Kinetics of the growth of spinel, MgAl 2 O 4 , on alumina particulate in aluminum alloys containing magnesium
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I.
INTRODUCTION
L O W cost metal matrix composites can be produced by incorporating ceramic reinforcing particles into melts of the matrix alloys of interest. In these processes, the melt temperatures can be relatively high and the processing times long. Under these conditions, most of the ceramic reinforcements of interest are thermodynamically unstable and undergo reaction at the interface to form undesirable compounds. 9 111Interface reactions often result in removing desirable elements from the alloy as well as increasing melt viscosity and reducing castability. Alumina reinforcement is thermodynamically stable in contact with pure molten aluminum. However, if the alloy contains sufficient magnesium, a reaction to form MgO or MgAlzO4 will proceed. a-A1203 fibers have been observed to react to form MgO and MgA1204 in ZE41A, a Mg-based alloy, t2-41 as well as in AI-4 pct Mg alloys.I5,6.7] Munitz e t al.[61 found that the reaction products were present as discrete crystals on the fibers and that some fiber surfaces remained unreacted. Particulate alumina also reacts. MgO was found on 8 0 / x m A1203 in A1-4.5 pct Mg alloys by Pal e t a l . t8j Hikosaka e t a l . Eg! found that spinel formed on A1203 in A1-3 pct Mg during vortex stirring under Ar. Reaction with a solid matrix has been observed by Suganuma et al. tl~ as well as by Watanabe and Saltoh. tlu Suganuma e t a l . found that A1203 fibers were completely covered with spinel after 5 hours at 853 K in AA-6061. Watanabe and Saitoh detected spinel on A1203 particulate after 20 hours at 873 K in AA-6061. On A1203, MgO and MgAlzO 4 form according to the reactions 3Mg + A1203 = 2A1 + 3MgO
[1]
3Mg + 4AlzO3 = 2A___l+ 3MgA1204
[2]
Using free energy of formation data from the JANAF tables (3rd ed.) and activity coefficient data for the A I / M g binary, t~21 the Mg concentration in equilibrium with A.D. McLEOD, Investigator, and C.M. GABRYEL, Technologist, are with Alcan International Limited, Kingston Research and Development Centre, Kingston, ON, Canada, K7L 5L9. Manuscript submitted February 4, 1991. METALLURGICAL TRANSACTIONS A
Reactions [1] and [2] for liquid alloys can be calculated. The result is shown as a stability diagram in Figure 1. Given the thermodynamic considerations, it is not surprising to find that A1103 is not stable in aluminum alloys that contain Mg. Calculations show that the reaction can also proceed in the solid state, as in Figure 2. Since A1203 is becoming more widely used as a reinforcement in aluminum alloy matrix composites, it was decided to carry out further investigations into the liquid state spinel formation reaction. The work carried out, measuring the rate of the spinel formation reaction, has not only provided a predictive model for Mg loss in liquid composites, but has also accounted for a better understanding of the kinetic mechanisms involved. II.
EXPERIMENTAL
First, a composite consisting of 15 vol pct alumina particulate in super purity aluminum was prepared. The size distributions of the three types of particulate use
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