Kinetics Of Pressureless Infiltration Of Al-Mg Alloys Into Al2O3 Preforms : A Non-Uniform Capillary Model

Abstract

Al-Mg alloys spontaneously infiltrate into porous ceramic preform in a nitrogenous atmosphere above 750 °C with Mg either pre-alloyed or introduced at the interface to initiate the process. The governing process variables are temperature, alloy composition, atmosphere and particle size of the porous preform. The present study investigates the flow kinetics of Al-Mg melts into porous Al2O3 preforms as a function of particle size of the preform from the standpoint of a physical phenomena fluid flow through a non-uniform capillary. Pressureless infiltration involves two major stages: (a) initiation associated with an incubation period and, (b) continuation where the melt infiltrates the preform. Long (~1 hr) and irreproducible incubation periods are typically observed in the Al- Mg/Al2O3 system when the samples are slowly heated in N2 atmosphere. Such lengthy periods prior to infiltration also lead to excessive Mg loss from the system. In order to accurately measure infiltration rates during the continuation stage, the incubation period was minimized by upquenching samples in air under self-sealing conditions. Interrupted experiments reveal that infiltration occurs within 5 mins. Different phenomena are expected to dictate the capillary rise kinetics through the porous ceramic post-incubation (more specifically, retard the melt movement) (a) triple-point ridging of the melt meniscus on the alumina surface (meniscus pinning) (b) interfacial reaction limited wetting and infiltration (c) pore size and distribution of the porous ceramic (d) melt (Al-Mg) / atmosphere (N2) reaction to form products inside the pore space (decrease in permeability) (e) time-dependent loss of Mg from the system (time-dependent contact angle) Some of the above phenomena viz., fluid flow inside the porous medium and chemical reaction of the melt with the reinforcement are invariably coupled in a complex manner. The contribution of each phenomenon to the kinetics of infiltration (a) and (e) was investigated separately. Triple-line ridging Al sessile drops on alumina substrate spread 4-5 orders of magnitude slower than that predicted by hydrodynamic equilibrium. The melt is pinned by ridges leading to spreading rates of 0.4-4 mm/hr in contrast to viscous drag controlled spreading rates of 1-10 mm/sec. In order to detect ridging in the Al-Mg/Al2O3 reactive couple, uniform Al2O3 capillaries were infiltrated. Experiments were conducted under sealed configuration with metal on both sides of the capillary and Mg turnings at the interface. The uniform capillary itself was placed inside an alumina preform and the assembly upquenched to 800-900 °C to minimize evaporative loss of Mg. Examination of the inner walls of the capillary after leaching away the infiltrated metal shows rough, granular features on the polycrystalline Al2O3 surface. No continuous ridges were seen. EDS of the granular phase suggested stoichiometry of spinel, MgAl2O4, formed as a result of the reaction between the melt and the capillary. From interrupted experiments the average infiltration rate inside the uniform capillary was calculated to be in the ballpark range of 2-6 µm/sec (which is a lower limit to the meniscus velocity), an order of magnitude faster than the spreading rates observed during triple-line ridging (0.1 – 1 µm/sec) indicating that the melt front pinning was not the operative mechanism for influencing infiltration kinetics. Pore size distribution of porous medium Additionally, infiltration was found to be faster in uniform channels (fractures in a preform, annular spaces and aligned pores in freeze-cast preforms) compared to the randomly packed bed itself. The effect of pore size on infiltration kinetics was studied by varying the particle size of the packed bed. Experiments were conducted for two systems (a) non-reactive liquid polyethylene glycol PEG 600 (b) reactive Al-Mg melts into packed alumina beds as a function of particle size and temperature. The PEG 600 / Al2O3 ‘model’ system was used to benchmark the effect of pore size and distribution of the particle bed on flow kinetics from a purely physical standpoint. Typically, a Washburn type of ‘parabolic’ kinetics was observed for the non-reactive couple and the ‘effective’ hydrodynamic radius, reff was extracted. (For a uniform capillary, reff and the physical radius of the capillary are the same). Surprisingly, the ‘Washburn’ radius was found to be 1-2 orders of magnitude smaller than the average pore size and even smaller than the minimum average pore size of the compact. The ‘Washburn’ radii for infiltration of Al-Mg melts was a further order of magnitude smaller than the corresponding values for infiltration of non-reactive PEG 600 through the same packed beds. Non-uniform capillary model To predict the infiltration kinetics through porous media, a sinusoidal capillary model was developed based on the pore size distribution. The input parameters for the model were the average pore neck size and average pore bulge size, which were extracted from the experimentally measured pore size distribution. The flow was assumed to be quasi-steady state and laminar. Hagen-Poiseuille’s equation was employed to calculate the total pressure drop, which was equated with the instantaneous pressure drop across the meniscus. The meniscus velocity within the non-uniform capillary was solved numerically based on the instantaneous pressure drop. The infiltration profile for the sinusoidal capillary displayed jumps associated rise in the narrow segments of the profile while the rise through the broad segment was considerably slow. The overall infiltration profile could be fitted by a parabolic Washburn-type equation. The ‘effective’ hydrodynamic radius of such a sinusoidal capillary was found to be 2-3 orders of magnitude smaller than the average capillary size and even smaller than the narrowest opening of the sinusoidal capillary. The overall kinetics was limited by flow through the broad segment of the profile where the capillary driving force is the lowest coupled with a large viscous retarding force due to the narrow feeding segment thereby leading to extremely slow flow rates. The calculated ‘effective’ radius of the sinusoidal capillary (reff = 0.03 µm) based on the pore size distribution of the 25-37 µm (1.4-10.8 µm) packed bed was similar to the experimentally observed ‘effective’ radius for flow in the non-reactive couple (reff = 0.06 µm) implying good agreement between experiments and modeling. The model was extended for the case of pressure infiltration of Al melts into SiC & TiC compacts reported in the literature, under conditions where chemical reactions are negligible. A good agreement to within a factor of 4 between the observed kinetics and the ones predicted by the current model is observed. In order to understand the origin of this ‘unphysical’ radius dictating capillary rise, the physics of flow through a stepped capillary was analysed. The kinetics of flow through the wide segment could be expressed by an ‘effective’ drodynamic radius r 4min based on geometrical parameters of the stepped capillary as: reff= r3max (Wetting situation) where rminand rmax are the radii of the narrow and broad segments of the capillary. The ‘effective’ radius from the above equation matched well with the numerically derived ‘effective’ radius for flow through the stepped capillary. A r 2 similar expression for flow under applied pressure was derived as: reff= min rmax (non- wetting situation) which is strictly correct for large values of applied pressure. Chemical reactions influencing infiltration kinetics: Upquenched samples (time-dependent contact angle due to Mg loss) The previous investigation of fluid flow in porous media from a purely physical standpoint reveals the dominant role of the pore size and distribution in the porous medium in controlling infiltration kinetics. This however, is accurate only if chemical factors are minimized. In case of the upquenched experiments for the Al-Mg/Al2O3 system, the ‘effective’ radius was determined to be an order of magnitude smaller than that for the PEG 600/Al2O3 couple implying additional chemical factors influencing flow kinetics in this reactive system. Experiments with Mg turnings mixed with the powder bed shows faster infiltration compared to the ones where the entire Mg was placed at the interface showing that local availability of Mg was responsible for slower infiltration kinetics. Diminishing Mg at the melt front, leads to increase of surface tension and increase in contact angle. This was modeled by incorporating a kinetics (time-dependent) contact angle into the sinusoidal capillary model developed for non-reactive infiltration. The infiltration kinetics was found to be retarded in the case of a kinetic contact angle. Thus, both flow retardation through a packed bed and time-dependent variations of contact angle due to Mg loss from the system are responsible for slow pressureless infiltration kinetics of Al-Mg melts inside Al2O3 preforms. The infiltration kinetics predicted by the sinusoidal capillary model thus defines an upper envelope to the rate of infiltration and subsequent composite formation for such a process governed by fluid flow; all other factors if present in effect, retard the kinetics further. Samples processed in N2 atmosphere (reduced permeability due to AlN formation) The more practical case of composite fabrication (PRIMEXTM process) by pressureless infiltration of Al-Mg melts in a flowing N2 containing atmosphere was also examined. The kinetics of infiltration of Al-Mg melts in a flowing N2-H2 atmosphere (pO2 ~ 10-20atm) for different particle sizes of the packed bed was investigated. A large scatter in the infiltrated heights was observed and the absolute infiltration rates could not be established. Moreover, incubation periods were seen to range from 1-2 hours for different particle sizes. Post-incubation, the infiltration kinetics for a wide range of particle sizes was found to be approximately an order of magnitude slower than that for the upquenched samples. Microstructural investigations of the etched samples revealed significant AlN formation at the start of the composite near the preform/billet interface. This reduced the cross-sectional area available for melt flow and possibly led to long incubation periods encountered in the process. AlN formation was also detected in the matrix on the particle surfaces as well as in the interior of the matrix. This reduced the permeability of the compact and increased the hydrodynamic resistance for flow through the porous compact leading to slower infiltration kinetics. Thus both AlN formation in the matrix and Mg loss from the melt retard capillary flow of the melt through the porous ceramic over and above the intrinsic hydrodynamic resistance for flow through the packed bed. Role of atmosphere on the pressureless infiltration process The role of atmosphere in promoting the pressureless infiltration process was examined by using different processing atmospheres such as vacuum, N2-H2 and Ar and combinations thereof. It is known that the pressureless infiltration of Al melts into porous Al2O3 preforms requires both N2 and a critical level of Mg in the system. Samples heated under vacuum and Ar to 900 °C under open conditions did not infiltrate. Rather these showed discoloration related to the formation of MgAl2O4 on the particle surface due to reduction of Al2O3 by Mg vapour. Moreover, samples heated in Ar upto 500 °C followed by heating up in N2-H2 till 900 °C did not infiltrate indicating irreversible changes. Interestingly enough, if the samples were heated in vacuum upto 700 °C followed by N2-H2 at 900 °C, infiltration was observed. Dewetted regions of the compact were seen too adjacent to the preform-billet interface. This indicated a minimum critical partial pressure of N2, which promotes infiltration. From an analysis of the different interfacial energies and their dependence on atmosphere, it was concluded that either an increase in the solid-vapour interfacial energy (~ 10%) or a decrease in the solid-liquid interfacial energy (~ 10%) would lead to a decrease in the contact angle, θ, by 10°, large enough to ensure wettability and infiltration in certain atmospheres. It was also established that Mg infiltrates into porous Al2O3 both in N2-H2 as well as Ar under sealed conditions. So the presence of a minimum partial pressure of N2 favouring wettability was specific to the Al-Mg/Al2O3 system. (pl see the original document for formulas)