Phase transformation kinetics and mechanism of polycrystalline Sr-hexacelsian to celsian

The isothermal transformation of Sr-hexacelsian to celsian in sintered bodies having chemical composition xSrO·Al2O3·2SiO2 (x = 1.0, 1.1 and 1.2) was analyzed using X-ray diffraction and JMA kinetics. All compositions showed rapid initial transformation followed by strong deceleration, and excess SrO accelerated the reaction. JMA plots exhibited two linear regions, indicating a mechanism change caused by crack formation at the advancing transformation front. Significant volume contraction, enhanced at high temperature due to thermal-expansion mismatch, generated internal stresses that promoted cracking and constrained later-stage growth. The Avrami index increased from 1.5 to about 2 with temperature, consistent with a shift from one-dimensional to two-dimensional growth. High activation energies (833–934 kJ/mol) were attributed to stress interactions between anisotropic hexacelsian grains.