Kinetics of Pressureless Infiltration of Al-Mg Melts into Porous Alumina Preforms
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- Patro, D. & Jayaram, V. Metall and Materi Trans B (2008) 39: 108. doi:10.1007/s11663-007-9111-9
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Effective “hydrodynamic” radii governing infiltration kinetics of reactive Al-Mg melts into alumina preforms were found to be three orders of magnitude smaller than the average pore size of the packed bed and also smaller compared with the kinetics for a nonreactive system. A sinusoidal capillary model was developed to predict flow kinetics within the packed bed. For the reactive system, two factors were ascribed for additional melt retardation: (1) different intrinsic wettabilities of the two liquids on alumina, thereby leading to significantly different “effective” local contact angles; and (2) local solute depletion from the meniscus, which was incorporated as a time-dependent contact angle.
Melt infiltration is a versatile technique for processing of both metal and ceramic-matrix composites and multiphase materials in powder metallurgy. The pressureless infiltration technique 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. It has been shown 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. 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 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. 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, and formation of interfacial reaction products during infiltration, which either lead to a reduction in permeability of the porous compact or change the melt chemistry and energetics of different interfaces in the system. Such chemical reactions additionally influence (in many cases retard)[13,14] the infiltration kinetics over and above that dictated by the physical flow of fluid through the compact. In such a process, reaction kinetics and solute transport are interactively coupled with the dynamics of fluid flow in a complex fashion due to the moving boundary nature of the process, with the slower of the two mechanisms controlling the infiltration rate. The activation energy determined from the temperature dependence of infiltration kinetics sheds insight into the rate-controlling mechanisms. Reported activation energies for many systems[15,16] are in the range of 100 to 400 kJ/mol indicating a chemical reaction-controlled kinetics.
The objective of this article is to understand infiltration kinetics in the reactive Al-Mg/Al2O3 system by investigating different aspects of the problem, viz. (1) the kinetics of infiltration from a fluid flow perspective and (2) coupling between chemical effects and fluid flow. To fulfill this, investigation and measurement of the kinetics were conducted in both a nonreactive as well as a reactive system. A phenomenological model and numerical analysis of infiltration process has been developed from a purely physical aspect of fluid flow and chemical effects are incorporated in the model.
Preparation of Porous Preforms and Alloys
Infiltration experiments were conducted with two different liquids (a) polyethylene glycol (PEG 600, M/s Merck, India) at room temperature and (b) Al-Mg melts at temperatures of 800 °C to 975 °C into porous packed alumina beds. Powder preparation and bed preparation were carried out meticulously for ensuring reproducibility of results as described previously. The particle sizes of fused alumina used in the study were 25 to 37, 63 to 75, and 90 to 125 μm. The Al-Mg alloys were prepared by standard casting technique. Billets of 8 mm diameter and a height of 10 to 20 mm were machined from the cast rods for use in infiltration experiments. The inductively coupled plasma–atomic emission spectroscopy (ICP-AES) analysis indicated the alloy composition to lie within 10 pct of the target value.
Configuration and Conditions
self-sealing conditions (to prevent ingress of air after melting of billet),
Mg disc equivalent to 3 wt pct Mg in the melt (as external initiator to reduce incubation period), and
upquenched experiments (to prevent excessive loss of Mg).
The pore size distribution of the porous compacts was measured using Hg porosimetry (Poremaster PM 60GT, Quantachrome Instruments, Florida, USA) and further corroborated with image analysis on polished sections of the composite. Furthermore, a relevant pore size distribution of 1.4 to 10.8 μm for the 25 to 37 μm bed, which accounts for the measured flow rates of polyethylene glycol (PEG 600), was extracted as described elsewhere. This was done by truncating the volumetric pore size distribution obtained from Hg porosimetry, to 95 pct, i.e., removing 2.5 pct by volume of the smallest pores and the largest pores since the remaining volume still controls the majority of the flow.
Effective Hydrodynamic Radii for Both Low-Temperature and High-Temperature Infiltration into Alumina Packed Beds in Microns
Average Pore Radius
Washburn Radius (PEG 600)
Washburn Radius (Al-3Mg)
25 to 37
0.06 ± 0.02
0.004 ± 0.001
63 to 75
0.25 ± 0.01
0.026 ± 0.005
90 to 125
0.27 ± 0.01
0.028 ± 0.005
Infiltration kinetics for the powder beds was seen to scale with the particle size. However, the 63 to 75 and 90 to 125 μm powder beds displayed similar infiltration kinetics. The infiltration profile for the different powder beds at 800 °C (Figure 3) was fitted to parabolic kinetics using the Washburn equation (Eq. ) to give hydrodynamic radii for capillary flow of Al-3 wt pct Mg melts into packed alumina beds (Table I, column 4). The relevant properties for Al-3 wt pct Mg obtained from literature are η = 1.04 × 10−3 Pa·s, γ = 0.795 N/m, and θavg = 85 deg (varies between 83 to 88 deg). The hydrodynamic radii for infiltration (Table I) were found to be two to three orders of magnitude smaller than the average pore size for all the particle size beds. Furthermore, the kinetics for reactive infiltration (Al-Mg/Al2O3 couple) was found to be an order of magnitude slower than that for nonreactive infiltration (PEG 600/Al2O3 couple) for similar particle sizes.
It is imperative to rationalize the extremely slow flow kinetics for both the nonreactive and reactive situations. A nonuniform capillary model (sinusoidal capillary) based on periodic changes in cross-sectional area of a flow path inside the porous compact has been developed to account for the origin of this unphysical radius governing infiltration kinetics. In addition, we shall highlight the significance of the effective local contact angle of the two liquids with different wettabilities inside the channels of the porous medium in influencing infiltration kinetics. However, this is a physical model that considers only capillary driving forces and viscous retarding forces. This model will be further extended to the case for reactive infiltration, which is associated with concomitant changes in melt chemistry due to interfacial reactions.
The wavelength, λ, represents the average length between a pore and pore neck and is typically found to be equal to the average particle size for porous media considered (based on observations of optical and SEM micrographs of polished sections) though statistically long and short wavelength flow paths exist in any random packed bed.
The packing inhomogeneity in the porous bed leads to local regions wherein λ < λcritical and such regions are not directly filled with the advancing meniscus but have to wait for the fluid to come around and fill these regions due to the 3-D interconnectivity. In other regions of the packed bed, there is a positive driving force for capillary flow. Fluid flow prefers the path of least resistance (i.e., preferential flow channels) within the porous medium and such channels control the volumetric flow rate. The macroscopic kinetics (as measured in the experiments with PEG 600) can thus be adequately represented by such nonuniform preferential flow paths, thereby leading to good agreement between observed values and modeled values, i.e., reff = 0.04 μm (for nonreactive flow).
The smaller hydrodynamic radius (and consequently much slower flow kinetics compared to PEG 600) can be explained based on the poor wettability of Al-3Mg on alumina (θ = 83 to 88 deg). The effective local contact angle oscillates between (θ + φ)min and (θ + φ)max with the local contact angle approaching 90 deg in the broader flow-controlling segment of the sinusoidal capillary. Thereby, the meniscus curvature-induced driving force drops to an extremely small value and leads to additional retardation of the melt over and above that due to the converging-diverging geometry of the sinusoidal capillary. The effective hydrodynamic radius of a given sinusoidal capillary (1.4 to 10.8 μm) is dependent on both the geometry (ro and A) as well as the effective local contact angle, particularly when the effective contact angle (θ + φ) is > 80 deg. Thus the reduction in effective capillary size from 0.04 μm for PEG 600 to a value between 0.04 and 0.006 μm for the Al-3Mg melt is a direct consequence of going from a wetting fluid to one whose effective contact angle is close to 90 deg.
Contact angle variations can additionally result from variations in the interfacial energetics for the reactive Al-Mg/Al2O3 system due to nonequilibrium effects arising from Mg solute variation, which will be examined next.
The major chemical reactions occurring during the infiltration process that can possibly influence melt flow kinetics are as follows: (a) evaporative loss of Mg from the melt leading to reduced wettability and (b) reaction of Al-Mg melt with reinforcement to form spinel, thereby altering wettability.
The XRD and SEM (Figure 5) indicate formation of MgAl2O4 on the surface of the particles ahead of the infiltration front. Thus, the Al-3 wt pct Mg melt wets and infiltrates a spinel surface.
The chemistry of the different interfaces in the presence of reactive solute (Mg) and their influence on the contact angle (θ) evolution during the infiltration/spreading process was considered.
Wettability and interfacial energetics
Solid-vapor and solid-liquid
The Mg reacts with the reinforcement to form MgAl2O4 ahead of the melt front. In all interrupted experiments, the bed ahead of the front was found to be discolored. The XRD of the composite (Figure 4) shows the presence of MgAl2O4 phase and SEM revealed formation of granular features on the particle surface (Figure 5).
Wetting angles of Al-3 wt pct Mg melt on Al2O3 and MgAl2O4 at 800 °C have been reported to be the same (83 ± 5 deg), implying that the quantity (γSV– γSL) for the Al-Mg/Al2O3 and Al-Mg/MgAl2O4 systems, are similar. Hence, wettability is not affected (even though the chemistry of the interfaces is changed).
Surface tension of the melt is strongly dependent on the local Mg level. The Mg continues to be depleted from the system due to evaporation ahead of the melt front. A decreasing Mg level increases the surface tension of the melt and the contact angle, θ, that leads to a reduction in the driving force and decelerates the melt front.
Infiltrated Lengths inside the 25 to 37 μm Alumina Bed for Different Melt Compositions and Different Location/Distribution of Mg
Al - (3Mg at interface) (800 °C)
20 to 50
Al - (3Mg at interface) + 3Mg mixed with alumina bed (800 °C)
15 to 35
Al-5Mg + (3Mg at interface) (900 °C)
45 to 110**
This difference in infiltration kinetics can be explained from considerations concerning the local Mg depletion at the melt front. Magnesium was replenished at the moving melt front more efficiently and frequently when Mg is mixed in the powder bed compared to the long-range diffusion from the bulk reservoir that is needed when Mg was placed either at the interface or alloyed with the billet. The diffusive velocity of Mg in the Al-Mg melt (based on reported values of D0 and Q) varies between 1.5 to 5 μm/s in the temperature range 800 °C to 975 °C, which is smaller than the observed melt velocity of 7 to 20 μm/s in the same temperature range. This suggests that Mg diffusion from the bulk reservoir to the instantaneous melt front is rate limiting. Similar effects have been observed in the Al-Si-Mg/SiC system as well.
The upper limit for infiltration kinetics for the Al-Mg melt is considered next, under a scenario where there is no Mg loss from the system (i.e., a hydrodynamic radius reff between 0.006 and 0.04 μm, Figure 7). For experiment 8(a), the kinetics for Al-3 wt pct Mg melt (not 6 wt pct, Mg since all of the Mg mixed with the powder bed is not available but dissolves progressively into the melt during infiltration) predicts an infiltration length of 15 to 35 mm in 15 minutes (Table II, 2). The actual infiltration length is 30 mm, which is within the predicted range. This implies that the infiltration kinetics lies close to the upper bound given by the scenario of nonreactive fluid flow through the packed bed when solute depletion is absent.
Progressive depletion of Mg from the melt meniscus leads to an increasing contact angle and this was incorporated as a time-dependent contact angle in the sinusoidal capillary model. Infiltration in uniform capillaries has been modeled using a time-dependent contact angle. Following the procedure previously described, the constants A and B (Eq. ) for the Al-3Mg/sapphire system were derived from reported wetting kinetics and are A = 0.148 and B = 0.0004. For the present case of infiltration of Al-Mg melts into alumina preforms, a similar kinetics of contact angle increase (from 85 to 89 deg) was assumed, as a result of depletion of Mg locally from the instantaneous front.
For infiltration in the reactive Al-Mg/Al2O3 system, the measured effective hydrodynamic radius was found to be three orders of magnitude smaller than the average capillary size and an order of magnitude smaller compared to the infiltration kinetics for a purely nonreactive system. This was attributed to two factors: (1) retardation effects due to a wall curvature, which results in a larger effective contact angle as a result of poor wettability of Al-Mg melt on alumina; and (2) Mg loss from the system resulting in a time-dependent contact angle θ leading to additional retardation due to an ever-decreasing driving force. Incorporation of such effects in the sinusoidal capillary model was able to rationalize the observed slow infiltration kinetics for the reactive Al-Mg/Al2O3 system.
We are thankful to Saswata Bhattacharyya (Department of Materials Engineering, IISc) for development of the numerical model and analysis. Financial support for this work was provided by the Volkswagen Foundation through a grant for a collaborative project with TU-Darmstadt, Germany.