Study Of Microstructure And Mechanical Properties And Stability Of Al₃Zr In Microalloyed Al-Cu-（Zr,Ti,V） Cast Alloy

Cast Al-Cu alloy has been widely used in aerospace,automobile and national defense industry due to the combination of high strength,good tensile properties and free-cutting.However,when the working temperature is higher than 200^oC,the main strengthening phaseθ″in Al-Cu alloy gradually changes into stable phase,reducing its strengthening effect and shortening the service life of the material,which restricts the further application of Al-Cu alloy.It is of great research significance to further improve the strength and heat resistance of Al-Cu alloy by trace addition of alloying elements to form nano-sized and dispersed thermal stability Al₃M phase in the matrix of Al-Cu alloy.In this research,by means of the scanning electron microscope（SEM）,transmission electron microscopy（TEM）,Spherical aberration correction electron microscope（Cs-corrected TEM）,electron probe microanalyzer（EPMA）,Energy Disperse Spectroscopy（EDS）,Microhardness and tensile tester and the first principles calculation,we focus on Al-Cu alloy and Al-Cu alloys with single addition or combined addition of Zr,Ti and V,and systematically investigate the effect of different heat treatment conditions on the mechanical properties and microstructure of the alloy and the effect of anti-phase boundary and elements segregation on the stability of precipitates.Multi-scale quantitative study of microalloying elements on the evolution of precipitats is one of key points in this work.The main conclusions are as follows:（1）Heating rate has effects on the miacrostructure and properties in Al-Cu and Al-Cu-Zr-Ti-V alloy.Compared the alloy heat treated with a 50℃/h heating rate,the alloys heated with a higher heating rate（300℃/h）exhibit a significant improvement of18.5%、8.1%in hardness,respectively.and a decrease about 5 h in the holding time for peak hardness.The strength did not change much at high temperature,but the elongation rate increased by 51.7%,63.6%,respectively.The heating rate is closely relative to the size and distribution of Al₃Zr precipitate.Higher heating rate induces the precipitates with larger mean size.In order to ensure the properties of the alloy,the short solid solution time should be chosen when the temperature rises rapidly and the long solid solution time should be chosen when the temperature rises slowly.（2）The best solution heat treatment condition of Al-Cu,Al-Cu-Zr-Ti,Al-Cu-Zr-Ti-V alloy in this work is heat treatment at 500^oC with holding time of 10 hours.The combined addition of Zr,Ti and V can more effectively promote the nucleation of Al₃Zr precipitated phase,reduce the precipitates coarsening rate,so improve thermal stability of the alloy after high temperature aging.After 184 days of exposure at 250^oC,the microhardness of the alloy with Zr,Ti,V and Zr,Ti decreased by about 5.3%,6.8%,respectively,while that of the base alloy decreased by about 34.9%.The Al₃Zr precipitates with heterogeneous nucleation at the dendrites edge gather together to form clusters with complex morphology,which was mainly caused by the continuous precipitation of Al₃Zr precipitates near the dislocation.（3）Two groups of anti-phase boundaries（APBs）in Al₃Zr precipitates are observed from<001>_Al zone axis.The anti-phase boundary（APB）located on the{100}planes is necessary factor for transforming occurrence between L1₂ and D0₂₃.The other type of APB situated on the{110}plane,which connects L1₂ and D0₂₃ structure or two mutually perpendicular{100}APBs.Both types have a displacement vector of a/2<110>.（4）Cu elements will access in the Al₃Zr precipitates in Al-Cu-Zr alloy,and substitute the Al sites to form the（Al,Cu）₃Zr precipitates,among which Al_2.5Cu_0.5Zr was the most stable.Cu elements would increase the stability of L1₂ structure and affect the precipitation behavior of Al₃Zr.During the transition of precipitation phase,three structures appear:L1₂,D0₂₃,and the intermediate phase with Cu rich.The intermediate phase appeared during the mutual transformation between D0₂₃ and L1₂,its cell parameters were a=b=4.086? and c=10.98?.