Study On The Prepartion And Property Of In-situ?TiB₂+ZrB₂? Nanoparticles Reinforced 7055 Alloy Matrix Composites

7055 aluminum alloy,a typical representative of super high strength aluminum alloy?7000Series?,was widely used as high-performance structural material in aviation,aerospace,military transportation and marine engineering due to its high strength,low density and excellent formability.However,with the increasing demand for high load to weight ratio,long voyage and low energy consumption in these areas,it is of great theoretical and practical significance to further enhance the strength of 7055 alloy,especially for the elastic modulus.In recent years,international investigators have been involved in developing compounding and microalloying technique to improve mechanical and functional properties of aluminum alloy,such as strength,modulus and thermal conductivity.In this research,the in-situ?TiB₂+ZrB₂?/7055Al nano composites with high strength and high modulus were prepared by combination of in-situ synthesis and alloying technique.The mechanism of the synergistic effect of in-situ?TiB₂+ZrB₂?nanoparticles and rare earth yittrium on the microstructure and mechanical properties of 7055aluminum alloy were studied.Meanwhile,the evolution,strengthening and toughening mechanism of hot rolling and heat treatment on the microstructure and mechanical properties of the composites were also investigated.The microstructure observation of the as-cast composites indicated that the in situ generated?TiB₂+ZrB₂?particles with the form of particle clusters showed a relative uniform distribution in the aluminium matrix.The particles separated from each other with the size from 20 to 100nm and average particle size was about 80 nm.Besides,the in-situ formed TiB₂ and ZrB₂particles exhibited hexagonal shape.Further observations indicated that the interface between the TiB₂ or ZrB₂ particles and the matrix was clean and these particles were well bonded to aluminium matrix.The matrix grains of the composites changed from coarse dendrites to fine equiaxed grains with the increase of in situ reinforced-particles.The average grain size of the composites was reduced to 28.2?m when the particle content is 2 vol.%.However,it is interesting that when the particle content was higher than 2 vol.%,there is no further refinement of grain size.The addition of rare earth element yittrium can significantly refined the grain size of the composites.The result showed that the grain size of the composites decreased firstly and then increased with the increase of yittrium content.The smallest grain size was obtained when the content of yittrium was 0.10 wt.%.Meanwhile,the particle clusters showed relative uniform distribution and the average size of particle decresed to 68 nm.What's more,the morphology and size of the particle became uniform.The microstructure observations of the hot-rolled and heat treatment composites is done,the results showed that there are few porosities and a large number of fine equiaxed grains due to dynamic recrystallization in hot-rolled composites.Meanwhile,the second phases and particle clusters distributed uniformly along the rolling direction.What's more,various rolling deformation and temperature have a obvious effect on the microstrure of the compoitses.After heat treatment,the dispersed nano-precipitates?MgZn₂?are formed and these nano-precipitates play an important role in improving mechanical properties of the composites.The tensile testing results of the compsites showed that the strength and Young's modulus of the composites increased significantly in comparison to the 7055 matrix alloy.The maximum ultimate tensile strength of the as-cast composites was about 361 MPa,with yield strength of323.1 MPa,Young's modulus of 76.8 GPa and elongation of 3.6%,which increased by 31.3%,28.7%,44.0%and 9.0%than that of the 7055Al alloy,respectively.It is worth noting that the Young's modulus of the composites with 2 vol.%particle content achieved 84.5 GPa,which increased by 20%than that of the 7055Al alloy.When mass fraction of yittrium was 0.10 wt.%,the maximum ultimate tensile strength is about 380.5 MPa,with elongation of 4.2%,which increased by 5%and 17%than that of the 7055Al alloy,respectively.The tensile testing results of the as-rolled composites indicated that the composites can obtain maximum ultimate tensile strength and elongation when the rolling deformation is 83%and rolling temperature is 420 ^oC.The maximum ultimate tensile strength is about 451.4 MPa,with elongation of 6.1%,which increased by 18.7%and 52.5%than as-cast composites.After heat treatment,compared with as-rolled composites,the ultimate tensile strength incresed significantly with a slight reduction in elongation of the composites.The maximum ultimate tensile strength was about 663.5 MPa,with elongation of 5.6%,which increased by 47.0%and decreased by 6.7%than as-rolled composites.Analysis of the strengthening and toughening mechanism of the composites demonstrates that the in-situ TiB₂ and ZrB₂ nanoparticles can refine the grain,impede the dislocation movement and enhance the bearing capacity of the materials.As a result,the strength of the composite is obviously improved.The possible strengthening mechanisms were identified and their contribution was confirmed as:?_CTE>?_Orowan>?_g.In the process of hot rolling,the fine recrystallized grain appears,which results increase of the grain boundary.The elimination of the defects such as bubbles and cracks can effectively reduce the number of crack sources and the strength of the material is further improved.After heat treatment,the dispersed nano precipitates?MgZn₂?enhance the strength of the materials and play a role of solid solution strengthening.