A comprehensive approach to crystallization sequence in novel MgO/CaO-Al2O3-SiO2 glass-ceramics by Ostwald's step rule and composition fluctuation model

An understanding of the crystallization sequence (the formation order of crystalline phases) of glasses is crucial for the processing and composition design of novel glass-ceramics. A comprehensive approach to the crystallization sequence in novel MgO/CaO-Al2O3-SiO2 (MAS/CAS) glass-ceramics was performed from the Ostwald's step rule and the model of the distribution of nanoscale composition fluctuations (DNCF) for the first time. The stoichiometric cordierite Mg2Al4Si5O18 glass exhibits the crystallization sequence predicted by the Ostwald's step rule and a narrow DNCF model, i.e., metastable β-quartz solid solutions (β-QSSs) → high-temperature type indialite → low-temperature type α-cordierite. A broad DNCF model was proposed to present in osumilite Mg2Al4Si11O30 and pyrope Mg3Al2Si3O12 glasses, resulting in the initial formation of β-QSSs or forsterite 2MgO·SiO2 prior to α-cordierite formation. The crystallization sequence in stoichiometric anorthite CaAl2Si2O8 glass was also understood along the Ostwald's step rule and a narrow DNCF model. The importance of nanoscale composition fluctuations, in particular the formation of MgO/CaO-rich fragile regions, was emphasized in the crystallization of MAS and CAS-based glasses having strong Al-O-Si linkages between AlO4 and SiO4 tetrahedra. The present article will contribute to the comprehensive design of new innovative aluminosilicate-based glass-ceramics.