Deformation And Damage Behaviors Of Solution Treated AM60B Magnesium Alloy Under High Velocity Impact

Deformation and damage behaviors of solution treated AM60B Mg alloy under high velocity impact have been investigated in this paper. High velocity impact experiments were carried out on power gun, one-stage and two-stage gas guns, and the cratering processes in Mg alloy target impacted by different projectiles under different impact velocities were studied. The deformed microstructure at different zones and depth levels adjacent to the crater under high velocity impact were characterized by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The mechanical properties of the materials near the crater after impact were measured by indentation test, Hopkinson bar and thermal simulation test, and the in-situ tensile tests were used to investigate the influence of defects induced by high velocity impact on the primary crack propagation.The results show that the cratering process in the Mg alloy target impacted by steel projectile was different from that impacted by Al projectile. With the impact velocity increasing, several crater morphologies including spherical cap, hemispherical, columned + hemispherical and hemispherical crater were experienced sequentially in Mg alloy target impacted by steel projectile, while the crater morphologies transformed from the spherical cap to hemispherical crater were experienced sequentially in Mg alloy target impacted by Al projectile.Under ballistic impact, the crater depth was the main penetration behaviors in the Mg alloy target impacted by steel projectile, while the crater volume was the main penetration behaviors in the Mg alloy target impacted by Al projectile. When the impact velocity was speeded up to the hypervelocity impact, the crater volume was the main penetration behaviors in Mg alloy target impacted by various projectiles. The impact cratering process was different from quasistatic compressive cratering, and kinetic energy of projectile dissipated by the impact cratering was always larger than the work dissipated by quasistatic compressive cratering. With the crater depth increasing, the dissipated energy gap between the impact and quasistatic cratering increased. The calculated impact cratering process shows that the formed shock pressure and temperature rising increased obviously at the interface between the projectile and target at impact moment as impact velocity increasing. When the impact velocity was reached 5000 m/s, the calculated shock pressure was two times magnitude higher than the materials strength, and shock temperature rising was exceeded the materials melting point, even boiling point. During the impact cratering process, the materials paralleled to the impact direction suffered the most severe plastic deformation, and the deformed levels of the materials 45°and vertical to impact direction decreased sequentially.The investigated deformed microstructures near the crater show that the microstructural distribution paralleled to the impact direction was widest, and the distribution scope 45°and vertical to impact direction decreased sequentially, and the ellipsoid-like microstructural distribution was formed. With the impact velocity increasing, the microstructural distribution zones near the crater became much wider. At almost same impact velocities, the microstructural distribution near the crater in Mg alloy target impacted by steel projectile was larger than that formed by Al projectile. Under ballistic impact, the deformed microstructure near the crater could be classified into three zones: high density twin zone, medium density twin zone and low density twin zone. Under hypervelocity impact, the ultrafine grain zone was appeared near the crater, and the deformed microstructure near the crater could be classified into four zones: ultrafine grain zone, ultrafine grain + high density twin zone, high density twin zone and low density twin zone. The low density twin zone was spread in the whole target with thickness of 30 mm. High velocity impact could provide a gradient variation of the strains and strain rates from the crater rim to the deep matrix, thus the ultrafine grain evolution adjacent to the crater could be obtained through the characterization of the deformed microstructure at different depth levels, and the physical model of the ultrafine grains formed near the crater was constructed.The investigated adiabatic shear bands show that the critical impact velocity was required for the formation of the adiabatic shear bands. With impact velocity increasing, the materials near the crater suffered uniform plastic deformation, strain localization (deformed bands), white-etching bands (transformed bands) and cracks sequentially, thus the deformed and transformed bands were the different stage products. The metallographic observation of the transformed bands etched by two etchants shows that the optical morphology of the transformed bands was closely related with etchants, thus the white-etching characteristic in metallographic observation to distinguish the transformed bands was inaccuracy. The different stain localization the materials suffered near the crater could lead to the formation of the deformed and transformed bands simultaneously in Mg alloy targets impacted at a critical velocity. The deformed bands composed of deformed, fragmental grains were confirmed, while the transformed bands composed of the ultrafine and equiaxed recrystallized grains were observed. The twinning and dislocation slipping played an important role for the formation of the ultrafine grains in the transformed bands, and its formation should be attributed to the twining-induced rotational dynamic recrystallization mechanism.The investigated typical microstructures near the crater show that the {1 0 12} extension twins with twinning direction along < 1011> and {1 0 11} contraction twins with twinning direction along < 1012> were the main twin modes in Mg alloy under high velocity impact. High stress and strain levels, low critical shear stress and small shear displacement should be responsible for the formation of two twin modes. High density dislocation structure have been confirmed in the ultrafine Mg grains formed near the crater under high velocity impact, which indicated that dislocation slipping was the main mechanism of the further plastic deformation of the ultrafine Mg grains. Under high velocity impact, high temperature and pressure the materials suffered at the interface between the projectile and target led to the formation of various molten-related microstructures, such as Al grains, MgAl compound and amorphous microstructure. The formation of the amorphous microstructure was a product of the melting and rapid solidification.The mechanical properties of the materials after impact show that dynamic yield strength of the materials near the crater after impact increased as the impact velocity increasing, and the maximum dynamic compressive strength of the materials was obtained at critical impact velocity. The critical values were obtained in Mg alloy target impacted by steel and Al projectiles at the velocities of 590 m/s and 2500 m/s, respectively. Beyond the critical values, the dynamic compressive strength of the materials after impact decreased as the impact velocity increasing. As the distance approaching to the crater rim, the dynamic yield strength of the materials after impact increased obviously, and the critical plastic deformation was existed for the dynamic compressive strength of the materials. Beyond the critical value, the maximum dynamic compressive strength of the materials was obtained at a certain distance from the crater rim. The situ tensile tests show that the microcracks, microvoids, ASBs and twin boundary induced by high velocity impact were the preferential sites for the nucleation and propagation of the primary cracks, and the formation of various defects led to the further deformability of the materials decreasing.