Sub-texture in adiabatic shear bands from arc additively manufactured stainless steel under dynamic loads

Adiabatic shearing is a common failure mechanism for additively manufactured metals and alloys under dynamic loads. Cylindrical samples ($\varnothing$4 mm×4 mm) along building and scanning directions were extracted from 316L stainless steel plate fabricated by cold metal transfer wire and arc additive manufacturing process (AM 316L). Cylindrical AM 316L samples were subjected to dynamic impacts to introduce adiabatic shear bands (ASBs) at high strain rates from 4000 to 6000 s⁻¹ by using a split Hopkinson pressure bar. Deformed AM 316L samples were cut along compression direction. Multiple methods including scanning electron microscope, electron-back-scatter diffraction, focused ion beam, transmission electron microscope, transmission kikuchi diffraction were applied to characterize the microstructure of ASBs. The dynamic flow stress of AM 316L increases with forward strain due to strain hardening at first, and then comes an obvious flat stage for the balance between adiabatic thermal softening and strain hardening followed by adiabatic shearing prevailing causing the last failure. The sub-grains in ASBs experienced a dynamic recrystallization process, present fully distinct equiaxed crystal morphology with high angle grain boundaries from the matrix, of which the grain size is about 200−300 nm. The complex thermal and mechanical processes during adiabatic shearing lead to the formation of duplex components in sub-texture, which conclude not only the <110>-fiber along the compression direction similar with the matrix, but also the crystallographic texture related to shear direction with plane (111) along shear plane and orientation <112> along shear direction. The residual large amount of Σ3 60° grain boundaries and twin-symmetry texture in ASBs prove that twinning recrystallization is the main dynamic recrystallization mechanism. The ASB propagating paths of AM 316L along different directions under dynamic loadings are the similar, which is that both ASBs successively extend along the symmetrical path of angles 35° with respect to the loading surface. These two paths are the locations of the maximum strain and thermal distribution during the dynamic loadings consistent with previous simulation work. In addition to the external physical conditions of the maximum strain and thermal field distribution in the sample under dynamic loading, the paths conform to the crystallographic condition that the intersection angle between the shear plane (111) and the matrix (110) is 35.2°. Accompanied with macro adiabatic shear bands, micro-strain localization bands are formed to accommodate more strain, wherein the sub-grains take distinct orientation from matrix.