Microstructure evolution during direct impact loading of commercial purity

Abstract

Commercially pure α-titanium powder was subjected to mechanical milling, after which each powder particle exhibited a core-shell-like structure consisting of coarse and fine grains, respectively. Subsequently, spark plasma sintering was used to consolidate the severe plastically deformed powder. Bulk samples made of a 3D network of continuously connected shells, hereafter referred to as harmonic structure, were obtained. The dynamic response of the processed samples was tested using a direct impact Hopkinson pressure bars (DIHPB) at initial strain rate of 15 500 s-1 corresponding to a velocity of 50 m.s-1, and at fixed axial strains of 20, 50 and 90%. For the strain of 20%, both compression and tensile twins dominate the coarse-grained core. At the same time, [0001] texture fiber developed in the core, while the grain in the shell rotated toward the \hbox{[2\Bar{1}\Bar{1}0]} axis parallel to the impact direction. At a strain of 50% the grains in the shell rotated further and reinforced the [0001] texture fiber, while in the same time twinning frequency saturated in the core. Increasing the strain to 90% resulted in quasi-disappearance of twins. Tortuous localization bands 25–30 μm wide were observed whose microstructural characteristics suggest dynamic recrystallization as a consequence of adiabatic heating