Research On The Microstructure And Mechanical Properties Of GNPs/Ti Matrix Composites By Powder Metallurgic Method

Recently,aerospace filed improved the requirements about the properties of high-end fasteners.Further,the key projects cried for superhigh strength fasteners of 1200 MP a～1500MPa,such as the new-generation aeromotor,mars probe and so on.The mechanical properties of Ti and Ti alloys were unable to meet the needs of fastener.Fortunately,Ti matrix composites were regard as a technology pathway to improve the tensile strength of Ti alloys.Due to its excellent mechanical properties,superlarge specific surface area and flexible feature,graphene nanosheets(GNPs)promoted outstandingly the mechanical properties of Ti matix composites(TMC s),and improved efficiently the plasticity of TMC s.Hence,GNPs/Ti matrix composites were significantly potential to meet the requirements for the material of super-high strength fasteners.However,during the preparation of GNPs/Ti matrix composites,the bottlenecks should be solved,such as the aggregation of GNPs and the severe interface reation between GNP s and Ti matrix.Therefore,the mature preparation technology of pre-dispersion and canned sintering and hot extrusion was adopted to fabricate GNPs/Ti matrix composites.Then,the distribution of GNP s and enhancement mechanism and fracture mechanism were researched.Meanwhile,focusing on the interface reaction.S ubsequently,the way of control the interface reaction by adding B elements was put forward.In order to reveal the interface reaction by B atoms,the contrastive study between canned sintering and spark plasma sintering(SPS)was deeply carried out.A new preparation technology predispersion and SPS and canned hot extrusion was refined.The problems of the aggregation of GNPs and the severe interface reation were soloved validly.A desired interface between GNPs and Ti matrix was obtained,resuting in super-high strength composites.To verify the plasticity of super-high strength composites,a preparation technology of hot upsetting and hot rolling-press was used to trial-manufacture studs.First of all,a preparation technology of pre-dispersion and canned sintering and hot extrusion was designed to fabricate GNPs/Ti matrix composites.the effects on microstructure and mechanical properties of composites with GNPs content and canned sintering temperature were systematically studied.Then,the enhancement mechanism and fracture mechanism were dicussed.Observations shown that the sizes of reinforcement increased with the inceasing of GNPs contents.As a result,the tensile strength of composites was improved,and the strain was reduced.In contrast,the canned sintering temperature increased,leading to severe interface reaction,resulting in the forming of coarse Ti C particles.Tensile tests shown that load transferring and dislocation strengthening of GNPs were main enhancing ways.The load transferring efficiency increased with the increasing of interfacial superficial area.Meanwhile,the interface bonding strength influenced fracture behaviour.The microcracks initiated and extended along the interface between GNPs and Ti matrix,leading to the fracture of composites.The decreasing of interface bonding strength expedited the extending speed,resuting that the strain of composites declined.In order to analyse deeply the interface reaction mechanism between GNPs and Ti matrix,the effects on microstructure and mechanical properties of composites were studied with extrusion temperature from900℃ to 1100℃.When the extrusion temperature increased to 1000℃,the severe interface reaction occurred,and the coarse Ti C particles formed,resulting that the GNPs were used out.Meanwhile,when the composites were extruded at 900℃,the layers of GNPs slided,and a new contact surface between inlayers of GNPs and Ti matrix generated,resulting in the forming the nano-Ti C plates.As for the severe interface reaction between GNPs and Ti matrix,the interface reaction control method was raised by adding B elements.The effects on microstructure and mechanical properties of composites with different contents of B were carried out.And the control mechanism of interface reaction by B atoms was discussed.Resulting that the B atoms preferentially distributed to the vacancy and bridge position of GNPs,the activities of C atoms in GNPs were reduced.Meanwhile,the diffusion rate of C atoms decreased by the hinder of B atoms.As a result,the severe interface reaction was controlled.Moreover,the B atoms which distributed at vacanvy combined with C atoms to form B-C bonds.The forming of B-C bonds optimized the structure of GNPs.Ultimately,The GNPs/Ti compostes were obtained with the ultimate tensile strength of 1350 MP a and elongation of 8.9%.In order to optimize the control mechanism of interface reaction by B atoms,a new technical line of pre-dispersion and SPS and canned hot extrusion was raised.B ased on the sintering mechanism of SPS,the effects on interface reaction and microstructure of composites were researched with SP S sintering parameters.R esulting that local high temperature field formed on the Ti powders surface,under the influence of heat from arcing and joule.The B atoms effectively and directly diffused to Ti matrix,and dissolved in Ti matrix at priority.As a result,the B atoms distributed at Ti matix and GNPs,and reduced the activities of C atoms.The diffusion of C atoms was hindered,and the severe interface reaction was inhibited.Furthermore,the composites bars were fabricated by the parameters of 800℃/10 min SPS and 1000℃ extrusion with a ultimate tensile strength of 1406 MPa and a strain of 5.5%.Then,aiming to high-end fasterners of 1400 MP a,the above-mentioned composites were selected to verify their plasticity.The M8×21-6h studs were successfully fabricated by the process flow of hot upsetting and hot rolling-press,which proved the feasibility of composites bars for 1400 MPa ultrahighstrength fasteners.