Study Of Structure And Properties Of ZrB₂/7055 And Sc-ZrB₂/7055 Composites

7055 aluminium alloy is widely used in industrial production due to its low density,high strength and good corrosion resistance.However,with the development of automobile,rail transportation,aerospace and other fields,people have put forward higher requirements for the lightness and strength of the material.Casting 7055 aluminium alloy due to its casting is easy to produce segregation,the existence of coarse dendrites,resulting in weak alloy performance,it is difficult to meet the needs of industrial production.Although the coarse dendrites in the material are transformed into equiaxed crystals after adding ceramic particles,the properties are greatly improved compared to traditional aluminium alloys,but the ceramic particles are limited by the high surface energy,easy agglomeration and insufficient grain refinement,thus the performance of the material is not widely used.For this reason,the addition of rare earth elements improves the distribution of ceramic particles and further refines the grain size in order to obtain a material with better properties.In this thesis,ZrB₂/7055 composites were prepared by the mixed salt reaction method,using 7055 aluminium alloy as the matrix alloy and choosing KBF₄ and K₂Zr F₆ mixed salt reaction systems.Using the same process,Sc-Zr B₂/7055 composites were prepared,with Sc elements introduced in the form of Al Sc₂ alloy;the organization and properties of the two composites in the as-cast and heat-treated states were The analysis was carried out.The main results are as follows:（1）The microstructure of 7055 aluminium alloy is coarse dendritic,after the addition of Zr B₂,it transforms into equiaxed crystals and the grain size decreases significantly;the addition of rare earth elements further refines the grains.7055 aluminium alloy’s main structure consists ofα-Al,Mg Zn₂,Al₂Cu Mg and Al₂Cu;after the addition of ceramic particles the material’s Sc has significantly improved the morphology and distribution of Zr B₂,which can effectively reduce the agglomeration size of Zr B₂ particles and promote a more uniform distribution of Zr B₂ particles.（2）The properties of the cast composites.The addition of ZrB₂ effectively improved the microhardness,strength,toughness,wear resistance and electrochemical properties of the material;the addition of element Sc further enhanced all properties of the material.The main mechanisms for toughening the composites were fine grain strengthening,load transfer strengthening,Zenner pegging strengthening,dispersion strengthening and coefficient of thermal expansion and elastic modulus mismatch strengthening.（3）Microstructure and properties of composites in the heat treated state.It was found that heat treatment does not change the morphology and distribution of Zr B₂ in the alloy.After heat treatment,the strength of Zr B₂/7055 was significantly enhanced,with higher enhancement of Sc-Zr B₂/7055 properties.Of the three heat treatments,the best strength is obtained by single-stage ageing,followed by regression re-aging and the worst by double-stage ageing,with the opposite electrochemical properties.The electrochemical properties of the material after heat treatment are lower than in the as-cast state due to the corrosion channels created by single-stage ageing.Regression re-aging results in a material with better overall properties.