High‐Temperature Rheology of Calcium Aluminosilicate (Anorthite) Glass‐Ceramics under Uniaxial and Triaxial Loading

Abstract

The high-temperature creep behavior of two fine-grained (ϳ3 m) anorthite-rich glass-ceramics was characterized at ambient pressure and under a confining pressure of ϳ300 MPa. Experiments were done at differential stresses of 15-200 MPa and temperatures of 1200°-1320°C. Of the two materials, one had a tabular (lathlike) grain structure with finely dispersed second phase of mullite, mostly in the form of ϳ3-5 m grains comparable to that of the primary anorthite phase, whereas the other had an equiaxed grain morphology with fine (ϳ400 nm) mullite precipitates concentrated at the anorthite grain boundaries. The results of creep experiments at ambient pressure showed that the material with the tabular grain structure had strain rates at least an order of magnitude faster than the equiaxed material. Creep in the tabular-grained material at ambient pressure was accompanied by a significant extent of intergranular 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. The equiaxed material, in contrast, showed a smooth transition from Newtonian (n ‫؍‬ 1) creep at low stresses to non-Newtonian behavior at high stresses (n > 2). Under the high confining pressure, the microstructures of both materials underwent significant changes. Grain-boundary mullite precipitates in the undeformed, equiaxed-grain material were replaced by fine (ϳ100 nm), intragranular precipitates of silliminate and corundum because of a pressure-induced chemical reaction. This was accompanied by a significant reduction in grain size in both materials. The substantial microstructural changes at high confining pressure resulted in substantially lower viscosities 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 ϳ300 MPa, possibly because of a grain-boundary sliding mechanism; the tabular-grained material showed Newtonian diffusional creep under similar conditions.

Superplasticity and ductile fracture of synthetic feldspar deformed to large strain

“…It is conceivable that temperature steps induce a transient period of enhanced dislocation activity. Possibly, the high activation energy may be attributed to cavitation since the formation of new surface requires additional energy as suggested by Nair et al [2001]. These authors deformed tabularshaped anorthite mullite ceramics and found a high creep activation energy and extensive cavitation.…”

“…Nair et al [2001] observed strong cavitation in synthetic anorthite aggregates with ≈10% mullite added when deformed at ambient pressure. Cavitation was suppressed at elevated pressures of 300 MPa.…”

“…This inference is based on the observation on metal alloys where cavity nucleation and growth are suppressed at hydrostatic pressures on the order of the flow stress [Bampton and Raj, 1982;Nicolaou and Semiatin, 2003;Pilling and Ridley, 1988;Zaki, 1998]. In silicates, cavitation was observed in compression experiments performed at atmospheric pressures to axial strains > 10-20% [e.g., Nair et al, 2001]. Cavitation at pressures significantly higher than the flow stress was first observed in synthetic fine-grained calcite samples using the split cylinder technique [Walker et al, 1990] and in synthetic fine-grained anorthite-diopside aggregates, deformed by grain boundary sliding in torsion [Dimanov et al, 2007].…”

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

“…[5][6][7][8][9][10][11] Furthermore, the application of CAS-based ceramic composites to a high temperature and high pressure environment was largely hindered by the relatively little knowledge of mechanical, microstructural and textural data of CAS and CAS-based composites from laboratory tests. 11,[13][14][15][16] (2) CAS crystal is triclinic with the lowest crystallographic symmetry. The relative activity of different slip systems and dynamic recrystallization in dislocation creeping CAS under varying physical and chemical conditions are still poorly known.…”

“…In this paper we present our experimental results on the mechanical properties and microstructures of monolithic calcium aluminosilicate (CAS or anorthite: CaAl 2 Si 3 O 8 ) aggregates, particulate and layered composites with equal volume fractions of quartz (SiO 2 ) and CAS, deformed in axial compression (σ 1 > σ 2 = σ 3 > 0, where σ 1 , σ 2 , σ 3 are the maximum, intermediate and least compressive principal stresses, respectively) at temperatures of 1173-1473 K, been carried out on monolithic CAS and other plagioclase feldspars under uniaxial compression (σ 1 > σ 2 = σ 3 = 0) at ambient pressure, [5][6][7][8][9][10][11] it remains uncertain if the results from small strain creep tests are able to be extrapolated to other conditions because a small amount of strain is usually insufficient for steady-state microstructure to occur. 12 Cavitation often occurs in the samples deformed at ambient pressure.…”