Microstructure and Mechanical Properties of Ultrafine Structured Al-4wt%Cu-(2.5-10) vol.%SiC Nanocomposites Produced by Powder Consolidation Using Powder Compact Extrusion

Ultrafine structured Al-4wt. %Cu(2.5-10) vol. % SiC nanocomposites were produced by high energy mechanical milling of a mixture of Al and Cu powders and SiC nano-powder to produce nanocomposites powders, followed by consolidation of the powders using powder compact extrusion (PCE). Scanning and transmission electron microscopy as well as tensile testing were used to characterize the extruded nanocomposite bars. Increasing the volume fraction of SiC nanoparticles from 2.5 to 5.0 causes the yield strength, ultimate tensile strength and microhardness of the nanocomposite to increase from 98 MPa, 168 MPa and 104 HV to 391 MPa, 400 MPa and 153 HV, showing the effectiveness of SiC nanoparticles and microstructural refinement in strengthening the material. However, the ductility decreases from 6.8% to 2%, possibly due to the existence of SiC nanoparticle agglomerates in the Al-4wt%Cu-5vol.%SiC nanocomposite. The ultrafine structured Al-4wt%Cu-(7.5 and 10)vol.%SiC nanocomposite bars fractured prematurely during tensile testing. The possible reason for this may be the existence of SiC nanoparticles agglomerates in their microstructure.


Introduction
Al alloy matrix composites (AMCs) synthesized using powder metallurgy have been studied widely over several decades, due to the potential of such materials to have excellent mechanical properties such as high wear resistance, high strength and

Abstract
Ultrafine structured Al-4wt.%Cu-(2.5-10)vol.% SiC nanocomposites were produced by high energy mechanical milling of a mixture of Al and Cu powders and SiC nano-powder to produce nanocomposites powders, followed by consolidation of the powders using powder compact extrusion (PCE).Scanning and transmission electron microscopy as well as tensile testing were used to characterize the extruded nanocomposite bars.Increasing the volume fraction of SiC nanoparticles from 2.5 to 5.0 causes the yield strength, ultimate tensile strength and microhardness of the nanocomposite to increase from 98 MPa, 168 MPa and 104 HV to 391 MPa, 400 MPa and 153 HV, showing the effectiveness of SiC nanoparticles and microstructural refinement in strengthening the material.However, the ductility decreases from 6.8% to 2%, possibly due to the existence of SiC nanoparticle agglomerates in the Al-4wt%Cu-5vol.%SiCnanocomposite.The ultrafine structured Al-4wt%Cu-(7.5 and 10)vol.%SiCnanocomposite bars fractured prematurely during tensile testing.The possible reason for this may be the existence of SiC nanoparticles agglomerates in their microstructure.__________________________________________________________________________ ______________ Amro A.Gazawi, Brian Gabbitas, Deliang Zhang, Charlie Kong, Paul Munroe (2015), Journal of Research in Nanotechnology, DOI: 10.5171/2015.928417improved modulus, as well as low density, which are all highly desirable for aerospace and automotive applications [1][2][3][4][5][6][7][8][9][10][11][12][13] .By reducing the sizes of ceramic particles in AMC`s to nanometer range (<100nm), there is a potential for offering higher strength and fracture toughness.In the fabrication of AMNCs, one of the challenges is the difficulty of dispersing nanometre sized ceramic particles homogeneously in the aluminium alloy matrix.One way of overcoming this difficulty is to produce AMNC powders with ceramic nanoparticles homogenously dispersed in the Al or Al alloy matrix of each of the powder particles by high energy mechanical milling (HEMM) of mixtures of metallic powders and the ceramic nanopowders 3,6,8,14 .The nanocomposite powders can be subsequently consolidated using severe plastic deformation processes such as powder compact forging and powder compact extrusion to produce near net shaped components or structural members 2,[7][8][9] .This approach has been used by Hesabi et al. 11 who synthesised nanostructured Al-5vol.%Al2O3nanocomposite powder by HEMM, and consolidated the milled nanocomposite powder into bars by powder compact extrusion.Their study showed that the ultrafine grained structure of the Al matrix coupled with the Al2O3 nanoparticles produced the nanocomposite bars with a strength of 356 MPa and good ductility.
On the other hand, Ogel et al. 13 used simple mixing of Al, Cu, and SiC powders and hot pressing to produce Al-5wt%Cu- (15,30)  vol.%SiC metal matrix composites reinforced with micrometer sized SiC particles.They found that the strength of the material was improved with increasing amounts of SiC particles, but the ductility of the material was clearly reduced.The microstructural examination also revealed that the SiC particles were not homogenously distributed in the Al alloy matrix, showing the difficulty with homogenously dispersing SiC particles even in the fabrication of AMCs using powder metallurgy.
In this study, we utilised the approach of synthesizing nanostructured Al-4wt.%Cu-(2.5-10)%SiCnanocomposite powders using HEMM, followed by consolidation of the milled nanocomposite powders by cold pressing and hot powder compact extrusion to obtain bulk ultrafine structured nanocomposite samples with a uniform distribution of SiC nano particles.The aim of the study is to elucidate the effect of the volume fraction of the SiC nanoparticles on the microstructure and mechanical properties of bulk ultrafine structured Al-4wt.%Cu-(2.5-10)%SiCnanocomposites.

Preparation of the nanocomposites
To produce the nanocomposite powders, the powders were first mixed under argon for 6 hours with a Restch PM100 planetary ball mill and a rotational speed of 100 rpm.Then, the powder mixture was milled for a net time of 12 hours with a rotational speed of 400rpm using the same ball mill.
The nanocomposite powders produced were compacted by using uniaxial pressing at room temperature for 5 minutes under a pressure of 1000 MPa with a cylindrical H13 steel die (internal diameter: 25mm).The powder compacts were heated to 500 ºC using induction heating under argon, and then extruded to produce cylindrical bars with diameter of 8 mm.The extrusion cylinder speed was 7.7mm/s.

Characterization
The density of the extruded bars was measured using Archimedes method.Flat dog-bone shaped tensile test specimens with a gauge length of 20 mm were cut from the extruded bars using an electrical discharge machine (EDM) wire cutter, and

Results and Discussion
The microhardness of samples produced by powder compact extrusion (PCE) was measured and an average was taken of 15 indents.The average microhardness of the Al-4wt.%Cu-(2.5-10)  vol.%SiC nanocomposite bars produced by PCE increased from 104 HV to 205 HV with increasing volume fraction of SiC nanoparticles from 2.5 to 10%.This can be compared with the microhardness of the milled nanocomposite powders, which increased from 122 HV to 192 HV, as shown in Fig. 1.During the process of milling mixtures of Al powder with 4wt%Cu and 2.5vol.%SiCnanoparticles together with 1wt%PCA, coarse powder particles with sizes in the range of 20-150 µm formed after 12 hours of milling.When the volume fraction of SiC nanoparticles increased to 5%, the nanocomposite powder particle sizes were in the range of 10-90 µm after 12 hours of milling.When the volume fraction of SiC nanoparticles increased to 7.5%, the nanocomposite powder particle sizes were in the range of 5-120 µm after 12 hours of milling.When the volume fraction of SiC nanoparticles increased to 10%, the nanocomposite powder particle sizes were in the range of 5-70 µm after 12 hours of milling, as can be seen in Figure 3.     7).Based on the Williamson-Hall method (Figure 7), the estimated average grain size and lattice strain of the Al-4wt%Cu-2.5vol.%SiCnanocomposites were 250 nm and 0.39%, respectively.With increasing volume fraction of SiC nanoparticles to 5 %, the grain size and lattice strain changed to 500 nm and 0.37%, respectively.For Al-4wt%Cu-7.5vol.%SiCnanocomposites, the grain size and lattice strain were 1250 nm and 0.36%, respectively.For Al-4wt%Cu-10vol.%SiC nanocomposites, the grain size and lattice strain were 1666 nm and 0.42%, respectively.The average grain size increased with increasing volume fraction of the SiC nanoparticles in the nanocomposite, while the lattice strain remained in the same range.This is due to the thermal stability of the microstructure of the Al-4wt%-(       increase of the yield strength.The high effectiveness of these two factors in strengthening the composite is also reflected by the observation that the yield strength of the ultrafine structured Al-4wt%Cu-5vol.%SiCnanocomposite (391 MPa) is more than 2.5 times higher than that of the coarse structured Al-4wt%Cu-10vol.%SiCcomposite with an average particle size of 23µm 17 .The significant decrease in ductility of the bulk nanocomposite samples with the increasing volume fraction of SiC nanoparticles, from 2.5 to 5%, may be due to two reasons: the existence of SiC nanoparticle agglomerates in the microstructure, which makes the formation of cavities under tensile stress easier, and the refinement of microstructure of the Al-4wt%Cu matrix which makes it easier to lose stability of deformation under tension 18 .Similarly, the premature fracture of the Al-4wt%Cu-(7.5 and 10)vol.%SiCnanocomposite samples during tensile testing may also be due to the existence of SiC nanoparticle agglomerates in their microstructure, which make it very easy to form cavities under tensile stress.With the total volume fraction of the SiC nanoparticles being at a high level of 7.5 or 10%, the number of such SiC nanoparticles agglomerates per unit volume of the sample can be quite high, so it is easy for cracks to form and propagate, causing fracture to occur before macroscopic yielding.
Gazawi, Brian Gabbitas, Deliang Zhang, Charlie Kong, Paul Munroe (2015), Journal of Research in Nanotechnology, DOI: 10.5171/2015.928417tested at room temperature using an Instron 4204 testing machine with a strain rate of 1.8x10-4 s -1 .Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffractometry (XRD) were used to characterize the microstructure of the extruded bars and fractured tensile test specimens.The microhardness of the extruded bars was measured using a Vickers microhardness tester with a load of 25-g and a loading duration of 15-s.

Figure 1 :
Figure 1: Microhardness for Al-4wt%Cu-(2.5-10)vol.%SiCnanocomposites bars produced by PCE.Fig.2 shows the relative density of the Al-4wt.%Cu-(2.5-10)vol.% SiC nanocomposite powder compacts and extruded bars produced as a function of the volume fraction of SiC nanoparticles.The theoretical density of the material components of the composite was used to calculate the relative density of the powder compacts and extruded bars using the rule of mixtures.From Fig.2, it can be seen that the relative density of the powder compacts decreased

Figure 4
Figure 4 shows typical SEM micrographs of the longitudinal sections of the extruded bars.It was clear that the extruded bars were almost fully dense, with the volume fraction of pores less than 1%.The clear discrepancy between the density of the

Figure 6 shows
Figure 6 shows XRD patterns of Al-4wt%Cu-(2.5-10)vol.%SiC nanocomposite cylindrical bars produced by PCE.The XRD patterns show strong Al peaks with weak SiC and Cu peaks, due to the small sizes and volume fractions of SiC and Cu particles.The average grain sizes and the lattice strain of the Al-4wt%Cu-(2.5-10)vol.%SiCnanocomposites cylindrical bars were estimated based on the broadening of the XRD peaks and using the Williamson-Hall method (Figure7).Based on the Williamson-Hall method (Figure7), the estimated average grain size and lattice strain of the Al-4wt%Cu-2.5vol.%SiCnanocomposites were 250 nm and 0.39%, respectively.With increasing volume fraction of SiC nanoparticles to 5 %, the grain size and lattice strain changed to 500 nm and 0.37%, respectively.For Al-4wt%Cu-7.5vol.%SiCnanocomposites, the

Figure 7 :
Figure 7: Grain size and lattice strain of the Al-4wt%Cu-(2.5-10)vol.%SiCnanocomposites bars produced PCE as a function of the fraction SiC nanoparticles.

Figure 10 shows
Figure10shows the tensile engineering stress-strain curves of specimens cut from the extruded bars.With 2.5 vol.%SiC, the extruded bar showed a yield strength, ultimate tensile strength (UTS) and plastic strain to fracture of 98 MPa, 168MPa and 6.8%, respectively.By increasing the volume fraction of SiC nanoparticles to 5%

Figure 10 :
Figure 10 : Tensile stress-strain curves of specimens cut from Al-4wt%Cu-(2.5-10)vol.% SiC bars produced by PCE.This work has shown that with increasing the volume fraction of SiC nanoparticles, the microstructure of the Al-4wt%Cu matrix of the bulk nanocomposite samples becomes fine.There are two reasons for this.The first reason is that an increase in the content of SiC nanoparticle content in the starting powder microstructure increases the effectiveness of HEMM, causing the microstructure of the nanocomposite powder produced by HEMM to be finer, as confirmed by TEM examination of the milled powder particles16 .The second reason is that the thermal stability of the microstructure of the Al-4wt%Cu matrix of the nanocomposites increases with the

Theta Al 111 Al 200 Al 220 Al 311 Al 222 SiC 311 SiC 220 Cu 111 SiC 111 Cu 200 10vol.%SiC 7.5vol.%SiC 5vol.%SiC 2.5vol.%SiC Cu 220 SiC 200 SiC
2.5-10)vol.%SiC matrix of the nanocomposite which increases with increasing volume fraction of the SiC nanoparticles.This enhances the Zenerdrag effect of nanoparticles to resist the movement of grain boundaries.This effect leads to a slower rate of microstructural coarsening with increased content of SiC nanoparticles during powder compact extrusion.Also, the carbide nanoparticles located throughout the composite __________________________________________________________________________ ______________ Amro A.Gazawi, Brian Gabbitas, Deliang Zhang, Charlie Kong, Paul Munroe (2015), Journal of Research in Nanotechnology, DOI: 10.5171/2015.928417