91D magnesium alloy formed by compound extrusion with different accumulated strains, (a) 0, point A; (b) 2.197, point B; (c) 2.801, point C; (d) 4.614, point D; (e) 6.427, point E; (f) 8.240, point F and (g) 9.146, point G. The location of point was labelled in Fig. 1.
The equivalent strain, εeq , is represented by
⎡
⎢ εeq = ⎣
2 ε2x + ε2y + ε2z + 3
2 + 2 + 2 xy yz zx
2
⎤1/2 ⎥ ⎦
(4)
In the present research, εx , εy , εz , yz and zx are 0. Therefore, the equivalent strain, εeq , can be expressed as
1 εeq = √ 2 cot /2 + ˚/2 + (R − r)/L 3
(5)
According to Eq. (5), in the section of modified ECAE, the equivalent strain of single pass depends on not only the include angle ˚ and the corner angle , but also inside transient arc radius r, lateral transient arc radius R and the diameter of billet L. When the billet√passes die “C”, the imposed strain of the billet increases by / 3. In the present research, the inside transient arc radius r, the lateral transient arc radius R and the diameter of billet L for compound extrusion are 13 mm, 26 mm and 13 mm, respectively. According to Eqs. (2) and (5), the accumulated strains of √ point A, B, C, D, E, F√and G are 0, 2.197 (ln 9), 2.801 (ln 9 + 3/9), 4.614 (ln 9 + 4 3/9), 6.427 (ln 9 +
Q. Chen et al. / Materials Science and Engineering A 528 (2011) 3930–3934
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√ √ √ 7 3/9), 8.240 (ln 9 + 10 3/9) and 9.146 (ln 9 + 23 3/18), respectively. 3.2. Optical microstructure Fig. 3(a) shows the microstructure of as-cast AZ91D magnesium alloy. As shown in Fig. 3(a), the as-cast AZ91D alloy consisted of ␣Mg matrix and eutectic (␣-Mg + -Mg17 Al12 ). In the as-cast AZ91 alloy, eutectic was located into interdendritic spaces. The presence of eutectic in the microstructure was attributed to non-equilibrium solidification caused by the fast cooling rate of conventional casting process. Although many grains already significantly refined after conventional extrusion, the grain structure was not homogeneous with very fine grains of 2–4 m as well as coarse grains of greater than 30 m (Fig. 3(b)). This so-called “bimodal” microstructure was also observed by Lin et al. [12]. Careful examination of Fig. 3(b) indicated that many fine grains were formed along the grain boundaries of those coarse grains. When the accumulated strain reached 2.801, recrystallisation was almost extended completely and most of old grains were replaced by fine and recrystallised grains (Fig. 3(c)). As shown in Fig. 3(d), the whole matrix of sample was taken up by recrystallised grains and average grain size was about 19 m. With increase in accumulated strain, recrystallised grains were refined and became more equiaxed (Fig. 3(e) and (f)). The fine grain of 6 m had been obtained when the accumulated strain approached 9.146 (Fig. 3(g)). The comparison of the microstructures in Fig. 3(b)–(g) revealed that the mean grain size tended to decrease as the accumulated strain increased from 0 to 8.240. However, when the accumulated strain exceeded 8.240, it was difficult to get more grain refinement further and the average grain size of compound extruded alloy reached to a final-equilibrium fine-grained size. With the increase of accumulated strain, the temperature at which the compound extrusion took place also had a prominent role to play. Since more heat caused by friction was generated during compound extrusion to a high accumulated strain than a low accumulated strain, the coarsening of recrystallised grains would be greater. This might counteract the effect alloy increased accumulated strain, resulting in the decrease of grain refinement ability during compound extrusion. During compound extrusion, once dynamic recrystallisation was completed, the further increase of accumulated strain provided no effect on grain refinement. 3.3. Hardness The mean HV values with the accumulated strain are shown in Fig. 4. It can be seen from Fig. 4 that the value of hardness dra-
Fig. 4. Influence of the accumulated strain on hardness of AZ91D alloy.
Table 1 Mechanical properties of as-cast and compound extruded AZ91D magnesium alloys. Conditions
YS (MPa)
UTS (MPa)
Elongation (%)
As-cast Compound extrusion
63.6 (2.7) 202.2 (4.5)
118.3 (8.3) 323.1 (7.6)
1.6 (0.4) 14.8 (1.6)
matically increased with increasing the accumulated strain from 0 to 4.614. However, when the accumulated strain approached 8.240, the hardness reached a saturated value and it was little influenced by increasing the accumulated strain on certain conditions. This result was in agreement with the microstructure evolution with increasing accumulated strain. The grain refinement and the increased grain boundary were attributed to increase in the value of hardness. 3.4. Mechanical properties and fractography Table 1 shows the tensile test results of the AZ91D alloy in both as-cast and compound extruded states. As shown in Table 1, the values of yield strength (YS) ultimate tensile strength (UTS) and elongation to fracture of the as-cast alloy were 63.6 MPa, 118.3 MPa and 1.6% respectively, and the values of YS, UTS and elongation to fracture of the compound extruded alloy were 202.2 MPa, 323.1 MPa and 14.8%, respectively. Fig. 5 shows typical fracture surfaces of as-cast and compound extruded AZ91D alloys. The SEM image of as-cast AZ91D in Fig. 5(a) shows a transcrystalline fracture, in which a few cracks could be observed. Moreover, the as-cast
Fig. 5. Fractography of as-cast and compound extruded AZ91D magnesium alloys: (a) as-cast alloy and (b) compound extruded alloy.
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AZ91 alloy contained coarse brittle intermetallic particles, which in-homogeneously distributed in the microstructure. Therefore, AZ91D alloy exhibited a little plastic deformation before fracture. It can be seen from Fig. 5(b), tiny and uniform dimples can be obviously observed on the fracture surface of the compound extruded alloy, corresponding to refined and equiaxed grains after compound extrusion. Comparison of Fig. 3(b)–(g) also shows that the higher accumulated strain gives a finer recrystallised grain size. The higher the accumulated strain, the greater the overall grain boundary and sub-grain boundary area. Since grain boundaries were effective obstacles to dislocation motion, and since fine-grained alloy would have a higher density of grain boundaries per unit volume, the strength of AZ91D alloy remarkably increased with decreasing grain size. The remarkably improved ductility was obtained in the compound extruded alloy because more fine grains contributed to the macroscopic deformation and the stress concentrations were accordingly reduced and spread over a wider area. Moreover, the compound extrusion process not only refined the grain size, but also changed the morphology and amount of intermetallic particles. As shown in Fig. 5(b), the breakage of intermetallic particles into smaller parts which facilitated the dislocation motion. Therefore, good ductility was obtained for compound extruded AZ91D alloy. 4. Conclusions By means of the new SPD method called compound extrusion, as-cast AZ91D magnesium alloy has been successfully applied to SPD up to the accumulated strain of 9.146 without the introduction of any significant damage. The fine grain of 6 m has been obtained after compound extrusion. Both the strength and the duc-
tility of the alloy are improved significantly after the processing due to grain refinement and to the homogeneous distribution of intermetallic particles. Furthermore, the value of hardness of AZ91D alloy increases obviously with increasing the accumulated strain from 0 to 8.240. However, the value of hardness increases a little with further increasing accumulated strain. Acknowledgements The authors are grateful for the support from the Natural Science Foundation of China (NSFC) under Grant No. 51005217. Dr. Chen is grateful for the support from China Postdoctoral Science Foundation Grant No. 20100480677. References ´ [1] K.N. Braszczynska-Malik, J. Alloys Compd. 477 (2009) 870–876. [2] S.M. Fatemi-Varzaneh, A. Zarei-Hanzaki, M. Haghshenas, J. Alloys Compd. 475 (2009) 126–130. [3] G. Ben Hamu, D. Eliezer, L. Wagner, J. Alloys Compd. 468 (2009) 222–229. [4] X.S. Huang, K. Suzuki, A. Watazu, I. Shigematsu, N. Saito, J. Alloys Compd. 470 (2009) 263–268. [5] M.Y. Zheng, S.W. Xu, K. Wu, S. Kamado, Y. Kojima, Mater. Lett. 61 (2007) 4406–4408. [6] W.J. Kim, C.W. An, Y.S. Kim, S.I. Hong, Acta Mater. 51 (2003) 3293–3307. [7] S.X. Ding, C.P. Chang, P.W. Kao, Metall. Mater. Trans. A 40A (2009) 415–424. [8] R.B. Figueiredo, T.G. Langdon, Mater. Sci. Eng. A 430 (2006) 151–156. [9] O. Kulyasova, R. Islamgaliev, B. Mingler, M. Zehetbauer, Mater. Sci. Eng. A 503 (2009) 176–180. [10] K. Máthis, J. Gubicza, N.H. Nam, J. Alloys Compd. 394 (2005) 194–199. [11] Y. Iwahashi, J. Wang, Z. Horita, M. Nemoto, T.G. Langdon, Scripta Mater. 35 (1996) 143–146. [12] J. Lin, Q. Wang, L. Peng, H.J. Rovenc, J. Alloys Compd. 476 (2009) 441–445.