Maximum weight gain rate as a function of Al,0, particle size, y8 B Table 2 as a function of Al,O3 particle size, for infiltration of Al-Mg filtration of Al-Mg alloy into ALO, preforms at 1450 K alloy into Al2O, preforms at 1450 K Average particle size of alumina Maximum weight gain rate Average particle size of alumina (um) Aw(mm2)k(mm) 0.192 Base allov stage corresponds to the rate of reaction enhanced vaporization of Mg [9]. The rate of reaction enhanced vaporization was, in turn, proportional to the diffusive ity of Mg vapor [9]. As the alumina particle size in the 3. 2. Growth stage preform decreases, the pore size correspondingly de creases(Table 1). The increase in the total number of The oxidation rates can be estimated from the slopes particles per unit volume could cause an increase in of the weight gain vs. time curves in Fig. 2. It is pore tortuosity (r). The diffusivity of Mg vapor observed that the weight gain rate continuously de rough the porous preform (Detr) is given by the creases with time. For parabolic kinetics, the weight following expression [22] gain per unit area, wt, would depend on oxidation time t. a (wt)=ct where d is the diffusivity of Mg vapor in free space and where c is the rate constant. The value of c indicates the is the volume fraction of pores in the preform. ac ate controlling mechanism. It follows from Eq.(2)that cording to eq.(1), an increase in the tortuosity of the the rate constant, c, can be obtained from the slope of preform with 0.53 um size particles would significantly a plot of(wt)- versus t as shown in Fig 4. It is observed duce the diffusivity of Mg vapor and, correspond that there is a single rate constant for the directed ingly, the rate of Mg vaporization. This lower evapora- oxidation of the base alloy. However, there appears to tion rate would reduce the vapor phase oxidation rate be two rate constants in the growth stage of oxidation of magnesium, and consequently, lower weight gain into preforms rate in the initial stage(amount of Mgo formed) Weight gain for the directed oxidation of the Al-Mg Formation of a continuous Mgo layer is essential for alloy into an alumina preform of 8 um particle size was bulk formation of Al2O3. The inability to form a con- also measured as a function of time tinuous Mgo layer at the top surface would prevent pressures(Fig. 5). It is seen from Figs. 2 and 5 that the formation of continuous Al, O3 layer in the growth weight gain rate decreases with time and is independent stage for the preform with 0.53 um sized particles. This of oxygen pressure. The presence of pre-existing Al2O behavior is consistent with the observed lack of weight particles introduces two effects that could alter the gain with oxidation time(Fig. 2). The result is also in growth kinetics of Al-Mg alloys: (a) It could provide agreement with that reported by Sindel et al. [23] and sites for preferential nucleation of Al2O; and(b)It Upadhyaya et al hese investigators [20, 23] ob- could limit liquid metal transport to the growth surface served that composite formation did not occur satisfac- The role of these two factors in the growth kinetics of torily for infiltration into Al2O, preforms of particle the Al-5wt%Mg alloy into alumina preforms are dis- size less than 2 um cussed bele Table 3 Data used in the calculation of oxygen transport through the near surface alloy layer operty Symbol value Reference Diffusivity of oxygen in molten aluminum"(m's-t Thickness of alloy layer (1 4.7×10 Oxygen concentration in the alloy film at the mgo/film interface b 44×10-5[25-27 Oxygen concentration in the alloy film at the Al,O / film interface fraction 2.0×10 [25-27 ivity of oxygen in molten aluminum is approximated by the diffusivity of aluminum in molten aluminum at 1450K culated at 1450 K for an Mg concentration of 0.22 mol. in the alloy filmIf. T’mugopalan, T. D&Roy / Murerials Science and Engineering A232 (1997) 39-46 43 Table 2 Maximum weight gain rate as a function of Al,O, particle size, for infiltration of Al-Mg alloy into A&O, preforms at 1450 K Table 4 A, as a function of A&O, particle size, for infiltration of Al-Mg alloy into Al,O; preforms at 1450 K Average particle size of alumina Maximum weight gain rate (wd (g mm2 s-r) 8 0.872 16 0.6 110 0.192 Base alloy 0.16 Average particle size of alumina (urn) 8 16 110 stage corresponds to the rate of reaction enhanced vaporization of Mg [9]. The rate of reaction enhanced vaporization was, in turn, proportional to the diffusiv￾ity of Mg vapor [9]. As the alumina particle size in the preform decreases, the pore size correspondingly de￾creases (Table 1). The increase in the total number of particles per unit volume could cause an increase in pore tortuosity (7). The diffusivity of Mg vapor through the porous preform (D,rr) is given by the following expression [22]: 3.2. Growth stage D,, = D ! (1) z The oxidation rates can be estimated from the slopes of the weight gain vs. time curves in Fig. 2. It is observed that the weight gain rate continuously de￾creases with time. For parabolic kinetics, the weight gain per unit area, wt, would depend on oxidation time, t, as: where D is the diffusivity of Mg vapor in free space and E is the volume fraction of pores in the preform. Ac￾cording to Eq. (l), an increase in the tortuosity of the preform with 0.53 nm size particles would significantly reduce the diffusivity of Mg vapor and, correspond￾ingly, the rate of Mg vaporization. This lower evapora￾tion rate would reduce the vapor phase oxidation rate of magnesium, and consequently, lower weight gain rate in the initial stage (amount of MgO formed). (wt)’ = et (2) where c is the rate constant. The value of c indicates the rate controlling mechanism. It follows from Eq. (2) that the rate constant, c, can be obtained from the slope of a plot of (wt)* versus t as shown in Fig. 4. It is observed that there is a single rate constant for the directed oxidation of the base alloy. However, there appears to be two rate constants in the growth stage of oxidation into preforms. Formation of a continuous MgO layer is essential for bulk formation of Al,O,. The inability to form a con￾tinuous MgO layer at the top surface would prevent formation of continuous Al,O, layer in the growth stage for the preform with 0.53 nm sized particles. This behavior is consistent with the observed lack of weight gain with oxidation time (Fig. 2). The result is also in agreement with that reported by Sindel et al. [23] and Upadhyaya et al. [20]. These investigators [20,23] ob￾served that composite formation did not occur satisfac￾torily for infiltration into Al,O, preforms of particle size less than 2 pm. Weight gain for the directed oxidation of the Al-Mg alloy into an alumina preform of 8 pm particle size was also measured as a function of time at different oxygen pressures (Fig. 5). It is seen from Figs. 2 and 5 that the weight gain rate decreases with time and is independent of oxygen pressure. The presence of pre-existing Al,O, particles introduces two effects that could alter the growth kinetics of Al-MS alloys: (a) It could provide sites for preferential nucleation of Al,O,; and (b) It could limit liquid metal transport to the growth surface. The role of these two factors in the growth kinetics of the Al-Swt%Mg alloy into alumina preforms are dis￾cussed below. Table 3 Data used in the calculation of oxygen transport through the near surface alloy layer & (mm’) k (mm’) 380 1.01 260 1.39 80 3.05 Property Symbol Value Reference Diffusivity of oxygen in molten aluminum” (In2 s-‘) Thickness of alloy layer (m) Oxygen concentration in the alloy film at the MgO/film interfaceb (mole fraction) Oxygen concentration in the alloy film at the Al,O,/tilm inrerfaceb (mole fraction) DO 1.3 x lo-* ~241 L 4.7 x 10-h 1131 XL 4.4 x 10-z [25-271 xg 2.0 x 10-G [25-271 “Tracer diffusivity of oxygen in molten aluminum is approximated by the diffusivity of aluminum in molten aluminum at 1450 K. bOxygen concentrations calculated at 1450 K for an Mg concentration of 0.22 mol.% in the alloy film