H. venugopalan, /. Deb Roy/Materials Science and Engineering 4232 (1997)39-46 interface decreases with particle size. Since the pore seen from Eq(7) that the rate of liquid metal trans- ize decreases with decreasing particle size, the inter- port is proportional to the square root of the particle particle spaces can be filled in a shorter time by tI size. It is seen fronn Table 5 that though the weight composite growing from the particle surfaces, and gain rate slightly increases with particle size, the in hence k would decrease with decrcasing particle size. crease is not proportional to the square root of the This decrease in k with decreasing particle size par- particle size. This indicates that at longer oxidation ially counteracts the increase in surface area per unit times, liquid mctal transport is not the only rate con- volume. Thus, although the weight gain rate(Table 2) trolling factor. It is seen from Table 2 that the rate increases with decreasing particle size, it does not ygen transport incrcascs with decrcasing change in proportion to the surface area per unit size. a joint control of the weight gain rate by oxy volume of the particles gen transport and liquid metal transport would ex The case for secondary nucleation and growth from plain the particle size dependence of the weight gain the Al2O, particles is further supported by observa- rate at extended oxidation times. It needs to be tions of the grain structure in the final composite. It noted, however, that the weight gain rates in the sec- is well established that oxidation without a preform ond period of the growth stage are small. Therefore leads to columnar grains, tens of micrometers across, the change in the mechanism cannot be defined con- growing with the same crystallographic orientation Al,O, composite grown into an Al2O, preform ex- two effects on the rate of directed oxidation of al Co over hundreds of micrometers [1, Il]. In contrast, the Thus, the presence of an Al2O, preform produces hibits the same orientation over smaller regions(< Mg alloys, In the initial period of growth, the mecha 50 um)[17]. In addition. the grain sizes of the nism of oxidation of Al-Mg alloy into the alumina alumina and the metal are refined [17]. Furthermore, preforms is similar to the mechanism of the oxidation one third to one half of the surface of each Al, O3 of the Al-Mg alloy into free space. The weight gain particle is directly bonded to the Al2O, matrix [17]. rate is controlled by oxygen transport through the These observations strongly support the role of A1,O, surface alloy layer and increases with increasing sec particles in the preform in increasing the weight gain ondary nucleation of Al,O3 on the Al,O3 particles. In rate by aiding secondary nucleation of Al, O3 this regime the weight gain rate decreases with in- 3. 4. Liquid metal transport time, liquid metal transport to slows down. The weight gain rate data in this regime It is observed from Fig. 4 that the rate constant seem to be consistent with a mixed transport control changes with increasing oxidation time. In addition the change of rate constant occurs earlier as the AlO, particle size decreases. However, no change of 4. Conclusions rate constant occurs for the directed oxidation of ti base alloy(Fig. 4)in the absence of a preform. The The kinetics of growth of Al-Mg alloys into Al, O3 change in the value of the rate constant indicates that preforms has been investigated as a function of oxy oxygen transport is no longer the rate controlling gen pressure, and preform particle size. The weight mechanism for oxidation into preforms. As oxidation gain rate decreased with time and was independent of proceeds into the preform, the oxidation front be- oxygen pressure. The presence of an A1,O, preform comes more extended and convoluted(Fig. 6). This, produces two effects on the rate of directed oxidation in turn, could slow down the rate of liquid metal of Al-Mg alloys. In the initial period of growth, the transport to the oxidation front. If the rate of liquid mechanism of oxidation of Al-Mg alloys into the metal transport decreased sufficiently, it would then alumina preforms was similar to the mechanism of become the rate controlling mechanism and would the oxidation of the Al-Mg alloys into free space correspond to the experimentally measured weight The weight gain rate was controlled by the transport gain rate. of oxygen through the surface alloy layer and in- Weight gain rates for the various particle sizes were creased with increasing secondary nucleation of A1,O3 measured at a time of 30000s(slope change has oc- on the Al2 O3 particles. In this regime, the weight gain curred)and are given in Table 5. The rate of liquid rate decreased with increasing Al2O3 particle size metal transport(J)is given by the expression [15] With increasing oxidation time, liquid metal transport R Lo the oxidation front decreased considerably. The (7) weight gain rate data in this regime seem to be con sistent with a mixed control inech olv where r denotes the pore radius, r denotes the parti- uid metal transport through the composite and cle size, and t denotes the timc of oxidation. It is oxygen transport across the alloy layerH. T’cmgopnlan, T. DebRoy / Marerinls Science and EngiueeCng A232 (1997) 39-46 45 interface decreases with particle size. Since the pore size decreases with decreasing particle size, the inter￾particle spaces can be filled in a shorter time by the composite growing from the particle surfaces, and hence k would decrease with decreasing particle size. This decrease in k with decreasing particle size par￾tially counteracts the increase in surface area per unit volume. Thus, although the weight gain rate (Table 2) increases with decreasing particle size, it does not change in proportion to the surface area per unit volume of the particles. The case for secondary nucleation and growth from the A120, particles is further supported by observa￾tions of the grain structure in the final composite. It is well established that oxidation without a preform leads to columnar grains, tens of micrometers across, growing with the same crystallographic orientation over hundreds of micrometers [I, 1 I]. In contrast, the Al,O, composite grown into an Al,O, preform ex￾hibits the same orientation over smaller regions ( = lo-50 urn) [17]. In addition, the grain sizes of the alumina and the metal are refined [17]. Furthermore, one third to one half of the surface of each A&O, particle is directly bonded to the A&O, matrix [17]. These observations strongly support the role of A&O, particles in the preform in increasing the weight gain rate by aiding secondary nucleation of Al,O,. 3.4. Lipid metal tmnsport It is observed from Fig. 4 that the rate constant changes with increasing oxidation time. In addition, the change of rate constant occurs earlier as the A120, particle size decreases. However, no change of rate constant occurs for the directed oxidation of the base alloy (Fig. 4) in the absence of a preform. The change in the value of the rate constant indicates that oxygen transport is no longer the rate controlling mechanism for oxidation into preforms. As oxidation proceeds into the preform, the oxidation front be￾comes more extended and convoluted (Fig. 6). This, in turn, could slow down the rate of liquid metal transport to the oxidation front. If the rate of liquid metal transport decreased sufficiently, it would then become the rate controlling mechanism and would correspond to the experimentally measured weight gain rate. Weight gain rates for the various particle sizes were measured at a time of 30000s (slope change has oc￾curred) and are given in Table 5. The rate of liquid metal transport (J) is given by the expression [15]: Jx[~l’:2K[~l”2 (7) where Y denotes the pore radius, R denotes the parti￾cle size, and t denotes the time of oxidation. It is seen from Eq. (7) that the rate of liquid metal trans￾port is proportional to the square root of the particle size. It is seen from Table 5 that though the weight gain rate slightly increases with particle size, the in￾crease is not proportional to the square root of the particle size. This indicates that at longer oxidation times, liquid metal transport is not the only rate con￾trolling factor. It is seen from Table 2 that the rate of oxygen transport increases with decreasing particle size. A joint control of the weight gain rate by oxy￾gen transport and liquid metal transport would ex￾plain the particle size dependence of the weight gain rate at extended oxidation times. It needs to be noted, however, that the weight gain rates in the sec￾ond period of the growth stage are small. Therefore, the change in the mechanism cannot be defined con￾clusively. Thus, the presence of an Al,O, preform produces two effects on the rate of directed oxidation of Al￾Mg alloys. In the initial period of growth, the mecha￾nism of oxidation of Al-Mg alloy into the alumina preforms is similar to the mechanism of the oxidation of the Al-ME alloy into free space. The weight gain rate is controlled by oxygen transport through the surface alloy layer and increases with increasing sec￾ondary nucleation of A&O, on the Al,O, particles. In this regime, the weight gain rate decreases with in￾creasing Al,O, particle size. With increasing oxidation time, liquid metal transport to the oxidation front slows down. The weight gain rate data in this regime seem to be consistent with a mixed transport control. 4. Conclusions The kinetics of growth of Al-Mg alloys into Al,O, preforms has been investigated as a function of oxy￾gen pressure, and preform particle size. The weight gain rate decreased with time and was independent of oxygen pressure. The presence of an A1203 preform produces two effects on the rate of directed oxidation of Al-ME alloys. In the initial period of growth, the mechanism of oxidation of Al-Mg alloys into the alumina preforms was similar to the mechanism of the oxidation of the Al-Mg alloys into free space. The weight gain rate was controlled by the transport of oxygen through the surface alloy layer and in￾creased with increasing secondary nucleation of Al,O, on the A&O, particles. In this regime, the weight gain rate decreased with increasing Al,O, particle size. With increasing oxidation time, liquid metal transport to the oxidation front decreased considerably. The weight gain rate data in this regime seem to be con￾sistent with a mixed control mechanism involving liq￾uid metal transport through the composite and oxygen transport across the alloy layer