Spatial Microstructural Response in a High-Strength Low-Alloy Steel Fabricated by Wire-arc Direct Energy Deposition Under Projectile Penetration

The spatial microstructural response under projectile penetration was investigated in a high strength low alloy (HSLA) steels fabricated by wire arc directed energy deposition (DED). Projectile penetration induces pronounced spatial heterogeneity because of severe local thermomechanical gradients. This heterogeneous response is controlled by the localized evolution of deformation substructures. Rapid dislocation multiplication promotes the accumulation of low angle grain boundaries (LAGBs). In regions where their further evolution is restricted, a high fraction of low angle grain boundaries is retained, leading to severe martensitic lath fragmentation at the impact surface and hierarchical lath elongation at the crater bottom. In contrast, extreme shear localization in the middle zone of the crater promotes the rotation of low angle grain boundaries into high angle grain boundaries (HAGBs), which triggers rotational dynamic recrystallization and the formation of adiabatic shear bands (ASBs) with refined equiaxed grains. At the same time, the highly heterogeneous deformation of the martensitic matrix constrains the reverted austenite (RA), resulting in distinct morphological responses including nanoscale subdivision, stacking fault rich continuous films, and strip like boundary retention. These findings link dislocation driven boundary evolution to adiabatic shear band formation and localized failure, and provide insight into the ballistic deformation of additively manufactured HSLA steels.