Microstructure And Properties Of AA6111 Matrix Composites Synergetically Reinforced By The Graphene Nanoplates And In-Situ ZrB₂ Nanoparticles

With the rapid development of modern industry,new requirements and challenges are put forward for the mechanical properties and functional characteristics of aluminum alloys,so as to meet the needs of aerospace,marine equipment,weapons and equipment,automobile industry,information technology and many other fields as much as possible.Therefore,it is urgent to produce a light weight,high strength,high stiffness and good thermal conductivity.Graphene nanoplates（GNPs）are considered as ideal reinforcements because of their excellent properties of high tensile strength and high thermal conductivity.The strength and thermal conductivity of aluminum matrix composites reinforced by GNPs have been improved,however,the plastic toughness has obviously decreased.In-situ ZrB₂ nanoparticles are thermodynamically stable ceramic phase with clean surface and good interface with matrix.High strength and high plasticity are typical characteristics of in-situ ZrB₂ nanoparticles reinforced aluminum matrix composites.In-situ nanoparticle strengthening is an effective method to improve the strength and plasticity of aluminum matrix composites.However,the thermal conductivity decreases slightly due to the introduction of ceramic nanoparticles.In this paper,a kind of GNPs-ZrB₂/AA6111 composite with discontinuous layered structure reinforced by GNPs and in-situ ZrB₂ nanoparticles was designed and prepared.The effects of the synergistic effect of GNPs and ZrB₂ nanoparticles on the microstructure of composites were studied.The effects of hot rolling deformation and GNPs content on the evolution of microstructure,mechanical properties and thermal conductivity of composites were analyzed.The synergistic strengthening-toughening mechanism of GNPs and ZrB₂nanoparticles and dislocation strengthening mechanism of discontinuous layered structure were revealed,and related models were established.The main conclusions are as follows:The microstructure of the composites:The microstructure of GNPs-ZrB₂/AA6111composites is discontinuous layered structure.The hot rolling deformation makes the randomly distributed GNPs in the grain boundaries directionally distributed along the rolling direction parallel to the rolling plane,forming a discontinuous layered strengthening structure with the ZrB₂ nanoparticles distributed in both sides of the GNPs.The diameter of GNPs was less than 5μm,and the average size of ZrB₂nanoparticles was 30 nm.Both GNPs and ZrB₂ particles refined the matrix grains,and the interface between the reinforcement and aluminum matrix was strengthened,including the strengthening of amorphous alumina interface between GNPs and Al matrix,the pinning effect of Al₂Cu particles generated on GNPs surface,and the strengthening of semi-coherent interface between ZrB₂ nanoparticles and Al matrix.The influence of hot rolling deformation and GNPs content on the microstructure evolution of GNPs-ZrB₂/AA6111 composites:With the increase of hot rolling deformation（75%～92%）,the sizes of ZrB₂ particle clusters and GNPs in the RD-ND plane of composites gradually decreased,and the trend of strip distribution was more obvious,andα-Al grains were refined,and the average grain size decreased from 7.1μm to 4.3μm,texture changed from simple to complex.When the hot rolling deformation was 83%,the composite was remained the most stable state,with the highest degree of recrystallization,the highest content of HAGBs was 51.8%,and the lowest dislocation density was 5.0×10¹⁵ m^-2.With the increase of GNPs content,the size of GNPs decreased and its distribution became more uniform,the interlayer spacing and average grain size of composites decreased,the number of high-angle grain boundaries and subgrain decreased,deformed grains and dislocation density increased.The effects of GNPs and ZrB₂ particles on dynamic recovery（DRV）and dynamic recrystallization（DRXs）:The dispersed ZrB₂ nanoparticles were located in the grains,hindering the dislocation slip and the recovery process.The dislocation packing groups near the particle clusters were generated by ZrB₂ particle clusters distributed at grain boundaries and large pieces of GNPs under the action of external forces,which would accelerate the recovery process.The dislocation storage energy and the driving force of crystal migration during deformation were improved by GNPs and ZrB₂ particle clusters distributed at grain boundaries.Therefore,the nucleation and growth of DRX could been promoted by GNPs and ZrB₂ particle clusters.However,dispersed ZrB₂nanoparticles would inhibit DRX due to pinning dislocations.The mechanical properties of GNPs-ZrB₂/AA6111 composites,and the influence law of deformation and GNPs content on the mechanical properties of composites:The UTS of GNPs-ZrB₂/AA6111 composites with 83%deformation enhanced by 0.1 wt%of GNPs and 3 wt%of ZrB₂ nanoparticles were significantly improved,reaching 426MPa,which is 33.5%higher than that of AA6111 matrix.YS and UTS of GNPs-ZrB₂/AA6111 composites increased with the increase of deformation（75%～92%）,but the highest El was obtained when the deformation was 83%,that was,it had the best plasticity.The YS and UTS of GNPs-ZrB₂/AA6111 composites with deformation of 83%were the highest when the content of GNPs was 0.3 wt%,but the elongation decreased continuously with the increase of the content of GNPs（0.1 wt%～0.5 wt%）.Synergistic strengthening-toughening mechanism of GNPs and ZrB₂ particles:The bonding strength between GNPs and Al matrix was improved due to the pinning effect of ZrB₂ and Al₂Cu particles and the interface strengthening of amorphous alumina.Combined with the four main strengthening mechanisms of composites,the contributions of fine grain strengthening,CTE strengthening,Orowan strengthening and load transfer strengthening were estimated respectively.The results showed that GNPs mainly played the role of load transfer,ZrB₂ particles mainly played the role of Orowan strengthening,and the CTE strengthening and fine grain strengthening caused by GNPs and ZrB₂ particles were relatively small.By improving strain hardening strength,increasing dislocation storage capacity and reducing stress concentration,the toughening effect can be achieved.Discontinuous layered structure induced dislocation strengthening mechanism transformation:after 92%hot rolling,the GNPs-ZrB₂/AA6111 composite showed a typical discontinuous layered structure,in which the local areas appeared nanolaminated structure.Due to the introduction of GNPs,in the GNP-Al-GNP sandwich nano-layered structure,GNPs confined dislocations to slip in the layer.And the dislocation movement mechanism was shifted from dislocation crossing mechanism to a confined layer slip（CLS）mechanism,which greatly increased the dislocation density.Under the synergistic effect of GNPs and ZrB₂,CLS dislocations induced by GNPs and pinned dislocations induced by dispersed ZrB₂ particles together formed a three-dimensional dislocation network,which greatly reduced the stress concentration in long grains,increased the dislocation storage energy and improved the ability of strengthening and toughening by dislocations.Thermal conductivity of composites:the thermal conductivity in the rolling plane of GNPs-ZrB₂/AA6111 composites was greater than that crossing through the rolling plane,and the thermal conductivity in-plane of composites was improved.Compared with the matrix AA6111,the effect of two-dimensional GNPs parallel to the rolling plane on improving the in-plane thermal conductivity was greater than that of ZrB₂particles on hindering.The in-plane thermal conductivity of composites increased with the increase of deformation.The better proportion of GNPs and ZrB₂ was 0.5 wt%GNPs and 1 wt%ZrB₂.The in-plane thermal conductivity,ultimate tensile strength and elongation of the composites were 240.481 W/m·K,435 MPa and 12.0%,which not only had relatively high thermal conductivity,but also ensured high strength over high plasticity.At the same time,the influence law of GNPs on the thermal conductivity of GNPs-ZrB₂/AA6111 composites was obtained.The higher the crystallinity of GNPs,the better the dispersion degree of GNPs,the higher the parallel arrangement degree of GNPs and the rolling surface,and the higher the interfacial bonding strength of GNPs and Al matrix,the more helpful it was to improve the thermal conductivity of the composites.