Research On The High Temperature Creep Properties Of A356.2 Matrix Composites Reinforced By In Situ Nanoparticle ZrB₂ And Er Rare Earth

A356.2 aluminum alloy has been used in aerospace,transportation,defense and other fields because of its excellent casting and mechanical properties.However,with the development of new aerospace vehicles,high-speed trains,energy saving vehicles,the traditional A356.2 alloy has been unable to meet the requirements of higher specific strength,specific modulus,creep resistance and fatigue resistance.Especially,in the harsh environment of high temperature and high load,the traditional aluminum alloy cannot be widely used because of its low temperature strength,poor creep resistance and short service life.It is an important way to improve the strength and creep resistance of alloys by introducing nanodispersions.In this research,nano ZrB₂ particle reinforced A356.2 matrix composites were prepared by in-situ synthesis technique.At the same time,rare earth of Er was used to modify and refine the matrix,as well as the dispersed precipitates were formed to further strengthen the composites.The microstructure,mechanical properties and creep properties of the composites were studied.Meanwhile,with the help of OM,XRD,SEM,TEM,electronic universal material testing machine,electronic high temperature creep testing machine,the effects of in situ nanoparticles,rare earth Er and the synergistic effect of nanoparticles and rare earth Er on the microstructure,mechanical properties and high temperature creep properties of A356.2 alloy were studied.The organizational structure analysis showed that in the ZrB₂/A356.2 composites synthesized from A356.2-KBF4-K2ZrF6 reaction system,the size of the reinforcing phases was 30-90 nm,the reinforcing phases showing the irregular quadrilateral,hexagonal,spherical and so on.And the primary ?-Al phase can be refined by the ZrB₂?But with the increase of the reinforcements,the aggregation of the reinforcements is increased.The addition of rare earth element Er can significantly refine the primary phase of A356.2 alloy,the grain changed from coarse dendrite to fine equiaxed crystal.At the same time,the eutectic silicon can be changed from the lath and long needle to small spherical or short rod.Moreover,Er can make the particle size of the composite material smaller and the distribute of the particles more evenly,When the content of Er was 0.10 wt.%,the effect of the composite was the best.The room temperature tensile strength of the material was improved after synthesized ZrB₂ particles in situ.And the tensile strength reached the maximum value of 192 MPa,when the mass fraction of ZrB₂ particles was 3wt.%,which was 24.68% higher than the matrix.The mechanical properties of A356.2 alloy at room temperature were improved after adding Er,the mechanical properties were the best when mass fraction of Er was 0.10 wt.%,the tensile strength was 207 MPa,the elongation was 7.5%,which were increased 34.42% and 59.57% compared with the A356.2 alloy.After the addition of Er,the mechanical properties of ZrB₂/A356.2 composites at room temperature were significantly higher than those without Er,when the mass fraction of Er was 0.10 wt.%,the mechanical properties of the composites were the best,the tensile strength was 225 MPa,the elongation was 8%,and the Vivtorinox hardness was 103.07 HV,which was increased by 17.19%,45.45% and 21.05% compared with those without Er.The mechanical properties of ZrB₂/A356.2 composites with different mass fraction of Er were treated by T6,and the mechanical properties of the composites were the same as those of the cast,when the mass fraction of Er was 0.10 wt.%,the mechanical properties of the composites were the best,the tensile strength of the material was 345 MPa,the elongation was 9.2%,and the Vivtorinox hardness was 121.83 HV,increased by 30.19%,37.31% and 21.94% compared with composites without Er.High temperature creep test showed that the creep properties of ZrB₂/A356.2 composites were affected by the mass fraction of Er,test temperature and applied stress.When the mass fraction of Er was 0.10 wt.%,the steady-state creep rate of the composites was the smallest,and the creep life was the longest,and the apparent stress index and the apparent activation energy was the largest.The results showed that the introduction of the nano reinforcements and the uniform dispersion of the matrix and the refinement of the eutectic grains were beneficial to the improvement of the high temperature creep resistance of the composites.The steady creep rate of the composites increases with the increase of creep temperature and applied stress.The linear regression of the steady-state creep rate and the logarithm of the applied stress were carried out,it was found that the true stress exponent was 5,which indicated that the creep was controlled by the lattice diffusion mechanism,and the improvement of the creep resistance of the composites was mainly due to the deformation mechanism of the second phase particles.The creep fracture mode of the composites without Er was quasi cleavage plus partial dimple fracture,but the creep fracture mode of the composites with Er was dimple fracture.This was closely related to the refinement of the matrix.