Exploring mechanical behavior of Mg–6Zn alloy reinforced with graphene nanoplatelets

Author’s Accepted Manuscript Exploring mechanical behavior of Mg-6Zn alloy reinforced with graphene nanoplatelets Muhammad Rashad, Fusheng Pan, Muhammad Asif www.elsevier.com/locate/msea

PII: DOI: Reference:

S0921-5093(15)30472-X http://dx.doi.org/10.1016/j.msea.2015.10.009 MSA32860

To appear in: Materials Science & Engineering A Received date: 11 August 2015 Revised date: 24 September 2015 Accepted date: 3 October 2015 Cite this article as: Muhammad Rashad, Fusheng Pan and Muhammad Asif, Exploring mechanical behavior of Mg-6Zn alloy reinforced with graphene n a n o p l a t e l e t s , Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2015.10.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Exploring mechanical behavior of Mg-6Zn alloy reinforced with graphene nanoplatelets Muhammad Rashad a,b,*, Fusheng Pan a,b,c,*, Muhammad Asif d a

College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China

b

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

c

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

d

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

*Corresponding authors: [email protected] (M.Rashad); [email protected] (F.S.Pan) (Tel.: +86-23-65112635; fax: +86-23-67300077) Abstract: Graphene and its derivatives have been extensively used as reinforcing agents owing to their high mechanical properties. In present work, an attempt is made to synthesize graphene nanoplatelets (GNPs) reinforced Mg-6Zn alloy using disintegrated melt deposition method. The effects of GNPs on microstructural and mechanical properties of alloy were investigated. The microstructural analysis revealed uniform dispersion of GNPs throughout composite matrix along with refined grain size. The results of strength measurements indicated that addition of GNPs lead to increase in micro hardness, tensile and compression strengths. The increased strength of synthesized composites may be attributed to grain refinement, uniform dispersion of GNPs, changes in basal textures and basic strengthening mechanisms. Moreover, comparison of synthesized composites with Mg-6Zn-CNTs composites revealed that GNPs have high potential to replace CNTs because GNPs are 4-6 times cheaper than CNTs. Keywords: A: Metal matrix composites, B: Mechanical properties, E: Disintegrated melt deposition method. 1. Introduction Magnesium and its alloys have been extensively investigated owing to their low density, high specific strength, and high damping capacities and easy recycle [1]. Due to ever increasing demand of lightweight materials in automotive and aerospace industries, mechanical properties of magnesium alloys were further improved by addition of reinforcement particles i.e. SiC, Al2O3, Ti, and CNTs [2-5]. Among these reinforcements, increasing consideration inclining to the addition of CNTs is more noticeable owing to their high mechanical strength [6]. During last decades, CNTs have been widely used to enhance the mechanical strength of pure Mg [7], Mg-6Zn alloy [8, 9], and AZ31/AZ81 alloys [10, 11]. However, an important issue of CNTs in metal matrix is their uniform distribution (due to strong Vander Waals forces). Although, many efforts [12, 13] have been made to achieve uniform dispersion of CNTs in composite matrix, still uniform dispersion of CNTs in matrix is big challenge for researchers, which forbid its practice in practical applications.

Recently, graphene has gained great research attention in scientific community owing to its excellent mechanical properties [14, 15]. Graphene nanoplatelets (GNPs) consist of 20-40 sheets of graphene with average diameter of 5-10µm. GNPs have high specific surface area and are very cheap as compared to CNTs. In past years, several researchers synthesized graphene nanoplatelets reinforced polymer composites [15], and works on graphene nanoplatelets reinforced metals have been explored rarely [16-21]. To the best of our knowledge, open literature reports so far suggested that no attempt is made to investigate the microstructure and mechanical properties Mg-6Zn alloys reinforced with GNPs. The aim of this work is to synthesize the GNPs reinforced Mg-6Zn alloys using disintegrated melt deposition (DMD) method. The synthesized composites were characterized for their microstructural and mechanical characteristics. 2. Experimental procedures Reinforcements, Graphene nanoplatelets (GNPs) with thickness and average diameter of 15-20nm and 5-10µm respectively, were purchased from Chengdu Organic Chemistry Co. Ltd., China. Pure Mg ingot and Zn granular were used as matrix materials. The Mg alloy and its composites were synthesized through DMD method. About 1300 grams Mg ingot was melted at 720°C under CO2-SF6 protective atmosphere in a graphite crucible using an electrical resistance furnace. About 78 gram of granular Zn was added into molten slurry. In next step, GNPs powder was added into Mg-6Zn alloy molten slurry and mixture was stirred for 0.5 min. After stirring, mixture was reheated to 720°C and kept at 720°C for 20 minutes. The molten mixture was poured into a preheated steel mold (300°C) and allowed to solidify normally. The casted billets were cut into Φ82mm 45mm samples and homogenized at 350°C for 12 hours. Homogenized billets were hot extruded using a 500T hydraulic press at 300°C with an extrusion ratio of 5.2:1 to obtain the rods of 16mm diameter. The RAM speed was set at 1m/min.

ⅹ

3. Characterizations Samples with Ф10×5mm dimensions were machined from extruded rods. Acidic solution was used to polish samples for microstructural characterizations. 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 in a 2θ range of 10-90°. The microstructures of composites were investigated using optical microscopy (OM), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Vickers hardness, tensile and compression tests were used to evaluate the mechanical properties of the synthesized composites. 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 1×10-3s-1. A minimum of five tests were conducted for each composition to obtain an average value. SEM was used to analyze the tensile and compression fracture images. 4. Results and discussions 4.1. Microstructure The microstructures of synthesized composites with different contents of GNP are shown in Figure 1. It can be observed clearly that primary α-Mg phase is efficiently refined with addition of GNPs. The grain size of Mg-6Zn alloy measure using linear intercept method show an average value of 19µm as shown in Figure 1(a). Figure 1(c-d) shows optical images of composite samples containing 0.5 and 1.5wt.% GNPs. Increasing GNP’s contents has reduced grain size (from 19 to 4µm) and increased the uniformity of grains. The composites with 0.5wt.% and 1.5wt.% GNP present finer and uniform microstructure with average grain size of 9 and 4µm respectively. Hence, GNP addition is able to contribute to the grain refinement in Mg-6Zn alloys. Figure 2 shows the SEM images of pristine alloy and its composites. It can be noticed that composite’s surfaces show the absence of microspores, blowholes and shrinkage voids which witness potential of processing parameters adopted during DMD method. The surface morphology of pristine alloy is shown in Figure 2(a). It can be observed those Mg-Zn phases (white precipitations) are present on the grain boundaries. The size of these Mg-Zn phases is several hundred nanometers. The surface morphology observations of composite samples revealed a great difference in the distribution of Mg-Zn phases which may be attributed to refined grain size as shown in Figure 2(b-c). It means that increasing GNP contents may affect the distribution of Mg-Zn phases. Figure 3 shows energy dispersive spectroscopic analysis of extruded Mg-6Zn-1.5GNPs composite surface. It can be observed that GNPs are uniformly dispersed throughout the composite matrix. Excellent bonding was observed between GNPs and Mg matrix without formation of gaps and voids. This is because wettability of GNP with molten Mg-6Zn alloy is excellent compared to the wettability of GNP with pure Mg particles processed by powder metallurgy methods [16]. The macroscopic observations of the materials revealed the absence of macro-pores, blowholes and shrinkage cavities, which indicated that the optimization of process parameters during DMD process had provided sufficient fluidity to the melt, resulting in defect-free castings. 4.2. Mechanical properties Vickers hardness results of as-extruded alloy and its composites measured parallel and perpendicular to extrusion directions are shown in Figure 4. It can be noticed that hardness of alloy increases with addition of GNPs and increase in GNP’s contents when measured perpendicular to extrusion directions (Transverse). The Mg-6Zn-1.5GNPs composite exhibited highest value of hardness (75.8) with +43.3%

improvements when compared to pure Mg-6Zn alloy. This improvement maybe attributed to grain refinement and inherent high strength of GNPs particles. These results are consistent with previous studies where GNPs may contribute towards higher hardness of metal matrix composites [4, 5, 19]. Hardness results of as-extruded alloy and its composites show different trend in longitudinal directions. It can be observed that hardness of alloy increases till the threshold of 0.5wt.% GNP is reached. With further increase in GNP’s contents (1.5wt.%), the hardness decreases. The decrease in hardness values of Mg-6Zn-1.5GNP composite may be due to inhomogeneous distribution of Mg-Zn phases as shown in Figure 2(c). Furthermore, it is interesting to note that hardness values (longitudinal directions) of Mg-6Zn alloy and Mg-6Zn-0.5GNP composite are about +18% and +19% higher compared to those in transverse directions. The possible reason for such difference is not known due to limited studies in this field. Room temperature tensile and compression properties of as-extruded alloy and its composites are shown in Figure 5 and Table 1. It can be seen that tensile yield strength (TYS), ultimate tensile strength (UTS) and fracture strain (FS) increases with increase in GNP’s contents. The Mg-6Zn-1.5GNP composite exhibited significant improvement in TYS (+34.5%) and FS (+23.5%) when compared to pure Mg-6Zn alloy. The results of compression tests showed different trend. The compressive yield strength (CYS) and ultimate compressive strength (UCS) increases while fracture strain (FS) decreases with increase in GNP’s contents. The Mg-6Zn-1.5GNP composite revealed significant improvement in CYS (+56.8%) when compared to pure Mg-6Zn alloy. The room temperature mechanical tests reveal significant improvement in yield strengths due to existence of GNP particles. Previous reports on magnesium based composites have revealed that nanoparticles randomly distributed in matrix would pinch the grain boundaries and serve as grain nucleation catalysts [22]. Thus, improvement in mechanical properties of composites may be attributed to grain refinement as shown by Hall-Petch relation [23]. Besides, coefficient of thermal expansion mismatch between matrix and GNPs may results in multidirectional thermal stress at matrix/GNP interface and induce dislocation densities which results in increased strength of composites. The dislocation density increases with the increase of particulates contents [24], therefore Mg-6Zn-1.5GNP composite exhibits high yield strength. The dislocation density due to coefficient of thermal expansion is given by [25]:

ρ=

A × VGNP × ( ∆CTE × ∆T ) b (1 − VGNP ) × t

(1)

where value of A is 8 for platelets, VGNP is volume fraction of GNPs, and t is smallest dimension of GNPs; t = 2rGNP . ∆CTE is difference between coefficient of thermal expansion of matrix (25×10-6 K-1) and GNPs (1×10-6 K-1) [17].

∆T is

difference between extrusion and testing temperatures. The increase in yield strength of composites due to dislocation densities is given by [25]:

∆σ = αµ b ρ

(2)

where α is geometric constant (its value is 1.25), µ is shear modulus of matrix and b is burger vector. According to phase diagram, there is no chemical reaction between Mg and carbon (GNPs). Therefore, GNPs are not soluble in Mg, thus in composite matrix, the matrix/reinforcement particle interface would be governed by mechanical bonding instead of chemical bonding. When material is subjected to external loading (Figure 6), load is transferred from soft magnesium matrix to hard reinforcement particles through load transfer mechanism [26]. This may lead to increase in strength of Mg-6Zn-GNP composites. It can be observed that tensile yield strengths of alloy and its composites are higher as compared to compressive yield strengths as shown in Table 1. Thus, all materials exhibit tension-compression yield asymmetry. The tension-compression yield asymmetry values of developed materials are shown in Figure 7. It can be noticed that yield asymmetry decreases with addition of GNP particles. Previous studies indicate that under tensile loading, high yield strength is due to activation of non-basal slip systems. Hence, tensile deformation behavior is slip dominant [27, 28]. However, in case of compression tests, stress-strain curves exhibit an upward concave profile after yield. This is attributed to twining in magnesium composites and due to crystallographic texture changes [29, 30]. In order to understand the yield behavior of pure Mg-6Zn alloy and its composites, x-ray diffraction analysis were conducted on the cross sections as shown in Figure 8 and Table 2. It can be noticed that dominant Mg peak are present in all materials. However, peaks corresponding to GNPs and Mg-Zn intermetallic phases could not be traced in all samples which might be attributed to relatively volume fraction of GNPs and Zn. Generally, reinforcement particle may affect the basal texture of magnesium crystal. Therefore, changes in basal texture of composites may confirm the existence of GNPs. The peaks at 2θ

{ }

{ }

equal to 32, 34, and 36º represents prism 1010 , basal {0002} , and pyramidal 1011

planes of hexagonal closed packed crystal structure of magnesium. It can be noticed that all samples show maximum prismatic plane intensity and basal plane intensity increases with increasing GNPs weight fractions. It means that addition of GNPs into alloy matrix have changed orientation of magnesium crystals and this change in orientation or crystallographic texture could be responsible for decrease in tension-compression yield asymmetry as shown in Figure 7. This is because addition of GNPs into alloy matrix resulted in crystallographic texture changes, which would toughen the ease of nucleation and growth of tensile twins. Thus, contribute towards increase in yield strength for plastic deformation. In general, size and shape of reinforcement particles plays critical role in

enhancing the mechanical properties of composites [31-33]. In case of carbonaceous reinforcements (i.e. CNTs and Graphene) morphology of inclusion particles also affect the resulting properties of composites. In 2011, Esawi et al. [34] reported the effect of multiwalled carbon nanotubes (MWCNTs) morphology on mechanical behavior of pure aluminum matrix. Short and long MWCNTs were dispersed in aluminum matrix using ball milling techniques followed by consolidation and extrusion. Experimental results revealed that long MWCNTs (with smaller diameters) bent and entangled. Therefore, more difficult to mix with increase in CNTs content compared to short MWCNTs (with large diameters), thus affect the mechanical properties of resulting composites. In another report, two different dimensions of graphene nanoplatelets with flake size of 5 and 25 microns were used to examine the effect of size on polymer composite properties [35]. Fracture toughness and three-point bending testing results revealed that larger flakes succeed in better reinforcement of the composite. Although, CNTs and graphene nanoplatelets (GNPs) have same molecular structure except their morphology. CNTs are one dimensional with thread like structures and GNPs are two dimensional with platelet morphology. Which of these materials is best to enhance the mechanical properties of composite matrix, Rafiee et al. [36] have synthesized CNTs and GNPs reinforced epoxy composites. Tensile and fracture toughness testing results proved that low content of GNPs outperform CNTs. This may be attributed to the good adhesion of GNPs with epoxy matrix, and two dimensional nature of GNPs. However, in case of metal matrix composites there is lack of such investigations. In present work, Mg-6Zn alloy is reinforced with GNPs using DMD method which involves mechanical stirring for only 0.5 min. Literature review revealed that mechanical properties of Mg-6Zn reinforced with CNTs are comparable to that of Mg-6Zn-GNP synthesized in present work [8]. However, synthesize of CNTs reinforced composites involve addition processes i.e. mechanical stirring for about 10 min and in addition ultra-sonication process at 500 W power level for 20 min, for uniform dispersion of CNTs. In addition, if we compare the cost of GNP and CNTs, we found that GNPs are five times cheaper than CNTs. Thus, it can be concluded that GNPs have high potential to replace the CNTs in metal matrix composites [19].

4.3. Fracture analysis Tensile fracture images of Mg-6Zn alloy and its composites are depicted in Figure 9(a-c). It can be observed that fracture surface of alloy exhibits a lot of cleavage steps, which support the observed fracture strain values. However, fracture images of composites revealed a lot of dimples and tear ridges which witness the increased tensile fracture strain values. The ductile fracture of composite samples is attributed due to uniform dispersion of GNP throughout the matrix [37, 38]. The compression fracture images of Mg-6Zn alloy and its composites showed formation of shear bands (Figure 9(d-f). It can be noticed that all samples show fracture at about 45º with respect to compression loading axis except Mg-6Zn-1.5GNP composite. The deformation behavior of magnesium alloy and its composites is governed by twining

at shown by shear bands.

5. Conclusions Disintegrated melt deposition method was successfully used to fabricate Mg-6Zn alloy reinforced with graphene nanoplatelets (GNP). Microstructural characterization revealed that addition of GNP serve as grain refiner for magnesium alloy. Scanning electron microscopic results showed uniform dispersion of GNP into composite matrix. Mechanical characterization showed that microhardness of alloy measured on cross sections showed an increase with increase in GNP contents. However, hardness measured along longitudinal direction showed increase till the threshold of 0.5wt.% GNP is reached. In addition, hardness values along longitudinal directions are about 18% higher compared to cross sectional values. Tensile testing revealed increase in yield strength, ultimate strength and ductility with addition of GNPs. On the other hand compression behavior showed that yield strength, ultimate compression strength increases while fracture strain decreases with increase in GNP contents. The increased mechanical properties of composites are attributed due to grain refinement, uniform dispersion of reinforcement particles, changes in basal texture, and basic strengthening mechanisms.

6. Acknowledgment 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).

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Figure 1. Optical microscopic images of as extruded (a) Mg-6Zn alloy, (b) Mg-6Zn-0.5GNPs, and (c) Mg-6Zn-1.5GNPs composites.

Figure 2. SEM images of as extruded (a) Mg-6Zn alloy, (b) Mg-6Zn-0.5GNPs, and (c) Mg-6Zn-1.5GNPs composites.

Figure 3. Energy dispersive spectroscopy (EDS) of extruded Mg-6Zn-1.5GNPs composite, showing dispersion of Zn and GNPs in the composite matrix.

Figure 4. Vickers hardness testing results of as-extruded Mg-6Zn alloy and its composites, measured in transverse and longitudinal directions.

Figure 5. Room temperature tensile (a) and compression (b) stress-strain curves of as-extruded Mg-6Zn alloy and its composites.

Figure 6. Sketch diagram showing load transfer from soft matrix to reinforcement particles during external loading.

Figure 7. Tension-compression yield asymmetry of synthesized materials.

Figure 8. XRD of as extruded (1) Mg-6Zn alloy, (2) Mg-6Zn-0.5GNPs, and (3) Mg-6Zn-1.5GNPs composites.

Figure 9. Tensile and compression fracture images of as extruded (a) Mg-6Zn alloy, (b) Mg-6Zn-0.5GNPs, and (c) Mg-6Zn-1.5GNPs composites.

Table 1. Room temperature mechanical properties of as-extruded Mg-6Zn alloy and its composites. Materials

Tensile properties

Compressive properties

0.2% Yield

Ultimate

Fracture

0.2%

strength

tensile

strain (%)

strength

Yield

Ultimate compressive

(MPa)

strength

(MPa)

strength (MPa)

Fracture strain (%)

(MPa) Mg-6Zn

159±5

276±7.0

17±1.5

109±4.5

426±6.1

21±1.7

Mg-6Zn-0.5GNP

171±4

295±3.5

18±1.9

131±5.1

435±5.4

20±2.0

Mg-6Zn-1.5GNP

214±2

313±5.2

21±1.1

171±2.4

448±4.7

16±2.9

Table 2. Texture results of Mg-6Zn alloy and its composite based on X-ray diffraction. Samples Mg-6Zn

Mg-6Zn-0.5GNP

Mg-6Zn-1.5GNP

Plane

I/Imax

{1010} Prism

0.800

{0002} Basal

0.018

{1011}Pyramidal

1.000

{1010} Prism

1.000

{0002} Basal

0.039

{1011}Pyramidal

0.930

{1010} Prism

1.000

{0002} Basal

0.045

{1011}Pyramidal

0.285

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