中文摘要：

利用Gleeble-3500试验机,在300~450℃和0.1~10 s~（-1）的变形条件下,研究了大规格铸锭晶粒尺寸的不均匀性对新型高强Al-7.68Zn-2.12Mg-1.98Cu-0.12Zr合金热变形行为的影响。SEM观察表明,大铸锭表层的晶粒尺寸比心层细小。热变形过程中,细晶组织（铸锭表层）的流变应力在高温和低应变速率条件下低于粗晶组织（铸锭心层）。计算得到表层和心层组织的热变形激活能分别为140和125.4 kJ/mol。基于位错密度理论,利用一种两阶段型本构方程分别预测了粗晶和细晶组织的流变应力,并建立了不同晶粒度组织的动态再结晶软化方程。电子背散射衍射（EBSD）观察表明,合金在300~400℃条件下发生动态回复,在450℃和低应变速率速（0.1 s~（-1））条件下出现动态再结晶（DRX）现象,动态再结晶晶粒在原始大角度晶界上形核。由于表层（细晶）组织的晶界密度高,因此其动态再结晶程度高于心层（粗晶）组织。

英文摘要：

The effects of grain size inhomogeneity in large size ingots on hot deformation behaviors of a new high strength Al-7.68Zn-2.12Mg-1.98Cu-0.12 Zr alloy were investigated using Gleeble-3500 test machine at 300~450 ℃ with strain rates from 0.1 to 10s~（-1）.SEM observation shows that the grain size in surface layer of the ingot is finer than that in central layer.During hot deformation,the flow stress of the sample with fine grain（surface layer） is lower than that of the sample with coarse grain（central layer） under high temperature and low strain rate conditions.The hot deformation activation energies（g） of 140 and 125.4 kJ/mol are obtained for fine grain micro structure and coarse grain micro structure,respectively.The flow stresses were predicted by a two-stage type constitutive model based on the dislocation density theory,and the dynamic recrystallization softening equations were established for the microstructures of fine grain and coarse grain.Electron Back-Scattered Diffraction（EBSD） observations reveal that the alloy exhibits dynamic recovery at temperatures ranging from 300 to 400 ℃ and dynamic recrystallization（DRX） at 450 ℃ with low strain rate（0.1 s~（-1））.The DRX grains nucleate at the original grain boundaries.As a result,the DRX fracture is higher in finer grain（surface layer） microstructure due to the higher grain boundary density.