Tensile Ductility of Superplastic Al ₂ O ₃ –Y ₂ O ₃ –Si ₃ N ₄ /SiC Composites

Abstract

Si 3 N 4 /SiC composites are ceramic materials that exhibit excellent performance for high‐temperature applications. Prepared from an ultrafine amorphous Si‐C‐N powder, sintered materials are constituted mainly of a β ‐Si 3 N 4 matrix with SiC inclusions and have a very small grain size (less than 1 μm). Such a microstructure is propitious for superplastic forming. Superplasticity has been studied in tension, from 1550° to 1650°C, under nitrogen atmosphere. Elongations over 100% have been achieved. In many cases, at the highest temperatures and slowest strain rates, materials are damaged by different processes, including microcracking, cavitation, and chemical decomposition. A map of the most suitable (strain‐rate/temperature) domain has been established. It allows the prevention of any structural alteration by selecting carefully the testing conditions. Since specimens suffered considerable strain‐induced hardening, sources for this phenomenon are examined. Although the experiments have involved high temperature and extensive strain, neither static nor dynamic grain growth has occurred. Crystallization of the amorphous grain‐boundary phase, which is reported in most cases, may be invoked. However, based on microstructural observations, it is not the unique origin for flow hardening.