DYNAMIC DEFORMATION OF POLYCRYSTALLINE ALUMINA

Using one-dimensional strain conditions, the dynamic stress-wave response of polycrystalline Al2O3 was measured with interferometry in both stress-wave loading and unloading to about 16 GPa and with slanted resistor gauges in loading to about 50 GPa. The stress-wave loading and unloading measurements were of high resolution and showed a 9.1-GPa elastic precursor wave (velocity 10.9 km/s) followed by a slower dispersive permanent deformation wave. Unloading was elastic in the stress range of these experiments. Both loading and unloading wave propagation were modeled well with a Maxwellian elastic-stress-relaxing model with a yield stress of 5.8 GPa and a relaxation time of 70 ns. The rate-dependent model correctly predicts both the dispersion of the permanent deformation wave and the unloading-wave behavior. The bulk pressure-volume behavior of alumina is given by the shock-velocity-particle-velocity relationship of Us=8.14 +1.28u p (km/s). Thermodynamic corrections to the dynamic bulk response yielded isothermal pressure-volume results which agreed well with direct hydrostatic determinations on polycrystalline Al2O3 and with results deduced from ultrasonic determinations on Lucalox. Permanent deformation of Al2O3 from a micromechanical standpoint appeared to be compatible with a model involving general microcracking throughout the volume of the material. This model is supported by the lack of an appreciable spall strength. When the yield process is ascribed to the onset of microfracture, which depends upon the initial flaw size and distribution, the earlier results on single crystals are phenomenologically related to the stress-wave behavior observed during this study on polycrystalline alumina.