High temperature mechanical behavior of AZ61 magnesium alloy reinforced with graphene nanoplatelets

    High temperature mechanical behavior of AZ61 magnesium alloy reinforced with graphene nanoplatelets Muhammad Rashad, Fusheng Pan, Dong Lin, Muhammad Asif PII: DOI: Reference:

S0264-1275(15)30679-1 doi: 10.1016/j.matdes.2015.10.101 JMADE 840

To appear in: Received date: Revised date: Accepted date:

26 August 2015 23 September 2015 18 October 2015

Please cite this article as: Muhammad Rashad, Fusheng Pan, Dong Lin, Muhammad Asif, High temperature mechanical behavior of AZ61 magnesium alloy reinforced with graphene nanoplatelets, (2015), doi: 10.1016/j.matdes.2015.10.101

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High temperature mechanical behavior of AZ61 magnesium alloy reinforced with graphene nanoplatelets

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Muhammad Rashad a,b,*, Fusheng Pan a,b,c,* , Dong Lin d, Muhammad Asif e College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China

National Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing 400044, China

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Chongqing Academy of Science and Technology, Chongqing , Chongqing 401123, China

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Department of Industrial and Manufacturing Systems Engineering, Kansas State University, Manhattan, KS, 66506, USA

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School of Materials Science and Engineering, Dalian University of Technology, Dalian116024, China

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*Corresponding authors: [email protected] (M.Rashad); [email protected] (F.S.Pan)

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Abstract: In present work, graphene nanoplatelets reinforced AZ61 magnesium alloy was synthesized by disintegrated melt deposition method. The synthesized materials were subjected to homogenization at 430°C for 24 hours and extruded at 350°C with the ratio of 5.2:1. Experimental results revealed that graphene nanoplatelets addition have significant effect on refining grain size and changing in basal textures due to their uniform distribution throughout the composite matrix, which results in significant improvement in room temperature micro hardness, tensile and compression strengths. In addition, tensile strength of as extruded graphene nanoplatelets-AZ61 composite was investigated at temperatures ranging from 75°C to 225°C with initial strain rate of 2×10-3 s-1. The results show that total fracture strain increases and tensile yield strength decreases with increasing testing temperature. The increased fracture strain at high temperature is mainly attributed to significant grain refinement and uniform particle distribution. The fracture surface analysis revealed that deformation possibly occurs through grain boundary sliding accommodated by diffusional transport. Keywords: Metal matrix composites, Mechanical properties, Disintegrated melt deposition method, Elevated temperatures. 1. Introduction World is facing energy crisis and lightweight materials have gained prodigious research consideration to reduce the fuel consumption in automotive and aviation industry. Among light materials, magnesium alloys have been regarded as ideal structural materials due to their low density, high specific strength and damping properties [1]. In order to further improve mechanical strength of magnesium alloys, the magnesium composites and conventional thermo mechanical process [2-4] (i.e. hot extrusion, hot rolling, and forging etc.) have been developed to achieve fine grains and uniformly dispersed reinforcement particles. Literature review shows that reinforcement particles i.e. SiC, Al2O3, TiO2, and carbon nanotubes (CNTs) have been extensively used to enhance the mechanical properties of Mg-6Zn alloy [5, 6], AZ31,

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and AZ81 [7, 8] magnesium alloys. The two dimensional graphene has become one of the most extensively investigated allotropes of carbon in recent years [9, 10]. Graphene can be used as reinforcement particles to enhance the mechanical strength of polymers and metals. Recently, graphene nanoplatelets (GNPs) (multilayer graphene) have been used to enhance the mechanical strength of pure magnesium [11] and its alloys [12-17] using powder metallurgy method. It was found that the mechanical properties of resulting composites were significantly improved. However, powder metallurgy method is very expensive and may result in damage of reinforcement particles (during ball milling process) which will finally affect the mechanical properties of resulting composites. On the other hand stir casting method is preferred due to several advantages over powder metallurgy method. For example, stir casting method is less expensive and porosity arising due to solidification shrinkage and hydrogen evolution is very small. In addition, wide range of shapes and size (of matrix and reinforcements) can be used. The reinforcement particles are free of damage and any kind of reinforcements/matrix can be used despite of their melting points.

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In addition, magnesium alloys exhibits low strength and formability at high temperatures. Several attempts have been made to increase high temperature strength and formability by alloying with suitable elements and developing composite materials. The creep resistance and high temperature strength of AZ91-3%La-0.3%Ca alloy was investigated by Wenwen et al. [18]. Similarly, Khomamizadeh et al. [19] explored that AZ91-2%RE magnesium alloy could retain its yield strengths at 140°C. Recently, K. Deng et al. [20] fabricated AZ91-1.5%SiC composites to examine the effect of strain rate on tensile properties at temperatures from 320°C to 370°C. Experimental results show that the higher plasticity was achieved at elevated temperatures. Besides, CNTs [21] and Al2O3 [22] reinforcements were found to be effective in improvement of formability of AZ31 magnesium alloys. However， literature review revealed that the room temperature strength and formability of AZ61 alloy reinforced with graphene nanoplatelets have never been reported. Accordingly, the effect of GNPs addition on room temperature and high temperature mechanical properties of AZ61 magnesium alloy is investigated. An attempt is made to replace the CNTs by GNPs. The composite materials were synthesized by disintegrated melt deposition method followed by homogenization and hot extrusion techniques. 2. Experimental procedures 2.1. Materials Reinforcements, graphene nanoplatelets with thickness and average diameter of 15-20nm and 5-10µm respectively, were purchased from Chengdu Organic Chemistry Co. Ltd., China. Pure Mg/Al ingots and Zn granular were used as matrix materials.

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2.2. Synthesis of composite The AZ61 alloy and its composites were synthesized through disintegrated melt deposition (DMD) method. About 1320 grams Mg ingot was melted at 740°C under CO2-SF6 protective atmosphere in a graphite crucible using an electrical resistance furnace. About 84 gram of Al and 14 grams of Zn were added into molten slurry. In next step, GNPs powder was added into AZ61 alloy molten slurry and mixture was stirred for 0.5 min. After stirring, mixture was reheated to 740°C and kept at 740°C for 5 minutes. The molten mixture was poured into a steel mold and allowed to solidify normally. The casted billets were cut into Φ82mmⅹ45mm samples and homogenized at 430°C for 24 hours. Homogenized billets were hot extruded using a 500T hydraulic press at 350°C with an extrusion ratio of 5.2:1 to obtain the rods of 16mm diameter. The RAM speed was set at 1m/min.

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2.3. Characterizations Bulk samples were machined from extruded bars and polished to examine their microstructures. X-ray diffraction (XRD) technique was used to analyze second phase peaks in synthesized materials. X-ray diffraction (XRD) analysis were conducted on extruded bulk samples using a Rigaku D/MAX-2500PC diffract meter with Cu Kα radiation at 40KV and 30mA, and a scan rate of 0.02º s-1 in a 2θ range of 10-90°. Crystallographic texture measurements were carried out using diffractometer with Cu-Ka radiation at 40KV and 34mA. Optical microscopy was used to analyze grain size. Scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) was used to examine the surface morphology of materials. The room temperature mechanical properties of materials were investigated in terms of Vickers hardness, tensile and compression tests. An automatic digital hardness tester (Shanghai HX-1000TM) was used to measure the microhardness under a load of 100 grams and 15 seconds dwell time. Tensile (sample with 25mm gauge length and 5mm diameter) and compression (sample with 12mm height and 8mm diameter) tests were carried out on bulk samples parallel to extrusion directions with strain speed of 1×10-3 s-1. A minimum of five tests were conducted for each composition to obtain an average value. High temperature tensile tests were carried out for AZ61-3GNP composite at temperatures ranging from 75°C to 225°C with initial strain rate of 2×10-3 s-1. SEM was used to analyze the tensile and compression fracture images. 3. Results and discussions 3.1. X-ray diffraction and Crystallographic texture measurements X-ray diffraction analysis of as-extruded AZ61 alloy and AZ61-3GNP composite taken in transverse and longitudinal direction of the samples is shown in Figure 1. It can be observe that peaks corresponding to pure magnesium and Mg17Al12 intermetallic phase are present in all samples. The peaks corresponding to GNP were not present which may be attributed due to low volume fraction of reinforcement particles. Generally, peaks at two theta equal to 32, 34, and 36º represents prism

1010, basal 0002, and pyramidal 1011 planes of hexagonal closed packed crystal

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structure of magnesium. The texture results derived from Figure 1 are summarized in Table 1. In pure AZ61 alloy and AZ61-3GNP composite, dominant textures in

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transverse and longitudinal directions were 1010 , 1011

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and 0002 , 1011

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respectively. However, very careful observation in transverse direction revealed that

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intensity of 0002 basal texture increases with addition of GNP. In order to confirm this, crystallographic texture analysis of samples were carried out using pole figures

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1010 Prism, 0002Basal, and 1011Pyramidal as shown in Figure 2. It is known

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that room temperature mechanical properties of magnesium composites are due to activity of basal plane which depends on crystalline alignment via Schmidt factor [23]. The change in crystallographic texture difference between pure alloy and composite reinforced with GNP may lead to improvement in mechanical strength of resulting materials. Figure 2 shows that pure AZ61 alloy have very smooth basal texture intensity distributed in the form of circles. However, addition of GNP into alloy has changed the intensity of basal texture. The basal textures were scattered and spread along the periphery of circle. The changes in basal texture intensities witness the increased strength of composite. The prismatic and pyramidal texture intensities of pure alloy and composites were changed to a small extent. 3.2. Microstructural characterization

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Optical microscopic images of as-extruded pure AZ61 alloy and its composite are shown in Figure 3 (a-b). It can be seen that primary α-Mg phase is refined with the addition of GNP particles with increased uniformity of grains. The initial grain size of pure AZ61 ally was reduced from 12 to 4µm, when adding 3wt.%GNP reinforcements. This implies that GNPs can act as nucleation sites and serve as grain refiner in magnesium composites. Figure 3 (c-d) show scanning electron microscopic images of pure AZ61 alloy and its composite. It can be observed that microstructures are very clear which inferred the potentiality of adopted DMD method. The microstructures of extruded samples consist of α-Mg and Mg17Al12 intermetallic phases. The Mg17Al12 intermetallic phases are produced due to chemical reaction between Mg and Al when temperature is less than 460ºC. It can be observed that dendrite network of Mg17Al12 intermetallic phases contains several agglomerations in pure AZ61 alloy as shown in Figure 3 (c). However, composite microstructure is free of agglomerations which might be attributed due to existence of GNPs which may help to dissolve Mg17Al12 intermetallic network. This is because carbonaceous particles regulate precipitation of Mg17Al12 phase. In general, when carbonaceous reinforcement particles (graphene nanoplatelets) are active in nano scale manipulation of the Mg17Al12 phase, the newly formed second phase particles move away and letting more second phase particles to be formed adjacent to the active carbonaceous reinforcement particle. Some of the second phase particles that moved away from the active carbonaceous reinforcement particle, begin to grow larger. The fraction of smaller and refined second phase particles increases when the rate of second phase particle formation from active

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carbonaceous reinforcement particle surpasses the rate of previously formed second phase particles. This may result in refinement of Mg17Al12 phases in composite matrix and assists in improving the mechanical properties [13]. Thus it can be concluded that GNPs have great effect on grain refinement and growth of Mg17Al12 intermetallic dendrite network in AZ61-3GNP composite.

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3.3. Room temperature mechanical properties

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Figure 4(a) shows tensile stress-strain curves of AZ61 alloy and AZ61-3GNP composite. The yield strength and ultimate tensile strength of composite showed 26% and 11.7% increase, compared to monolithic alloy. In addition the tensile fracture strain of composite was 10.7%. The compression stress-strain curves of AZ61 alloy and its composite are shown in Figure 4(b). It can be observed that compressive yield strength and ultimate compressive strengths of composite were increased from 170 to 226 MPa (an increase of 32.9%) and 461 to 480 MPa (an increase of 4.12%) respectively, with slight reduction in compressive fracture strain values. The Vickers hardness of composite with 3wt.% GNPs was approximately 15.5% greater than pure AZ61 alloy as shown in Table 2. In general, strengthening mechanism of metal matrix composites can be described in terms of theoretical models for example Orowan looping, which is governed by dislocation densities generated due to addition of reinforcements [24]. In addition, improved mechanical properties of composite may be attributed due to grain refinement (as shown in Figure 3) which can be explained by Hall-Petch model [25, 26]. This model explains the effect of grain size on mechanical strength of resulting composite. The mechanical strength of composite can be increased by grain refinement according to following relationship [27]. 1

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 c   0  K (d )

where  c is yield strength of composite materials,  0 and K are constant related to material, and d is the mean grain size. Literature review revealed that several studies were carried out to investigate the influence of carbon nano-materials on mechanical properties of metal matrices [28]. It was found that graphene nanoplatelets (GNP) have great influence on mechanical strength owing to their large surface area [16]. Load transfer efficiency is high due to large contact area between GNP reinforcement/matrix [29-31]. During external loading, at first matrix is strained and later load is transferred from soft and strained matrix to hard reinforcement particles. In next step strength of reinforcement particles is contributed against fracture which results in increased strength of composite materials. Thus, load transfer of AZ61-3GNP composite, model considering the interfacial areas and cross-section areas of GNP is given by following equation [32].  S m      mVm  A  2 

 c  VGNP 

(2)

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where  c and  m are yield strength of composite material and matrix respectively.

VGNP and Vm are volume fraction of GNPs and matrix respectively. S is interfacial

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given by

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area, A is cross-section area of GNP and  m is the shear stress of matrix which is

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Literature review revealed that few attempts have been made to increase the strength of AZ61 alloy with carbonaceous reinforcements. In 2011, Fukuda et al. [33] fabricated the AZ61-CNT composites using powder metallurgy method which includes wet process using isopropyl alcohol based zwitterionic surfactant solution to un-bundle CNTs. The experimental results revealed that in situ formed Al2MgC2 phase have significantly increased the elongation of composites with slight decrease in strength. In another report [34], similar method was adopted to enhance the mechanical strength of Mg-6Al alloy with addition of different contents of CNTs. Microstructural characterization showed formation of Al2MgC2 phase at the interface of CNT and matrix. The tensile strength of resulting composites were increased, however the elongations were adversely affected. The reduced strength and elongation of composite maybe attributed due to poor dispersion of CNTs in composite matrices. The uniform dispersion of CNTs is difficult to achieve, because of strong Van der Waals attractions between carbon atoms which may results in sudden agglomeration [35, 36]. In addition, interfacial bonding between carbonaceous reinforcements and metal matrix is very week due to poor wettability. To overcome these problems, several techniques have been used to disperse CNTs in metal matrices, such as stir melting, ultra-sonication, ball milling, chemical coating with different surfactants etc. [37-41]. However, these strategies are not effective on industrial scale owing to complex synthesis processes. Therefore, there is need to find a reinforcement having mechanical strength close to CNTs but different structure, that's graphene. Graphene is single layer of carbon atoms arranged in hexagonal form. Graphene can be found in the form of graphene oxide, reduced graphene oxide, few layer graphene and multilayer graphene. All these allotropes of graphene are very expensive and can only be used on nano scale. Among these, multilayer graphene which is commonly known as graphene nanoplatelets, are cheap and easy to produce in bulk quantities on industrial scale. Therefore, multilayer graphene or graphene nanoplatelets are useful for composite materials and maybe used to enhance the mechanical strength of matrices. Graphene nanoplatelets possess high specific surface area and are cheaper as compared to carbon nanotubes. Comparison of present study with previous studies (described above) revealed that mechanical properties of AZ61 alloy were significantly improved with slight reduction in fracture strain. No Al2MgC2 phase was observed as shown in Figure 1. The method adopted in present work was quite simple and easy. Thus, GNPs have great potential to replace the CNTs in metal

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matrix composites [16]. 3.4. Fracture images and dispersion of GNPs

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The fracture images of synthesized AZ61 alloy and its composite under tensile and compressive loading are depicted in Figure 5. It confirms the fracture strain values after mechanical testing as shown in Table 2. It can be observed that tensile image of AZ61 alloy (Figure 5(a)) revealed mixed mode fracture with cleavage steps showing evidence of ductile material. Similarly, composite tensile fracture (Figure 5(b)) also significant plastic deformation along with several micro cracks and cavities which may be responsible for reduced fracture strain values compared to AZ61alloy. These micro crack observed in composite fracture images maybe attributed due to high dislocation densities at GNP-matrix interface. The compressive fracture analysis revealed that shear bands are present in pure AZ61 alloy and it’s composite as shown in Figure 5(c-d). In addition, it was observed that all fractures occurred at 45º angle with respect to compressive loading. The shear bands observed in case of compressive fracture attributed due to work hardening behavior and heterogeneous deformation because work hardening rate is higher in case of samples failed by shear bands [42].

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It is known that distribution of reinforcement particles play critical role in improving the mechanical strength and ductility of resulting composite [43-45]. Presence of large and sharp reinforcement particles may result in agglomeration in composite matrix, which may affect the ductility of resulting composites by enhancing the micro cracks and cavities [45]. On the other hand, small reinforcements maybe easily distributed in composite matrices and have great contribution toward enhanced strength and ductility. In order to estimate the dispersion of GNP particles in composite matrix, SEM-EDS analysis was carried out on tensile fracture image of AZ61-3GNP composite as shown in Figure 6. It can be observed that GNP particles were effectively distributed in composite matrix as shown in Figure 6(e). The uniform dispersion of GNP is attributed due to adopting efficient strategy while preparing the composite sample. Furthermore, it can be observed that few Mg elements cover the same area corresponding to the GNP elements. Therefore, it can be concluded that GNP/matrix interface is free of intermetallic phase, which may lead to poor compatibility between matrix and GNPs.

3.5. High temperature deformation behavior of AZ61-3GNP composite High temperature tensile testing results of composite reinforced with GNP are shown in Figure 7 and Table 3. It can be observed that GNP particles have significantly exacerbated the softening of matrix alloy with increase in temperature. The stress-strain curves up to 150º are governed by work-hardening. However, the curve at 225º (highest temperature in present study) revealed decreasing stress almost immediately after straining. The yield stress of composite was decreased slowly from room temperature (25º) to 150º. When testing temperature increases from 150 to 225º,

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there is sudden reduction in yield stress. Since, yield stress decreased significantly only at 225º, therefore thermal stability of AZ61-3GNP composite is maintained up to 150º. The dramatic softening at 225º may be attributed due to recrystallization and grain coarsening. The recrystallization depends on testing temperature and applied strain in case of AZ alloys. However, in present composite, the reinforcements GNPs propelled the recrystallization process.

where,

 FS

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 FS

 152080    1 9  RT   19.23  sinh 5.96 10 e   

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Several theoretical models were proposed to estimate the flow stress of materials at high temperatures. These models revealed that variation of flow stress is dependent on testing temperature. The comparison between experimental flow stress and predicted flow stress calculated with the help of theoretical model is made. According to Maksoud et al. [46], peak flow stress of material is given as follow.

is the peak flow stress, R is molar gas constant, and T is testing

temperature. Another, model was proposed by Takuda et al. [47] which can also be

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 FS

   K    0  n

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used to estimate the peak flow stress,  FS of materials as given below.

   3.24 105   105  1 Where, K     406 ,  0  1s , n  A log    B , m     0.303 , t  t     0   62.0  , A  0.016 , and B     0.053 .  t 

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1( K )

In above equations, K is almost constant and depends on dimensional less temperature, t ,  is strain rate, and T is temperature. The n and m are strain rate sensitivity exponents. A is a constant parameter, while parameter B depends on temperature. The flow stress of composite, calculated by above two proposed models is shown in Figure 8. It can be observed that theoretical flow stress of AZ61-3GNP composite is very close to experimental values. The theoretical predictions revealed the potential of GNP reinforcement to improve the softening effect of temperature in AZ61 matrix. The fractographic analysis of AZ61-3GNPs composite at room and high temperatures is shown in Figure 9. It can be observed that AZ61-3GNP composite exhibits ductile fractures with a lot of dimples and tear ridges, which are overwhelmingly dominated at testing temperature of 150 and 225º. Furthermore, it can be noticed that several micro pores were appeared at fracture surface tested at high temperature (225º). This

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4. Conclusions

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analysis revealed that GNP improved the secondary processing potential of AZ61 matrix incredibly [48, 49]. There are limited reports on high temperature formability of magnesium alloys reinforced with nano and ceramic reinforcements. Hassan et al. [21] have made successful attempt to examine the high temperature formability of CNT-AZ31 composites. It was found that elongation-to-failure tensile tests (conducted up to 250°C) of synthesized composites exhibited an incredible increment in ductility which may be attributed to softening of alloy matrix due to presence of CNTs. In another work [22], Al2O3-AZ31 composites were tested at high temperatures. Experimental results revealed that yield strength decrease and ductility increase with increase in testing temperature. Recently Deng et al. [50] synthesized SiC-AZ91 composites using stir casting and forging techniques. The tensile tests at elevated temperatures were carried out with different stain rates. It was found that strain rate significantly affect the yield strength of composite at high temperatures. The grain refinement and uniform dispersion of SiC particles led to impressive increase in plasticity of composites. Comparison of above discussed literature with present studies revealed that like other reinforcement particles, GNPs may also contributed towards high plasticity of magnesium alloys at elevated temperatures. Due to limited research in this field, there is still long way to achieve full potential of GNPs as reinforcement particles in magnesium composites.

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In present work, AZ61 alloy was reinforced with 3wt.% GNP using disintegrated meld deposition method. The extruded and homogenized materials were characterized for their microstructural and mechanical properties. The following conclusions can be drawn from present work:

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1. The disintegrated meld deposition technique is a suitable method to incorporate carbonaceous reinforcement, GNPs during liquid state processing to fabricate AZ61-GNP composite with uniform dispersion of reinforcement particulates. 2. Microstructural characterization of synthesized materials revealed that addition of GNP into AZ61 alloy matrix has refined grain size and increased the basal texture intensities without formation of intermetallic phase between reinforcement and matrix. Furthermore, GNP helps to dissolve the Mg17Al12 intermetallic phases in composite matrix. 3. Room temperature mechanical characterization of materials revealed a significant increase in micro hardness, tensile and compression yield strengths with addition of GNP. Thus, GNP has high potential to improve mechanical properties of magnesium alloys. The increased mechanical properties of composite is attributed due to grain refinement, uniform dispersion of GNP, changes in basal texture intensity, and efficient load transfer from soft matrix

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to hard two dimensional GNPs. 4. High temperature deformation behavior of AZ61-GNP composite revealed that

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GNP led to enhanced softening with incredible increase in fracture strain of AZ61 matrix. This may be attributed due to complete recrystallization of composite matrix during high temperature testing. Furthermore, fracture mode of composite changes to ductile mode with increase in testing temperatures. Acknowledgments

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The present work was supported by the National Natural Science Funds of China (No. 50725413), the Ministry of Science and Technology of China (MOST) ( No. 2010DFR50010 and 2011FU125Z07), and Chongqing Science and Technology Commission (CSTC2013JCYJC60001). References

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[27] Sanaty-Zadeh A. Comparison between current models for the strength of particulate-reinforced metal matrix nanocomposites with emphasis on consideration of Hall–Petch effect. Materials Science and Engineering: A. 2012;531:112-8. [28] Tjong SC. Recent progress in the development and properties of novel metal matrix nanocomposites reinforced with carbon nanotubes and graphene nanosheets. Materials Science and Engineering: R: Reports. 2013;74:281-350. [29] Tsai J-L, Lu T-C. Investigating the load transfer efficiency in carbon nanotubes reinforced nanocomposites. Composite Structures. 2009;90:172-9. [30] Schadler LS, Giannaris SC, Ajayan PM. Load transfer in carbon nanotube epoxy composites. Applied Physics Letters. 1998;73:3842-4. [31] Thostenson ET, Li WZ, Wang DZ, Ren ZF, Chou TW. Carbon nanotube/carbon fiber hybrid multiscale composites. Journal of Applied Physics. 2002;91:6034-7. [32] Shin SE, Choi HJ, Shin JH, Bae DH. Strengthening behavior of few-layered graphene/aluminum composites. Carbon. 2015;82:143-51. [33] Fukuda H, Kondoh K, Umeda J, Fugetsu B. Fabrication of magnesium based composites reinforced with carbon nanotubes having superior mechanical properties. Materials Chemistry and Physics. 2011;127:451-8. [34] Fukuda H, Kondoh K, Umeda J, Fugetsu B. Interfacial analysis between Mg matrix and carbon nanotubes in Mg–6 wt.% Al alloy matrix composites reinforced with carbon nanotubes. Composites Science and Technology. 2011;71:705-9. [35] Pérez-Bustamante R, Gómez-Esparza CD, Estrada-Guel I, Miki-Yoshida M, Licea-Jiménez L, Pérez-García SA, et al. Microstructural and mechanical characterization of Al–MWCNT composites produced by mechanical milling. Materials Science and Engineering: A. 2009;502:159-63. [36] Shimizu Y, Miki S, Soga T, Itoh I, Todoroki H, Hosono T, et al. Multi-walled carbon nanotube-reinforced magnesium alloy composites. Scripta Materialia. 2008;58:267-70. [37] Tucho WM, Mauroy H, Walmsley JC, Deledda S, Holmestad R, Hauback BC. The effects of ball milling intensity on morphology of multiwall carbon nanotubes. Scripta Materialia. 2010;63:637-40. [38] Darsono N, Yoon D-H, Kim J. Milling and dispersion of multi-walled carbon nanotubes in texanol. Applied Surface Science. 2008;254:3412-9. [39] Habibi MK, Hamouda AMS, Gupta M. Enhancing tensile and compressive strength of magnesium using ball milled Al+CNT reinforcement. Composites Science and Technology. 2012;72:290-8. [40] Rashad M, Pan F, Guo W, Lin H, Asif M, Irfan M. Effect of alumina and silicon

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carbide hybrid reinforcements on tensile, compressive and microhardness behavior of Mg–3Al–1Zn alloy. Materials Characterization. 2015;106:382-9. [41] Li Q, Viereckl A, Rottmair CA, Singer RF. Improved processing of carbon nanotube/magnesium alloy composites. Composites Science and Technology. 2009;69:1193-9. [42] Rashad M, Pan F, Tang A, Asif M. Effect of Graphene Nanoplatelets addition on mechanical properties of pure aluminum using a semi-powder method. Progress in Natural Science: Materials International. 2014;24:101-8. [43] Wang Z, Song M, Sun C, He Y. Effects of particle size and distribution on the mechanical properties of SiC reinforced Al–Cu alloy composites. Materials Science and Engineering: A. 2011;528:1131-7. [44] Slipenyuk A, Kuprin V, Milman Y, Spowart JE, Miracle DB. The effect of matrix to reinforcement particle size ratio (PSR) on the microstructure and mechanical properties of a P/M processed AlCuMn/SiCp MMC. Materials Science and Engineering: A. 2004;381:165-70. [45] Ahmed S, Jones FR. Effect of particulate agglomeration and the residual stress state on the modulus of filled resin. Part II: Moduli of untreated sand and glass bead filled composites. Composites. 1990;21:81-4. [46] Maksoud IA, Ahmed H, Rödel J. Investigation of the effect of strain rate and temperature on the deformability and microstructure evolution of AZ31 magnesium alloy. Materials Science and Engineering: A. 2009;504:40-8. [47] Takuda H, Morishita T, Kinoshita T, Shirakawa N. Modelling of formula for flow stress of a magnesium alloy AZ31 sheet at elevated temperatures. Journal of Materials Processing Technology. 2005;164–165:1258-62. [48] Asif M, Tan Y, Pan L, Li J, Rashad M, Usman M. Thickness Controlled Water Vapors Assisted Growth of Multilayer Graphene by Ambient Pressure Chemical Vapor Deposition. The Journal of Physical Chemistry C. 2015;119:3079-89. [49] Rashad M, Pan F, Asif M, She J, Ullah A. Improved mechanical proprieties of “magnesium based composites” with titanium–aluminum hybrids. Journal of Magnesium and Alloys. 2015;3:1-9. [50] Deng K, Wang C, Wang XJ, Wu K, Zheng M. Microstructure and elevated tensile properties of submicron SiCp/AZ91 magnesium matrix composite. Materials and Design. 2012;38:110-14.

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Figure captions:

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Figure 1. X-ray diffraction patterns of as-extruded AZ61 alloy and AZ61-3GNP composite taken (a) in transverse direction and (b) in longitudinal direction of the samples.

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Figure 2. Pole figures 0002 , 1010 , and 1011 of as-extruded AZ61 alloy and

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AZ61-3GNP composite taken along cross section direction of the samples.

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Figure 3. Optical images showing grain morphology of as-extruded (a) AZ61 alloy and (b) AZ61-3GNP composite, SEM images of AZ61 alloy (c) and AZ61-3GNP composite (d) showing presence of second phases.

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Figure 4. Tensile (a) and compression (b) stress-strain curves of AZ61 alloy and it’s composite.

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Figure 5. Tensile fracture surface of extruded (a) AZ61 alloy and (b) AZ61-3GNP composite; and compressive fracture surface of extruded (c) AZ61 alloy and (d) AZ61-3GNP composite.

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Figure 6. SEM-EDS analysis of tensile fracture surface of extruded AZ61-3GNP composite (a); (b) magnesium, (c) aluminum, (d) zinc, (e) carbon (GNP), and (f) oxygen. Figure 7. Typical tensile stress-strain curves of AZ61-3GNP composite at different temperatures.

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Figure 8. (a) Effect of testing temperature on flow stress and fracture strain of AZ61-3GNP composite, and (b) comparison of experimental flow stress with theoretical flow stress. Figure 9. SEM images of tensile fracture surface of extruded AZ61-3GNP composites tested at (a) 25ºC, (b) 75ºC, (c) 150ºC, and (d) 225ºC.

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Figure 1. X-ray diffraction patterns of as-extruded AZ61 alloy and AZ61-3GNP composite taken (a) in transverse direction and (b) in longitudinal direction of the samples.

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Figure 2. Pole figures 0002 , 1010 , and 1011 of as-extruded AZ61 alloy and

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AZ61-3GNP composite taken along cross section direction of the samples.

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Figure 3. Optical images showing grain morphology of as-extruded (a) AZ61 alloy and (b) AZ61-3GNP composite, SEM images of AZ61 alloy (c) and AZ61-3GNP composite (d) showing presence of second phases.

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Figure 4. Tensile (a) and compression (b) stress-strain curves of AZ61 alloy and it’s composite.

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Figure 5. Tensile fracture surface of extruded (a) AZ61 alloy and (b) AZ61-3GNP composite; and compressive fracture surface of extruded (c) AZ61 alloy and (d) AZ61-3GNP composite.

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Figure 6. SEM-EDS analysis of tensile fracture surface of extruded AZ61-3GNP composite (a); (b) magnesium, (c) aluminum, (d) zinc, (e) carbon (GNP), and (f) oxygen.

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Figure 7. Typical tensile stress-strain curves of AZ61-3GNP composite at different temperatures.

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Figure 8. (a) Effect of testing temperature on flow stress and fracture strain of AZ61-3GNP composite, and (b) comparison of experimental flow stress with theoretical flow stress.

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Figure 9. SEM images of tensile fracture surface of extruded AZ61-3GNP composites tested at (a) 25ºC, (b) 75ºC, (c) 150ºC, and (d) 225ºC.

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Table captions:

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Table 1. Texture results of AZ61 alloy and its composite based on X-ray diffraction. Table 2. Room temperature mechanical properties of AZ61 alloy and it’s composite.

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Table 3. High temperature tensile properties of AZ61-3GNP composite.

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Table 1. Texture results of AZ61 alloy and its composite based on X-ray diffraction. Samples Section Plane I/Imax AZ61 magnesium Transverse 1010 Prism 1.000 alloy

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AZ61-3GNP composite

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1011Pyramidal

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1010 Prism

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0002 Basal

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1011Pyramidal

1.000

1010 Prism

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0002 Basal

0.093

1011 Pyramidal

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1010 Prism

0.220

0002 Basal

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Imax is maximum intensity from either prism, basal or pyramidal planes.

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Table 2. Room temperature mechanical properties of AZ61 alloy and it’s composite Compressive properties 0.2% Ultimate Yield compressi strength ve (MPa) strength (MPa) 170±5.1 461±6.8

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Tensile properties 0.2% Ultimat Fractur Yield e e strain strength tensile (%) (MPa) strength (MPa) AZ61 Alloy 75.7± 184±5. 300±7. 11.5±1. 2.5 5 1 9 AZ61-3GN 87.5± 232±4. 335±6. 10.7±2. P 1.8 9 6 1 (~15.5 (~26%) (~11.7 %) %)

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226±4.7 (~32.9%)

480±5.6 (~4.12%)

Fractu re strain (%) 16.7± 2.1 15.1± 3.5

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Table 3. High temperature tensile properties of AZ61-3GNP composite. Tensile properties Ultimate tensile strength (MPa) 335±9.1 318±7.5 280±8.9 197±8.5

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Temperatur e (°C)

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025 075 150 225

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0.2% Yield strength (MPa) 232±5.5 212±7.1 194±4.9 168±5.8

Fracture strain (%) 10.7±5.5 17.4±6.1 38.1±5.9 68.1±6.9

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GNP stretching under tensile loading in AZ61-GNP composite

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Graphical abstract

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Highlights  GNP reinforced AZ61 magnesium alloy was synthesized by DMD method.  GNP addition leads to grain size refining and changing in basal textures.  Room temperature strength was increased with addition of GNP.  Fracture strain increases and tensile YS decreases with increasing testing temperatures.