METAL MATRIX NANOCOMPOSITES; POTENTIALS, CHALLENGES AND FEASIBLE SOLUTIONS

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METAL MATRIX NANOCOMPOSITES; POTENTIALS, CHALLENGES AND FEASIBLE SOLUTIONS

This thesis focuses on the processing and properties of three different metal matrix nanocomposites, Al and copper matrix composites produced by classical powder metallurgy techniques and magnesium (Elektron21) matrix composites fabricated by an ultrasound assisted casting method in the frame of EXOMET project. In fact, aluminum and copper matrix composites are reinforced by graphene nanoplatelets (GNPs) whereas magnesium matrix composites are reinforced by some nano-ceramic particles such as aluminum nitride and aluminum oxide. Regarding the Al and copper matrix composites, the targets were to obtain a uniform dispersion of graphene within the metallic matrix, to avoid the undesirable reaction between GNPs and the metal matrix, to obtain a strong interfacial bonding, to improve the mechanical properties, decrease the coefficient of thermal expansion and improve the thermal and electrical conductivities. The experimental results showed that fabrication of MMNCs reinforced by graphene is not an easy task and it is vital to take into account all the possible issues in order to exploit all the potentials of graphene in the composites. In General, the aim was to fabricate composite materials and determine the relationship between the production, microstructure and final properties of the bulk composites. Regards to the dispersion of reinforcement within the matrix, it is found out in the case of aluminum and copper matrix nanocomposites the wet mixing method has a great potential to disperse the graphene nanoplatelets not only homogeneous but also without damaging the structure of graphene. However, the dispersion of graphene nanoplatelets at higher contents is still puzzling. On the other hand, although AlN and Al2O3-AlOOH nanoparticles were dispersed uniformly within the molten electron 21 alloy by means of ultrasonication, but during the solidification, particles were pushed and agglomerated after solidification. On the whole, it can be concluded that despite all the efforts that have been undertaken during this work, and although some promising results were obtained at low graphene content, the fabrication of MMNCs reinforced by high graphene content is still challenging. In any case, it was proved that the wet mixing method which was developed during this research can be easily and successfully applied to the fabrication of MMNCs reinforced with low graphene content and allowed to achieve improved thermal and mechanical properties. Moreover, in El21/AlN and El21/Al2O3-AlOOH several complex phenomena such as particle pushing, particle agglomeration, reaction between the particle and the matrix and in-situ formation of some particles have been identified. Microstructural analysis showed that the grain size slightly reduced through the addition of AlN particles. All mechanical properties of El21-AlN at room temperature such as hardness, compression and tensile strength, did not show any improvement with respect to the unreinforced alloy. However, a significant improvement was observed in the creep resistance of El21-AlN so that at low stresses the minimum creep rate of composite is one order of magnitude lower than base alloy. Nevertheless, this minimum creep rate is almost equal to the minimum creep rate of El 21 after solution treated followed by ageing up to the maximum hardness. In the case of El21-AlN composites some aluminum was transferred to the melt, while zirconium reacts with the nanoparticles, originating some complex interfacial reaction that could form strong interfacial bondings between the matrix and the reinforcement. On the other hand, in El21-Al2O3 composites a reaction between alumina and magnesium leads to the in-situ formation of MgO particles and to the presence of Al in the alloy. The presence of Al in the alloy can bring to reactions with rare earth, with the formation of highly stable phases (i.e. Al2Nd), whose dissolution seems to be impossible even at high solution treatment temperature. The overall trend of precipitation hardening of composites was completely different from the base alloy that shows the effect of nanoparticles on the dissolution of precipitates and kinetics of precipitation during the ageing treatment. Finally, it can be concluded that the fabrication of metal matrix composites either by powder metallurgy or by casting faces with several challenges. . In particular, some of these difficulties can be attributed to the nature of the involved materials, while some of them are related to the preparation technique. It seems that these latter can be often solved by changing the production process or by using post-processing techniques. More challenging, instead are the problems related to the composition of matrix and reinforcement, their reactivity and to the dispersion of particles, in particular if nano-sized. These topics still bring a significant challenge to the materials scientists, and it would be worth to say that fabrication of MMNCs with uniform dispersion of reinforcement, strong interfacial bonding, without detrimental reactions and improved isotropic properties is still a puzzling issue.

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