Deformation mechanisms and damage in α-alumina under hypervelocity impact loading

Deformation mechanisms in alpha-alumina under hypervelocity impact are investigated using molecular dynamics simulations containing 540x10(6) atoms. A cylindrical projectile impacting normal to the (0001) surface at 18 km/s generates large temperature and pressure gradients around the impact face, and consequently local amorphization of the substrate in a surrounding hemispherical region is produced. Away from the impact face, a wide range of deformations emerge and disappear as a function of time under the influence of local stress fields, e.g., basal and pyramidal slips and basal and rhombohedral twins, all of which show good agreement with the experimental and theoretical results. New deformation modes are observed, such as twins along {0 (1) over bar 11}, which propagate at a roughly constant speed of 8 km/s and nucleate a large amount of defects where subsequent fractures initiate. The relation between deformation patterns and local stress levels is investigated. During unloading, we observe that microcracks nucleate extensively at the intersections of previous deformations within an hourglass-shaped volume that connects top and bottom free surfaces. From the simulation, the fracture toughness of alumina is estimated to be 2.0 +/- 0.5 MPa root m. The substrate eventually fails along the surface of the hourglass region during spallation when clusters of substrate material are ejected from both free surfaces. (c) 2008 American Institute of Physics.