Thermodynamics and Kinetics of the Formation of Al 2 O 3 / MgAl 2 O 4 /MgO in Al-Silica Metal Matrix Composite
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FOR the past many years, the different interfacial reaction phases forming in metal matrix composites (MMCs) such as Al2O3, MgAl2O4, and MgO in Al/Al2O3,[1–12] Al/oxidized SiC,[13–17] and Al/SiO2[21–25] are widely studied. These interfacial products or fine hard ceramic particles improve the wetting and prevent the degradation of reinforcement by direct reactions. Interestingly, in recent times, research on MMCs is largely concentrated on the in-situ formation of these particles as reinforcements at high percentage (in-situ MMCs), because they are the ceramic materials bearing high-temperature capability cum special properties and capable of improving the properties of resulting composites. Among the different processes established until now, the displacement reactions between the liquid metal and the ceramic oxides have been found to gain interest, because they satisfy the thermodynamic criterion at the processing conditions, i.e., high negative Gibbs free energy of the reactions. The materials such as SiO2, ZnO, CuO, TiO2, alumino silicate, glass, and mullite, V.M. SREEKUMAR and K.R. RAVI, Senior Research Fellows, R.M. PILLAI, Head, Material and Minerals Division, and B.C. PAI, Assistant Director, are with the National Institute for Interdisciplinary Science and Technology (CSIR), Thiruvananthapuram, India 695 019. Contact e-mail: rmpillai@rediff.com M. CHAKRABORTY, Deputy Director, is with the Indian Institute of Technology, Kharagpur, India 721 302. Manuscript submitted May 2, 2007. Article published onlined February 12, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
kaowool, flyash, etc. are recognized as simpler and cheaper ceramic oxides amenable for the in-situ generation of Al2O3, MgAl2O4, and MgO in Al alloy.[23–32] The different important displacement reactions possible during the reduction of SiO2 in Al-Mg alloy are given subsequently:[33] SiO2ðsÞ þ 2MgðlÞ ! 2MgOðsÞ þ SiðlÞ DG1023K ¼ 268:22 kJ/mol 2SiO2ðsÞ þ MgðlÞ þ 2AlðlÞ ! MgAl2 O4ðsÞ þ 2SiðlÞ DG1023K ¼ 449:632 kJ/mol 3SiO2ðsÞ þ 4AlðlÞ ! 2Al2 O3ðsÞ þ 3SiðlÞ DG1023K ¼ 556:44 kJ/mol
½1
½2
½3
3SiO2ðsÞ þ 2MgOðsÞ þ 4AlðlÞ ! 2MgAl2 O4ðsÞ þ 3SiðlÞ DG1023K ¼ 631:084 kJ/mol
½4
Because the standard Gibbs free energy of the reactions is highly negative, the formation of Al2O3, MgAl2O4, and MgO phases is thermodynamically feasible or spontaneous. Various experimental studies carried out on the formation of Al2O3, MgAl2O4, and MgO are tabulated with respect to the different composite systems and the alloying elements (particularly Mg) in the alloy (Table I). It can be observed that the type of VOLUME 39A, APRIL 2008—919
Table I.
Comparison of Various Studies Carried Out and Composite Systems on Al2O3, MgAl2O4, and MgO Formations Phases Identified
System
References
Al/Al2O3
1 through 4 5 4, 6, 7 8 9 10 11 12 23 24 25 26 27 28 9 29 13 14 15 16 17 18 19 19 19 20 21 22 30 31 32
Al/Saffil Al/Alumino silicate Al/Flyash Al/SiC/Flyash Al/Kaowool, Al/saffil, Al/mullite (all silica binded) Al/Al2O3 Al/MgO Al/ox.SiC
Al/SiO2 (glass)
Al/SiO2 (silica gel,
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