H. Venugopalan, T. Deb Roy/ Materials Science and Engineering 4252(1997)39-40 Table 1 Description of the characteristics of the alumina preforms used in the directed oxidation of Al-5wt%Mg alloy 100 (b) Average particle size Estimated pore Surface area per unit size(uin) volume x 10-6(m") 0.22 16 11 00091 supplied by Sumitomo Chemical Supplied by ALCOA "Supplied by ALFA-AESAR. 02004006008001000 Time. s found to be proportional to the surface area of the directed oxidation of Al-Mg alloy at 1450 K into A1, 0, preforms of Al2O, particle. It is clear from these experimental ob- different particle sizes:(a)8 um, (b)16 um, (c)110 um, and (d)0.53 servations that while AlO3 preforms can provide sites um. The weight gain per unit area vs time plot in the initial stage for for the epitaxial nucleation of fresh Al,2O3, the loose the base alloy (e)is also shown. Data correspond to the initial stage preform can also provide a tortuous path for metal in Fig. 2. wicking to the growth surface and, thus, limit liquid letal transport. Thus, the effect of preform particle size 2. Experimental procedure on the interplay between secondary nucleation and liquid metal transport, and their effects on the com The thermogravimetric set-up, used for studying re- posite growth rate, need to be examinee action kinetics during the directed oxidation of Al-Mg In this paper, we examine the directed oxidation of alloys, consisted of a Cahn model 1000 Al-Mg alloys through AL, preforms. The oxidation recording electric balance, a high temperature silicon kinetics were studied by thermogravimetry. The weight carbide tube furnace, and a gas flow and pressur gain in the growth stage was monitored as a function of control system. The balance had a sensitivity of 0.5ug oxygen pressure, and particle size. The results show and the measurement accuracy was 0. 1% of the range that the oxidation kinctics of Al alloys into Al2O The quartz reaction tube was of 4& mm internal diame- preforms can be tailored by appropriate selection of the ter. The furnace had a 25 mm equi-temperature zone at the center. The furnace was equipped with an electronic temperature controller that regulated the temperature Cylindrical samples 14 mm in diameter and 8 mm in length of an Al 5056 alloy (5 wt% Mg, 0.10 wt%Cu 0. 40 wt% Fe.0.10 wt% Zn. 0.10 wt% Mn and balance l61 AI)were placed at the bottom of Al2O, crucibles 14.2 mm in diameter and 27 mm in length. and then covered with Al,O3 powder. Powders of average particle sizes Base alloy ranging from 0.53 to 110 um were used. For the 8 Alm 100 Al,, preform, composite growth was also at various partial pressures of oxygen. In all experi- 0.53 ments, the total weight of the AlO powder in preform was 500 mg. The Al2O, layer thickness was 0100002000030040000500 about 3.5 mm. Composite growth occurred upward. away from the alloy surface, infiltrating the preform Time. s The presence of the Al2O, preform reduces the cross- Tig. 2. Weight gain per unit arca v3. timc for directed oxidation of sectional arca of the melt exposed to the oxidizing Al-Mg alloy at 1450 K into Al2O, preforms of different particle atmosphere. The reduction in cross-sectional area can izes. The weight gain per unit area vs, time plot for the base alloy is vn. The c be dctcrmincd from the packing density of the prefs rate were maintained constant at 85092.93303 Pa and 8333 mms-1 The packing density of the Al2O, preform was esti- STP, respectively. ted, by assuming a random loose packing ofH. Venugopalan, T. DebRoy /Materials Science and Engineering -A232 (1997) 39-46 41 Table 1 Description of the characteristics of the alumina preforms used in the directed oxidation of Al-Swt%Mg alloy Average particle size Estimated pore Surface area per unit of alumina (urn) size (urn) volume x 10m6 (m-l) 0.53” 0.22 1.9 Sb 3.3 0.125 16’ 6.6 0.0625 110’ 45.5 0.0091 “Supplied by Sumitomo Chemical. bSupplied by ALCOA. cSupplied by ALFA-AESAR. found to be proportional to the surface area of the A&O, particle. It is clear from these experimental ob￾servations that while A&O, preforms can provide sites for the epitaxial nucleation of fresh Al,O,, the loose preform can also provide a tortuous path for metal wicking to the growth surface and, thus, limit liquid metal transport. Thus, the effect of preform particle size on the interplay between secondary nucleation and liquid metal transport, and their effects on the com￾posite growth rate, need to be examined. In this paper, we examine the directed oxidation of Al-Mg alloys through Al,O, preforms. The oxidation kinetics were studied by thermogravimetry. The weight gain in the growth stage was monitored as a function of oxygen pressure, and particle size. The results show that the oxidation kinetics of Al alloys into A&O3 preforms can be tailored by appropriate selection of the preform particle size. 400 8 pm 300 - 200 - loo￾O- 0.53 pm -100, ’ * t 0 10000 20000 30000 40000 50000 Time, s Fig. 2. Weight gain per unit area vs. time for directed oxidation of Al-Mg alloy at 1450 K into A&O, preforms of different particle sizes. The weight gain per unit area vs. time plot for the base alloy is also shown. The oxygen pressure, total pressure and the total gas flow rate were maintained constant at 85092, 93303 Pa and 8333 mm3 s - ’ STP, respectively. 60- W (4 -207 8 4 a I ? 0 200 400 600 800 1000 Time, s Fig. 3. Weight gain per unit area vs. time in the initial stage of directed oxidation of Al-Mg alloy at 1450 K into Also, preforms of different particle sizes: (a) 8 urn, (b) 16 urn, (c) 110 urn, and (d) 0.53 pm. The weight gain per unit area vs time plot in the initial stage for the base alloy (e) is also shown. Data correspond to the initial stage XFig. 2. 2. Experimental procedure The thermogravimetric set-up, used for studying re￾action kinetics during the directed oxidation of Al-Mg alloys, consisted of a Cahn model 1000 automatic recording electric balance, a high temperature silicon carbide tube furnace, and a gas flow and pressure control system. The balance had a sensitivity of 0.5 ug and the measurement accuracy was 0.1% of the range. The quartz reaction tube was of 48 mm internal diame￾ter. The furnace had a 25 mm equi-temperature zone at the center. The furnace was equipped with an electronic temperature controller that regulated the temperature to i 5 K. Cylindrical samples 14 mm in diameter and 8 mm in length of an Al 5056 alloy (5 wt% Mg, 0.10 wt% Cu, 0.40 wt% Fe, 0.10 wt% Zn, 0.10 wt% Mn and balance Al) were placed at the bottom of Al,O, crucibles 14.2 mm in diameter and 27 mm in length, and then covered with A&O, powder. Powders of average particle sizes ranging from 0.53 to 110 urn were used. For the 8 urn Al,O, preform, composite growth was also carried out at various partial pressures of oxygen. In all experi￾ments, the total weight of the Al,O, powder in the preform was 500 mg. The A&O3 layer thickness was about 3.5 mm. Composite growth occurred upward: away from the alloy surface, infiltrating the preform from beneath. The presence of the A1,03 preform reduces the cross￾sectional area of the melt exposed to the oxidizing atmosphere. The reduction in cross-sectional area can be determined from the packing density of the preform. The packing density of the A&O, preform was esti￾mated, by assuming a random loose packing of