H, Venugopalan, I, DebRoy Materials Science and Engineering A232(1997)39-46 the growth stage [10]. Composite formation in the port in the oxidation kinetics in the growth stage, growth stage starts when the near surface aluminum DebRoy et al. [14] carried out directed oxidation exper alloy becomes depleted in Mg and reaches a concentra- iments of an Al-Mg alloy in which platinum wires were tion where Al,O, formation becomes more favorable positioned inside the alloy so that the wires would than MgAl,, [iO]. During growth, bulk oxidation of extend through the composite matrix and the top Mgo Al to Al,O, occurs epitaxially on the spinel [11 transport. They [14] ob- Several models have becn proposed to explain the served that the rate of oxidation in the growth stage kinetics of oxidation of Al to Al,O3 in the growth stage was independent of the presence or absence of Pt wires, [8, 12]. It has been suggested that during the growth indicating that the transport of elcctronic specics does stage of the directed oxidation of Al-Mg alloys, a not control the oxidation kinetics of Al-Mg alloys that continuous MgO film exists at the top of the alumina do not contain silicon. It has been shown by Venugo matrix with a thin aluminum alloy film separating the palan et al. [15] that oxygen transport in the near two layers[11, 13](Fig. 1). The presence of this continu- surface alloy layer controls the rate of alumina forma ous MgO film restricts the formation of a protective tion in the growth stage of directed oxidation of binary alumina layer on the surface. At the MgO/Al-alloy film Al-Mg alloys. An examination [16] of the transport interface, Mgo dissociates and oxygen dissolves in the processes involved in the growth stage indicates that Al-alloy film. The magnesium ions formed by dissocia- silicon additions to Al-Mg alloys increase the rate of tion of mgo diffuse through the mgo layer to the oxygen transport through the alloy film and decrease MgO/air interface where they are oxidized to regenerate the rate of oxygen transport through Mgo, respec MgO. During the outward transport of magnesium ions vely. As a result of the reduced rate of electro through the MgO, electrical neutrality is maintained by transport through MgO, this step becomes more impor- the simultaneous transport of electrons [12]. The oxy- tant in the oxidation of Al-Mg-Si alloys than that of dissolved in the alloy film is transported from the Al-Mg alloys. MgOalloy film interface to the alloy film/Al,O, inter- Although the directed oxidation of Al alloys into free pace has been investigated extensively, little informa The supply of aluminum to the alloy film/ALO, inter- tion is available about directed oxidation of Al alloys face is thought to be sustained by the wicking of metal nto Al,O, preforms. Breval et al. [17 examined the through channels in the alumina. One or more of the structure of Al2O3/Al composites produced by directed ahove mentioned reaction steps could be the rate con xidation of an Al alloy into Al,O, preforms. They trolling ding to Hagelberg et al. [12]. the rate of oxida- mechanism in the growth stage observed that the preform refines the alumina grain size and the size of the metal regions. growth into a tion of Al-Mg-Si alloys in the growth stage is con- preform also leads to a more randomly oriented Al,, trolled by the electronic conductivity of the external unlike the preferred orientation [1, 7(0001 axis of Mgo layer. To investigate the role of electronic trans- AlO3 parallel to the growth direction)obtained with out an Al2O, preform. Even though the preform effec tively disrupted the columnar Al,O, matrix structure, Gas phase the Mgo layer on the top surface of the composite was retained [17. In addition, the presence of a thin metal layer beneath the Mgo layer was also observed Mg e Watan et al. [18] investigated the growth of AlO3/Al composites into Al-O, preforms. They measured the thickness of the composite as a function of oxidation Alloy layer O time and temperature. It was observed that the thick ness of the Al,O3/Al composite increased at a constant rate with time at 1473 K. At 1523 K. the thickness AlO/Al composite proportional to the square root of the pore size. How- ever,no explanation was advanced for the observed variation of thickness with time. Nagelberg [19] ob served that the rate of oxidation of aluminum alloys into AL,O, preforms initially increased rapidly to a Al-Mg alloy aximum luc and subsequently decreased as matrix growth proceeded through the preform. Upadhyaya et al. [20] observed that the oxidation rate of Al-2.5 wt% Mg alloy into Al,O3 preforms increased with decrea ig. 1. Schematic diagram of the composite structure AL, Os particle size. However, the oxidation rate wa40 H. Vemgopalm, T. DebRoy /Murerials Science and Etzgineedng A232 (1997) 39-46 the growth stage [IO]. Composite formation in the growth stage starts when the near surface aluminum alloy becomes depleted in Mg and reaches a concentra￾tion where Al,O, formation becomes more favorable than MgAI,O, [lo]. During growth, bulk oxidation of Al to Al,O, occurs epitaxially on the spine1 [I 11. Several models have been proposed to explain the kinetics of oxidation of Al to Al,O, in the growth stage [8,12]. It has been suggested that during the growth stage of the directed oxidation of Al-Mg alloys, a continuous MgO &lm exists at the top of the alumina matrix with a thin aluminum alloy film separating the two layers [11,13] (Fig. 1). The presence of this continu￾ous MgO film restricts the formation of a protective alumina layer on the surface. At the MgO/Al-alloy film interface, MgO dissociates and oxygen dissolves in the Al-alloy film. The magnesium ions formed by dissocia￾tion of MgO diffuse through the MgO layer to the MgO/air interface where they are oxidized to regenerate MgO. During the outward transport of magnesium ions through the MgO, electrical neutrality is maintained by the simultaneous transport of electrons [12]. The oxy￾gen dissolved in the alloy film is transported from the MgO/alloy film interface to the alloy film/Al,O, inter￾face where composite growth takes place epitaxially. The supply of aluminum to the alloy film/A1,03 inter￾face is thought to be sustained by the wicking of metal through channels in the alumina. One or more of the above mentioned reaction steps could be the rate con￾trolling mechanism in the growth stage. According to Nagelberg et al. [12], the rate of oxida￾tion of Al-Mg-Si alloys in the growth stage is con￾trolled by the electronic conductivity of the external MgO layer. To investigate the role of electronic trans￾Gas phase 1 d2 t MgO te- I Alloy layer Al2 OS/Al composite Al-Mg alloy Fig. 1. Schematic diagram of the composite structure. port in the oxidation kinetics in the growth stage, DebRoy et al. [14] carried out directed oxidation exper￾iments of an Al-Mg alloy in which platinum wires were positioned inside the alloy so that the wires would extend through the composite matrix and the top MgO layer to facilitate electronic transport. They [14] ob￾served that the rate of oxidation in the growth stage was independent of the presence or absence of Pt wires, indicating that the transport of electronic species does not control the oxidation kinetics of Al-Mg alloys that do not contain silicon. It has been shown by Venugo￾palan et al. [15] that oxygen transport in the near surface alloy layer controls the rate of alumina forma￾tion in the growth stage of directed oxidation of binary Al-Mg alloys. An examination [16] of the transport processes involved in the growth stage indicates that silicon additions to Al-Mg alloys increase the rate of oxygen transport through the alloy film and decrease the rate of oxygen transport through MgO, respec￾tively. As a result of the reduced rate of electronic transport through MgO, this step becomes more impor￾tant in the oxidation of Al-Mg-Si alloys than that of Al-Mg alloys. Although the directed oxidation of Al alloys into free space has been investigated extensively, little informa￾tion is available about directed oxidation of Al alloys into A&O, preforms. Breval et al. [17] examined the structure of A120,/Al composites produced by directed oxidation of an Al alloy into Al,O, preforms. They observed that the preform refines the alumina grain size and the size of the metal regions. Growth into a preform also leads to a more randomly oriented A1,03, unlike the preferred orientation [1.7] (0001 axis of A&O, parallel to the growth direction) obtained with￾out an Al,O, preform. Even though the preform effec￾tively disrupted the columnar A&O3 matrix structure, the MgO layer on the top surface of the composite was retained [17]. In addition, the presence of a thin metal layer beneath the MgO layer was also observed. Watari et al. [ 181 investigated the growth of Al,O,/Al composites into Al?O, preforms. They measured the thickness of the composite as a function of oxidation time and temperature. It was observed that the thick￾ness of the Al,O,/Al composite increased at a constant rate with time at 1473 K. At 1523 K, the thickness increased as a parabolic function of time, and was proportional to the square root of the pore size. How￾ever, no explanation was advanced for the observed variation of thickness with time. Nagelberg [19] ob￾served that the rate of oxidation of aluminum alloys into Al,O, preforms initially increased rapidly to a maximum value and subsequently decreased as matrix growth proceeded through the preform. Upadhyaya et al. [20] observed that the oxidation rate of Al-2.5 wt% Mg alloy into A&O, preforms increased with decreasing Al,O, particle size. However, the oxidation rate was not