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

The microstructure and tensile properties of the as-received and heat-treat ed Nextel 720 fibre have been studied. During its fabrication the Nextel 72 0 fibre is pyrolysed at a temperature lower than 1400 degreesC for a very s hort time which does not allow the microstructure to be stabilised. A pseud o-tetragonal metastable alumina rich mullite is formed which crystallises i n the form of mosaic grains containing low angle boundaries. These mosaic g rains enclose some rounded and elongated alpha -alumina grains. The evoluti on of the mullite to the stable orthorhombic symmetry is seen from 1200 deg reesC for post heat treatments lasting several hours. For longer heat treat ments at 1200 degreesC or at 1300 degreesC and higher temperatures, the mos aic grains begin to recrystallise into single grains and the alumina reject ed from the mullite contributes to the growth of elongated alpha -alumina g rains which suggests diffusion through an intergranular liquid silicate pha se. At 1400 degreesC the mullite has the 3:2 composition and after 24 h the growth of the elongated alpha -alumina grains leads to a reduction of the room temperature tensile strengths. The alpha -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 degreesC. (C) 2001 Els evier Science Ltd. All rights reserved.