Abstract The Hugoniot Elastic Limit (HEL) and spall strength of an ultrahigh strength Al-Zn-Mg-Cu alloy in natural aging (NA), peak aging (PA) and over aging (OA) states have been measured under different shock stresses. It is found that the HEL and the spall strength of this alloy following these three heat treatments are independent of peak stress (or strain rate). The PA alloy shows the highest HEL ranging from 1.35±0.03 to 1.40±0.11 GPa, followed by the OA alloy, and the NA alloy exhibits the lowest value. On the other hand, the NA alloy has the highest spall strength ranging from 1.73±0.07 GPa to 1.84±0.03 GPa, while the spall strengths for PA and OA alloys are quite comparable, about 11 % lower than NA. The free surface velocity profiles beyond pullback minima are analyzed, showing that although spall strength and growth rate of voids for the PA and OA alloys are almost identical, the OA alloy shows a higher pullback peak, indicating more damage is produced in the OA alloy at the peak stress of 1.9 GPa. The post-impact characterization of the microstructure of these samples reveals a mixed mode of intergranular and transgranular fracture. It is demonstrated that the Al7Cu2Fe inclusions act as the main nucleation sites for voids and determine the overall spall strength. The distribution of equilibrium precipitates along grain boundaries is detrimental to the spall resistance. Hence, the absence of grain boundary (GB) precipitates is responsible for the higher spall strength for the NA heat treatment state. However, the size and amount of GB precipitates has negligible effect on spall response, especially when the peak stress exceeds 2.4 GPa.

中文翻译：

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时效状态对超高强度Al-Zn-Mg-Cu合金冲击诱发剥落行为的影响

摘要 测量了超高强度 Al-Zn-Mg-Cu 合金在不同冲击应力下在自然时效 (NA)、峰值时效 (PA) 和过时效 (OA) 状态下的 Hugoniot 弹性极限 (HEL) 和剥落强度。发现经过这三种热处理后该合金的 HEL 和剥落强度与峰值应力（或应变率）无关。PA 合金的 HEL 最高，范围从 1.35±0.03 到 1.40±0.11 GPa，其次是 OA 合金，NA 合金表现出最低值。另一方面，NA 合金的剥落强度最高，范围从 1.73±0.07 GPa 到 1.84±0.03 GPa，而 PA 和 OA 合金的剥落强度相当，比 NA 低约 11%。分析了超出回拉最小值的自由表面速度剖面，表明尽管 PA 和 OA 合金的剥落强度和空隙的增长速度几乎相同，但 OA 合金显示出更高的回拉峰值，表明在 1.9 GPa 的峰值应力下，OA 合金产生了更多的损伤。这些样品微观结构的冲击后表征揭示了晶间和穿晶断裂的混合模式。结果表明，Al7Cu2Fe 夹杂物是空洞的主要形核点，决定了整体的剥落强度。平衡析出物沿晶界分布不利于抗剥落性。因此，不存在晶界 (GB) 析出物是 NA 热处理状态较高的剥落强度的原因。然而，GB 沉淀物的大小和数量对剥落响应的影响可以忽略不计，