Directionally solidified eutectic and off-eutectic mullite–zirconia fibres

Eutectic and off-eutectic mullite–zirconia fibres were grown by LFZ (laser floating zone) directional solidification. The microstructure of the mullite–zirconia eutectic fibres varies from planar coupled eutectic (1 mm/h pulling rate) through mullite columnar growth with coarse zirconia inclusions (10 mm/h pulling rate) to faceted mullite eutectic dendrites, which enclose a dispersion of fine zirconia fibrils (100–500 mm/h pulling rates). Near-equilibrium conditions determine the crystallization of monoclinic zirconia and the absence of any amorphous phase, whereas for higher speeds the tetragonal structure is retained and a residual liquid is kept after the eutectic solidification. Similar structural and morphological characteristics are displayed by the mullite-rich off-eutectic composition added to the development of prismatic crystals of mullite primary phase. In opposition, heavy constitutional supercooling takes place in the case of the zirconia-rich off-eutectic fibres, where equiaxed zirconia dendrites soon form as a primary phase, leaving a non-equilibrium mixture of alumina and sillimanite as interdendritic constituent.