SUPERPLASTIC FORMING OF SIALON CERAMICS

Superplastic SiAlON's of the nomial composition Ym/3-Si12-(m + n)Alm + nOnN16-n are reported in this study using a transient-phase-forming approach. They encompass the fields of single-phase alpha'-SiAlON, single-phase beta'-SiAlON, and their two-phase mixtures. Excellent formability is obtained at 1550-degrees-C for the beta' and alpha' + beta' materials, and at 1600-degrees-C for the alpha' material. Typically in the nonequilibrium state before deformation, these fine-grained materials undergo dramatic phase and microstructure evolutions during superplastic forming. In particular, the stress-biased alpha-Si3N4 --> beta'-SiAlON reaction is found to result in elongated and aligned grains with fiber-strengthening effect and excellent formability.