Superplasticity of ceramic materials—II. Effect of initial porosity and doping on the superplastic behaviour of alumina

Abstract

The present work has demonstrated the ability of fine grained alumina containing small additions of copper oxide or nickel oxide to undergo superplastic tensile flow. The effect of small levels of porosity on superplastic behaviour has been examined using specimens subjected to HIP. This provides an effective way of controlling the density of ceramic materials while avoiding appreciable grain growth. Results showed that low density specimens tended to have higher superplastic elongations to failure than higher density specimens. In the latter case, long cracks developed during deformation and grew perpendicular to the tensile axis; this led to premature failure. The uniformly distributed pores in the lower density specimens provided sites for both crack initiation and crack arrest, leading to a large number of short cracks, and to higher failure strains. TEM observations gave evidence of significant dislocation activity in grain boundary regions of the deformed materials and this is tentatively interpreted in terms of the electronic contributions of the dopants to dislocation mobility.