Study On Microstructure And High-cycle Fatigue Performance Of In-situ ZrB₂/AA6111 Aluminum Matrix Nanocomposites

At present,the requirements for energy saving and emission reduction of automobiles are becoming higher and higher,and the development of new energy vehicles has encountered problems of insufficient battery life.These problems have restricted the further development of the automobile industry,and automobile lightweight has become the optimal solution to solve these problems.Due to its excellent comprehensive mechanical properties,AA6111 aluminum alloy has become the material of choice for automotive lightweighting,but with the continuous improvement of automotive performance,it also puts forward higher requirements for its safety and reliability,which requires the material to have a higher high cycle fatigue performance.In this paper,on the basis of AA6111 aluminum alloy,a novel anti-fatigue in-situ ZrB₂ nanoparticle reinforced AA6111 aluminum matrix composite material was prepared by in-situ reaction technology.The effect of particle content on the microstructure and tensile properties of in-situ ZrB₂ nanoparticles reinforced AA6111aluminum matrix composites as-cast and after extrusion deformation were studied;The heat treatment strengthening phenomenon of composites was studied,and the optimal heat treatment parameters were determined.On this basis,the high-cycle fatigue properties and fracture mechanism of T6 composites with different particle contents were investigated.Research on the microstructure of ZrB₂/AA6111 composite materials showed that:The addition of ZrB₂ particles increases the heterogeneous nucleation sites in the composite,and the ZrB₂ particles on the grain boundaries hinder the growth of the crystal grains,while inhibiting the formation of network precipitates and dendrites,resulting in grain refinement and equiaxation.With the increase of particle content,the grain refinement effect of composite materials is getting better and better,but the particle agglomeration size shows an increasing trend.After hot extrusion deformation,the ZrB₂ particles in the composite are distributed in the matrix along the extrusion direction.With the increase of the particle content,the number and width of the ZrB₂particle bands increase significantly.The overall particle agglomeration size is significantly reduced compared with before extrusion,and the distribution is more uniform along the extrusion direction.The composite material dynamically recrystallized during the extrusion process to form fine equiaxed crystals.During the hot extrusion process,the hard ZrB₂ particles will become the nucleation sites of the grain boundaries,which promotes the recrystallization process.Therefore,as the content of ZrB₂ particles increases,the number of recrystallized grains increases rapidly,making the average grain size obvious decrease.The mechanical properties of ZrB₂/AA6111 composite materials showed that:As the content of particles in the composite increases,the tensile strength of the as-cast and extruded composites continues to increase,and the elongation tends to increase first and then decrease.It is found that the elongation of the composite is inversely related to the average particle agglomeration size.The hardness of the composite material is higher than that of the matrix alloy,and it increases as the content of reinforcing particles increases.With the increase of aging time,the hardness change trend of the composite material is:increase?stable?decrease,the reason is that the precipitation phase changes with the increase of aging time.After T6 heat treatment of the composite extruded material,it was found that the ultimate tensile strength was significantly improved,but the elongation decreased.The high-cycle fatigue properties of ZrB₂/AA6111 composite materials showed that:The composite with a particle content of 2 vol.%has the highest fatigue limit of125 MPa,which is an increase of 31.6%relative to the matrix alloy.According to the composite material S-N curve and its mechanical property test results,the engineering Goodman straight line,Gerber parabola and Soderberg straight line of the composite material with different particle content are obtained,and the fatigue life engineering formula and the equivalent life curve engineering formula of the composite material with a particle content of 2 vol.%are calculated based on the experimental data.And their characteristics are analyzed separately,which provides an important basis for the fatigue life prediction of composite materials in practical applications.Observation of fatigue fractures shows that the fatigue crack growth process of matrix alloys and composite materials can be divided into four stages:The stage of crack initiation,the stage of microcrack propagation,the stage of stable crack growth and the stage of unstable fracture.Observation of the crack initiation zone in the fractures of the matrix alloy and composite materials shows that the crack initiation is due to the expansion of the resident slip zone near the sample surface.The width of the fatigue striations of the composite material are smaller than that of the matrix alloy,indicating that the crack propagation in the composite material is slower.The crack propagation in the matrix alloy is represented by the fracture of Mg and Si compounds,and this phenomenon is rarely found in composite materials.The cracks in the composite material will be hindered by the reinforcing particles during the propagation process and change their original propagation path.This is one of the important reasons for the cracks to grow slowly in the composite material.The research on the anti-fatigue mechanism of ZrB₂/AA6111 composite material shows that:The reasons leading to the improvement of the fatigue life of composite materials include:?1?The strengthening effect of the particles makes the composite material produce less strain under the action of cyclic stress,causing a smaller degree of slip band expansion,thereby extending its fatigue life.?2?ZrB₂ particles delay the propagation of cracks by hindering the movement of dislocations.?3?The more obvious crack closing effect of composite materials leads to slower crack growth.?4?Due to the addition of ZrB₂ particles,the grain size of the composite material is refined,which increases the grain boundary density and hinders the crack propagation.