Directionally solidified eutectic and off-eutectic mullite–zirconia fibres

Journal of the European Ceramic Society, Volume 33, Issue 5, May 2013, Pages 953-963.
R.G. Carvalho, M.S. Pires, A.J.S. Fernandes, R.F. Silva, F.M. Costa.

 

I3N, Department of Physics, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal and

CICECO, Department of Ceramic and Materials Engineering, University of Aveiro, Campus de Santiago, 3810-193 Aveiro, Portugal.

 

 

Abstract

 

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.

 

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