Effect of aging state on shock induced spall behavior of ultrahigh strength Al-Zn-Mg-Cu alloy

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.