高温压缩变形Al-Zn-Mg-Cu合金动态再结晶后的{111}/{111}近奇异晶界

{111}/{111}Near Singular Boundaries in a Dynamically Recrystallized Al-Zn-Mg-Cu Alloy Compressed at Elevated Temperature

提高{111}/{111}近奇异晶界比例可增强Al-Zn-Mg-Cu合金晶界腐蚀抗力,为了解此类晶界形成机理并探索其调控方法,本工作将470℃、12 h和520℃、6 h双级固溶及冷轧后再结晶的Al-Zn-Mg-Cu合金样品分别在450、480和520℃进行应变速率为0.001 s^(-1)、真应变为1.20的压缩变形,压缩后立即水冷。采用电子背散射衍射和基于五参数分析的晶界界面匹配定量表征方法对上述3个样品的显微组织和晶界特征分布进行观察分析。结果表明,高温压缩后的显微组织不均匀,存在间隔分明的细晶组织和粗晶组织,其中细晶组织中的晶界以小角度晶界(LAGB)为主,粗晶组织中的晶界以大角度晶界(HAGB)为主;粗、细晶组织中的{111}/{111}近奇异晶界({111}/{111}-NSB)比例均随压缩温度的升高而增大,其中经520℃压缩的试样,其LAGB中的{111}/{111}-NSB占总晶界的比例为8.77%,HAGB中的{111}/{111}-NSB占总晶界的比例为4.53%。进一步考察各试样的应力-应变曲线以及450℃压缩30%时的显微组织特征可见,应变介于0.05~0.70为稳态流变阶段,主要发生初次动态再结晶(DRX),其主要由粗晶及其HAGB构成。应变介于0.70~1.20为二次硬化阶段,主要发生二次DRX,其中不连续DRX(DDRX)和连续DRX(CDRX)同时进行,且优先发生在某些微区;不论是DDRX因形核和核心长大生成的HAGB,还是CDRX因亚晶合并引入的LAGB,均使对应微区的晶粒细化,导致样品总体流变应力急剧上升。二次DRX阶段,CDRX行为随压缩温度的提高而增强,{111}/{111}-NSB比例也随之增加,特别是LAGB中的{111}/{111}-NSB比例快速增加。

Increasing the fraction of{111}/{111}near-singular boundaries({111}/{111}-NSBs)has been reported as a primary solution to intergranular corrosion failure in Al-Zn-Mg-Cu alloys.The authorsprevious work demonstrates that continuous static recrystallization resulting from a specific prestrain and annealing is conducive to the formation of{111}/{111}-NSBs in Al-Zn-Mg-Cu alloys.Therefore,the development of such boundaries in the alloys during dynamic recrystallization(DRX),particularly during discontinuous DRX(DDRX)and continuous DRX(CDRX)at elevated temperatures,should be elucidated.In the present work,an Al-Zn-Mg-Cu alloy containing 7.79%Zn,1.53%Mg,and 1.68%Cu(mass fraction)was selected as the experimental material.A hot-rolled plate of the alloy was subjected to a two-stage solution treatment at 470oC for 12 h and 520oC for 6 h followed by cold rolling and recrystallization annealing.Three parallel samples cut from the recrystallized plate were compressed at 450,480,and 520oC at a strain rate of 0.001 s^(-1)to a true strain of 1.20.The samples were water quenched immediately after the compression.Electron backscatter diffraction and grain boundary inter-connection measurement based on five-parameter analysis were performed to examine the microstructures and grain boundary character distributions of the compressed samples.The results indicate that the microstructures of the samples were uneven,exhibiting fine-and coarse-grained regions.Low-angle grain boundaries are dominant in the fine-grained regions,whereas high-angle grain boundaries are dominant in the coarse-grained regions.The fraction of{111}/{111}-NSBs increases with the compression temperature in fine-and coarsegrained regions.In the sample compressed at 520oC,the{111}/{111}-NSBs from the low-angle grain boundaries constitute 8.77%of all grain boundaries,while those from the high-angle grain boundaries constitute 4.53%.The stress-strain curves and the microstructures of the sample compressed at 450oC to a true strain of 0.36 show that primary DRX occurs at strains from 0.05 to 0.70.Furthermore,the coarsegrained microstructures and high-angle grain boundaries develop during the stage involving steady-state flow.When the strain increases from 0.70 to 1.20,secondary DRX(including DDRX and CDRX)occurs in some regions,leading to dramatic grain refinement and a sharp increase in flow stress.In this stage,CDRX intensifies with increasing compression temperature,and{111}/{111}-NSBs in the low-angle grain boundaries increase rapidly.