Improving dynamic tensile strength without sacrifice of final elongation in aluminum by gradient microstructures

Gradient microstructures (GMs) help to improve simultaneously the strength and ductility of metallic materials; therefore, GMs have attracted more and more interest in recent years. The current researches on GMed metals mainly focus on the quasi-static mechanical properties, whereas the dynamic mechanical behavior of GMed metals remains mysterious. Here, GMed pure aluminum was prepared by cryogenic pre-torsion. Split Hopkinson tensile bar (SHTB) tests show that GMs can improve the dynamic tensile strength without the sacrifice of final elongation. Quantitative microstructural characterization indicates that the gradients of dislocation density, grain size and precipitate volume fraction contribute to the excellent dynamic mechanical properties in GMed aluminum. A modified Johnson-Cook model based on multiple GMs was proposed to describe the dynamic mechanical behavior of GMed metallic materials. Systematical finite element simulations were further conducted to reveal the underlying mechanisms during SHTB tests of GMed aluminum bar. The gradient of yield flow stress contributes the improvement of dynamic tensile strength. The extra hardening from multiple GMs and the occurrence of dynamic recrystallization (DRX) promote the increase of final elongation.