Kinetics of Pressureless Infiltration of Al-Mg Melts into Porous Alumina Preforms
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ration is a versatile technique for processing of both metal and ceramic-matrix composites[1] and multiphase materials in powder metallurgy.[2] The pressureless infiltration technique[3] has been employed for making near net-shaped composites by infiltrating Al-Mg melts into SiC, Al2O3, and Si3N4 in a nitrogen atmosphere. The presence of both Mg in the system and a minimum partial pressure of N2 were shown to be crucial for this process.[3] It has been shown[4] that Mg has a twofold role in the process: initiation and continuation of infiltration. Termination of infiltration was associated with the local depletion of Mg from the melt front leading to formation of passivating products, either Al2O3 or MgAl2O4 under open conditions (where the infiltration front is exposed to the atmosphere). Using a different self-sealing configuration, Al could be infiltrated into Al2O3 preforms in air with Mg present externally at the billet-preform interface.[5] With this configuration, infiltration was shown to continue for longer periods of time, consequently producing greater thickness of composite. Composite formation rates for the Al/Al2O3 system are found to be extremely slow in the range mm/h to cm/h, which is also found in other systems[6–8] processed through the melt infiltration technique. Such rates are found to be two to three orders of magnitude slower than that predicted by fluid flow using a uniform capillary bundle model based on the average of the pore size distribution (~mm/s).[5,8] From a physical standpoint of flow through porous media, more realistic models of nonuniform capillaries DEBDUTT PATRO, formerly Research Scholar, with the Department of Materials Engineering, Indian Institute of Science, Bangalore, is Research Engineer, Material Mechanics Lab, John F. Welch Technology Center, General Electric, Bangalore 560066, India. Contact e-mail: [email protected] VIKRAM JAYARAM, Professor, is with Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India. Manuscript submitted September 28, 2007. Article published online December 20, 2007. 108—VOLUME 39B, FEBRUARY 2008
have been proposed[9,10] to explain the slow rise of liquids. Recently, a sinusoidal capillary model has been developed for infiltration of nonreactive liquids in porous ceramics.[11] Flow under such conditions was shown to be governed by an ‘‘effective hydrodynamic’’ radius, one to two orders of magnitude smaller than the average pore radius. The origin of this ‘‘unphysical’’ radius was rationalized based on a consideration of the driving forces on the meniscus while moving through the larger, rate-limiting segment. Capillary pressure is dictated by rmax, while viscous drag is determined by rmin (rmax and rmin being the diameter of the pore and throat, respectively), thereby leading to slow flow rates. However, high-temperature melt-infiltrated system are reactive associated with dissolution of the ceramic in the melt, depletion of reactive species from the melt,[12] and formation of interfacial reaction products during infiltration
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