STRESS-RELAXATION AND SOLID-SOLUTION HARDENING OF CUBIC ZRO2 SINGLE-CRYSTALS

Solid solution hardening in cubic ZrO2 single crystals of varying Y2O3 contents (12.7, 15.2, 17.7, and 20.5 mol %) oriented for easy {100} [ 011] slip has been studied at 1400 degrees C. Strain rate cycling and stress relaxation experiments have been performed to characterize the thermally-activated deformation processes. The strain rate sensitivity is very low at small strains but increases with increasing strain; th e values measured by stress relaxation are greater than those derived from the strain rate cycling experiments, and the relaxation curves sh ow ''inverse'' curvature at small strains. The athermal component of t he flow stress originating. from long-range dislocation interactions w as estimated from dislocation densities obtained from etch pit microgr aphs. The dislocation density increases with increasing Y2O3 concentra tion, but the densities are too small to cause the appreciable atherma l component of the flow stress; we believe that significant recovery m ust have occurred during cooling. The stress relaxation data can be in terpreted by assuming that the deformation itself is mainly athermal, but that thermally-activated recovery takes place during the deformati on; the Y2O3 solute may cause hardening by decreasing the diffusion ki netics. Alternatively, it is possible that the flow stress is controll ed by the intrinsic lattice resistance of secondary slip systems.