| Aluminium matrix composites(AMCs)reinforced with medium or high volume fraction of Si C particles have been widely used in the field of the defense equipments,aerospace filed and precision instruments for their good dimensional stability,high specific strength and modulus as well as good wear resistance.However,due to the large difference in chemical and physical properties between Si C particles and the Al matrix,issues,such as weak interface bonding,limited plasticity and difficult processing have been aroused.Appropriate selection of reinforcements and a good interface bonding between the components can be an effective way to improve the comprehensive properties of the composites.High-entropy alloys(HEA)have been a good candidate serving as reinforcements in AMCs because of their excellent mechanical properties and good interface with the matrix.This thesis focuses on using nanocrystalline high-entropy alloy(NHEA)particles as a new type of reinforcement in the aluminum matrix composites.Starting from the effects of types,volume fraction,interface thickness and distribution of HEA particles on microstructural evoluation and mechanical properties of AMCs,a high-performance NHEA particle reinforced aluminum matrix composite was fabricated,and the strengthening/toughening mechanism of the composite was clarified.Based on the above findings,the influence of NHEA particles and Si C particles on the properties of composites was comparatively studied.Finally,hybrid AMCs reinforced by Si C particles and NHEA particles were produced.The main conclusions of the thesis are as follows:(1)The mechanical properties of HEA particle reinforced 2024 aluminum matrix composites prepared by powder metallurgy with different HEA particle types(atomized powder and mechanical alloyed powder),different volume fractions(0～30 %)and different interface thicknesses(100 nm～1 μm)are compared.The results show that when the volume fraction of particles is not over 15 %,NHEA particle reinforced Al matrix composites with an interface thickness of ～100 nm have better mechanical properties.Among them,when the volume fraction of NHEA particles is 7.5 %,the tensile yield strength of the composite is 419 MPa,and the fracture strain is still ～ 8 %,which is superior to mechanical properties of the reported HEA particle reinforced aluminum matrix composites.(2)By controlling the milling time,assisted by hot pressing and hot extrusion processes,three typical NHEA particle distributions were constructed in the composites.They were particle agglomeration mode,uniform distribution mode and banded structure mode,respectively.It is found that the mechanical properties of particle agglomerated composites are the worst,and the fracture strain(7.1 %)of the composite with uniform particle distribution is the best.The composite with a high proportion(45 vol.%)of banded structure has the best tensile yield strength(464.9 MPa)but low fracture strain(2.2 %).However,the composite with a proper proportion(19 vol.%)of banded structure has excellent combination of yield strength(445.2 MPa)and fracture strain(5.4 %).The results of mechanical tests indicate the strengthplasticity could be adjusted by controlling particle distribution and the proportion of the banded structure.(3)The micro-size zone near the interface between reinforcements and the matrix is the main reason for the difference in mechanical properties of Si C particle reinforced AMCs and NHEA particle reinforced AMCs.Si C particles have better strengthening effect than NHEA particles,due to the higher density thermal mismatch dislocations at the Si C/Al interface.However,NHEA particle reinforced AMCs have better plasticity than that of Si C particle reinforced AMCs,because the damage tolerance of the composite is improved due to the passivation of cracks caused by high-strength NHEA particles and good interface bonding with low mismatch.In addition,NHEA particle reinforced AMCs have better plastic deformation ability and turning performance than that of Si C particle reinforced AMCs.(4)A hybrid particle reinforced 7075 aluminum matrix composite with high strength and good wear resistance was developed by the optimization of components(the matrix and reinforcements combination)and solution time(1～4 h).Compared with Si C particle reinforced7075 Al composites with the same particle volume fraction(45 vol.%),the hybrid reinforced composite exhibits excellent comprehensive properties,such as higher strength(712 MPa)and plasticity as well as better wear resistance,i.e.The plastic microzones with low density dislocations formed around NHEA particles significantly reduce the density of fractured Si C particles nearby and improve the bearing capacity of the composites,which is the reason for the improved strength and wear resistance of the composite.The inhibition of NHEA particles on crack propagation promotes fracture absorption energy,which is the main reason for the improved plasticity of the hybrid composite. |
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