SUPERPLASTIC FORMING OF AN ALPHA-PHASE RICH SILICON-NITRIDE

The deformation behavior of fine-grained, alpha-phase rich silicon nit ride materials has been studied between 1823 and 1923 K, both in compr ession and in tension. It is first shown that the higher the alpha-pha se content, the better the superplastic forming ability. A large tensi on-compression flow asymmetry was evidenced. For instance, shear-thick ening flow shows up in compression whereas shear-thinning is observed in tension. Furthermore, much higher flow stresses and hardening rates are reported in compression than in tension. Elongations of mon than 80% were achieved for strain rates between 2.5 and 5 x 10(-5) s(-1). I n the light of our results and of the abundant literature dealing with the high temperature deformation in silicon nitride, a sketch of the different deformation stages is proposed, which emphasizes the tension -compression asymmetry. Starting from the promising results obtained a t the laboratory scale, the feasibility for net-shaping of a real part was demonstrated by hot-forging of a parabolic shell.