Microstructural stability and room temperature mechanical properties of the Nextel 720 fibre

The microstructure and tensile properties of the as-received and heat-treated Nextel 720 fibre have been studied. During its fabrication the Nextel 720 fibre is pyrolysed at a temperature lower than 1400°C for a very short time which does not allow the microstructure to be stabilised. A pseudo-tetragonal metastable alumina rich mullite is formed which crystallises in the form of mosaic grains containing low angle boundaries. These mosaic grains enclose some rounded and elongated α-alumina grains. The evolution of the mullite to the stable orthorhombic symmetry is seen from 1200°C for post heat treatments lasting several hours. For longer heat treatments at 1200°C or at 1300°C and higher temperatures, the mosaic grains begin to recrystallise into single grains and the alumina rejected from the mullite contributes to the growth of elongated α-alumina grains which suggests diffusion through an intergranular liquid silicate phase. At 1400°C the mullite has the 3:2 composition and after 24 h the growth of the elongated α-alumina grains leads to a reduction of the room temperature tensile strengths. The α-alumina creation coupled to the phase transformation and dissolution of mullite leads to an increase of the Young's modulus after heat treatments from 1200°C.