A thermogravimetric study of the oxidative growth of Al 2 O 3 /Al alloy composites
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O. Salas Graduate Student Researcher, University of California, Santa Barbara, California 93106 H. Ni Assistant Specialist, University of California, Santa Barbara, California 93106
V. Jayaram Assistant Professor, Indian Institute of Science, Bangalore, India
C. G. Levi Associate Professor of Materials and Mechanical Engineering, University of California, Santa Barbara, California 93106
R. Mehrabian President, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 (Received 22 May 1989; accepted 16 May 1991)
The oxidation of liquid A l - M g - S i alloys at 900-1400 °C was studied by thermogravimetric analysis (TGA). The development of a semi-protective surface layer of MgO/MgAl 2 O 4 allows the continuous formation of an Al2O3-matrix composite containing an interpenetrating network of metal microchannels at 1000-1350 °C. An initial incubation period precedes bulk oxidation, wherein A12O3 grows from a near-surface alloy layer by reaction of oxygen supplied by the dissolution of the surface oxides and Al supplied from a bulk alloy reservoir through the microchannel network. The typical oxidation rate during bulk growth displays an initial acceleration followed by a parabolic deceleration in a regime apparently limited by Al transport to the near-surface layer. Both regimes may be influenced by the Si content in this layer, which rises due to preferential Al and Mg oxidation. The growth rates increase with temperature to a maximum at ~1300 °C, with a nominal activation energy of 270 kJ/mole for an Al-2.85 wt. % Mg-5.4 wt. % Si alloy in O2 at furnace temperatures of 1000-1300 °C. An oscillatory rate regime observed at 1000-1075 °C resulted in a banded structure of varying Al2O3-to-metal volume fraction. I. INTRODUCTION Directed metal oxidation is a new technique for the fabrication of ceramic matrix composites that advantageously utilizes the high temperature oxidation of liquid metals.1'2 (The technology is known commercially as DIMOX™ and was developed by the Lanxide Corporation of Newark, DE.) Suitable alloying enables the growth of a bulk oxide matrix containing a network of fine microscopic channels through which liquid metal is continuously supplied to the reaction interface. Commercial application may utilize a ceramic preform (particulate or fiber) placed upon the metal reservoir at the start of the process. The oxide/metal composite grows through the preform, away from the melt surface, yielding a ceramic-matrix composite with two interpenetrating strengthening phases: the ceramic preform and the ductile metal. For the particular case of A12O3/A1 matrix composites, the process is typically carried out between 1100 and 1400 °C using specific dopants such 1982 http://journals.cambridge.org
J. Mater. Res., Vol. 6, No. 9, Sep 1991 Downloaded: 13 Mar 2015
as Mg and Si, which may be added as alloying elements or externally as oxides (MgO and SiO2) to the alloy/preform interface.1 A companion study by the present authors3 discussed the nucleation and growth mechanisms during composite formation from two al
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