PHASE-TRANSFORMATIONS IN DICALCIUM SILICATE .2. TEM STUDIES OF CRYSTALLOGRAPHY, MICROSTRUCTURE, AND MECHANISMS

The crystallography, microstructures, and phase transformation mechanisms in dicalcium silicate (Ca2SiO4) were studied by TEM. Three types of superlattice structures were observed in the alpha(L)' and beta-phases. Almost all beta-grains were twinned and strained. Symmetry-related domain structures inherited from previous high-temperature transformations were observed in beta-grains. Both the alpha-->alpha(H)' and alpha(L)'-->beta-transformations were considered to be ferroelastic, and spontaneous strains were calculated. In terms of the crystal structures, the major driving force for the beta-->gamma transformation is proposed to be strains and cation charge repulsions in the beta-structure. This mechanism can be displacive, but it needs to overcome a comparatively high energy barrier.