High-temperature rheology of calcium aluminosilicate (anorthite) glass-ceramics under uniaxial and triaxial loading

The high-temperature creep behavior of two fine-grained (similar to3 mum) a northite-rich glass-ceramics was characterized at ambient pressure and unde r a confining pressure of similar to 300 MPa. Experiments were done at diff erential stresses of 15-200 MPa and temperatures of 1200 degrees -1320 degr eesC. Of the two materials, one had a tabular (lathlike) grain structure wi th finely dispersed second phase of mullite, mostly in the form of similar to3-5 mum grains comparable to that of the primary anorthite phase, whereas the other had an equiaxed grain morphology with fine (similar to 400 nm) m ullite precipitates concentrated at the anorthite grain boundaries. The res ults of creep experiments at ambient pressure showed that the material with the tabular grain structure had strain rates at least an order of magnitud e faster than the equiaxed material. Creep in the tabular-grained material at ambient pressure was accompanied by a significant extent of intergranula r cavitation: pore-volume analysis before and after creep in this material suggested that > 75% of the bulk strain was due to growth of these voids. T he equiaxed material, in contrast, showed a smooth transition from Newtonia n (n = 1) creep at low stresses to non-Newtonian behavior at high stresses (n > 2). Under the high confining pressure, the microstructures of both mat erials underwent significant changes. Grain-boundary mullite precipitates i n the undeformed, equiaxed-grain material were replaced by fine (similar to 100 nm), intragranular precipitates of silliminate and corundum because of a pressure-induced chemical reaction. This was accompanied by a significan t reduction in grain size in both materials. The substantial microstructura l changes at high confining pressure resulted in substantially lower viscos ities for both materials. The absence of mullite precipitates at the grain boundaries changed the behavior of the equiaxed material to non-Newtonian ( n = 2) at a pressure of similar to 300 MPa, possibly because of a grain-bou ndary sliding mechanism; the tabular-grained material showed Newtonian diff usional creep under similar conditions.