Effects Of Zn Contents On Microstructures And Mechanical Properties Of Accumulative Orthogonal Extrusion Processed TiB₂/Al-Zn-Mg-Cu Composites

Nano-particle reinforced Al matrix composites have broad application prospects in the aerospace industry due to their light weight,high strength and high modulus.In-situ process is used to introduce the particle reinforcement phase,which is due to the better wettability,the stronger bonding strength and the more nano-scale particles formed between particles and matrix interface than traditional external addition method.However,nanoparticles tend to agglomerate at the grain boundary in order to reduce the Van der Waals'force among particles due to their high suiface energy.When the composites are exerted external load,the stress concentration is easy to cause the formation of void and arise the fracture of materials,which restricts the improvement of material properties.In order to effectively improve the distribution of reinforcing particles,in-situ TiB₂ reinforced Al-Zn-Mg-Cu composites are studied in this paper.Two kinds of schemes are used to achieve particle dispersion:1)Elemental regulation is used in the solidification stage of materials to improve the particle dispersibility.This research increases Zn content in the composition of the composite to reduce the solid-liquid interfacial energy between the ceramic particles and the Al flux to achieve particle dispersion;2)Severe Plastic Deformation?SPD?process is used to improve the dispersibility of the particles.The Accumulative Orthogonal Extrusion Process?AOEP?is developed by writer's research group to disperse the particles by intense shear strain.This experiment studies the particle dispersibility and mechanical properties of the material under three processing conditions:As Cast-Extrusion Process?EP?-AOEP state.The research results are found that:?1?The effects on Zn content in particle dispersion of composites:1)The increase of Zn content promotes the dispersion of TiB₂ particles.The reason is that during the solidification process of the material,the increase of Zn content reduces the interfacial energy between the solid phase TiB₂particles and the liquid phase Al matrix,which promotes the engulfment of the TiB₂ particles by the solidification front.Therefore,the TiB₂ particles at the grain boundary are reduced and the particle dispersibility is enhanced;2)SPD promotes the dispersion of TiB₂ particles.The SPD process exerts severe plastic deformation on the material.Due to the inhomogeneous deformation between particles and the matrix material,the material is subjected to severe shear strain,which promotes the fragmentation of the TiB₂ particle agglomerate and finally achieves particle dispersion.?2?The effects on Zn content in the grain of composites:1)In the As Cast state,the increase of Zn content causes the increase in the grain size of the material.This is mainly due to the fact that during the solidification process,more TiB₂ particles in the high Zn composite material are phagocytized into the interior of the grain,and the TiB₂ particles in the grain boundary are reduced.Therefore,the ability to pin the grain boundary is weakened,resulting in as-cast microstructure grain size growing up;2)In the EP state,the increase of Zn content causes the decrease in the grain size of the material.This is mainly because the SPD promotes the dislocation propagation inside the grains of the composite material,while the high Zn material has higher ability to contribute to dislocation propagation due to the higher TiB₂ content inside the grains.This behavior promotes the generation of the small-angle grain boundaries and the transformation to the large-angle grain boundary,and finally achieves grain refinement;3)In the AOEP state,the grain size of high Zn sample changes a liitle because the grain refinement has been achieved in the EP state.For the low Zn content sample,SPD promotes further grain refinement of the material.Therefore,high Zn samples can achieve grain refinement at the lower strain level.?3?The effects on Zn content in mechanical properties of composites:It is found that the dispersion of TiB₂ particles in TiB₂/Al-Zn-Mg-Cu composites is different from the traditional alloy strengthening mechanism.After the improvement of TiB₂ particles dispersion,the strength of the material is significantly improved.This is mainly due to the fact that the agglomerates of TiB₂ in the grain boundary contribute less to the Orowan strengthening.When the particle dispersion is strengthened,the TiB₂nano-sized particles dispersed inside the grains increase,and the contribution to the strength of the Orowan bypass strengthening mechanism increases.In addition,precipitation strengthening,grain boundary strengthening and solid solution strengthening also play an important role in the strengthening behavior of materials.Moreover,the fracture behavior of composite in EP after T6 heat treatment is dominated by intergranular fracture,while the fracture behavior of composite in AOEP after T6 heat treatment is dominated by transgranular fracture,which indicates that AOEP can effectively improve the plasticity of the material.