Mechanical and microstructural characterization of calcium aluminosilicate (CAS) and SiO2/CAS composites deformed at high temperature and high pressure

We performed axial compression experiments on polycrystalline calcium aluminosilicate (CAS or anorthite) and on particulate and layered composites with equal volume fractions of CAS and SiO2 (quartz) at a confining pressure of 300 MPa, temperatures of 1173-1473 K, and strain rates of 10−5 to 10−4 s−1. The dense samples were fabricated from quartz crystalline and CAS glass powders by hot isostatic pressing (HIP). Under the experimental conditions, triclinic CAS, regardless in monolithic aggregates or composites, deforms by dislocation creep as indicated by TEM microstructures, intensive grain boundary migration recrystallization and strong crystallographic preferred orientation (CPO). Dislocation creep of CAS is characterized by dominant glide on a single slip system (0 1 0)[1 0 0] while mechanical twinning, anisotropic growth and recrystallization play an role to relieve the strain incompatibilities which would otherwise result from such limited slip systems. Particulate and particularly layered composites are significantly stronger than monolithic CAS aggregates, indicating that quartz is an effective reinforcement to the CAS matrix even when the material is used at high temperature and high pressure. Under layer-normal compression, the flow strength of layered composites increases remarkably with decreasing the thickness of the layers, and the thin-layered composites are significantly stronger than particulate counterparts with the same composition. The observed layering-induced stiffening is due to constraint effects of rigid quartz on plastic flow of CAS.