Tensile and Compressive Responses of Ceramic and Metallic Nanoparticle Reinforced Mg Composites

In the present study, room temperature mechanical properties of pure magnesium, Mg/ZrO2 and Mg/(ZrO2 + Cu) composites with various compositions are investigated. Results revealed that the use of hybrid (ZrO2 + Cu) reinforcements in Mg led to enhanced mechanical properties when compared to that of single reinforcement (ZrO2). Marginal reduction in mechanical properties of Mg/ZrO2 composites were observed mainly due to clustering of ZrO2 particles in Mg matrix and lack of matrix grain refinement. Addition of hybrid reinforcements led to grain size reduction and uniform distribution of hybrid reinforcements, globally and locally, in the hybrid composites. Macro- and micro- hardness, tensile strengths and compressive strengths were all significantly increased in the hybrid composites. With respect to unreinforced magnesium, failure strain was almost unchanged under tensile loading while it was reduced under compressive loading for both Mg/ZrO2 and Mg/(ZrO2 + Cu) composites.


Introduction
Magnesium is the lightest metallic construction material which is ~35% lighter than aluminum. Owing to its low density, magnesium offers high specific mechanical properties. In addition, magnesium has other favorable advantages including high damping capacity, high dimensional stability, good machinability, good electromagnetic shielding characteristics and recyclability. With these beneficial properties, magnesium becomes an attractive material for manufacturing lighter components/products for diverse applications. In practical and commercial applications, magnesium is mostly used in the form of alloys [1,2]. With the advent of composite technology, researchers have also made extensive studies on the development of high performance magnesium composites. Magnesium composites that were synthesized mostly contained micron-sized particles comprising of ceramic reinforcements such as carbides, oxides, nitrides and borides and metallic reinforcement such as Ti, Cu and Ni [3][4][5][6]. While the strengths in magnesium composites can be improved using micron particles, the ductility was inevitably decreased. Previous studies [7][8][9][10][11] on magnesium nanocomposites showed the ability of nano particulate reinforcements on enhancing strength and/or ductility of magnesium. Commonly, the nanocomposites are synthesized using single ceramic [7][8][9] or metal reinforcements [10,11]. Reviews of work on particle reinforced magnesium based nanocomposites can be found in recent publications by Dieringa [12] and Ferguson et al. [13]. In addition, recent investigations were also made on the magnesium composites containing hybrid particle reinforcements [14][15][16][17][18][19][20][21]. Hybrid reinforcements were prepared by using different combinations such as "ceramic + ceramic", "ceramic + CNT" and "ceramic + metal" in Mg matrix. Among those combinations, as reported in the previous investigations [12][13][14][15][16][17][18][19][20][21], "ceramic + metal" hybrid reinforcements in Mg matrix offered the best improvement in mechanical properties in the related composite systems.
Accordingly, the present study focused on the synthesis of magnesium composites using single (ZrO 2 ) and hybrid (ZrO 2 + Cu) reinforcements in nano length scale. The aim is to investigate the effect of addition of nano ZrO 2 and different combination of hybrid reinforcements on the mechanical properties of pure magnesium. Materials synthesis was carried out using the microwave assisted powder metallurgy route. Characterizations on microstructure, hardness, tensile and compressive properties were done on the extruded samples. Particular emphasis was placed to study the effect of single and hybrid reinforcements on the variation in microstructure and mechanical properties of magnesium. Furthermore, the use of different extrusion ratio on the properties of Mg/ZrO 2 composite was also investigated.

Grain Size and Reinforcement Distribution
Grain size measurement results from Table 1 show that there was no significant change in grain size when ZrO 2 reinforcement was present in Mg matrix regardless of the amount of the reinforcement being added. It was realized that, especially for fine particle reinforced composites, grain refinement can only be achieved by using a sufficiently high amount of the reinforcement particles in the matrix material [22]. However, it can be noticed from the current results that matrix grain refinement not only depends on the amount of the reinforcement but also on the particle distribution (Table 1 and Figure 1).
When the ZrO 2 content increased from 0.3 to 1.0 vol % in Mg matrix, the reinforcing particles tend to form larger clusters/agglomerates maintaining the same distribution pattern without dispersing them throughout the matrix (Figure 1a,b). This indicates that the uniformity of particle distribution is insufficient to provide the grain size reduction in all Mg/ZrO 2 compositions. Consequently, the grain size in Mg/ZrO 2 composites remained the same when compared to that of Mg. Also, applying high extrusion ratio (from 20.25:1 to 26:1) has no effect on the grain size variation showing the similar grain size in case of Mg/1.0ZrO 2 composite. But the use of increased extrusion ratio provided better reinforcement distribution as indicated by the smaller space between the clustered nanoparticles shown in Figure 1c. In case of Mg/(ZrO 2 + Cu) hybrid composites, a significant reduction in grain size was observed. The reduction was about one third when compared to that of pure Mg and Mg/ZrO 2 composites. This indicates the usefulness of copper as hybrid reinforcement assisting in the matrix grain refinement. Having limited solid solubility in magnesium, the presence of copper causes Mg 2 Cu intermetallics formation [10,16,23]. The existence of more obstacles (ZrO 2 , Cu and Mg 2 Cu intermetallics) can effectively pin the grain boundary which contributes to the grain size reduction in the hybrid composites. Unlike Mg/ZrO 2 composite, reinforcement distribution was globally finer with increasing presence of second phases in Mg/(ZrO 2 + Cu) composites as observed in the micrograph ( Figure 1). The morphology of the Mg/ZrO 2 material shows a more or less continuous film of clustered/agglomerated ZrO 2 nanoparticles at the grain boundaries, whereas in the Cu-containing composite the ZrO 2 film is discontinuous and punctuated with Cu particles. In case of Mg/ZrO 2 composite, the ZrO 2 particles were mostly in the clustered/agglomerated form with minimal particle dispersion within the matrix (Figure 2a).  Figure 3 shows the X-ray diffraction patterns of Mg and Mg composites. In case of Mg/1.0ZrO 2 composites, an additional peak of ZrO 2 was revealed when compared to Mg and Mg/(ZrO 2 + Cu) composites. In case of Mg/(ZrO 2 + Cu) composites, the peak related to ZrO 2 was not present. However, the peaks matching to Cu and Mg 2 Cu were found in the XRD patterns. From the related studies [9,24], the peaks related to the ceramic reinforcement particles which are in nano length scale do not generally appear in the composites. This is due to either the amount of reinforcement being too small (less than 2 vol %) to be detected by the XRD diffractometer or the presence of fine  formly distr dies [25,26]

Hardness
Hardness measurement results in Table 1 show a minimal change in both macro-and micro-hardness values in the case of pure Mg and Mg/ZrO 2 composites. Results suggest that the presence of single ZrO 2 reinforcement is ineffective in increasing hardness of Mg matrix. This may primarily be attributed to poor reinforcement distribution of ceramic reinforcements. To achieve high hardness level in the composites, clustering of second phases is not preferable due to a number of facts including larger intercluster spacing and the weak bonding among brittle ceramic particulates at the clustered particles region. No variation in hardness can also be attributed to the similar grain size between Mg and Mg/ZrO 2 composites. On the other hand, an improvement in both macro-and micro-hardness was observed in the hybrid composites when compared to both Mg and Mg/ZrO 2 composites. This may primarily be attributed to the grain refinement resulting from the distribution of second phases (Table 1 and Figure 1). The presence of hard Mg 2 Cu intermetallics can additionally provide an increase in hardness level of Mg matrix [27].

Tensile Properties
The results of room temperature tensile properties are shown in Figure 4 and summarized in Table 2. Not only the presence but also the increasing amount of ZrO 2 reinforcement shows no effect on the variation of 0.2% yield and ultimate tensile strengths showing similar strength levels of pure Mg and composite samples. In fact, the strengths in Mg/0.3ZrO 2 and Mg/1.0ZrO 2 composites tend to decrease when compared to pure magnesium. Presence of clustered ZrO 2 particulates in Mg matrix (Figures 1a,b and 2a) could be the prime reason for this decrement. The reduction in strengths originates from weak adhesion among ceramic particulates within clusters and/or between matrix and clustered particulates [4,28]. This also indicates the lack of load transfer from the matrix to ceramic reinforcement clusters and thus yielding in the Mg/ZrO 2 composites follows the matrix yielding behavior that contains defects. The use of high extrusion ratio (20.25:1 to 26:1) provides improvement in both 0.2% yield and ultimate tensile strengths to some extent. This is in line with the improved reinforcement distribution in Mg/1.0ZrO 2 composite with the use of higher extrusion ratio, 26:1 ( Figure 1c). However, a significant strength increment was not achieved due to partial break down of reinforcement clusters [29] and lack of grain refinement after applying high extrusion ratio (26:1) in Mg/1.0ZrO 2 composition. Expecting to acquire better tensile properties, Cu was added as hybrid reinforcement to Mg/ZrO 2 compositions. To fix the total reinforcement amount to be 1 vol %, 0.7 vol % and 0.4 vol % Cu were added to the Mg/0.3 vol % ZrO 2 and Mg/0.6 vol % ZrO 2 compositions, respectively. From the tensile test results (Table 2), a significant improvement in both 0.2% yield and ultimate tensile strengths were observed in the hybrid composites. The improvement in strengths can mainly be attributed to: (a) the combined presence of ZrO 2 , Cu and additional Mg 2 Cu intermetallics; (b) effective load transfer from matrix to the reinforcements/second phases conforming uniform reinforcement/second phase distribution; (c) grain refinement as indicated earlier in Table 1. The reduction in grain size is primarily responsible for strength improvement in magnesium hybrid composites. Additionally, the strength increment may be supported by the effective load transfer mechanism. Irrespective of enhanced tensile strengths shown by the hybrid composites, failure strain is similar to that of pure Mg and Mg/ZrO 2 composites.

Tensile Fractography
Tensile fractographs can be seen in Figure 5. From the failure analysis, the same fracture features revealing rough fracture surfaces and cleavage fracture were observed in pure Mg and Mg/ZrO 2 composites. Owing to HCP crystal structure, magnesium's deformability is limited due to the lack of sufficient slip activity. Although the size of clustered reinforcements was in micron length scale, premature failure was not observed in the Mg/ZrO 2 composites observing similar failure strain when compared to magnesium ( Table 2). This is different from micron size particle reinforced magnesium composites in which failure strain reduction was commonly found when compared to that of unreinforced magnesium. The premature failure in these composites is mainly due to the debonding between the matrix and reinforcement particles, and rapid particle cracking [4]. In the current study, there were two possibilities for Mg/ZrO 2 composite eventual failure: (a) the matrix failure could control the fracture behavior in the composites observing the similar fracture features and (b) failure due to intergranular cracking initiated from micro-cracking of the reinforcement clusters at the grain boundary (Figure 5b). In case of hybrid composite, fine and homogenous fracture features were

Materia
The mate ranging from ZrO 2 ) of 99 with an ave were used.

Process
The

Microstructure Characterization
Microstructural characterization studies were conducted to determine the grain size and distribution of reinforcements. OLYMPUS metallographic optical microscope, Scion Image Analyzer and HITACHI S-4300 Field Emission Scanning Electron Microscope (FESEM) equipped with energy dispersive X-ray spectroscopy (EDS) were used for this purpose.

X-ray Diffraction Studies
X-ray diffraction analysis was carried out on the polished extruded Mg and Mg composite samples using automated Shimadzu LAB-X XRD-6000 diffractometer. The samples were exposed to CuK α radiation (λ = 1.54056 Å) at a scanning speed of 2 °C/min. The Bragg angle and the values of the interplanar spacing (d) obtained were subsequently matched with the standard values for Mg, ZrO 2 , Cu and related phases.

Mechanical Testing
The mechanical behavior of both monolithic and composite samples was quantified in terms of hardness, tensile and compressive properties. Macrohardness measurements were made using Future-Tech FR-3 Rockwell Type Hardness Tester. The test was conducted under conditions of 2 s dwell time and a test load of 15 kgf using steel ball indenter (1.588 mm) in accordance with ASTM E18-02. Microhardness measurements were performed on the magnesium matrix of the polished samples using Shimadzu-HMV automatic digital microhardness tester. The test was done using a Vickers indenter under a test load of 25 gf and a dwell time of 15 s in accordance with the ASTM standard E384-99.
The tensile properties of the as-extruded monolithic magnesium and its composite counterparts were determined in accordance with procedures outlined in ASTM standard E8M-01. The tensile tests were conducted on round tension test specimens (5-mm gage diameter and 25-mm gage length) on MTS 810 automated servo-hydraulic mechanical testing machine at a crosshead speed set at 0.254 mm/min.
Compression tests were performed on cylindrical monolithic and composite samples according to ASTM E9-89a using MTS 810 automated servo-hydraulic mechanical testing. Extruded rod of 8 mm diameter was cut into 8 mm length samples for compression tests to provide the aspect ratio (l/d) of unity. Samples were tested at a strain rate of 5 × 10 −3 min −1 and the compression load was applied parallel to the extrusion direction.

Fracture Behavior
To investigate the failure mechanism during tensile and compressive loadings, the tensile and compressive fractured surfaces of pure Mg and its composite specimens were characterized using scanning electron microscope (SEM).

Conclusions
The major conclusions are as follows: 1. Magnesium based nanocomposites and hybrid composites can be successfully synthesized using microwave sintering assisted powder metallurgy approach. 2. The formation of ZrO 2 reinforcement clusters in Mg matrix resulted in the lack of grain refinement in all Mg/ZrO 2 composite compositions. On the other hand, the use of hybrid reinforcements (ZrO 2 + Cu) realized significant grain size reduction in magnesium hybrid composites. 3. Both macro-and micro-hardness, tensile, and compressive strengths were improved only in the Mg/(ZrO 2 + Cu) hybrid composites primarily as a result of their finer grain size. 4. Tensile failure strain remained the same whereas compressive failure strain was decreased in all composites when compared to pure Mg. 5. Tensile failure analysis revealed the similar rough fracture features in the case of Mg and Mg/ZrO 2 composites. Refined fracture features were observed in the hybrid composite. 6. Under compressive loading, the presence of shear bands was minimally observed from the fracture surfaces of Mg and Mg/ZrO 2 composites. In case of hybrid composite, a large amount of shear band formation was observed.