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nano-particles composite via thermomechanical processing
Materials Science & Engineering A 595 (2014) 284–290
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Mechanical properties improvement of cast AZ80 Mg alloy/nano-particles composite via thermomechanical processing H. Khoshzaban Khosroshahi a, F. Fereshteh Saniee b, H.R. Abedi c,n a
Mechanical Engineering Department, Islamic Azad University, Takestan, Iran Mechanical Engineering Department, Bu-Ali Sina University, Hamedan, Iran c School of Metallurgical and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran b
art ic l e i nf o
a b s t r a c t
Article history: Received 9 November 2013 Received in revised form 7 December 2013 Accepted 7 December 2013 Available online 16 December 2013
Three different nano-composites with cast AZ80 magnesium alloy matrix and nano-particles reinforcement, including Al2O3, TiO2 and SiC, have been prepared via stir casting method. In order to refine the microstructure and improve the mechanical properties, the monolithic AZ80 as well as AZ80/nano-particles composites were thermomechanically processed employing forward extrusion at 200 1C. The obtained microstructures of extruded nano-composites materials indicated significant grain refinement due to the progression of dynamic recrystallization (DRX); however, the extruded monolithic material depicts a typical necklace structure with limited DRX grain. To evaluate the mechanical properties, micro-hardness and tensile tests were conducted. The results showed a remarkable increase in hardness of extruded nano-composites because of adding harder particles along with outstanding grain refinement. Furthermore, tensile tests results indicated a remarkable improvement in yield and ultimate strength as well as room temperature ductility for extruded composites compared with cast AZ80 Mg alloy. Finally, it is concluded that adding SiC nano-particles to the cast AZ80 Mg alloy resulted in the best combination of strength and ductility in comparison with Al2O3 and TiO2 particles. This may be attributed to the homogenous distribution of SiC particles and the lowest level of agglomeration during the extrusion process. & 2013 Elsevier B.V. All rights reserved.
Keywords: Magnesium matrix composite Nano-particles Grain refinement Forward extrusion
1. Introduction Magnesium alloys, being among the lightest structural materials, are very attractive in many applications. Their excellent strength to weight ratio predestines Mg-based alloys for applications as structural components in automobile and aircraft industries. However, magnesium alloys exhibit poor room temperature formability because of their hexagonal close-packed crystal structure with a limited number of operative basal slip systems. Accordingly, numerous studies have focused on eliminating this restriction; nonetheless, all these efforts can be categorized into two main groups. The first, the researches which have employed the different thermomechanical processing routes [1–3]; and the second, those of fabricated magnesium alloy matrix composites via adding sub-micron reinforcement particles [4–6]. As is well established the thermomechanical treatment may effectively improve the mechanical properties of cast AZ-series, as the most common magnesium alloys [7–9]. It has been found that the ductility and strength values are significantly improved due to the modification of γ-Mg17Al12 phase morphology and also due to the occurrence of grain refinement. It has been reported n
Corresponding author. Tel.: þ 98 21 82084116; fax: þ 98 21 88006076. E-mail address: [email protected] (H.R. Abedi).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.12.029
that employing forward extrusion at 270 1C leads to a considerable grain refinement through dynamic recrystallization where the presence of small γ eutectic particles along newly formed grain boundaries is evident. These dynamically form during processing of the alloy or through the separation from the coarse network of elongated phases [10]. Apparently, associated microstructures have resulted in the improvement of tensile properties of AZ91 magnesium alloy. Since magnesium matrix composites exhibit many advantages over the monolithic magnesium or magnesium alloys, such as high elastic modulus, high strength, superior creep and wear resistances at elevated temperatures [11,12], numerous studies have been also made on fabricating magnesium matrix composites. Gupta et al. [13] investigated the microstructure and mechanical properties of AZ81 magnesium alloy reinforced with Al2O3particulates. The related results showed that the AZ81/Al2O3 nano-composite exhibited higher tensile and compressive energy absorbed until fracture compared to the monolithic AZ81 alloy. Ferkel and Mordike [14] have employed powder metallurgy to strengthen the magnesium with SiC nanoparticles, and have reported the improvement of creep resistance and tensile properties of the obtained materials. Enhanced physical and mechanical properties of the Mg–Y2O and Mg–Al2O3 nano-composite prepared by the disintegrated melt deposition method have been also characterized by Goh et al. and Gupta et al., respectively [11,15].
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Considering all of these advantages, it seems that applying these two methods (fabricating nano-composites and employing thermomechanical processes) simultaneously may be more influential in the improvement of mechanical properties compared to the employing of these methods individually. Despite the extensive conducted researches focused on each approach, there are just a few studies which have been directed towards thermomechanical processing of Mg matrix nano-composites. In this regard, Deng et al. [16] investigated the microstructure and tensile properties of forged AZ91/SiC nano-composites with different volume fractions of SiC reinforcement particles. They reported that the presence of submicron-SiC particulates assisted in improving the thermal stability, micro-hardness, elastic modulus and yield strength. In addition, Paramsothy et al. [17] evaluated the mechanical properties of AZ91/AZ31 hybrid magnesium alloy with Si3Ni4 nanoparticles after hot extrusion process. Their results showed that compared to the monolithic hybrid alloy, the nano-composite exhibited higher yield strength, ultimate tensile strength, failure strain and work of fracture simultaneously. In line with these efforts, the aim of this paper is to study the effect of adding different nano-particles reinforcement, namely Al2O3, TiO2 and SiC particles, to the cast AZ80 magnesium alloy on the mechanical properties of fabricated composite processed via the forward extrusion method. The microstructure-processing-properties relationships have been properly addressed through this manuscript.
2. Experimental procedure 2.1. Material and casting process Cast AZ80 magnesium alloy was selected as the matrix alloy, the chemical composition of which is shown in Table 1. The microstructure of cast AZ80 Mg alloy is depicted in Fig. 1. As is shown, the microstructure of the AZ80 Mg alloy characterized by the α-Mg matrix and γ (Mg17Al12) intermetallic compound distributed in the grain interior and along the grain boundaries. There different metal matrix composites were fabricated by adding 1%wt. TiO2, SiC and Al2O3 nano-particles to the matrix of AZ80 Mg alloy. The amount of purity as well as average size of nanoparticles is tabulated in Table 2. To fabricate the composites, stir casting method was employed. Towards this direction, the AZ80 magnesium alloy was molten at 750 1C and then the particles were added to the matrix. As the nano-particles were added, the molten metal was stirred for 5 min using a stainless steel rod to provide a homogeneous composition. The melt was poured into a preheated steel mold (450 1C) and solidified under 150 MPa pressure to reduce the porosity of composite ingots. The whole fabricating process was conducted under the protective atmosphere of MgCl2 and NaCl to avoid oxidation. The cast rods were prepared in dimensions of Φ40 mm H100 mm. The cylindrical specimens for forward extrusion were extracted from the center of cast rods in the size of 28 mm diameter and 40 mm height. 2.2. Thermomechanical process The cast AZ80 Mg alloy and metal matrix composites were processed using the forward extrusion method. The extrusion process was conducted at 200 1C on the cylindrical specimens in Table 1 The chemical composition of the experimental AZ80 Mg alloy.
Fig. 1. The microstructure of cast AZ80 magnesium alloy.
Table 2 The purity and average size of reinforced nano-particles. Particle
Purity (%)
Average particle size (nm)
TiO2 SiC Al2O3
99.5 99 99
21 40–50 80
the size of 28 mm diameter and 40 mm height. The work-pieces were heated to the predetermined temperatures in a resistant furnace and held for 10 min to equalize the temperature. All the extrusion tests were performed using the extrusion ratio of 3. Forward extrusion processing was successfully carried out with no danger of crack or discontinuity. To minimize friction during the process, MoS2 lubricant was pasted to the surface of the workpieces and dies. After deformation, to preserve the obtained microstructure, the processed specimens were immediately quenched into water. In order to examine the microstructures, the processed work-pieces were cut parallel to the extrusion direction. The obtained cross-section was mechanically polished and etched for 7 s using a solution of 70 ml ethanol, 4.2 g picric acid, 10 ml acetic acid and 10 ml water. The obtained microstructures were examined by a MEIJI optical microscope (OM). 2.3. Mechanical testing
Element
Al
Zn
Mn
Si
Fe
Cu
Ni
Mg
wt%
7.4
0.48
0.026
0.066
0.029
0.02
0.0081
Balance
Uniaxial tensile test was employed to evaluate the mechanical properties of extruded AZ80 Mg alloy and nano-composites. The tensile specimens were prepared from processed and cast materials
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with a gauge length of 30 mm according to ASTM E8 [18]. The tensile tests were carried out at room temperature using an Instron4208 universal testing machine under an initial strain rate of 0.001 s 1. To justify the deformation behavior of the processed composites, the obtained fracture surfaces were examined by Scanning Electron Microscopy (SEM). In addition, the Vickers micro-hardness measurements were also performed throughout the cross-section of the extruded composites using a load of 10 g force with a loading time of 30 s.
3. Results and discussion 3.1. Microstructural characterization 3.1.1. Initial microstructure As is shown in Fig. 1, the initial microstructure consists of coarse cast grains and a coarse continuous network of γ (Mg17 Al12)-eutectic phase, which significantly destructs the mechanical properties of the experimental alloy. The initial grain size of cast AZ80 magnesium alloy is measured to be 5073μm. 3.1.2. Microstructure of extruded materials Fig. 2 depicts the final microstructure of monolithic AZ80 Mg alloy after extrusion. As is observed, the initial coarse grains have been elongated along the extrusion direction, significantly. In addition, the continuous network of γ (Mg17Al12)-eutectic phase has been broken to smaller particles. The latter is attributed to the effect of high shear strain imposed during the extrusion process. It is noteworthy to mention that the volume fraction of γ
(Mg17Al12)-eutectic phase in the microstructure of extruded alloy becomes lower compared with cast AZ80 magnesium alloy. In fact, during the extrusion process remarkable volume fraction of γ (Mg17Al12)-eutectic phase has been dissolved to the α-Mg matrix phase. The strain induced dissolution of eutectic phase has already been reported during thermomechanical processing [19], particularly during severe plastic deformation of Mg–Al alloys [20]. For further clarification, the final microstructures of the processed monolithic AZ80 Mg alloy in higher magnification have been illustrated in Fig. 2b. As is evident, some new fine grains were formed at the initial grain boundaries by dynamic recrystallization (DRX) phenomenon during the extrusion process. This led to the formation of typical necklace structures with bimodal grains size. Fig. 3 shows the microstructures of extruded reinforced AZ80 Mg alloys by Al2O3, TiO2 and SiC nano-particles. As is observed, the sub-micron reinforcement particles have a significant effect on the microstructural evolution of the experimental alloy during the extrusion process. The final grain size of the extruded material decreases with the addition of nano-particles compared to the unreinforced alloy. This matter can be attributed to the following two main reasons. (i) Nano-particulates may induce recrystallization of the magnesium matrix through particle stimulation of nucleation (PSN) mechanism at the interfaces of particles and the AZ80 matrix alloy [21]. (ii) Added reinforcement particles could restrict the grain growth of new recrystallized grains formed during the extrusion process [16]. In the case of AZ80 Mg alloy reinforced by Al2O3, TiO2 (Fig. 3a and b), the trace of initial elongated grains is still evident; however, microstructure with equiaxed grains has been achieved after extrusion of AZ80 Mg alloy reinforced by SiC particles (Fig. 3d).The agglomeration of particles can be seen in some regions for all types of composites; nonetheless, it is relatively less pronounced for SiC particles. For the sake of clarity, the grain size of extruded composites has been measured and the results tabulated in Table 3. As the obtained results show, the minimum grain size was achieved after extrusion of the AZ80/SiC composite. Hence, it can be concluded that SiC nano-particles have a more pronounced effect on the grain refinement of AZ80 magnesium alloy compared with Al2O3, TiO2 reinforcement particles. As is obviously shown in Fig. 3, adding all types of nanoparticles leads to grain refinement of the AZ80 magnesium alloy; however, different grain sizes have been achieved in the case of each system. As was mentioned, the nano-particles induce more nucleation sites for new dynamic recrystallized grains via the PSN mechanism; hence, particles agglomeration in the microstructure leads to reducing of these favorable sites for nucleation of new grains and coarser grain size obtained during the extrusion process. Therefore, according to this fact agglomeration of SiC particles was less pronounced compared to the two other types of particles (Al2O3 and TiO2); thus, obtaining the finest grain size in the microstructure of extruded AZ80/SiC composite can be reasonable. 3.2. Mechanical properties
Fig. 2. The microstructure of extruded monolithic AZ80 magnesium alloy.
3.2.1. Micro-hardness The results of micro-hardness measurements are illustrated in Fig. 4. As is evident, the extrusion process resulted in higher hardness values for unreinforced AZ80 magnesium alloys which can be attributed to the high dislocations density produced during the thermomechanical processing. Furthermore, the added nanoparticles increase the hardness of AZ80 Mg alloy significantly. As is well known, the submicron-reinforcement particulates are harder than magnesium alloy and render their inherent property of hardness to the soft matrix. Furthermore, these particulates act as obstacles to the motion of dislocations. The results also show
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Fig. 3. The microstructure of extruded AZ80 matrix composites reinforced by (a, d) Al2O3, (b, e) TiO2 and (c, f) SiC nano-particles.
Table 3 The average grain size of extruded materials. Materials
Average grain size (μm)
AZ80 AZ80/Al2O3 AZ80/TiO2 AZ80/SiC
32 17 20 15
that among all the composites, AZ80/SiC depicts the maximum micro-hardness, which can be attributed to the significant grain refinement that occurred after extrusion.
3.2.2. Tensile properties The engineering stress–engineering strain curves obtained from room temperature uniaxial tension tests of cast AZ80 as well
Fig. 4. The measured values of Vickers micro-hardness related to the cast AZ80 alloy and AZ80/nano-particles composites.
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Fig. 5. The engineering stress–engineering strain curves of cast AZ80 and extruded nano-composites obtained from room temperature tensile tests.
as extruded unreinforced AZ80 and nano-composites are plotted in Fig. 5. As is clearly observed, reinforcement particles have an outstanding effect on the tensile behavior of AZ80 magnesium alloys. For the sake of clarity, the variations of yield strength (YS), ultimate strength (UTS) and elongation (E) of processed samples as well as cast AZ80 Mg alloy extracted from the related curves are presented in Fig. 6. As is shown, the higher level of YS, UTS and elongation that has been achieved for unreinforced extruded AZ80 may be because of the following reasons: (i) lower grain size compared to the cast material; and (ii) dissolution of significant volume fraction of γ-Mg17Al12 eutectic phase. In addition, the results reveal remarkable improvement of YS and UTS of the AZ80 magnesium alloy due to the presence of nano-particles. The increase in these parameters can primarily be attributed to grain refinement that occurred via dynamic recrystallization and the strong multidirectional thermal stress at the nano-particles/ Mg interface, as well as small particles size and low degree of porosity, which lead to effective transfer of applied tensile load to the reinforcement particulates. As is evident in Fig. 6d, significant improvement of elongation to fracture of extruded composites has been detected compared to the cast material. This is because of the formation of fine recrystallized grains during the extrusion process; in addition, there is breakage and dissolving of the coarse continuous network of the γ (Mg17Al12)-eutectic phase. Among the three prepared composites, AZ80/SiC shows the highest YS and elongation to fracture. As shown in Fig. 3, this may be attributed to the fine and equiaxed microstructure of AZ80/SiC after extrusion. However, in the case of AZ80/Al2O3 and AZ80/TiO2, the presence of elongated grains and particles agglomeration leads to lower elongation. 3.3. Fracture surface The fracture surfaces of the extruded composites achieved by room temperature tensile tests are presented in Fig. 7. As is observed in lower magnification figures, all types of composites show ductile fracture surfaces with quite a few dimples which are marked in Fig. 7. However, under higher magnification it is clearly seen the main reasons of higher ductility of AZ80/SiC composites compared with AZ80/Al2O3 and AZ80/TiO2. In the case of AZ80/ Al2O3, a crack initiated from the interface of Al2O3 and matrix and propagated to the α magnesium phase (Fig. 7d). It can be explained as follows. Extrusion process at high temperature induces heavily built multidirectional thermal stress at the particulate–matrix interface at grain boundaries owing to the large difference of
Fig. 6. The variation of yield and ultimate strength as well as elongation to fracture of cast AZ80 and extruded nano-composites achieved from tensile flow curves.
the coefficient of thermal expansion between matrix and reinforcement, and induces high dislocation density which promotes crack initiation [22]. As Fig. 7e illustrates, the agglomeration of TiO2 particles led to the initiation of a deep crack and fracture from this region. On the other hand, from Fig. 7f it could be found that the submicron-SiC particulates distribute relatively homogeneous in the composite matrix and there is no trace of agglomeration in the fracture surface of AZ80/SiC composite. This, along with effective grain refinement, may the main reasons of the best mechanical behavior of AZ80/SiC composite compared with AZ80/Al2O3 and AZ80/TiO2.
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Fig. 7. The fracture surfaces of extruded AZ80 matrix nano-composites after room temperature tensile test.
As depicted in Fig. 5, different extruded composite materials exhibit different mechanical properties during room temperature tensile tests. In the case of AZ80/Al2O3 composite, the lowest level of yield strength as well as ductility can be attributed to the two main reasons: (i) no existence of good wetting between nanoAl2O3 particles and AZ80 matrix which may promote crack initiation at the interface of reinforcement particles and magnesium alloy matrix (Fig. 7), (ii) no presence of strong interfacial bonding between AZ80 matrix and Al2O3 particles which leads to the separation of nano-particles from the magnesium matrix (see Fig. 7). According to the fracture surface of extruded AZ80/TiO2 composite, the nano-particles show good bonding and wetting with the magnesium matrix; however, TiO2 particles agglomeration caused a decrease in yield strength and elongation to fracture of the composite compared to the extruded AZ80/SiC composite.
As is clearly seen in Fig. 7, in addition to the existence of good bonding between the SiC particles and the AZ80 matrix, homogenous distribution of nano-particles without any pronounced agglomeration is the main reason for the high level of yield strength and the high level of ductility of extruded AZ80/SiC composites compared with the two other composite systems.
4. Conclusion The present study deals with the microstructure evolution and mechanical properties of AZ80 magnesium alloy matrix composites reinforced by three different nano-particles processed via forward extrusion at elevated temperatures. In this purpose, three different composites have been prepared by adding nano-particles
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including Al2O3, TiO2 and SiC to the cast AZ80 magnesium alloy through the stir casting method. Then, these composites as well as monolithic AZ80 Mg alloys were processed via forward extrusion at 200 1C. The microstructural investigations depicted that the extruded monolithic AZ80 magnesium alloy showed a typical necklace structure because of restricted occurrence of DRX process at the initial grain boundaries. However, in the case of extruded nano-composites, it is found that added reinforcement particles have a significant effect on the grain refinement of the material. The obtained grain sizes after extrusion of the nano-composites were lower compared to the monolithic AZ80 Mg alloy. In addition, it is concluded that the extruded AZ80/SiC composite illustrated minimum grain size compared with AZ80/Al2O3 and AZ80/TiO2 composites. The micro-hardness measurements results indicated increasing hardness for nano-composites compared to the monolithic alloy after the extrusion process. Furthermore, significant improvement of tensile properties including yield and ultimate strength as well as room temperature ductility has been achieved for nano-composites after the extrusion process. References [1] [2] [3] [4] [5]
P. Changizian, A. Zarei-Hanzaki, A.A. Roostaei, Mater. Des. 39 (2012) 384–389. G. Wu, Y. Zhang, W. Liu, W. Ding, J. Magnes. Alloy. 1 (2013) 39–46. S.M. Arab, A. Akbarzadeh, J. Magnes. Alloy. 1 (2013) 145–149. M. Paramsothy, J. Chan, R. Kwok, M. Gupta, Nanomaterials 2 (2012) 147–162. Z. Wang, X. Wang, Y. Zhao, W. Du, Trans. Nonferr. Met. Soc. China 20 (2010) 1029–1032.
[6] H. Dieringa, J. Mater. Sci. 46 (2010) 289–306. [7] H.R. Abedi, A. Zarei-Hanzaki, S.M. Fatemi-Varzaneh, A.A. Roostaei, Mater. Des. 31 (2010) 4386–4391. [8] H.R. Abedi, A. Zarei-Hanzaki, S. Khoddam, Mater. Des. 32 (2011) 2181–2190. [9] P. Changizian, A. Zarei-Hanzaki, H.R. Abedi, Mater. Sci. Eng. A 558 (2012) 44–51. [10] Z. Li, J. Dong, X.Q. Zeng, C. Lu, W.J. Ding, Mater. Sci. Eng. A 466 (2007) 134–139. [11] S.F. Hassan, M.J. Tan, M. Gupta, Mater. Sci. Eng. A 486 (2008) 56–62. [12] M. Habibnejad-Korayem, R. Mahmudi, W.J. Poole, Mater. Sci. Eng. A 519 (2009) 198–203. [13] M. Paramsothy, X.H. Tan, J. Chan, R. Kwok, M. Gupta, J. Alloys Compd. 545 (2012) 12–18. [14] H. Ferkel, B.L. Mordike, Mater. Sci. Eng. A 298 (2001) 193–199. [15] C.S. Goh, M. Gupta, J. Wei, L.C. Lee, J. Compos. Mater. 41 (2007) 2325–2335. [16] K.K. Deng, K. Wu, Y.W. Wu, K.B. Nie, M.Y. Zheng, J. Alloy. Compd. 504 (2010) 542–547. [17] M. Paramsothy, J. Chan, R. Kwok, M. Gupta, Mater. Sci. Eng. A 528 (2011) 6545–6551. [18] E. Karimi, A. Zarei-Hanzaki, M.H. Pishbin, H.R. Abedi, P. Changizian, Mater. Des. 49 (2013) 173–180. [19] P. Changizian, A. Zarei-Hanzaki, M. Ghambari, A. Imandoust, Mater. Sci. Eng. A 582 (2013) 8–14. [20] X. Li, W. Qi, K. Zheng, N. Zhou, J. Magnes. Alloy. 1 (2013) 54–63. [21] X.J. Wang, K. Wu, H.F. Zhang, W.X. Huang, H. Chang, W.M. Gan, M.Y. Zheng, D. L. Peng, Mater. Sci. Eng. A 465 (2007) 78–84. [22] A. Mazahery, H. Abdizadeh, H.R. Baharvandi, Mater. Sci. Eng. A 518 (2009) 61–64.
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Mechanical properties improvement of cast AZ80 Mg alloy/nano-particles composite via thermomechanical processing H. Khoshzaban Khosroshahi a, F. Fereshteh Saniee b, H.R. Abedi c,n a
Mechanical Engineering Department, Islamic Azad University, Takestan, Iran Mechanical Engineering Department, Bu-Ali Sina University, Hamedan, Iran c School of Metallurgical and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran b
art ic l e i nf o
a b s t r a c t
Article history: Received 9 November 2013 Received in revised form 7 December 2013 Accepted 7 December 2013 Available online 16 December 2013
Three different nano-composites with cast AZ80 magnesium alloy matrix and nano-particles reinforcement, including Al2O3, TiO2 and SiC, have been prepared via stir casting method. In order to refine the microstructure and improve the mechanical properties, the monolithic AZ80 as well as AZ80/nano-particles composites were thermomechanically processed employing forward extrusion at 200 1C. The obtained microstructures of extruded nano-composites materials indicated significant grain refinement due to the progression of dynamic recrystallization (DRX); however, the extruded monolithic material depicts a typical necklace structure with limited DRX grain. To evaluate the mechanical properties, micro-hardness and tensile tests were conducted. The results showed a remarkable increase in hardness of extruded nano-composites because of adding harder particles along with outstanding grain refinement. Furthermore, tensile tests results indicated a remarkable improvement in yield and ultimate strength as well as room temperature ductility for extruded composites compared with cast AZ80 Mg alloy. Finally, it is concluded that adding SiC nano-particles to the cast AZ80 Mg alloy resulted in the best combination of strength and ductility in comparison with Al2O3 and TiO2 particles. This may be attributed to the homogenous distribution of SiC particles and the lowest level of agglomeration during the extrusion process. & 2013 Elsevier B.V. All rights reserved.
Keywords: Magnesium matrix composite Nano-particles Grain refinement Forward extrusion
1. Introduction Magnesium alloys, being among the lightest structural materials, are very attractive in many applications. Their excellent strength to weight ratio predestines Mg-based alloys for applications as structural components in automobile and aircraft industries. However, magnesium alloys exhibit poor room temperature formability because of their hexagonal close-packed crystal structure with a limited number of operative basal slip systems. Accordingly, numerous studies have focused on eliminating this restriction; nonetheless, all these efforts can be categorized into two main groups. The first, the researches which have employed the different thermomechanical processing routes [1–3]; and the second, those of fabricated magnesium alloy matrix composites via adding sub-micron reinforcement particles [4–6]. As is well established the thermomechanical treatment may effectively improve the mechanical properties of cast AZ-series, as the most common magnesium alloys [7–9]. It has been found that the ductility and strength values are significantly improved due to the modification of γ-Mg17Al12 phase morphology and also due to the occurrence of grain refinement. It has been reported n
Corresponding author. Tel.: þ 98 21 82084116; fax: þ 98 21 88006076. E-mail address: [email protected] (H.R. Abedi).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.12.029
that employing forward extrusion at 270 1C leads to a considerable grain refinement through dynamic recrystallization where the presence of small γ eutectic particles along newly formed grain boundaries is evident. These dynamically form during processing of the alloy or through the separation from the coarse network of elongated phases [10]. Apparently, associated microstructures have resulted in the improvement of tensile properties of AZ91 magnesium alloy. Since magnesium matrix composites exhibit many advantages over the monolithic magnesium or magnesium alloys, such as high elastic modulus, high strength, superior creep and wear resistances at elevated temperatures [11,12], numerous studies have been also made on fabricating magnesium matrix composites. Gupta et al. [13] investigated the microstructure and mechanical properties of AZ81 magnesium alloy reinforced with Al2O3particulates. The related results showed that the AZ81/Al2O3 nano-composite exhibited higher tensile and compressive energy absorbed until fracture compared to the monolithic AZ81 alloy. Ferkel and Mordike [14] have employed powder metallurgy to strengthen the magnesium with SiC nanoparticles, and have reported the improvement of creep resistance and tensile properties of the obtained materials. Enhanced physical and mechanical properties of the Mg–Y2O and Mg–Al2O3 nano-composite prepared by the disintegrated melt deposition method have been also characterized by Goh et al. and Gupta et al., respectively [11,15].
H. Khoshzaban Khosroshahi et al. / Materials Science & Engineering A 595 (2014) 284–290
285
Considering all of these advantages, it seems that applying these two methods (fabricating nano-composites and employing thermomechanical processes) simultaneously may be more influential in the improvement of mechanical properties compared to the employing of these methods individually. Despite the extensive conducted researches focused on each approach, there are just a few studies which have been directed towards thermomechanical processing of Mg matrix nano-composites. In this regard, Deng et al. [16] investigated the microstructure and tensile properties of forged AZ91/SiC nano-composites with different volume fractions of SiC reinforcement particles. They reported that the presence of submicron-SiC particulates assisted in improving the thermal stability, micro-hardness, elastic modulus and yield strength. In addition, Paramsothy et al. [17] evaluated the mechanical properties of AZ91/AZ31 hybrid magnesium alloy with Si3Ni4 nanoparticles after hot extrusion process. Their results showed that compared to the monolithic hybrid alloy, the nano-composite exhibited higher yield strength, ultimate tensile strength, failure strain and work of fracture simultaneously. In line with these efforts, the aim of this paper is to study the effect of adding different nano-particles reinforcement, namely Al2O3, TiO2 and SiC particles, to the cast AZ80 magnesium alloy on the mechanical properties of fabricated composite processed via the forward extrusion method. The microstructure-processing-properties relationships have been properly addressed through this manuscript.
2. Experimental procedure 2.1. Material and casting process Cast AZ80 magnesium alloy was selected as the matrix alloy, the chemical composition of which is shown in Table 1. The microstructure of cast AZ80 Mg alloy is depicted in Fig. 1. As is shown, the microstructure of the AZ80 Mg alloy characterized by the α-Mg matrix and γ (Mg17Al12) intermetallic compound distributed in the grain interior and along the grain boundaries. There different metal matrix composites were fabricated by adding 1%wt. TiO2, SiC and Al2O3 nano-particles to the matrix of AZ80 Mg alloy. The amount of purity as well as average size of nanoparticles is tabulated in Table 2. To fabricate the composites, stir casting method was employed. Towards this direction, the AZ80 magnesium alloy was molten at 750 1C and then the particles were added to the matrix. As the nano-particles were added, the molten metal was stirred for 5 min using a stainless steel rod to provide a homogeneous composition. The melt was poured into a preheated steel mold (450 1C) and solidified under 150 MPa pressure to reduce the porosity of composite ingots. The whole fabricating process was conducted under the protective atmosphere of MgCl2 and NaCl to avoid oxidation. The cast rods were prepared in dimensions of Φ40 mm H100 mm. The cylindrical specimens for forward extrusion were extracted from the center of cast rods in the size of 28 mm diameter and 40 mm height. 2.2. Thermomechanical process The cast AZ80 Mg alloy and metal matrix composites were processed using the forward extrusion method. The extrusion process was conducted at 200 1C on the cylindrical specimens in Table 1 The chemical composition of the experimental AZ80 Mg alloy.
Fig. 1. The microstructure of cast AZ80 magnesium alloy.
Table 2 The purity and average size of reinforced nano-particles. Particle
Purity (%)
Average particle size (nm)
TiO2 SiC Al2O3
99.5 99 99
21 40–50 80
the size of 28 mm diameter and 40 mm height. The work-pieces were heated to the predetermined temperatures in a resistant furnace and held for 10 min to equalize the temperature. All the extrusion tests were performed using the extrusion ratio of 3. Forward extrusion processing was successfully carried out with no danger of crack or discontinuity. To minimize friction during the process, MoS2 lubricant was pasted to the surface of the workpieces and dies. After deformation, to preserve the obtained microstructure, the processed specimens were immediately quenched into water. In order to examine the microstructures, the processed work-pieces were cut parallel to the extrusion direction. The obtained cross-section was mechanically polished and etched for 7 s using a solution of 70 ml ethanol, 4.2 g picric acid, 10 ml acetic acid and 10 ml water. The obtained microstructures were examined by a MEIJI optical microscope (OM). 2.3. Mechanical testing
Element
Al
Zn
Mn
Si
Fe
Cu
Ni
Mg
wt%
7.4
0.48
0.026
0.066
0.029
0.02
0.0081
Balance
Uniaxial tensile test was employed to evaluate the mechanical properties of extruded AZ80 Mg alloy and nano-composites. The tensile specimens were prepared from processed and cast materials
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with a gauge length of 30 mm according to ASTM E8 [18]. The tensile tests were carried out at room temperature using an Instron4208 universal testing machine under an initial strain rate of 0.001 s 1. To justify the deformation behavior of the processed composites, the obtained fracture surfaces were examined by Scanning Electron Microscopy (SEM). In addition, the Vickers micro-hardness measurements were also performed throughout the cross-section of the extruded composites using a load of 10 g force with a loading time of 30 s.
3. Results and discussion 3.1. Microstructural characterization 3.1.1. Initial microstructure As is shown in Fig. 1, the initial microstructure consists of coarse cast grains and a coarse continuous network of γ (Mg17 Al12)-eutectic phase, which significantly destructs the mechanical properties of the experimental alloy. The initial grain size of cast AZ80 magnesium alloy is measured to be 5073μm. 3.1.2. Microstructure of extruded materials Fig. 2 depicts the final microstructure of monolithic AZ80 Mg alloy after extrusion. As is observed, the initial coarse grains have been elongated along the extrusion direction, significantly. In addition, the continuous network of γ (Mg17Al12)-eutectic phase has been broken to smaller particles. The latter is attributed to the effect of high shear strain imposed during the extrusion process. It is noteworthy to mention that the volume fraction of γ
(Mg17Al12)-eutectic phase in the microstructure of extruded alloy becomes lower compared with cast AZ80 magnesium alloy. In fact, during the extrusion process remarkable volume fraction of γ (Mg17Al12)-eutectic phase has been dissolved to the α-Mg matrix phase. The strain induced dissolution of eutectic phase has already been reported during thermomechanical processing [19], particularly during severe plastic deformation of Mg–Al alloys [20]. For further clarification, the final microstructures of the processed monolithic AZ80 Mg alloy in higher magnification have been illustrated in Fig. 2b. As is evident, some new fine grains were formed at the initial grain boundaries by dynamic recrystallization (DRX) phenomenon during the extrusion process. This led to the formation of typical necklace structures with bimodal grains size. Fig. 3 shows the microstructures of extruded reinforced AZ80 Mg alloys by Al2O3, TiO2 and SiC nano-particles. As is observed, the sub-micron reinforcement particles have a significant effect on the microstructural evolution of the experimental alloy during the extrusion process. The final grain size of the extruded material decreases with the addition of nano-particles compared to the unreinforced alloy. This matter can be attributed to the following two main reasons. (i) Nano-particulates may induce recrystallization of the magnesium matrix through particle stimulation of nucleation (PSN) mechanism at the interfaces of particles and the AZ80 matrix alloy [21]. (ii) Added reinforcement particles could restrict the grain growth of new recrystallized grains formed during the extrusion process [16]. In the case of AZ80 Mg alloy reinforced by Al2O3, TiO2 (Fig. 3a and b), the trace of initial elongated grains is still evident; however, microstructure with equiaxed grains has been achieved after extrusion of AZ80 Mg alloy reinforced by SiC particles (Fig. 3d).The agglomeration of particles can be seen in some regions for all types of composites; nonetheless, it is relatively less pronounced for SiC particles. For the sake of clarity, the grain size of extruded composites has been measured and the results tabulated in Table 3. As the obtained results show, the minimum grain size was achieved after extrusion of the AZ80/SiC composite. Hence, it can be concluded that SiC nano-particles have a more pronounced effect on the grain refinement of AZ80 magnesium alloy compared with Al2O3, TiO2 reinforcement particles. As is obviously shown in Fig. 3, adding all types of nanoparticles leads to grain refinement of the AZ80 magnesium alloy; however, different grain sizes have been achieved in the case of each system. As was mentioned, the nano-particles induce more nucleation sites for new dynamic recrystallized grains via the PSN mechanism; hence, particles agglomeration in the microstructure leads to reducing of these favorable sites for nucleation of new grains and coarser grain size obtained during the extrusion process. Therefore, according to this fact agglomeration of SiC particles was less pronounced compared to the two other types of particles (Al2O3 and TiO2); thus, obtaining the finest grain size in the microstructure of extruded AZ80/SiC composite can be reasonable. 3.2. Mechanical properties
Fig. 2. The microstructure of extruded monolithic AZ80 magnesium alloy.
3.2.1. Micro-hardness The results of micro-hardness measurements are illustrated in Fig. 4. As is evident, the extrusion process resulted in higher hardness values for unreinforced AZ80 magnesium alloys which can be attributed to the high dislocations density produced during the thermomechanical processing. Furthermore, the added nanoparticles increase the hardness of AZ80 Mg alloy significantly. As is well known, the submicron-reinforcement particulates are harder than magnesium alloy and render their inherent property of hardness to the soft matrix. Furthermore, these particulates act as obstacles to the motion of dislocations. The results also show
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Fig. 3. The microstructure of extruded AZ80 matrix composites reinforced by (a, d) Al2O3, (b, e) TiO2 and (c, f) SiC nano-particles.
Table 3 The average grain size of extruded materials. Materials
Average grain size (μm)
AZ80 AZ80/Al2O3 AZ80/TiO2 AZ80/SiC
32 17 20 15
that among all the composites, AZ80/SiC depicts the maximum micro-hardness, which can be attributed to the significant grain refinement that occurred after extrusion.
3.2.2. Tensile properties The engineering stress–engineering strain curves obtained from room temperature uniaxial tension tests of cast AZ80 as well
Fig. 4. The measured values of Vickers micro-hardness related to the cast AZ80 alloy and AZ80/nano-particles composites.
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Fig. 5. The engineering stress–engineering strain curves of cast AZ80 and extruded nano-composites obtained from room temperature tensile tests.
as extruded unreinforced AZ80 and nano-composites are plotted in Fig. 5. As is clearly observed, reinforcement particles have an outstanding effect on the tensile behavior of AZ80 magnesium alloys. For the sake of clarity, the variations of yield strength (YS), ultimate strength (UTS) and elongation (E) of processed samples as well as cast AZ80 Mg alloy extracted from the related curves are presented in Fig. 6. As is shown, the higher level of YS, UTS and elongation that has been achieved for unreinforced extruded AZ80 may be because of the following reasons: (i) lower grain size compared to the cast material; and (ii) dissolution of significant volume fraction of γ-Mg17Al12 eutectic phase. In addition, the results reveal remarkable improvement of YS and UTS of the AZ80 magnesium alloy due to the presence of nano-particles. The increase in these parameters can primarily be attributed to grain refinement that occurred via dynamic recrystallization and the strong multidirectional thermal stress at the nano-particles/ Mg interface, as well as small particles size and low degree of porosity, which lead to effective transfer of applied tensile load to the reinforcement particulates. As is evident in Fig. 6d, significant improvement of elongation to fracture of extruded composites has been detected compared to the cast material. This is because of the formation of fine recrystallized grains during the extrusion process; in addition, there is breakage and dissolving of the coarse continuous network of the γ (Mg17Al12)-eutectic phase. Among the three prepared composites, AZ80/SiC shows the highest YS and elongation to fracture. As shown in Fig. 3, this may be attributed to the fine and equiaxed microstructure of AZ80/SiC after extrusion. However, in the case of AZ80/Al2O3 and AZ80/TiO2, the presence of elongated grains and particles agglomeration leads to lower elongation. 3.3. Fracture surface The fracture surfaces of the extruded composites achieved by room temperature tensile tests are presented in Fig. 7. As is observed in lower magnification figures, all types of composites show ductile fracture surfaces with quite a few dimples which are marked in Fig. 7. However, under higher magnification it is clearly seen the main reasons of higher ductility of AZ80/SiC composites compared with AZ80/Al2O3 and AZ80/TiO2. In the case of AZ80/ Al2O3, a crack initiated from the interface of Al2O3 and matrix and propagated to the α magnesium phase (Fig. 7d). It can be explained as follows. Extrusion process at high temperature induces heavily built multidirectional thermal stress at the particulate–matrix interface at grain boundaries owing to the large difference of
Fig. 6. The variation of yield and ultimate strength as well as elongation to fracture of cast AZ80 and extruded nano-composites achieved from tensile flow curves.
the coefficient of thermal expansion between matrix and reinforcement, and induces high dislocation density which promotes crack initiation [22]. As Fig. 7e illustrates, the agglomeration of TiO2 particles led to the initiation of a deep crack and fracture from this region. On the other hand, from Fig. 7f it could be found that the submicron-SiC particulates distribute relatively homogeneous in the composite matrix and there is no trace of agglomeration in the fracture surface of AZ80/SiC composite. This, along with effective grain refinement, may the main reasons of the best mechanical behavior of AZ80/SiC composite compared with AZ80/Al2O3 and AZ80/TiO2.
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Fig. 7. The fracture surfaces of extruded AZ80 matrix nano-composites after room temperature tensile test.
As depicted in Fig. 5, different extruded composite materials exhibit different mechanical properties during room temperature tensile tests. In the case of AZ80/Al2O3 composite, the lowest level of yield strength as well as ductility can be attributed to the two main reasons: (i) no existence of good wetting between nanoAl2O3 particles and AZ80 matrix which may promote crack initiation at the interface of reinforcement particles and magnesium alloy matrix (Fig. 7), (ii) no presence of strong interfacial bonding between AZ80 matrix and Al2O3 particles which leads to the separation of nano-particles from the magnesium matrix (see Fig. 7). According to the fracture surface of extruded AZ80/TiO2 composite, the nano-particles show good bonding and wetting with the magnesium matrix; however, TiO2 particles agglomeration caused a decrease in yield strength and elongation to fracture of the composite compared to the extruded AZ80/SiC composite.
As is clearly seen in Fig. 7, in addition to the existence of good bonding between the SiC particles and the AZ80 matrix, homogenous distribution of nano-particles without any pronounced agglomeration is the main reason for the high level of yield strength and the high level of ductility of extruded AZ80/SiC composites compared with the two other composite systems.
4. Conclusion The present study deals with the microstructure evolution and mechanical properties of AZ80 magnesium alloy matrix composites reinforced by three different nano-particles processed via forward extrusion at elevated temperatures. In this purpose, three different composites have been prepared by adding nano-particles
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including Al2O3, TiO2 and SiC to the cast AZ80 magnesium alloy through the stir casting method. Then, these composites as well as monolithic AZ80 Mg alloys were processed via forward extrusion at 200 1C. The microstructural investigations depicted that the extruded monolithic AZ80 magnesium alloy showed a typical necklace structure because of restricted occurrence of DRX process at the initial grain boundaries. However, in the case of extruded nano-composites, it is found that added reinforcement particles have a significant effect on the grain refinement of the material. The obtained grain sizes after extrusion of the nano-composites were lower compared to the monolithic AZ80 Mg alloy. In addition, it is concluded that the extruded AZ80/SiC composite illustrated minimum grain size compared with AZ80/Al2O3 and AZ80/TiO2 composites. The micro-hardness measurements results indicated increasing hardness for nano-composites compared to the monolithic alloy after the extrusion process. Furthermore, significant improvement of tensile properties including yield and ultimate strength as well as room temperature ductility has been achieved for nano-composites after the extrusion process. References [1] [2] [3] [4] [5]
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