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Microstructure and mechanical properties of SPD-processed an as-cast AZ91D+Y magnesium alloy by equal channel angular extrusion and multi-axial forging
Materials and Design 30 (2009) 4557–4561
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Microstructure and mechanical properties of SPD-processed an as-cast AZ91D+Y magnesium alloy by equal channel angular extrusion and multi-axial forging Zude Zhao, Qiang Chen *, Chuankai Hu, Dayu Shu Southwest Technique and Engineering Institute, Chongqing 400039, PR China
a r t i c l e
i n f o
Article history: Received 17 February 2009 Accepted 16 April 2009 Available online 22 April 2009
a b s t r a c t The aim of this work is to study the microstructure and mechanical properties of an as-cast AZ91D+Y magnesium alloy processed via two different severe plastic deformation techniques, equal channel angular extrusion (ECAE) and multi-axial forging (MAF). The grains were significantly refined after only one pass for both ECAE and MAF processed billets. However, the homogeneity of the SPD-processed microstructure increased with increasing number of passes. Micro-hardness and tensile tests showed that billets processed by ECAE and MAF techniques followed a same behaviour. With the increase of the number of processing passes (accumulated strain), the values of micro-hardness, yield strength, ultimate tensile strength and elongation were observed to increase. Grain refinement caused by dynamic recrystallization was introduced to explain the effects of the number of processing passes (accumulated strain) on the microstructure and mechanical properties of AZ91D+Y magnesium alloys processed by ECAE and MAF techniques. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
2. Experimental procedures
Magnesium alloys have been received much attention as important materials in aerospace, automobile and construction industries due to their low density, high specific strength and stiffness, good electromagnetic shielding characteristics and good machinability [1]. However, their applications are often restricted due to their inherent deficiencies, such as low ductility and strength at ambient temperature. An effective method to overcome these drawbacks of magnesium alloys is to develop magnesium alloys with ultra-fine grained microstructures [2,3]. Several special severe plastic deformation (SPD) methods such as equal channel angular extrusion (ECAE) [4–6], high pressure torsion (HPT) [7,8] and multi-axial forging (MAF) [9–11] have succeeded in producing ultra-fine grained metallic materials. Although many studies [4–6,10,11] exploring the efficiency of these grain refinement methods for magnesium alloys have been published in the past decade, to the best of our knowledge the comparison of ECAE and MAF for AZ91D+Y magnesium alloys has not been reported. The aim of this work is to investigate the microstructure and mechanical properties of an as-cast AZ91D+Y magnesium alloy prepared by ECAE and MAF techniques.
The experiments were conducted using an as-cast magnesium alloy with yttrium (Y) addition, AZ91D+Y (Mg–8.92%Al–1.18%Zn– 0.09%Mn–0.46%Y), where the compositions were given in wt.%. The material was supplied in the form of as-cast rods with diameters of 70 mm. Some of these rods were cut into billets with diameters of 59 mm and lengths of 60 mm for ECAE technique. The billets processed with ECAE technique were conducted at 300 °C using a hydraulic press of 200-tonnes capacity, with a pressing speed of 1 mm/s. MoS2 lubricant was used between the billets and the die walls. Both the included angle / and the angle of curvature u of the outside corner were 90°. Therefore, the ECAE die configuration imposed the equivalent strain (ee) of 0.907 on billets during each pass according to the following equation [12]
* Corresponding author. E-mail address: [email protected] (Q. Chen). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.04.023
1
e ¼ pffiffiffi 2 cot 3
1 1 1 1 / þ u þ u csc /þ u 2 2 2 2
Billet of the alloy was extruded through multiple number of passes using processing route Bc where the billets were rotated by 90°in the same sense between each pass [12]. The billet was successfully extruded for up to four passes. Before MAF, some of as-cast rods were cut into billets with dimensions of 60 mm 60 mm 120 mm. These billets were held at 300 °C for 30 min and then formed through the MAF die preheated to 300 °C, with a speed of 1 mm/s. MoS2 was used as a lubricant during MAF. The MAF die
4558
Z. Zhao et al. / Materials and Design 30 (2009) 4557–4561
Table 1 Details of SPD processing. SPD technique
Strain per pass
Number of passes
Total strain
ECAE MAF
0.907 0.693
4 5
3.628 3.465
configuration imposed the equivalent strain (ee) of 0.693 on billets during each pass according to the following equation:
e ¼ j lnðh0 =hÞj where h0 and h are the initial and final height of billets, respectively. The details of the used ECAE and MAF processes are tabulated in Table 1. It can be seen that the as-cast magnesium alloys were processed through the ECAE and MAF techniques to approximately the same accumulated strain. Vickers micro-hardness was measured on the plane perpendicular to the extrusion axis of the ECAE-processed billets and on the plane perpendicular to the axis of the last compression for the MAF-processed billets. Hardness values were measured at 50 g load for 20 s using a Vickers micro-hardness tester. Each hardness value was the average of seven measurements. The
Fig. 1. Optical photograph of the microstructure of as-cast AZ91D+Y magnesium alloy.
Fig. 2. Optical photographs of AZ91D+Y after ECAE (a) one pass, (b) two passes, (c) three passes and (d) four passes.
Z. Zhao et al. / Materials and Design 30 (2009) 4557–4561
mechanical properties were measured according to ASTM B557 on cylindrical samples with a reduced section of 6 mm and a gauge length of 50 mm using an Instron 5569 testing machine at a cross head speed of 1 mm/min. Each tensile value was the
4559
average of three measurements. Microstructure of the samples were examined with optical microscopy after polishing and etching at room temperature using a solution of 100 ml ethanol, 10 ml H2O, 6 g picric and 5 ml acetic acid.
Fig. 3. Optical photographs of AZ91D+Y after MAF (a) one pass, (b) two passes, (c) three passes, (d) four passes and (e) five passes.
4560
Z. Zhao et al. / Materials and Design 30 (2009) 4557–4561
3. Results
3.2. Micro-hardness and tensile properties The Vickers micro-hardness values of the ECAE and MAF processed Mg alloys are plotted in Fig. 4. The error bars for the billets show the range of hardness values obtained for seven measurements for each pass. As shown in Fig. 4, the variations of microhardness in both ECAE and MAF processed billets exhibited a similar trend. Hardness increased notably for the one pass, reaching a maximum at four passes (ee = 3.628) for the ECAE processed billets and at five passes (ee = 3.465) for the MAF processed billets in the investigated range.
76
ECAE MAF
Microhardness (Vickers)
74
3 passes 2 passes
72 70
1 pass
68
4 passes
5 passes 4 passes
66
1 pass
60 58 -0.5
0 pass 0.0
0.5
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Accumulated strain 11 10
(b)
ECAE MAF
4 passes
3 passes 2 passes
9 8
5 passes
1 pass
4 passes
7
3 passes
6
2 passes
5 1 pass
4 3 -0.5
0 pass 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Accumulated strain Fig. 5. Yield strength, ultimate tensile strength (a) and elongation (b) vs. accumulated strain for an as-cast AZ91D+Y magnesium alloy SPD processed by ECAE and MAF techniques.
Fig. 5 shows the yield strength (YS), ultimate tensile strength (UTS) and elongation of ECAE and MAF processed billets as a function of the equivalent strain imposed into the billets. The error bars for the billets show the range of tensile values obtained for three measurements for each pass. As shown in Fig. 5a, a significant increase in the UTS was observed for the one pass followed by a gradual increase for the subsequent passes. However, the YS and Elongation were observed to increase significantly for the one pass, followed by a rise with decreasing rate of the same mechanical parameters for the subsequent passes. Fig. 5b shows that the elongation values of ECAE and MAF processed billets followed the same trend. The most striking difference between the ECAE processed billets and the MAF processed billets was after the one pass, the ductility of ECAE processed billets increased notably, compared to the slight increase in the ductility of MAF processed billets. 4. Discussion
62
56
0.0
3 passes 2 passes
64
-0.5
Elongation (%)
Fig. 1 shows that the microstructure of as-cast AZ91D+Y magnesium alloy was composed of the matrix (a-Mg) and the precipitate of Mg17Al12, which were mainly distributed at grain boundaries. Fig. 2 shows optical photographs of as-cast billets after ECAE processing for (a) one pass, (b) two passes, (c) three passes and (d) four passes. As shown in Fig. 2a, a large number of fine grains appeared at the initial grain boundaries, especially at triple junctions, which indicated the occurrence of dynamic recrystallization during ECAE. Furthermore, non-recrystallized regions of old grains remained and the mean grain size was about 30 lm. Therefore, the refinement of grains was initially not uniform, as there were coarse grains surrounded by fine ones. This is ‘‘bimodal” microstructure and was also observed by Chang et al. [13]. With further ECAE deformation (Fig. 2b), the microstructure became more homogeneous by successive breaking-up of coarse grains. Fine grained and reasonably homogeneous microstructure was obtained after three passes (Fig. 2c). It was believed that dynamic recrystallisation occurred during ECAE. However, after three passes, it was difficult to get further grain refinement (Fig. 2d). A similar case has been reported by Pérez-Prado et al. [14] and Guo et al [11]. Once a critical minimum grain size was achieved, subsequent passes did not have any noticeable refining effect. Fig. 3 shows optical photographs of as-cast billets after MAF for (a) one pass, (b) two passes, (c) three passes, (d) four passes and (e) five passes. As shown in Fig. 3a, the grain structure was not homogeneous, but a mixed structure of coarse grains and fine grains. However, a homogeneous microstructure was obtained after five passes and the area fraction of dynamically recrystallized grains reached 90% (Fig. 3e). The refined equiaxed grains give evidence that dynamic recrystallization took place.
Strength (MPa)
3.1. Microstructure observations
340 ECAE-UTS 320 (a) 3 passes 4 passes ECAE-YS 2 passes 300 5 passes MAF-UTS 280 4 passes MAF-YS 3passes 260 2 passes 240 4 passes 1 pass 220 1 pass 3 passes 2 passes 200 180 160 0 pass 5 passes 1 pass 4 passes 140 3 passes 120 2 passes 100 0 pass 1 pass 80
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Accumulated strain Fig. 4. Micro-hardness vs. accumulated strain for an as-cast AZ91D+Y magnesium alloy SPD processed by ECAE and MAF techniques.
In magnesium alloys that contained high level of aluminum, e.g. >3 wt.% Al, the grain boundaries were often obscured by the eutectic phase. The eutectic phase, Mg17Al12, is created by non-equilibrium solidification. Processing temperature was within the single-phase field a-Mg for the investigated AZ91D+Y magnesium alloys and the intermetallic Mg17Al12 got partially dissolved [15]. However, the time was too short to allow complete homogenisation during processing, i.e. segregation was only reduced but not eliminated [15]. On cooling from the processing temperature (300 °C), the intermetallic Mg17Al12 occurred only in the areas of high Al-content since
Z. Zhao et al. / Materials and Design 30 (2009) 4557–4561
diffusion of Al in Mg was rather slow. As shown in Figs. 2d and 3e, after multi pass SPD processes, intermetallic Mg17Al12 phase was appeared in the microstructure of Mg alloy. The grain size of AZ91D+Y magnesium alloys was strongly affected by the number of processing passes (accumulated strain). After only one pass, the grains of ECAE and MAF processed billets were significantly refined. Comparing Fig. 2 with Fig. 3, it is clear that the higher accumulated strain billet gave a finer recrystallised grain of the Mg alloy. Kim et al. [5] suggested that with the increase of the number of passes leads to the misorientation angles between neighboring grains. This leads to an increase of the amount of stored energy in the Mg alloy and increases the driving force towards recrystallisation. Another effect of processing the billet to a higher accumulated strain is the influence on the overall grain boundary and sub-grain boundary areas. The higher the accumulated strain, the greater the overall grain boundary and subgrain boundary areas. This leads to greater potential for the development of recrystallisation nuclei, and therefore a finer recrystallised grain size [16]. It can be noticed that the microstructure of the ECAE processed AZ91D+Y alloy (Fig. 2a) and the MAF processed AZ91D+Y alloy (Fig. 3a) possessed a bimodal grain size distribution after the one pass. With increase of the number of passes, subgrains continued to form from the grains with progressive conversion of their low angle into high angle boundaries [5]. As a result of this continuous dynamic recrystallization, a high volume fraction of fine and equiaxed grains with high angle boundaries was achieved after severe deformation [5]. As shown in Figs. 4 and 5, severely deformed AZ91D+Y magnesium alloy through ECAE and MAF techniques showed similar trend in hardness and tensile properties. The values of hardness and tensile properties increased with increasing number of passes. According to the Hall–Petch relationship [17], the remarkable refinement of primary a-Mg grain was attributed to the increasing number of passes. These results are in full agreement with the findings of Guo et al. [10]. These investigators studied the effect of MAF process on the microstructure and mechanical properties of an ascast AZ80 magnesium alloy. The hardness, YS, UTS and elongation of MAF processed as-cast AZ80 was found to increase with increasing number of passes. The seven-pass MAF-formed billet had the maximum hardness, YS and UTS of 87.3HB, 258.78 and 345.04 MPa values. Therefore, it seems that the tensile properties of as-cast magnesium alloys were mainly controlled by the strengthening due to grain refinement. 5. Conclusions An as-cast AZ91D magnesium alloy with yttrium addition was SPD-processed by ECAE and MAF techniques with the same initial
4561
conditions to approximately the same accumulated strain. The grains of this alloy were significantly refined after only one pass for both ECAE and MAF processed billets. The microstructure of SPD (ECAE and MAF) processed billets was initially non-uniform with a bimodal grain size distribution but became more homogeneous with increasing number of passes. The values of hardness, YS, UTS and ductility of ECAE and MAF processed billets increased with increasing number of passes. References [1] Zheng MY, Wu K, Liang M, Kamado S, Kojima Y. The effect of thermal exposure on the interface and mechanical properties of Al18B4O33w/AZ91 magnesium matrix composite. Mater Sci Eng A 2004;372:66–74. [2] Máthis K, Gubicza J, Nam NH. Microstructure and mechanical behavior of AZ91 Mg alloy processed by equal channel angular pressing. J Alloys Compd 2005;394:194–9. [3] Cherukuri B, Nedkova TS, Srinivasan R. A comparison of the properties of SPDprocessed AA-6061 by equal-channel angular pressing, multi-axial compressions/forgings and accumulative roll bonding. Mater Sci Eng A 2005;410–1:394–7. [4] Kim WJ, An CW, Kim YS, Hong SI. Mechanical properties and microstructures of an AZ61 Mg Alloy produced by equal channel angular pressing. Scripta Mater 2002;47:39–44. [5] Kim WJ, Hong SI, Kim YS, Min SH, Jeong HT, Lee JD. Texture development and its effect on mechanical properties of an AZ61 Mg alloy fabricated by equal channel angular pressing. Acta Mater 2003;51:3293–307. [6] Kim HK, Kim WJ. Microstructural instability and strength of an AZ31 Mg alloy after severe plastic deformation. Mater Sci Eng A 2004;385:300–8. [7] Hebesberger T, Stüwe HP, Vorhauer A, Wetscher F, Pippan R. Structure of Cu deformed by high pressure torsion. Acta Mater 2005;53:393–402. [8] Vorhauer A, Hebesberger T, Pippan R. Disorientations as a function of distance. A new procedure to analyze local orientation data. Acta Mater 2003;51:677–86. [9] Han BJ, Xu Z. Grain refinement under multi-axial forging in Fe-32%Ni alloy. J Alloys Compd 2008;457:279–85. [10] Guo Q, Yan HG, Chen ZH, Zhang H. Effect of multiple forging process on microstructure and mechanical properties of magnesium alloy AZ80. Acta Metal Sin 2006;42:739–44. [11] Guo Q, Yan HG, Chen ZH, Zhang H. Grain refinement in as-cast AZ80 Mg alloy under large strain deformation. Mater Charact 2007;58:162–7. [12] Iwahashi Y, Furukawa M, Horita Z, Langdon TG. Microstructural characteristics of ultrafine-grained aluminum produced using equal-channel angular pressing. Metal Mater Trans A 1998;29:2245–52. [13] Chang SY, Lee SW, Kang KM, Kamado S, Kojima Y. Improvement of mechanical characteristics in severely plastic-deformed Mg alloys. Mater Trans 2004;45:488–92. [14] Pérez-Prado MT, del Valle JA, Ruano OA. Grain refinement of Mg–Al–Zn alloys via accumulative roll bonding. Scripta Mater 2004;51:1093–7. [15] Kleiner S, Beffort O, Uggowitzer PJ. Microstructure evolution during reheating of an extruded Mg–Al–Zn alloy into the semisolid state. Scripta Mater 2004;51:405–10. [16] Atkinson HV, Burke K, Vaneetveld G. Recrystallisation in the semi-solid state in 7075 aluminium alloy. Mater Sci Eng A 2008;490:266–76. [17] Smith WF, Hashemi J. Foundations of materials science and engineering. 4th ed. USA: McGraw-Hill Companies, Inc.; 2006. p. 242.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Short Communication
Microstructure and mechanical properties of SPD-processed an as-cast AZ91D+Y magnesium alloy by equal channel angular extrusion and multi-axial forging Zude Zhao, Qiang Chen *, Chuankai Hu, Dayu Shu Southwest Technique and Engineering Institute, Chongqing 400039, PR China
a r t i c l e
i n f o
Article history: Received 17 February 2009 Accepted 16 April 2009 Available online 22 April 2009
a b s t r a c t The aim of this work is to study the microstructure and mechanical properties of an as-cast AZ91D+Y magnesium alloy processed via two different severe plastic deformation techniques, equal channel angular extrusion (ECAE) and multi-axial forging (MAF). The grains were significantly refined after only one pass for both ECAE and MAF processed billets. However, the homogeneity of the SPD-processed microstructure increased with increasing number of passes. Micro-hardness and tensile tests showed that billets processed by ECAE and MAF techniques followed a same behaviour. With the increase of the number of processing passes (accumulated strain), the values of micro-hardness, yield strength, ultimate tensile strength and elongation were observed to increase. Grain refinement caused by dynamic recrystallization was introduced to explain the effects of the number of processing passes (accumulated strain) on the microstructure and mechanical properties of AZ91D+Y magnesium alloys processed by ECAE and MAF techniques. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
2. Experimental procedures
Magnesium alloys have been received much attention as important materials in aerospace, automobile and construction industries due to their low density, high specific strength and stiffness, good electromagnetic shielding characteristics and good machinability [1]. However, their applications are often restricted due to their inherent deficiencies, such as low ductility and strength at ambient temperature. An effective method to overcome these drawbacks of magnesium alloys is to develop magnesium alloys with ultra-fine grained microstructures [2,3]. Several special severe plastic deformation (SPD) methods such as equal channel angular extrusion (ECAE) [4–6], high pressure torsion (HPT) [7,8] and multi-axial forging (MAF) [9–11] have succeeded in producing ultra-fine grained metallic materials. Although many studies [4–6,10,11] exploring the efficiency of these grain refinement methods for magnesium alloys have been published in the past decade, to the best of our knowledge the comparison of ECAE and MAF for AZ91D+Y magnesium alloys has not been reported. The aim of this work is to investigate the microstructure and mechanical properties of an as-cast AZ91D+Y magnesium alloy prepared by ECAE and MAF techniques.
The experiments were conducted using an as-cast magnesium alloy with yttrium (Y) addition, AZ91D+Y (Mg–8.92%Al–1.18%Zn– 0.09%Mn–0.46%Y), where the compositions were given in wt.%. The material was supplied in the form of as-cast rods with diameters of 70 mm. Some of these rods were cut into billets with diameters of 59 mm and lengths of 60 mm for ECAE technique. The billets processed with ECAE technique were conducted at 300 °C using a hydraulic press of 200-tonnes capacity, with a pressing speed of 1 mm/s. MoS2 lubricant was used between the billets and the die walls. Both the included angle / and the angle of curvature u of the outside corner were 90°. Therefore, the ECAE die configuration imposed the equivalent strain (ee) of 0.907 on billets during each pass according to the following equation [12]
* Corresponding author. E-mail address: [email protected] (Q. Chen). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.04.023
1
e ¼ pffiffiffi 2 cot 3
1 1 1 1 / þ u þ u csc /þ u 2 2 2 2
Billet of the alloy was extruded through multiple number of passes using processing route Bc where the billets were rotated by 90°in the same sense between each pass [12]. The billet was successfully extruded for up to four passes. Before MAF, some of as-cast rods were cut into billets with dimensions of 60 mm 60 mm 120 mm. These billets were held at 300 °C for 30 min and then formed through the MAF die preheated to 300 °C, with a speed of 1 mm/s. MoS2 was used as a lubricant during MAF. The MAF die
4558
Z. Zhao et al. / Materials and Design 30 (2009) 4557–4561
Table 1 Details of SPD processing. SPD technique
Strain per pass
Number of passes
Total strain
ECAE MAF
0.907 0.693
4 5
3.628 3.465
configuration imposed the equivalent strain (ee) of 0.693 on billets during each pass according to the following equation:
e ¼ j lnðh0 =hÞj where h0 and h are the initial and final height of billets, respectively. The details of the used ECAE and MAF processes are tabulated in Table 1. It can be seen that the as-cast magnesium alloys were processed through the ECAE and MAF techniques to approximately the same accumulated strain. Vickers micro-hardness was measured on the plane perpendicular to the extrusion axis of the ECAE-processed billets and on the plane perpendicular to the axis of the last compression for the MAF-processed billets. Hardness values were measured at 50 g load for 20 s using a Vickers micro-hardness tester. Each hardness value was the average of seven measurements. The
Fig. 1. Optical photograph of the microstructure of as-cast AZ91D+Y magnesium alloy.
Fig. 2. Optical photographs of AZ91D+Y after ECAE (a) one pass, (b) two passes, (c) three passes and (d) four passes.
Z. Zhao et al. / Materials and Design 30 (2009) 4557–4561
mechanical properties were measured according to ASTM B557 on cylindrical samples with a reduced section of 6 mm and a gauge length of 50 mm using an Instron 5569 testing machine at a cross head speed of 1 mm/min. Each tensile value was the
4559
average of three measurements. Microstructure of the samples were examined with optical microscopy after polishing and etching at room temperature using a solution of 100 ml ethanol, 10 ml H2O, 6 g picric and 5 ml acetic acid.
Fig. 3. Optical photographs of AZ91D+Y after MAF (a) one pass, (b) two passes, (c) three passes, (d) four passes and (e) five passes.
4560
Z. Zhao et al. / Materials and Design 30 (2009) 4557–4561
3. Results
3.2. Micro-hardness and tensile properties The Vickers micro-hardness values of the ECAE and MAF processed Mg alloys are plotted in Fig. 4. The error bars for the billets show the range of hardness values obtained for seven measurements for each pass. As shown in Fig. 4, the variations of microhardness in both ECAE and MAF processed billets exhibited a similar trend. Hardness increased notably for the one pass, reaching a maximum at four passes (ee = 3.628) for the ECAE processed billets and at five passes (ee = 3.465) for the MAF processed billets in the investigated range.
76
ECAE MAF
Microhardness (Vickers)
74
3 passes 2 passes
72 70
1 pass
68
4 passes
5 passes 4 passes
66
1 pass
60 58 -0.5
0 pass 0.0
0.5
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Accumulated strain 11 10
(b)
ECAE MAF
4 passes
3 passes 2 passes
9 8
5 passes
1 pass
4 passes
7
3 passes
6
2 passes
5 1 pass
4 3 -0.5
0 pass 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Accumulated strain Fig. 5. Yield strength, ultimate tensile strength (a) and elongation (b) vs. accumulated strain for an as-cast AZ91D+Y magnesium alloy SPD processed by ECAE and MAF techniques.
Fig. 5 shows the yield strength (YS), ultimate tensile strength (UTS) and elongation of ECAE and MAF processed billets as a function of the equivalent strain imposed into the billets. The error bars for the billets show the range of tensile values obtained for three measurements for each pass. As shown in Fig. 5a, a significant increase in the UTS was observed for the one pass followed by a gradual increase for the subsequent passes. However, the YS and Elongation were observed to increase significantly for the one pass, followed by a rise with decreasing rate of the same mechanical parameters for the subsequent passes. Fig. 5b shows that the elongation values of ECAE and MAF processed billets followed the same trend. The most striking difference between the ECAE processed billets and the MAF processed billets was after the one pass, the ductility of ECAE processed billets increased notably, compared to the slight increase in the ductility of MAF processed billets. 4. Discussion
62
56
0.0
3 passes 2 passes
64
-0.5
Elongation (%)
Fig. 1 shows that the microstructure of as-cast AZ91D+Y magnesium alloy was composed of the matrix (a-Mg) and the precipitate of Mg17Al12, which were mainly distributed at grain boundaries. Fig. 2 shows optical photographs of as-cast billets after ECAE processing for (a) one pass, (b) two passes, (c) three passes and (d) four passes. As shown in Fig. 2a, a large number of fine grains appeared at the initial grain boundaries, especially at triple junctions, which indicated the occurrence of dynamic recrystallization during ECAE. Furthermore, non-recrystallized regions of old grains remained and the mean grain size was about 30 lm. Therefore, the refinement of grains was initially not uniform, as there were coarse grains surrounded by fine ones. This is ‘‘bimodal” microstructure and was also observed by Chang et al. [13]. With further ECAE deformation (Fig. 2b), the microstructure became more homogeneous by successive breaking-up of coarse grains. Fine grained and reasonably homogeneous microstructure was obtained after three passes (Fig. 2c). It was believed that dynamic recrystallisation occurred during ECAE. However, after three passes, it was difficult to get further grain refinement (Fig. 2d). A similar case has been reported by Pérez-Prado et al. [14] and Guo et al [11]. Once a critical minimum grain size was achieved, subsequent passes did not have any noticeable refining effect. Fig. 3 shows optical photographs of as-cast billets after MAF for (a) one pass, (b) two passes, (c) three passes, (d) four passes and (e) five passes. As shown in Fig. 3a, the grain structure was not homogeneous, but a mixed structure of coarse grains and fine grains. However, a homogeneous microstructure was obtained after five passes and the area fraction of dynamically recrystallized grains reached 90% (Fig. 3e). The refined equiaxed grains give evidence that dynamic recrystallization took place.
Strength (MPa)
3.1. Microstructure observations
340 ECAE-UTS 320 (a) 3 passes 4 passes ECAE-YS 2 passes 300 5 passes MAF-UTS 280 4 passes MAF-YS 3passes 260 2 passes 240 4 passes 1 pass 220 1 pass 3 passes 2 passes 200 180 160 0 pass 5 passes 1 pass 4 passes 140 3 passes 120 2 passes 100 0 pass 1 pass 80
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Accumulated strain Fig. 4. Micro-hardness vs. accumulated strain for an as-cast AZ91D+Y magnesium alloy SPD processed by ECAE and MAF techniques.
In magnesium alloys that contained high level of aluminum, e.g. >3 wt.% Al, the grain boundaries were often obscured by the eutectic phase. The eutectic phase, Mg17Al12, is created by non-equilibrium solidification. Processing temperature was within the single-phase field a-Mg for the investigated AZ91D+Y magnesium alloys and the intermetallic Mg17Al12 got partially dissolved [15]. However, the time was too short to allow complete homogenisation during processing, i.e. segregation was only reduced but not eliminated [15]. On cooling from the processing temperature (300 °C), the intermetallic Mg17Al12 occurred only in the areas of high Al-content since
Z. Zhao et al. / Materials and Design 30 (2009) 4557–4561
diffusion of Al in Mg was rather slow. As shown in Figs. 2d and 3e, after multi pass SPD processes, intermetallic Mg17Al12 phase was appeared in the microstructure of Mg alloy. The grain size of AZ91D+Y magnesium alloys was strongly affected by the number of processing passes (accumulated strain). After only one pass, the grains of ECAE and MAF processed billets were significantly refined. Comparing Fig. 2 with Fig. 3, it is clear that the higher accumulated strain billet gave a finer recrystallised grain of the Mg alloy. Kim et al. [5] suggested that with the increase of the number of passes leads to the misorientation angles between neighboring grains. This leads to an increase of the amount of stored energy in the Mg alloy and increases the driving force towards recrystallisation. Another effect of processing the billet to a higher accumulated strain is the influence on the overall grain boundary and sub-grain boundary areas. The higher the accumulated strain, the greater the overall grain boundary and subgrain boundary areas. This leads to greater potential for the development of recrystallisation nuclei, and therefore a finer recrystallised grain size [16]. It can be noticed that the microstructure of the ECAE processed AZ91D+Y alloy (Fig. 2a) and the MAF processed AZ91D+Y alloy (Fig. 3a) possessed a bimodal grain size distribution after the one pass. With increase of the number of passes, subgrains continued to form from the grains with progressive conversion of their low angle into high angle boundaries [5]. As a result of this continuous dynamic recrystallization, a high volume fraction of fine and equiaxed grains with high angle boundaries was achieved after severe deformation [5]. As shown in Figs. 4 and 5, severely deformed AZ91D+Y magnesium alloy through ECAE and MAF techniques showed similar trend in hardness and tensile properties. The values of hardness and tensile properties increased with increasing number of passes. According to the Hall–Petch relationship [17], the remarkable refinement of primary a-Mg grain was attributed to the increasing number of passes. These results are in full agreement with the findings of Guo et al. [10]. These investigators studied the effect of MAF process on the microstructure and mechanical properties of an ascast AZ80 magnesium alloy. The hardness, YS, UTS and elongation of MAF processed as-cast AZ80 was found to increase with increasing number of passes. The seven-pass MAF-formed billet had the maximum hardness, YS and UTS of 87.3HB, 258.78 and 345.04 MPa values. Therefore, it seems that the tensile properties of as-cast magnesium alloys were mainly controlled by the strengthening due to grain refinement. 5. Conclusions An as-cast AZ91D magnesium alloy with yttrium addition was SPD-processed by ECAE and MAF techniques with the same initial
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conditions to approximately the same accumulated strain. The grains of this alloy were significantly refined after only one pass for both ECAE and MAF processed billets. The microstructure of SPD (ECAE and MAF) processed billets was initially non-uniform with a bimodal grain size distribution but became more homogeneous with increasing number of passes. The values of hardness, YS, UTS and ductility of ECAE and MAF processed billets increased with increasing number of passes. References [1] Zheng MY, Wu K, Liang M, Kamado S, Kojima Y. The effect of thermal exposure on the interface and mechanical properties of Al18B4O33w/AZ91 magnesium matrix composite. Mater Sci Eng A 2004;372:66–74. [2] Máthis K, Gubicza J, Nam NH. Microstructure and mechanical behavior of AZ91 Mg alloy processed by equal channel angular pressing. J Alloys Compd 2005;394:194–9. [3] Cherukuri B, Nedkova TS, Srinivasan R. A comparison of the properties of SPDprocessed AA-6061 by equal-channel angular pressing, multi-axial compressions/forgings and accumulative roll bonding. Mater Sci Eng A 2005;410–1:394–7. [4] Kim WJ, An CW, Kim YS, Hong SI. Mechanical properties and microstructures of an AZ61 Mg Alloy produced by equal channel angular pressing. Scripta Mater 2002;47:39–44. [5] Kim WJ, Hong SI, Kim YS, Min SH, Jeong HT, Lee JD. Texture development and its effect on mechanical properties of an AZ61 Mg alloy fabricated by equal channel angular pressing. Acta Mater 2003;51:3293–307. [6] Kim HK, Kim WJ. Microstructural instability and strength of an AZ31 Mg alloy after severe plastic deformation. Mater Sci Eng A 2004;385:300–8. [7] Hebesberger T, Stüwe HP, Vorhauer A, Wetscher F, Pippan R. Structure of Cu deformed by high pressure torsion. Acta Mater 2005;53:393–402. [8] Vorhauer A, Hebesberger T, Pippan R. Disorientations as a function of distance. A new procedure to analyze local orientation data. Acta Mater 2003;51:677–86. [9] Han BJ, Xu Z. Grain refinement under multi-axial forging in Fe-32%Ni alloy. J Alloys Compd 2008;457:279–85. [10] Guo Q, Yan HG, Chen ZH, Zhang H. Effect of multiple forging process on microstructure and mechanical properties of magnesium alloy AZ80. Acta Metal Sin 2006;42:739–44. [11] Guo Q, Yan HG, Chen ZH, Zhang H. Grain refinement in as-cast AZ80 Mg alloy under large strain deformation. Mater Charact 2007;58:162–7. [12] Iwahashi Y, Furukawa M, Horita Z, Langdon TG. Microstructural characteristics of ultrafine-grained aluminum produced using equal-channel angular pressing. Metal Mater Trans A 1998;29:2245–52. [13] Chang SY, Lee SW, Kang KM, Kamado S, Kojima Y. Improvement of mechanical characteristics in severely plastic-deformed Mg alloys. Mater Trans 2004;45:488–92. [14] Pérez-Prado MT, del Valle JA, Ruano OA. Grain refinement of Mg–Al–Zn alloys via accumulative roll bonding. Scripta Mater 2004;51:1093–7. [15] Kleiner S, Beffort O, Uggowitzer PJ. Microstructure evolution during reheating of an extruded Mg–Al–Zn alloy into the semisolid state. Scripta Mater 2004;51:405–10. [16] Atkinson HV, Burke K, Vaneetveld G. Recrystallisation in the semi-solid state in 7075 aluminium alloy. Mater Sci Eng A 2008;490:266–76. [17] Smith WF, Hashemi J. Foundations of materials science and engineering. 4th ed. USA: McGraw-Hill Companies, Inc.; 2006. p. 242.