Study On Solution And Ageing Behaviors Of In-situ TiB₂/2009 Composites

Controlling the microstructure of particle reinforced aluminum matrix composites by heat treatment is an important way to improve the material properties of the composites.Compared to aluminum alloys,the solution and ageing bahaviors of the composites are changed due to the changes of a variety of solution and ageing conditions caused by the addition of reincforce particles,such as interfaces,stresses and dislocations.However,there are great controversies about the influence of reinforcements on solution and ageing behaviors in both experiments and theories,which hinders the development of particle reinforced aluminum matrix composites.Moreover,compared to traditional composite prepared with non in-situ particles,there are obvious features of the particles in size,distribution and interface with the matrix in the in-situ TiB₂ particle reinforced aluminum matrix composites.Thus,there are special effects of particles on the microstructures and properties of the composites during heart treatment.In order to further understand the influence of in-situ particles on solution and ageing behaviors of aluminum matrix composites,and then to optimize the heart treatment crafts to improve the mechanical properties,the solution and ageing behaviors of in-situ TiB₂/2009 composites is studied.In the study of solution treatment,we discovered that the solution of the composites was inhibited in the clusters of particles and second phases,and we firstly provide a new way of solution treatment of the composites that high temperature solution treatment under the temperature over the eutectic point.The results show:the second phases in the clusters are difficult to solution in the treatment below the eutectic point,the long time solution treatment below the eutectic point leads larger grain sizes,which reduce the mechanical properties of the composites;the disolution of the second phases in the in-situ TiB₂/2009 composites is promoted through increasing the solution treatment temperature above the eutectic point,and better solution effect is obtained,leads to higher mechanical properties,such as tensile properties,fatigue limit and impact toughness;the larger size of clusters,the better effect obtains through high temperature solution treatment;the melting regions are confined inside the clusters due to the particles,which reduce the damage of the melting.Base on the diffusion theory,the solution model of the composites is derived through building a particle layer between the second phase and the matrix.This model analyses the physical essence of the solution of the second phase in the composites:the time needed to dissolve the second phase is much longer in the composites than in the alloys,the solution atom concentration outside the clusters is lower,so the temperature need to melt is higher.In the study of ageing treatment of the composites,a two step ageing behavior was discovered.After quenching from solution treatment temperature,there are high dislocation density regions around the particles due to the difference in coefficient of thermal expansion between the particles and the matrix.This leads to the formation of“particle induced rich/poor precipitate zones”in the composites.Base on the diffusion transformation theory,the precipitation model of the composites is derived through building particle induced interfaces and dislocations.This modle analyses the process and principle of the formation of the“particle induced rich/poor precipitate zones”:the high dislocation density accelarate the precipitation in the regions around the particles;the precipitation is inhibited due to low solute atoms concentration and low vacancy concentration in the farther regions.This kind of structrures does harm to the mechanical properties of the composites through the poor precipitate zones.The nucleation rate in the composites can be increased by high temperature solution treatment,thus the density of the precipitates is increased,and the“particle induced rich/poor precipitate zones”structures are optimized,leading to higher mechanical properties.Increasing the solution temperature from 500?to 510?,the tensile properties of the composites aged at 180?for 14 h increase from 459 MPa to 476 MPa in yield strength,from 564 MPa to 597 MPa in ultimate tensile strength,from 8.8%to10.7%in elongation.The dislocation density induced by difference in coefficient of thermal expansion is increased through cryogenic treatment,leading to more obvious“particle induced rich/poor precipitate zones”structures,which reduces the ultimate tensile strength and elongation of the composites.With pre-strech treatment,homogeneous dislocations generate in the composites,leading to homogeneous precipitates,thus the“particle induced rich/poor precipitate zones”structures disappear,and the mechanical properties of the composites increase.With 3%pre-stretch,ageing at 180?for 1 h,the composites obtain high strength and best elongation,the yield strength is 509 MPa,the ultimate strength is 620 MPa and the elongation is 11.1%.