β→α Transformation in Polycrystalline SiC: I, Microstructural Aspects

Abstract

The effects of annealing conventionally sintered, hot‐pressed, and reaction‐sintered Sic have been investigated by optical and transmission electron microscopy. In conventionally sintered and hot‐pressed bodies, “composite” grains consisting of α‐SiC plates sandwiched between recrystallized β‐SiC envelopes grow rapidly into the fie‐grained β matrix; thickening of these α plates within the p envelopes is relatively sluggish. The existence of the composite plates can be explained by the extreme anisot‐ropy of the interfacial energy between β‐ and α‐Sic: {111)β(000l) α interfaces have energies several orders of magnitude lower than random β/α interfaces. In reaction‐sintered Sic, this anisotropy is manifested by rapid growth of α seeds along their basal planes into the epitaxial β reaction product; slow growth occurs perpendicular to the basal planes.