Fabrication And Properties Of Graphene Reinforced Titanium Alloy Matrix Nanocomposites With Network Architecture

Discontinuous reinforced titanium matrix composites(DRTMCs)have broad application prospects in aerospace,automobile manufacturing,weaponry and other fields.Among DRTMCs,composites with network distribution of reinforcement can achieve high strength improvement while having less ductility loss.Graphene(GR)is a rational reinforcement material due to its excellent mechanical properties.In the research of GR reinforced DRTMCs,the reinforcement are mostly uniformly distributed,which brings serious ductility loss.At the same time,the dispersion issue of GR and the reaction between GR and titanium matrix restrict the performance of the composites.In this paper,few-layered GR(<10 layers)is used as reinforcement materials,large-sized Ti6AI4V spherical powder(TC4,100-150?m)is used as matrix materials to prepare GR/TC4 composites with network distribution of reinforcement.The phase composition,microstructure,mechanical properties,friction properties and thermal conductivity of the network structured GR/TC4 composites with different interface states and different GR additions(0.05-1.5 wt.%)are discussed.First of all,the preparation technology to achieve network distribution of reinforcement is explored by chemical crosslinking method and spark plasma sintering at high temperature and low pressure(SPS,1050?,60 MPa).GR is uniformly distributed on the surface of TC4 powder by PVA modification and chemical crosslinking.PVA removal and densification are obtained by SPS sintering in one step,then GR/TC4 composites with three-dimensional network distribution of reinforcement are successfully prepared.The matrix structure of the composites are?+?lamellar structure,and grain refinement occur compared with TC4.The reinforcement is composed of Ti C formed by in-situ reaction of GR with TC4 and a small amount of unreacted GR.The ratio of GR reaction is about 80-90%.The maximum Ti C particle size is about 300-500 nm.The reinforcement is mainly distributed along the network boundary,the matrix inside the network structure is hardly strengthened and still maintains good ductility.The average microhardness of the composites gradually increases with the increase of GR addition(0.25-1.5 wt.%),and the maximum increase was 18%(1.5 wt.%GR).The maximum compressive yield strength of the composites increased by 27.3%(1.0 wt.%GR).All the samples showed good ductility with fracture strain higher than 28%.0.25 wt.%GR/TC4 composite has the best tensile properties,with an increase in yield strength of 7.1%,while still maintaining a high elongation of 10%.On the basis of chemical cross-linking and high-temperature-low-pressure SPS,the preparation process of the network structured composites is further optimized.The three-dimensional rotating mechanical mixing method is used to disperse the GR on the surface of the TC4 powder.The high-pressure-low-temperature SPS sintering(700-950?,150-500 MPa)is used to control the reaction between GR and the matrix.The matrix structure of the composites is?+?lamellar structure above850?,and the size of the lamellar structure increases with the increase of temperature.Below 900?,the composites exhibit poor interfacial bonding and tensile properties.Above 900?,good interfacial bonding can be achieved.When sintered at 900?,250 MPa,the strength of the composites don't change significantly with the extension of the holding time(5-15 min),but the ductility increase.When the temperature is further increased to 950?,the ductility of the composites decrease and the strength don't change significantly.Among the various sintering conditions explored,the mechanical properties of the composites are best at 900?,250 MPa,10-15 min.The reinforcement under this conditions can meet the requirements of interface strength while retaining adequate GR to exert excellent mechanical properties.The reinforcement in the composite is a"core+shell"structure with Ti C encapsulating GR.The size of Ti C particles gradually increases from the GR side to the TC4 side,with a maximum Ti C size about 200-300 nm.The microstructure and properties of composites with different GR contents(0.05-0.25 wt.%)are discussed.The average microhardness of the composite increases with the increase of GR addition.The microhardness of 0.25 wt.%GR/TC4 composite(355HV)is 9%higher than that of TC4.The tensile strength and yield strength increase with the increase of GR addition.There is almost no loss of ductility when the GR addition?0.1 wt.%.When the GR addition exceeds 0.1 wt.%,the ductility decreases significantly.0.1 wt.%GR/TC4 composites showed the best tensile properties.The yield strength and the tensile strength are increased by 23.0%and18.1%,respectively,while high elongation is maintained at 15.2%.The excellent mechanical properties come from the synergistic enhancement of GR/Ti C and the quasi-continuous network distribution of reinforcement,which give full play to the ductility advantages of the matrix.In terms of friction properties,the friction coefficient of GR/TC4 composites are lower than that of TC4,and it decreases with the increase of GR addition.The friction coefficient of 0.2 wt.%GR/TC4 composite(0.41)is 25.4%lower than TC4.The reduction of the friction coefficient is mainly due to the lubrication effect of the residual GR.The wear mechanism of the composites is mainly abrasive wear,accompanied by slight peeling wear and oxidative wear.In terms of thermal conductivity,the thermal conductivity of GR/TC4 composites decreases first and then increases with the increase of GR addition.The thermal conductivity of 0.15 wt.%GR/TC4 composite is the lowest,which is 7.016 Wm^-1K^-1.0.25 wt.%GR/TC4 composite possess the highest thermal conductivity at7.124 Wm^-1K^-1.When the amount of GR added is very small,the thermal conductivity decreases mainly due to the increase in the interface thermal resistance.As the amount of GR addition further increases,the leading factor of thermal conductivity gradually changes to GR with excellent thermal conductivity.