High temperature behaviour of polycrystalline aluminosilicate fibres with mullite bulk composition. I. Microstructure and strength properties

Abstract

Temperature-dependent microstructure/strength relations of commercial aluminosilicate fibres with nearstoichometric mullite bulk composition are investigated. As-received Nextel 440 ™ fibres (70 wt% Al 2O 3, 28 wt% SiO 2, 2 wt% B 2O 3), consisting of nanometresized γ-Al 2O 3 and a minor amount of amorphous phase, have tensile strength of 2100 MPa. Heat treatment at 1200 °C causes transformation into fine-grained mullite (mean grain size: 80 nm). The remaining tensile strength is 1600 MPa. Heat treatment at higher temperatures leads to considerable reduction of strength (1100 MPa after firing at 1400 °C) attributed to mullite grain coarsening (mean grain size at 1400 °C: 135 nm). Altex 2K fibres (72 wt% Al 2O 3, 28 wt% SiO 2) consist of single-phase mullite with a mean grain size of 115 nm. Tensile strength is comparatively low (1250 MPa). Annealing at 1000 °C causes a significant increase in strength (1600 MPa). Above 1200 °C mullite coarsening and strength reduction are similar to those of Nextel 440 fibres.