Study On Effects Mechanisms Of Zr And V In 7xxx Series Al Alloy

7150 aluminum alloy is a representative ultra high strength 7xxx series aluminum alloy,one of its evolutionary features is the use of trace Zr instead of Mn,Cr as grain refine and recrystallization inhibit element.However,the size and distribution of Al₃Zr dispersoids form in ingots of this alloy which subject to conventional homogenization around 470℃ directly are heterogeneous.This will cause seriously partial recrystallization in material after final heat treatments and will affect its applying performance.In addition,V also has effects on grain refinement and recrystallization inhibiting,and trace addtion of V can form large number of dispersoids,but study on the effects of trace addition of V in ultra high strength 7xxx series aluminum alloy is limited.As part research of Ministry of Science and Technology support program "New Toughening Heat Treatment Technology Reaserch",the purposes of present study are to study the effects of pretreatments which before convential homogenization on precipitation law of Zr-containing and V-containing dispersoids,recrystallization resistance,microstructure evolution during plastic working process and recrystallization law during annealing in ultra high strength 7150 aluminum alloy.Use classical precipitation nucleation,growth and coarsening theory to reveals the precipitation mechanism of dispersoids,and use nucleation,growth theory of recrystallization to reveals the mechanism of recrystallization resistance increasing due to pretreatments.Lay the foundations for developing new homogenization heat treatment process and further improving combination properties of ultra high strength 7150 aluminum alloy.The equilibrium solubilities of Zr and V in Al-6.4Zn-2.3Mg-2.2Cu（wt.%）alloy were calculated using phase diagram,and the effects of pretreatments（250℃ to 400℃）which before conventional homogenization at 470℃ for 24h on the precipitation law of Zr-containing and V-containing dispersoids in 7150 type aluminum alloy were studied.The results show that,the equilibrium solubilities of Zr and V at temperature between 250℃ and 400℃ are range from 0.00062%to 0.017%and from 0.000042%to 0.041%,respectively.After the ingots of 7150 type aluminum alloy containing trace of Zr（about 0.1 wt%）and containing trace of V（0.11 wt%）subject to conventional homogenization at 470℃ for 24h,fine spherical Al₃Zr dispersoids which coherent with Al matrix and having a volume fraction of between 0.28%and 0.33%are form in the former,and rod-like,triangular shape and nearly spherical Al21V2 dispersoids which incoherent with Al matrix and having a volume fraction of between 1.82%and 2.09%are form in the latter.But pretreatments does not change the morphologies and the interface relationship with Al matrix of dispersoids,also they have insignificant effect on the volume fraction of dispersoids.When holding times of pretreatments are within 24h,the holding time have obvious effects on the precipitation of dispersoids.But,when pretreated at 250℃ and above 300℃ the variation trends of size and density of Al₃Zr after conventional homogenization are opposite.When pretreated at 250℃,the size of Al₃Zr dispersoids decrease and the density increase with holding time of pretreatments prolonged.When pretreated above 300℃,the size of Al₃Zr dispersoids increase and the density decrease with holding time of pretreatments prolonged.The reason for this is that,the diffusion capacities of Zr in Al is very low at 250℃,long distance diffusion and coarsen are incapable,but the number of Al₃Zr dispersoids nuclei which can further coarsen at 470℃ are increase with holding time of pretreatments prolonged.When pretreated obove 300°C,the diffusion capacity of Zr in Al increase significantly,long distance diffusion and coarsen are capable,the numbers of Al₃Zr dispersoids nuclei which can coarsen at 470’C are decrease with holding time of pretreatments prolonged.Therefore,with the holding time of pretreatments prolonged,the variation trends of Al₃Zr dispersoids are opposite when pretreated at relative low temperature（250 ℃）and relative high temperature（above 300 ℃）,respectively.For trace V containing（about 0.1 wt.%）7150 type aluminum alloy ingots which subject to preatreatments followed by conventional homogenization at 470℃ for 24h,when pretreated at 250℃,the size of Al21V2 dispersoids decrease and the density increase with holding time of pretreatments prolonged.When pretreated above 300℃,the variations of size and number density of Al21V2 dispersoids with holding time of pretreatments prolonged are unobvious.The effects of pretreatments（250℃ to 40℃）which before conventional homogenization at 470℃ for 24h on recrystallization resistance,microstructure evolution during plastic working process and recrystallization law during annealing in Zr containing and V containing 7150 type aluminum alloys.were studied.The results show that,pretreatments can significantly increase recrystallization resistance in 7150 aluminum alloy containing trace of Zr,but the increased degree is relate to Zr contents.When Zr contents is about 0.1 wt.%,microstructure in the alloy with pretreatments after post-deformation solution treatment is mainly recovery,recrystallization is almost completely inhibited,but the microstructure in the alloy without pretreatments is partial recrystallization.In 7150 aluminum alloy with 0.13 wt.%Zr containing,obvious recrystallization occurs in the alloy without pretreatment during post hot-rolling inter-annealing at 400℃ for 2h,but the microstructure in the alloy with pretreatment is mainly recovery.Pretreatments can increase the recrystallization temperature of this alloy during post cold rolling annealing by 70℃.Pretreatments can increase recrystallization resistance some extent in 7150 type aluminum alloy containing trace of V,but its effect is not as strong as with same Zr content.In 7150 type aluminum alloy with 0.12 wt.%containing,the microstructure of this alloy without pretreatment during post hot-rolling inter-annealing at 400℃ for 2h is mainly recrystallized grains,but the recrystallized fraction is decreased some extent in the alloy with pretreatment at 300℃for 24h,pretreatment increase the recrystallization temperature of post cold-rolling annealing in this alloy by 20℃.The reason for pretreatments at temperature between 250℃ and 400 ℃ which before conventional homogenization at 470℃ for 24h increase the pinning pressure Pz of Al₃Zr and Al21V2 dispersoids exert on grain boundaries is mainly that pretreatments can refine Al₃Zr and Al21V2 dispersoids.But the pinning pressure Pz of dispersoids exert on grain boundaries is also strongly affected by the interface relationship of dispersoids with Al matrix.Al₃Zr dispersoids coherent with AI matrix results a higher pinning pressure Pz,but AI21V2 dispersoids incoherent with Al matrix results a much lower pinning pressure Pz.Adding same contents Zr or V into 7150 type aluminum alloy,although V can form much higher volume fraction of dispersoids than that of Zr,but the pinning pressure Pz produce by Al21V2 dispersoids is much lower than that of produce by Al₃Zr dispersoids.Hence,inhibiting effect of Al21V2 dispersoids which incoherent with Al matrix is significantly lower than that of Al₃Zr dispersoids which coherent with Al matrix on recrystallization.The results further confirm that,using P_z = 6γ_GB（V_f/r）that Hallam H proposed instead of using P_z = 3γ_GB/2（V_f/r）to calculate pinning pressure Pz which produce by dispersoids coherent with Al matrix is more reasonable.