A thermogravimetric study of the oxidative growth of Al₂O₃/Al alloy composites

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.

Effect of dopants on the phase formation in microwave processed Al–Al2O3 composites

“…During microwave heating protective Al 2 O 3 layer was formed at the surface of the undoped Al powder compact, which restricted the diffusion of oxygen towards the bulk side [4]. Therefore, Al-Al 2 O 3 composite growth rate was very slow.…”

“…The oxygen needed for the oxidation of Al was supplied by the continuous dissolution of one or both of the Mg-rich surface oxides into the Mg-lean alloy than by direct dissolution of oxygen from air [3,4]. MgO was thermodynamically less stable than MgAl 2 O 4 in the Mg-lean alloy (<7.24% Mg).…”

“…Al-Al 2 O 3 composites have been prepared from aluminium and its alloys by various routes, e.g. high temperature (∼900-1400 • C) oxidation of liquid Al-Mg and Al-Mg-Si alloys [3][4][5], directed metal oxidation [6], microwave heating [7], reactive melt infiltration [8], squeeze casting [9][10], in situ method [11], friction stir processing [12], powder metallurgy [13][14] etc.…”

“…During high temperature oxidation of liquid Al-Mg and Al-Mg-Si alloys in the temperature range of 1100-1300 • C initial oxidation followed by an incubation period of limited oxide growth and then bulk Al-Al 2 O 3 composite formation was observed [3][4][5]. A duplex oxide layer in which MgO was the upper layer and MgAl 2 O 4 containing metal microchannels was the lower layer, was formed at the surface and got continuously thickened during incubation.…”

“…The bulk oxide growth was preceded by the decrease in MgO layer thickness. Continuous formation of MgO at the surface, reaction of some of the MgO with the melt to form MgAl 2 O 4 , dissolution of the MgO and MgAl 2 O 4 in the Al-rich alloy, diffusion of dissolved oxygen through the metal layer and epitaxial growth of Al 2 O 3 resulted in bulk oxide growth [3][4][5]. Growth of Al-Al 2 O 3 composites by directed metal oxidation technique occurred by continuous infiltration of a preform (reinforcement of filler) consisted of Al 2 O 3 powder through outward oxidation of Mg, MgO and Si doped molten Al metal [6].…”