Journal of Materials Research
- Journal of Materials Research / Volume 6 / Issue 09 / 1991, pp 1982-1995
- Copyright © Materials Research Society 1991
- Published online: 31 January 2011
- DOI: http://dx.doi.org/10.1557/JMR.1991.1982 (About DOI)
Articles
A thermogravimetric study of the oxidative growth of Al2O3/Al alloy composites
K.C. Vlacha1, O. Salasa2, H. Nia3, V. Jayarama4, C.G. Levia5 and R. Mehrabiana6
a1 Assistant Research Engineer, University of California, Santa Barbara, California 93106
a2 Graduate Student Researcher, University of California, Santa Barbara, California 93106
a3 Assistant Specialist, University of California, Santa Barbara, California 93106
a4 Assistant Professor, Indian Institute of Science, Bangalore, India
a5 Associate Professor of Materials and Mechanical Engineering, University of California, Santa Barbara, California 93106
a6 President, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213
Abstract
The oxidation of liquid Al–Mg–Si alloys at 900–1400 °C was studied by thermogravimetric analysis (TGA). The development of a semi-protective surface layer of MgO/MgAl2O4 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 Al2O3 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.
(Received May 22 1989)
(Accepted May 16 1991)
