H. Venugopalan, T. Deb Roy/Materials Science and Engineering A232(1997)39-46 monodisperse AlO3 particles in the preform. Hence, the packing density of the Al,O3 preform used in this c400 study was assumed to be 55%[21]. Similarly, for ran- dom loose packing, the pore size in the Al2O, preform 300 was estimated to be 0. 414 times the particle size [21] 日 b) The characteristics of the preforms used in this study are given in Table I 200 The crucible containing the alloy was suspended by a platinum wire from the balance and positioned within 100 the equi-temperature zone of the furnace. Prior to conducting each experiment, the reaction tube was evacuated and purged with argon. The samples were then heated to the test temperature at a heating rate of 01000020003000040000 0.33 Ks-I in an argon(99.9% purity) atmosphere. The Time. s directed oxidation experiments were carried out at 1450 K and at an oxygen pressure of 85092 Pa. Experiments Fig. 5. Weight gain per unit area vs time for directed oxidation of were repeated to check the reproducibility of the weight different oxygen pressures: (a)21273 Pa,(b)85092 Pa. The total gain data. The total gas flow rate was kept constant 8333 mm3sI STP (298K and 101.3 kPa). a typical pressure and the total gas flow rate were maintained constant at 93303 Pa and 8333 mm 's STP, respectively scatter of 1-5% was observed in the weight gain mea sured during oxidation. The weight of the sample was alloy and for the composites containing alumina pre- continuously recorded using a computer assisted data forms were normalized by the net cross-sectional area acquisition system. Subsequently, the recorded data perpendicular to the macroscopic growth direction were differentiated numerically to obtain the weight When the oxidation was carried out without any pre- gain rate. The zero on the time axis was taken as the form. this responded to the internal cross-scc instant when oxygen of the desired partial pressure was tional area of the crucible(154 mm2). When a preform introduced in the reactor. It takes about 75 s for the gas was used, this area was taken as the void space between to reach the crucible. Correction of the time axis was the alumina particles. For random loose packing, this ignored since the total oxidation time was of the order void area fraction is <0.45 [21]. Thus, the cross-sec. of 50 ks tional area of the void space is 0. 45 x 154=69.3 mm ther tha 3. Results and discussion net area available for growth of the reaction product the normalized weight gain curves in Fig. 2 should in The weight gain vs. time curves for directed oxida- posites grown into A1, 0, preforms (except for preform g (Al-5056 alloy)into of 0.53 um particle size)exhibit a weight gain per unit alumina preforms of varying particle sizes are presented area. corrected for the presence of particles, higher than in Fig. 2. The weight gain VS. time plot for the base preform. Thus, the differences in the oxidation rates of 80000 Al-Mg alloys, with and without the preforms, cannot be completely attributed to the reduction in the melt area exposed to oxidation. The differences in the nof 日6000 16 um malized weight gain VS time data in Fig. 2 are analyzed s4000 3. 1. Initial stage 20000 Base alloy The weight gain vs. time plots in the initial stage for oxidation with and without the preform are shown in 0100002003000040000500 Fig. 3. The data show that when a bed of 0.53 um Time. s diameter particles is uscd, oxidation did not occur at appreciable rates. When the particle size was increased Fig. 4. Parabolic kinetics in the growth stage of directed oxidation of to 8 um or larger, the initial stage oxidation rates with Al-Mg alloy at 1450 K into Al2O, preforms of different particle or without the preform exhibited minor differences. It ne growth stage in Fig. 2. can be shown that the weight gain rate in the initial42 H. Vemrgopalan, T. DebRoy /Materials Science and Engineering 6232 (1991) 39-46 monodisperse Al,O, particles in the preform. Hence, the packing density of the A1,03 preform used in this study was assumed to be 55% 1211. Similarly, for ran￾dom loose packing, the pore size in the A&O, preform was estimated to be 0.414 times the particle size 1211. The characteristics of the preforms used in this study are given in Table 1. The crucible containing the alloy was suspended by a platinum wire from the balance and positioned within the equi-temperature zone of the furnace. Prior to conducting each experiment, the reaction tube was evacuated and purged with argon. The samples were then heated to the test temperature at a heating rate of 0.33 K s- ’ in an argon (99.9% purity) atmosphere. The directed oxidation experiments were carried out at 1450 K and at an oxygen pressure of 85092 Pa. Experiments were repeated to check the reproducibility of the weight gain data, The total gas flow rate was kept constant at 8333 mm3 s-’ STP (298 K and 101.3 kPa). A typical scatter of l-5% was observed in the weight gain mea￾sured during oxidation. The weight of the sample was continuously recorded using a computer assisted data acquisition system. Subsequently, the recorded data were differentiated numerically to obtain the weight gain rate. The zero on the time axis was taken as the instant when oxygen of the desired partial pressure was introduced in the reactor. It takes about 75 s for the gas to reach the crucible. Correction of the time axis was ignored since the total oxidation time was of the order of 50 ks. 3. Results and discussion The weight gain vs. time curves for directed oxida￾tion of the Al - 5 wt% Mg alloy (Al-5056 alloy) into alumina preforms of varying particle sizes are presented in Fig. 2. The weight gain vs. time plot for the base "0 10000 20000 30000 40000 50000 Time, s Fig. 4. Parabolic kinetics in the growth stage of directed oxidation of AI-Mg alloy at 1450 K into A1,0, preforms of different particle sizes. Data correspond to the growth stage in Fig. 2. 3 400 (4 -g 300- / (b) h *g 200- &II ii loo- ‘22 I g 07,“)“’ 0 10000 20000 30000 40000 Time, s Fig. 5. Weight gain per unit area vs time for directed oxidation of Al-Mg alloy at 1450 K into A120, preform of 8 pm particle size at different oxygen pressures: (a) 21273 Pa, (b) 85092 Pa. The total pressure and the total gas Row rate were maintained constant at 933Q3 Pa and 8333 mm3 s - ’ STP, respectively. alloy and for the composites containing alumina pre￾forms were normalized by the net cross-sectional area perpendicular to the macroscopic growth direction. When the oxidation was carried out without any pre￾form, this area corresponded to the internal cross-sec￾tional area of the crucible ( 154 nm’). When a preform was used, this area was taken as the void space between the alumina particles. For random loose packing, this void area fraction is z 0.45 1211. Thus, the cross-sec￾tional area of the void space is 0.45 x 154 = 69.3 mm’. If the preform had no effect other than to reduce the net area available for growth of the reaction product, the normalized weight gain curves in Fig. 2 should, in principle, overlap. However. it is clear that the com￾posites grown into A&O, preforms (except for preform of 0.53 pm particle size) exhibit a weight gain per unit area, corrected for the presence of particles, higher than that of the oxidation rate of the alloy without the preform. Thus, the differences in the oxidation rates of Al-Mg alloys, with and without the preforms, cannot be completely attributed to the reduction in the melt area exposed to oxidation. The differences in the nor￾malized weight gain vs. time data in Fig, 2 are analyzed below. 3.1. I&id stage The weight gain vs. time plots in the initial stage for oxidation with and without the prefoml are shown in Fig. 3. The data show that when a bed of 0.53 pm diameter particles is used, oxidation did not occur at appreciable rates. When the particle size was increased to 8 pm or larger, the initial stage oxidation rates with or without the preform exhibited minor differences. It can be shown that the weight gain rate in the initial