Investigation On Microstructure And Mechanical Properties Of Mg-matrix Composites

Due to ever-increasing demand for structural strength and energy efficientmaterials, the light weight magnesium metal and its composites have attractedsignificant research interest in aerospace and automotive industry. Magnesium basedcomposites reinforced with nano-size (i.e, SiC, Cu, Al2O3and CNTs) and hybridparticles have been explored extensively to achieve the optimum strength. However,further research is required to achieve high strength to weight ratio for these composites.In present work, carbonaceous material graphene nanoplatelets (GNPs) have beenused as reinforcement particles to enhance the mechanical strength of magnesiummatrix composites. Along with individual GNPs particles, the effects of metal-grapheneand metal-metal hybrid reinforcement particles addition on microstructure andmechanical behavior of pure magnesium were also investigated. Addition of smallamount of aluminum metal particle along with the graphene nanoplatelets isadvantageous to obtain good compatibility between graphene and matrix magnesium.The main contents can be summarized below:Firstly, the effect of individual GNPs addition to pure magnesium was investigated.Experimental results revealed improvement in tensile strength of pure magnesium butductility of the composite was adversely affected. It was observed that improvement intensile strength is low which may be attributed to the poor compatibility of GNPs withmagnesium matrix. In order to improve the compatibility between graphene andmagnesium matrix, small amount of aluminum was added in the composite.Experimental results showed that when aluminum content is kept constant at1.0wt.%and GNPs content was varied from0.09to0.3wt.%, both tensile strength and ductilitywas increased simultaneously. This is because more and more GNPs align along theextrusion direction and resist composite rupture. On the other hand when GNPs contentwas kept constant at0.18wt.%and aluminum particle contents were varied from0.5to1.5wt.%, then tensile strength and ductility of resulting composite increase till thethreshold of1.0wt.%. Improved mechanical strength of the composites is attributed tothe basic strengthening mechanisms, mismatch in coefficient of thermal expansionbetween reinforcement and matrix, Orowan looping and load transfer mechanism.Secondly, synergetic effect of graphene and carbon nanotubes was investigated inthe magnesium matrix. Mechanical characterization revealed that composite reinforced with hybrid (GNPs+CNTs)-Aluminum particles exhibited higher tensile failure strainrelative to those reinforced with individual GNPs and MW-CNTs. The impressiveincrease in tensile failure strain (%) confirmed the significant synergetic effect betweenGNPs and MW-CNTs. This improvement in failure strain can be attributed to:(a) therapid aggregation of two-dimensional GNPs can be inhibited by intercalating one-dimensional multi-walled carbon nanotubes (MW-CNTs) and (b) long and flexibleMW-CNTs bridge adjacent GNPs to form three dimensional hybrid structures whichprevent their aggregation, thus resulting in a high contact area between CNTs+GNPshybrid structure and the matrix.Thirdly, the effect of GNPs addition on mechanical properties of magnesium-10wt.%Titanium alloy was investigated. Room temperature tensile results revealed thataddition of Ti and Ti+GNPs into monolithic Mg lead to increase in both tensile strengthand failure strain. In addition, effect of1Cu-GNPs hybrids on mechanical strength ofpure magnesium was investigated. Experimental results shows that strength and failurestrain increase with increase in GNPs contents from0.18to0.36wt.%. However, whenGNPs contents increase from0.36to0.54wt.%then failure strain start to decrease,which is due to GNPs clustering.Fourthly, effect of graphene nanoplatelets addition on tensile strength of Mg-1Al-1Sn alloy was also investigated using powder metallurgy method. Tensile resultsrevealed that addition of0.18wt.%GNPs to the Mg-1Al-1Sn alloy matrix leads toincrease in tensile strength. However, the ductility of the resulting composite wasadversely affected. Increased tensile strength of the composite is attributed to the basicstrengthening mechanisms.Fifthly, effect of different metallic hybrid particles addition into pure magnesiumwas investigated. Pure Mg reinforced with10%Ti and10%Ti-1%Al hybrid particulatesrevealed improvement in both tensile strength and ductility. This can be attributed to thebetter compatibility between Ti particle and matrix due to presence of Al as alloyingelement. Furthermore, effect of Al-Cu particulates hybrids addition into pure Mg wasalso examined. The synthesized composites exhibited homogeneous dispersion of Cuparticles in the matrix, therefore leading to enhancement in tensile strength and ductilityby addition of1.0wt.%Al-0.6wt.%Cu hybrid particles. On the other hand, when Cucontent is kept constant and Al content was varied from1to9wt.%, experimentalresults showed improvement in hardness, tensile and compressive strength of the hybridcomposites. Increase in aluminum content led to increase in Vickers harness, strength (both in tension and compression). However, tensile failure strain of compositesincreases till the threshold value of3wt.%Al is reached. Decreasing trend of tensilefailure strain for the composites with6and9wt.%Al contents can be attributed to theformation of brittle intermetallic phases Mg17Al12. The increased mechanical strength ofmetallic particulate reinforced magnesium composites is caused by mismatch incoefficient of thermal expansion (between matrix and reinforcement particles) whichresults in punching of dislocations at the interface, Orowan looping and load transferfrom soft matrix to hard reinforcements or second phases.