Dynamic Failure Investigation in Ultrafine Grained AA2219: Mechanical and Microstructural Analysis

In this study, the high strain rate behavior of ultrafine grained (UFG) AA2219 alloy processed via multi axial forging at cryogenic temperature was investigated. Room temperature forged sample was used as a reference to determine the effects of significant grain size refinement on the dynamic response of the materials. The initial microstructure characterization indicated that severe plastic deformation in the cryogenically process alloy resulted in its grain size reduction to ~ 270 nm and the second phase breakage to finer particles. The results of the dynamic impact tests show that the strain hardening and thermal softening are substantially less significant in the UFG materials, whereas the maximum flow stress and the strain rate sensitivity increased. Furthermore, the grain size reduction led to the absorption of higher portion of the deformation energy and an increase in the toughness of the fabricated UFG material when compared to the conventionally forged samples. This improvement is approximately 56% at a strain rate of 4000 s⁻¹ obtained via the grain structure refinement. Microstructure analysis of the post-deformed samples revealed two fully transformed adiabatic shear bands (ASBs) in the coarser grained material due to intense localized strain and thermal instability during the impact tests which caused the pushing off of the second phases and cracks formation inside the ASBs. However, low-intensity deformed ASBs and a notable enhancement in crack initiation strength were observed by morphology and final configuration of the post-deformed UFG samples. In addition, no considerable hardness variations were experienced in the impacted UFG material due to the saturation of grain size during the cryogenically forging process. In contrary to the UFG alloys, significant hardness increase was observed in the deformed coarse grained material which was associated with softening in the adjacent regions providing a zone prone to cracks initiation.