Microstructural Characterization of a High-Strength Aluminum Alloy Subjected to High Strain-Rate Impact

The deformation and damage modes associated with the high strain-rate behavior of a high-strength aluminum alloy Al 2139 were analyzed. The microstructure was characterized at different physical scales to determine how the strengthening and toughening mechanisms of the alloy can inhibit and resist failure modes, such as shear localization and bending tensile failure, which occur due to high strain-rate impact. Grain morphology, precipitates (Omega and theta'), and Mn-bearing dispersed particles and inclusions were characterized by optical microscopy (OM), orientation imaging microscopy (OIM), energy dispersive spectroscopy (EDS), transmission electron microscopy/high-resolution transmission electron microscopy (TEM/HRTEM), selected area diffraction (SAD), and scanning electron microscopy (SEM) investigations of a 38-mm plate impacted by 4340 steel projectiles. Large grain sizes reduce grain boundary (GB) area and allow for more precipitation in the matrix, and these precipitates are shown to play a critical role in the toughening and strengthening of the alloy. Dispersed particles are associated with ductile failure, and inclusions are associated with ductile failure and shear failure. Different deformation modes were observed for the nanoscale precipitates, which affected overall behavior at size scales spanning the nano to the macro.