The Effects Of TiC/TiB₂ Nanoparticles On The Microstructure And Properties Of Rolling Al-Mg-Si Alloy

Nanoparticles can increase the strength and ductility of cast Al alloys simultaneously,and they can provide a more obvious strengthening effect than micron-sized particles.Thus,Al matrix composites(AMCs)reinforced with nanoparticles have potential applications in the aerospace field.It is known that agglomeration is easy to form when the volume fraction of nanoparticles is high.This is still a problem and can lead to a decrease in strength and ductility,thus,limiting the applications of AMCs reinforced with nanoparticles.Plastic deformation can improve the distribution of nanoparticles,and severe plastic deformation is more effective.The cold rolling process is one of the major methods to produce Al sheets in industrial fields.However,few reports are found to study the effects of nanoparticles on the microstructure evolution,mechanical properties and corrosion behavior of cold rolling Al sheets.Thus,it has important significance and applied prospect to use the severe plastic deformation to improve the distribution of nanoparticles,further improving the mechanical properties and corrosion resistance of AMC sheets.In this paper,the“master alloy+stirring casting+cold rolling”process was adopted to fabricate the TiC_p/Al-Mg-Si composite,the TiB_2p/Al-Mg-Si composite and the(TiC_p-TiB_2p)/Al-Mg-Si composite sheets.ARB and CARB processes were used to prepared Al-Mg-Si alloy,3.0 wt%TiC_p/Al-Mg-Si composite and“Al-Mg-Si alloy+3.0 wt%TiC_p/Al-Mg-Si composite”ARB and CARB sheets.The effects of nanosized TiC_p,TiB_2p and(TiC_p-TiB_2p)on the microstructure evolution and tensile properties of Al-Mg-Si matrix alloy were investigated.The effects and mechanisms of nanosized TiC_p and nanosized TiB_2p on the corrosion behavior of cold rolling Al-Mg-Si alloy in the accelerated corrosion test,the electrochemical corrosion test and the immersion corrosion test were investigated.The formation of the hierarchically heterogeneous structure with a high content of nanosized TiC_p(3.0 wt%),bimodal-sized grains and bimodal-sized??precipitates was investigated,and its effect and mechanism on the strength anisotropy and was revealed.The novelty is shown as below:1)The effect and strengthening mechanism of nanosized TiC_p,TiB_2p and(TiC_p-TiB_2p)on the microstructure evolution and mechanical properties at room temperature were revealed:i)It was revealed that nanosized TiC_p,TiB_2p and(TiC_p-TiB_2p)significantly reduced the size of as-cast?-Al grains and cold-rolled+T6 recrystallized?-Al grains of the Al-Mg-Si matrix alloy.The order of refining effect from strong to weak was:nanosized (TiC_p-TiB_2p)?nanosized TiB_2p?nanosized TiC_p.The grain refinement of as-cast?-Al was mainly attributed to:nanosized TiC_p could act as the heterogeneous nucleated core of?-Al;nanosized TiB_2p could impede the dendrite growth of?-Al.ii)It was revealed that nanosized TiC_p,TiB_2p and(TiC_p-TiB_2p)significantly reduced the size of?"precipitates.The order of refining effect from strong to weak was:nanosized(TiC_p-TiB_2p)?nanosized TiB_2p?nanosized TiC_p.The?"precipitates refinement was mainly attributed to the grain refinement which could improve the diffuson of solution atoms.It caused the uniform precipitation of?"precipitates,further decreasing the size of?"precipitates.iii)It was revealed that nanosized TiC_p,TiB_2p and(TiC_p-TiB_2p)increased the ultimate tensile strength and yield strength of the Al-Mg-Si matrix alloy 20 MPa and 13 MPa,34 MPa and 29 MPa,49 MPa and 59 MPa,respectively.The order of strengthening effect from strong to weak was:nanosized(TiC_p-TiB_2p)?nanosized TiB_2p?nanosized TiC_p.The strengthening mechanism included three aspects:1 the refinement of?"precipitates;2 the recrystallized grain refinement;3 the impediment of nanosized TiC_p and TiB_2p on dislocations.2)The effects and mechanism of nanosized TiC_p and nanosized TiB_2p on the passivation effect,intergranular corrosion(IGC)and corrosion resistance fo the matrix Al-Mg-Si alloy during the corrosion process were revealed:i)It was revealed that the corrosion resistance from high to low during the accelerated corrosion test,the electrochemical corrosion test and the immersion corrosion test was:the 0.5 wt%TiC_p/Al-Mg-Si composite sheet?the Al-Mg-Simatrix alloy sheet?the 0.5 wt%TiB_2p/Al-Mg-Si composite sheet.ii)The periodic corrosion behavior was found in the 3.5 wt%Na Cl solution,and the IGC was the main factor to influence the corrosion behavior of the Al-Mg-Si alloy.There were two corrosion stages in the first cycle.Stage I:the Al₂O₃ passivation film was formed at the beginning of corrosion process.Then,the IGC caused the severe corrosion till the exfoliation of Al₂O₃ film,reaching the maximum of corrosion weight loss.Stage II:the corrosion was reduced when the new Al₂O₃ film was formed.The Al₂O₃ film formed and consumed with IGC.When the consumption rate of Al₂O₃ film was over the formation rate,stage II ended,and another corrosion cycle was started.iii)It was revealed that the more second phases at grain boundaries,the more seriousIGC was.The addition of nanosized TiC_p could reduce the number of second phases at grain boundaries and increase the corrosion resistance.The number of second phases at grain boundaries from least to most was:the 0.5 wt%TiC_p/Al-Mg-Si composite sheet?the Al-Mg-Simatrix alloy sheet?the 0.5 wt%TiB_2p/Al-Mg-Si composite sheet.iv)It was revealed that the density of Al₂O₃ film was affected by two reasons:1 the grain refinement was beneficial for the formation of Al₂O₃ film.2 IGC was slight when the number of second phases at grain boundaries reduced.3)The formation and mechanism of the hierarchically heterogeneous structure with the layered distribution of nanosized TiC_p,bimodal-sized grains and bimodal-sized?? precipitates on the strength anisotropy were revealed:i)It was revealed that the YS and UTS of 32-layered 3.0-TiC_p composite were 50 MPa and 63 MPa,and 106 MPa and 109 MPa higher than those of the primary 3.0-TiC_pcomposite along RD and TD,respectively.The YS and UTS anisotropy were 56 MPa and 46 MPa along RD and TD,respectively.The uniform elongation decreased 3.9%and 3.1%along RD and TD,respectively.ii)It was revealed the uniform elongation along RD increased from 4.2%to 5.0% in the 3.0-TiC_p/M composite,compared with the 32-layered 3.0-TiC_p composite.The YS,UTS and the uniform elongation along TD increased from 380 MPa,431 MPa and 5.0%to 443 MPa,501 MPa and 6.4%,respectively.iii)It was revealed that the enhancement in the 3.0-TiC_p/M composite was attributed to the impediment of the fine??precipitate,uniformly distributed nanosized TiC_p and refined recrystallized grains on dislocations.iv)The strength anisotropy of the 3.0-TiC_p/M composite was eliminated by CARB processes.The strength differences were 2 MPa and 3 MPa along RD and TD in CARB_i sheet,respectively.The strength differences were 2 MPa and 4 MPa along RD and TD in CARB_ii sheet,respectively.v)It was revealed that the distribution of nanosized TiC_p was uniform with the help of 90°rotation and the interfacial shear force;a new hierarchically heterogeneous structure with the layered distribution of nanosized TiC_p,bimodal-sized grains and bimodal-sized??precipitates was constructed.vi)It was revealed that the strengthening mechanism of CARB processes was attributed to:the hierarchically heterogeneous structure with the layered distribution of nanosized TiC_p,bimodal-sized grains and bimodal-sized??precipitates led to the uniform microstructure and tensile properties.This study provides a new method to improve the tensile properties,strength anisotropy and corrosion resistance of deformed Al alloy,providing an experimental basis,technical and theoretical reference for the development of nanoparticle reinforced AMCs sheets with excellent mechanical properties and corrosion resistance.