Microstructure And Fatigue Properties Of In-situ(ZrB₂+Al₂O₃) Nanoparticle Reinforced AA6016 Matrix Composites

With the continuous increase of car ownership in our country,the energy crisis and the increasingly severe environmental pollution situation,automobile lightweight has become the most direct and effective way to save energy and reduce emissions.Aluminum alloy is currently the most widely used light alloy material in the automotive manufacturing field due to its low density,high specific strength and specific rigidity,and good formability.The development of new energy vehicles and lightweight technology require that under the premise of ensuring vehicle safety,the weight of structural parts should be reduced as much as possible,and the service life should be high.However,the traditional aluminum alloy has been unable to meet its higher strength,excellent impact resistance,anti-fatigue performance requirements.In-situ nano-particle reinforced aluminum matrix composites have a unique in-situ nano-reinforcement effect,the size of the reinforced particles is nanometer and the interface between the reinforced particles and the matrix is well,which is expected to greatly improve the fatigue life of aluminum alloy.In this paper,AA6016 regarded as the matrix,the in-situ(ZrB₂+Al₂O₃)_np/AA6016 composites were prepared via the Al-Zr-B-O reaction system.The effects of the content of ZrB₂ and Al₂O₃ particles on the microstructure and tensile properties of the composites before and after hot extrusion were investigated.The high cycle fatigue behavior and fracture mechanism of the composites were studied,which provided theoretical basis and technical basis for the promoting industrial application of particle reinforced aluminum matrix composites.The microstructure of in-situ(ZrB₂+Al₂O₃)_np/AA6016 composites showed that:the ZrB₂ and Al₂O₃ were mainly distributed along grain boundaries in the form of particle agglomeration due to the"nano-size effect"and"particle pushing effect".With the increase of(ZrB₂+Al₂O₃)nano-particles content,the grain size of the composite material is gradually refined into equiaxed crystals,and the grain refining effect is obvious After hot extrusion deformation,the coarse AlFeSi phase and(ZrB₂+Al₂O₃)particle clusters were effectively broken and uniformly distributed in a streamlined direction along the extrusion direction.The number and area of the particle clusters were significantly reduced.With the increase of the volume fraction of nano-particles(ZrB₂+Al₂O₃),the area of"matrix-enriched area"in the composite decreased significantly,the number of elongated particle bands increased and the spacing between particle bands decreased.During the process of hot extrusion,dynamic recrystallization took placed firstly in the vicinity of ZrB₂ and Al₂O₃ particles,and with the increase of particle content,the recrystallized grains distributed near the particles increase and the size becomes smaller.The tensile properties of in-situ(ZrB₂+Al₂O₃)_np/AA6016 composites showed that:before and after hot extrusion,with the increase of the volume fraction of nano(ZrB₂+Al₂O₃)particles,the tensile strength and elongation of the composites both first increased and then decreased.Among them,when the particle content was 3 vol.%,the tensile performance was the best.The tensile strength and elongation of the hot-extruded composite were 398.1 MPa and 28.3%,respectively,which were 28%and 68%higher than those before extrusion.In the T6 state,the high-cycle fatigue behavior of the in-situ(ZrB₂+Al₂O₃)_np/AA6016 composites with different particle content after hot extrusion showed that:the fatigue limit of the composite was higher than that of the matrix alloy,and it first increased and then decreased with the increase of particle content.When the particle content of ZrB₂ and Al₂O₃ was 3 vol.%,the fatigue limit of the composite was the highest,130 MPa,which was 44%higher than that of the matrix alloy.Therefore,the addition of ZrB₂ and Al₂O₃ particles improved the fatigue crack initiation resistance,this was mainly attributable to the in-situ nano-ZrB₂ and Al₂O₃ reinforcement effect,load transfer and the blocking effect of particles on dislocations.The fatigue crack growth behavior of composites showed that:in the steady growth stage,the ZrB₂ and Al₂O₃ particle bands would hinder the fatigue crack propagation,fatigue cracks cannot bypass the particle bands but selectively deflect to the area with the lowest particle density,thereby reducing the fatigue crack growth rate;in the rapid expansion stage,the stress intensity factor in the plastic zone was much higher than that in the steady growth stage.The fracture of coarse brittle precipitates and the internal cracking of coarse particle clusters accelerated the fatigue crack growth rate,which made the fatigue crack growth rate of the composite higher than that of the matrix alloy.The main reasons for improving the fatigue crack growth resistance of in-situ(ZrB₂+Al₂O₃)_np/AA6016 composites were as follows:(1)ZrB₂ and Al₂O₃particles can slow down fatigue crack growth by hindering the movement of dislocations.(2)The addition of ZrB₂ and Al₂O₃ particles can refine the grain size and increase the grain boundary density.The grain boundary can inhibit the propagation of short cracks,while the grain boundary with larger grain boundary orientation difference tended to cause crack deflection,thus reducing the fatigue crack propagation rate.