Influence of precipitate distributions on adiabatic shear band formation and flow behavior of heat-treated 7050 aluminum alloy

Thermal/Heat treatment of structural metals such as 7050 Al alloy is known to improve their mechanical properties and makes them most suitable for applications in aerospace and automobile. Existing microstructural inhomogeneities such as precipitates tend to influence the high strain rate mechanical behavior and formation of adiabatic shear bands (ASBs) of structural metals during service. Therefore, in this study, the effect of initial microstructure such as grain boundary (GB) precipitates on the initiation and nucleation of the adiabatic shear band (ASB) and plasticity of heat-treated 7050 Al Alloy was investigated at high strain rates. The high strain rate impact testing was carried out on 7050 Al cylindrical specimens at increasing impact momentums of 35 kgm/s, 43 kgm/s, and 59 kgm/s (up to 3025 s−1), after heat-treating using T6-temper and high-temperature pre-precipitation (HTPP) conditions. Strain localizations and distributions were also characterized in-situ using digital image correlation (DIC) and thermal camera. The HTPP condition resulted in higher ductility but lower peak stress at impact relative to the T6-temper condition, due to increased static recrystallization and copper (Cu) content after heat treatment. Also, the initial microstructure dictates the nucleation’s heat rise during impact. In-situ analysis showed that the temperature rise was less than the homologous temperature and did not play a major role in the formation of ASB within 7050 Al alloys. It was also demonstrated that morphology and compositional redistribution of GB precipitates reduce the susceptibility to ASB nucleation in 7050 Al alloy. This was attributed to a ~70% increase in aspect ratios (AR), Cu contents of GB precipitates, and alloy matrix which dictate the formation of ASBs. The T6-temper heat treatment condition resulted in agglomeration and interlocking of continuously distributed Al-Zn-Mg precipitates (ф1), while the Al-Cu precipitates (ф2) disintegrate within the ASBs leaving interstitial plots, which increases the propensity of ASB with distinct plastic flows. Meanwhile, after HTPP heat treatment, there were discontinuous distributions of the Al-Zn-Mg precipitates (ф1) with an increase in AR (~70%) and large spacings (PFZs-precipitation-free zones). The Al-Zn-Mg precipitates (ф1) became globular and were intersected by the Al-Cu precipitates (ф2), which seemed to enter solution at ASB regions, with less distinct plastic flows. It is apparent that precipitates within the initial microstructure of 7050 Al prior to deformation undergo fragmentation and/or dissolution as well as the reduction in aspect ratios during the occurrence of ASBs.