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

Abstract

Deformation mechanisms in ␣-alumina under hypervelocity impact are investigated using molecular dynamics simulations containing 540ϫ 10 6 atoms. A cylindrical projectile impacting normal to the ͑0001͒ 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 ͕0111͖, 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 2.0Ϯ 0.5 MPaͱ 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.

Response of seven crystallographic orientations of sapphire crystals to shock stresses of 16–86 GPa

“…This assumption implies that mechanisms of nucleation of fractures and of plastic shears involve common phenomena, such as twinning. Nucleation of cracks from the intersection of two twin systems was observed in molecular dynamic simulations [22] and explains the loss of strength of plastically deformed sapphire.…”

“…The particle velocity histories in Fig. 2 are "noisy" and their reproducibility is much less than the case of alumina ceramic [20,22] whose waveforms are smooth and well reproducible. The strong irregular oscillations and low reproducibility of the waveforms of c-cut sapphire are a consequence of intrinsic heterogeneity of its inelastic deformation.…”

“…Similar oscillation, although not as strong as that for sapphire, were recorded on the waveforms in experiments with titanium [27], shock compression of which is accompanied by intense twinning. Twins can grow with extremely high speed -up to 8 km/s in sapphire [22]. The formation of each twin makes an essential contribution to the plastic strain and thereby causes stress relaxation.…”

“…Graham and Brooks then measured the Hugoniots of c-cut, a-cut (11)(12)(13)(14)(15)(16)(17)(18)(19)(20), and n-cut (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23) sapphire crystals [4]. The normals to the flat surfaces of a-cut and of n-cut crystals are parallel to the a axis and n axis, respectively.…”

“…Chen and Howitt [21] have found that the partial dislocations are the twinning dislocation for basal twins. A large volume of information about deformation mechanisms, including basal and pyramidal slips and basal and rhombohedral twins, in sapphire under hypervelocity impact has been obtained by means of molecular dynamic simulations [22].…”