Abstract
The present work deals with studies on the dynamic behavior of ultrafine grained AA2519 alloy synthesized via cryogenic forging (CF) and room temperature forging (RTF) techniques. A split-Hopkinson pressure bar was used to perform high strain rate tests on the processed samples and the microstructures of the samples were characterized before and after impact tests. Electron backscatter diffraction (EBSD) maps demonstrated a significant grain size refinement from ~740 nm to ~250 nm as a result of cryogenic plastic deformation showing higher dislocation densities and stored strains in the CF sample when compared to the RTF sample. This microstructure modification caused the increase of dynamic flow stress in this alloy. In addition, the aluminum matrix of the CF alloy is more densely populated with fragmented particles than the RTF alloy due to the heavier plastic deformation applied to the cryogenically forged alloy. The results obtained from the stress–strain curve for the RTF sample showed intense thermomechanical instabilities in the RTF sample which led to a severe thermal softening and the subsequent sharp drop in the flow stress. However, no significant decrease was observed in the stress–strain curve of the CF alloys with ultrafine grains which means that thermal softening would probably not be the most effective failure mechanism. Furthermore, higher level of sensitivity of CF alloys to strain rates was observed which is ascribed to transition of rate-controlling plastic deformation mechanisms. In the post-mortem microstructure investigation, deformed and transformed adiabatic shear bands (ASBs) were identified on the RTF alloy when the strain rate is over 4000 s−1 at which it had experienced a significant thermal softening. On the other hand, circular path and aligned split arcs are the various shapes of the deformed ASB seen at no earlier than 4500 s−1 in the CF alloys. This is associated with the crack failure caused by grain boundary sliding.
Highlights
Strain-hardenable aluminum alloys such as AA2519 have progressively been considered as one of the prime engineering materials for aircraft and automotive structural components due to their unique mechanical properties such as high strength-to-weight ratio, excellent ductility, good machinability, and high corrosion resistance [1,2]
The results of the microstructural characterization of the two room temperature forging (RTF) and cryogenic forging (CF) of AA2519 via optical microscope (OM), samples were also characterized using microscopy (SEM), and Electron backscatter diffraction (EBSD) prior to dynamic mechanical loading are highlighted in Figures 2–4, respectively
More shear stress has been generated during the cryogenic forging which leads to the breaking down of the second phase particles
Summary
Strain-hardenable aluminum alloys such as AA2519 have progressively been considered as one of the prime engineering materials for aircraft and automotive structural components due to their unique mechanical properties such as high strength-to-weight ratio, excellent ductility, good machinability, and high corrosion resistance [1,2]. The microstructures of the room temperature forged (RTF) and the cryoforged (CF) AA2519 as well as the deformed samples after the high strain rate compression tests have been investigated in order to analyze the failure mechanisms such as shear bands and cracks. These experimental data include important information on the dynamic yield and flow stresses, the strain rate sensitivity (SRS) and microstructural evolution of the alloys which can be applied in the formulation and validation of robust constitutive models for simulating the material’s response at high strain rates
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