Microstructure Regulation And Strengthening Mechanism Of Powder Metallurgy Fabricated GNPs/Al Composites

While the modulus and strength significantly increased,the plasticity decreases sharply,which makes it difficult to achieve a good strength-ductility match in traditional aluminum matrix composites（AMCs）reinforced with ceramic particles and whiskers.Using flexible graphene nanoplatelets（GNPs）as the reinforcement of AMCs is expected to solve this problem.In the present thesis,the microstructure evolution of GNPs/Al composites powders during the ball milling process was investigated to balance the dispersion and destruction of GNPs for a large-scale production purpose.Cold drawing was employed to regulate the distribution of GNPs for improved strengthening effect.Annealing was employed to modify the interface microstructure of the composites.The effect of anneal temperature on interface microstructure and the mechanical property of GNPs/Al composites was discussed in detail.Finally,the relationship between GNPs morphology,distribution and GNP/Al interface microstructure on the mechanical property and fracture behavior of GNPs/Al composites was analyzed.The strengthening mechanism of GNPs was revealed as well.Results shows that the composite powders undergo multiple cold-welding during the ball milling process.Each cold-welding process includes powder deformation stage and powder welding stage.In powder deformation stage,Al powder changed from spherical to flake.While the powder welding stage include repeated crush,cold-welding and crush behavior of powders.The duration of each cold-welding process reduces with increasing ball milling speed or prolong ball milling time.The microstructure evolution of GNPs during ball milling process can be divided into GNPs fragment stage and GNPs dispersion stage.The GNPs fragment occurred during powder deformation stage and GNPs dispersion occurred during the stable cold-welding stage.A sharp decrease in GNPs plate diameter and thickness could be found during the GNPs fragment stage,while it was gently during GNPs dispersion stage.The GNPs fragment behavior during ball milling process could be influenced by the relative size between GNPs plate diameter and Al powder diameters.When Al powder diameter is bigger than GNPs plate diameter,the occurrence of cold-welding could be delayed but the cold-welding process can be accelerated.At the same time,under the buffering effect of large-diameter-Al powders,the structural damage to GNPs was reduced and a large GNPs diameter was obtained.After cold compaction and hot extrusion,the composites were compacted with a relative density higher than 99.5%.Finally,a good strength-ductility match was obtained in 5vol.%GNPs/Al composites with an UTS of 344.2MPa and strain to failure of 10.2%.The cold drawing process could significantly improve the distribution of GNPs.The dispersion of GNPs clusters can be ascribed to the additional shear strain layer in the deformation zone.The GNPs dispersion mechanism changed from GNPs exfoliation to GNPs crack during the cold drawing process with the decrease of GNPs cluster thickness.Under drawing equivalent strain of 6.0,most the GNPs clusters in 1vol.%and 2vol.%GNPs/Al composites can be eliminated.The UTS improvement by GNPs in 1vol.%and2vol.%GNPs/Al composites increased from 6MPa and 21MPa in as-extruded composites to 75MPa and 114MPa in as-drawn composites.And the GNPs strengthening efficiency can reach 52 and 40 in 1vol.%and 2vol.%GNPs/Al composites,respectively.The GNP/Al interface microstructure depends on the annealing temperature:when annealed at 600℃,no Al₄C₃ was detected.With increasing temperature between 620℃and 650℃,interface reaction degree increased with temperature.As the annealing temperature increases,the UTS of the composite increases as well.When part of the interface reaction occurs,the bonding strength of the GNP/Al interface increased,which is favored for load-transfer strengthening effect.The UTS of the composites and the strengthening efficiency of GNPs increase simultaneously.At the same time,the failure mode of the composite changed from GNPs debonding to GNPs fracture.Combining the tensile test results and the calculation results,the strengthening mechanisms that operated in GNPs/Al composites are grain refinement,load-transfer and Orowan strengthening.With increasing GNPs plate diameter,the primary strengthening mechanism changed from Orowan strengthening to load transfer mechanism.Meanwhile,the failure mode of the composites changed from GNPs debonding to GNPs fracture.The strengthening and toughening behavior of GNPs can be affected by its plate diameter.With increasing GNPs plate diameter,the load transfer effect of GNPs can be improved,leading to enhanced UTS of composites.GNPs with larger plate diameter can prevent the movement of dislocation more effectively at the early stage of tensile test,leading to enhanced strength of composites and working harden rate,which thus improves the uniformity of deformation and delays the formation of necking.As a consequence,a good strength-ductility match can be obtained in GNPs/Al composites.