GRAIN MICROSTRUCTURE EVOLUTION OFMg (AM50A)/SiCp METAL MATRIX COMPOSITES

Henry Hu*Institute of Magnesium Technology (ITM), Inc., Ste-Foy, Quebec, Canada, G1P 4N7

(Received November 10, 1997)(Accepted July 6, 1998)

Introduction

The influence of the matrix grain structure on the mechanical and physical properties of metal matrixcomposites (MMCs) has been emphasized with a great deal of investigation [1–9]. It is almostimpossible to apply rules developed for microstructural control in the solidification of unreinforcedmetals directly to MMCs, due to the fact that solidification behaviors of the matrix of MMCs are oftenmodified with the presence of reinforcement. As reported in the literature, extensive research work hasbeen performed on understanding of matrix grain structure evolution of aluminum-based MMCs. Theprevious studies have shown that the reinforcement can reduce the grain size of the matrix significantlyif it catalyses heterogeneous nucleation of the primary metal phase. A typical example of matrix grainrefinement is that of hypereutectic aluminum-silicon alloys where the silicon primary phase nucleatespreferentially on the surface of graphite, SiC, SiO2, and Al2O3. However, experimental evidence hasalso confirmed that thea aluminum phase in hypoeutectic aluminum-silicon alloys has a tendency toavoid the reinforcement, and does not nucleate on its surface. Mortensen and Jin [3] have even indicatedthat the grain size of the composite castings is likely to be somewhat larger than that of an identicalcasting of the unreinforced matrix as the reinforcement does not induce nucleation of the primary phaseof the matrix.

Recently, the demand for reduced weight and increased stiffness in advanced materials applicationhas generated strong interest in research and development of light metal matrix composite components.With their advantages of reduced density, magnesium-based MMCs have great potential for broadacceptance in the automotive and aerospace industries. To date, only limited information [10] isavailable on the development of matrix grain structure of magnesium-based MMCs.

The objective of this study was to investigate the development of matrix grain structure during thesolidification of SiC particulate-reinforced magnesium (AM50A) composites. Computer-based thermalanalysis, optical and scanning electron microscopy (SEM) techniques were employed to examine theoccurrence of nucleation and grain refinement involved.

* Current affiliation: Global Technology Center, Meridian Magnesium, 112 McNab St., Strathroy, Ontario, Canada N7G 1H4.

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Scripta Materialia, Vol. 39, No. 8, pp. 1015–1022, 1998Elsevier Science Ltd

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Experimental Procedures

Materials and Processing

Mg-4.9wt.%Al-0.39wt.%-0.2wt.%Zn alloy AM50A was used as the matrix for the composite. Highpurity silicon carbide (98.5%b-SiC) particulates with an average diameter of 13mm was selected asthe reinforcement for the composite. Composite specimens with 5 vol.% SiC particulates were preparedby using a mixing and casting process [11] which has been developed at the Institute of MagnesiumTechnology (ITM). Basically, the processing of magnesium composites consists of mixing pre-heatedSiC particulates with magnesium, melt stirring and ingot casting. In each batch, 1.5 kg of the compositemelt was prepared in an electric resistance furnace using a steel crucible under the protection of aSF6/CO2 gas blend. The composite melt was held at 700°C for a half hour, stirred for 10 minutes, andthen cast at 700°C into a steel mold to produce ingots 60380320 mm. The unreinforced AM50A wasalso cast at the same condition.

Thermal Analysis

For each thermal analysis, about 400 grams of melt sample were taken from the well-stirred alloy orcomposite melt at 700°C into a small steel crucible. A chromel-alumel (K-type) thermocouple protectedby a thin steel sheath was positioned at a distance of 0.02 m from the bottom of the crucible center, andwas connected to a computer(Macintosh)-based data acquisition system to measure the temperaturevariation. In thermal analysis, the temperature of the solidifying AM50A alloy and composite sampleswas recorded by the data acquisition system at a regular interval of 500 ms as they cooled from thecompletely liquid state, through the solidification range, to become fully solid. The melt samples wereprotected with SF6/CO2 gas blend during the entire measurement period. The acquired temperature (T)vs. time (t) data were processed and cooling curves (T vs. t) were plotted using the Microsoft Excelspreadsheet software. Several duplicate runs on each melt were conducted to ensure an uncertainty of60.1%.

Microstructural Analysis

Specimens were cut from the center of the as-cast ingots and prepared following the standardmetallographic procedures. Due to indistinct grain boundaries in the as-cast structure of AM50A alloyand AM50A/SiC composites, which are disguised by theb intermetallics (Mg17Al12), it was necessaryto subject the as-cast specimens to an anti-germination solution heat treatment (T4), which dissolves theb intermetallics and reveals the grain boundaries. A LECO 300 optical microscope and a JEOL 840scanning electron microscope were employed to characterize the microstructure of the specimens. Thegrain-size determination was conducted on the solution-treated specimens based on ASTM Standard E112–88. At least five fields were randomly selected for each specimen in order to ensure an uncertaintyof 65%.

Results and Discussion

Thermal Analysis

Figure 1 represents the typical result of thermal analysis for AM50A alloy. Examination of the coolingcurve shown in Figure 1 indicates two distinct stages during the solidification process of the AM50A

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alloy. The nucleation of primary magnesium phase happened in stage [1], from which the nonequilib-rium liquid temperature was recorded as 622.5–623.4°C. Stage [2] is the occurrence of the eutecticreaction, i.e., L3 Mg (a) 1 Mg17Al12, where the nonequilibrium solidus temperature was determinedas 428.2°C. The nonequilibrium liquidus temperature (622.5–623.4°C) measured is slightly higher thanthe value of 620°C reported in the Norsk Hydro Magnesium’s data sheet [12]. The nonequilibriumsolidus temperature (428.2°C) is in good agreement with the data existing in the literature [12]. Theenlarged stage [1] of the cooling curve given in Figure 1(b) evidently shows a considerable differencein the degree of supercooling (DT 5 0.9°C) at the liquidus temperature plateau. The typical coolingcurve of AM50A/SiC composite is given in Figure 2. Based on the cooling curves, no appreciabledifference is present in the liquidus and solidus temperatures between AM50A alloy and AM50A/SiCcomposite. For the AM50A/SiC composite, however, no apparent supercooling is observed in Figure2(b), which illustrates the enlarged stage [2] of the cooling curve. The difference in supercoolingappearance between the AM50A alloy and the AM50A/SiC composite can be directly attributed to themechanism of the primary phase nucleation, and consequently, the extent of grain refinement.

Microstructure

The as-cast microstructure of unreinforced AM50A alloy and SiC particulate-reinforced AM50A/SiCcomposite is depicted in Figures 3 and 4. It was anticipated that the as-cast microstructure of bothmaterials displays obscure grain boundaries which are considerably disguised by theb intermetallics(Mg17Al12) as illustrated in Figures 3 and 4. But, an evident difference in grain sizes betweenunreinforced AM50A alloy and SiC particulate-reinforced AM50A/SiC composite can be seen inFigures 3 and 4. This implies that a finer grain structure in the composite could result from the additionof SiC particulates. Figures 5 and 6 distinctly reveal the grain boundaries of the unreinforced AM50Aalloy and the SiC particulate-reinforced AM50A/SiC composite in the T4 condition. As can be seen, thegrains in the matrix of the AM50A/SiC composite are significantly refined compared with those of the

Figure 1. (a) typical cooling curve and (b) enlarged liquidus temperature region of AM50A.

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AM50A alloy. Figure 7 presents the grain size measurements from the heat treated specimens for boththe unreinforced AM50A alloy and the AM50A/SiC composite. It is worth noting that the grain size ofthe AM50A alloy matrix is reduced by more than two times in the composite due to the grain refinementeffect of the SiC particulates on the matrix.

It has been observed [13] that supercooling of magnesium and aluminum alloys can be correlated totheir grain structure. Melts that would give coarse grains yields high supercooling, and melts with low

Figure 2. (a) typical cooling curve and (b) enlarged liquidus temperature region of AM50A/SiCp composite.

Figure 3. Optical micrograph showing as-cast microstructure of AM50A alloy.

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or no super cooling give rise to fine grains. The reason is that the grain nucleation in a solidifying liquidalloy is difficult to initiate when heterogeneous nuclei are absent. As such, the melt needs to supercooluntil appropriate nuclei form. At the beginning of the nucleation, the melt temperature increases and thegrain growth occurs at the normal equilibrium temperature. With the reinforcement addition, thenumber of nuclei and the nucleation rate increase. As a result, small or no supercooling appears on thecooling curve as the fine grained structure is achieved. The results of thermal analysis acquired in thisstudy conforms to the principles of the nucleation theory. The unreinforced AM50A alloy which has

Figure 4. Optical micrograph showing as-cast microstructure of AM50A/SiCp composite.

Figure 5. Optical micrograph showing grain structure of AM50A alloy in T4 condition.

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coarse grains gives 0.9°C of supercooling on its cooling curve. Meanwhile, no supercooling is exhibitedby the particulate-reinforced AM50A/SiC composite, in which the grain refinement takes place.

Grain Refinement Mechanisms for AM50A/SiCp Composite

Mechanisms for grain refinement of the matrix of metal matrix composites have been thoroughlyinvestigated [1–10]. Primarily, grain refinement results from two separate processes: nucleation of newcrystals from the melt, and subsequent growth of the new crystals to a limited size [14]. Althoughheterogeneous nucleation enhances the rapid creation of primary crystals, the onset of primary phasenucleation should not be followed by rapid crystal growth.

Examination of the microstructure of the AM50A/SiC composite via the SEM manifests the SiCparticulate distribution in the composite as shown in Figure 8. The presence of SiC particulates inside

Figure 6. Optical micrograph showing grain structure of AM50A/SiCp composite in T4 condition.

Figure 7. Average grain sizes of AM50A/SiCp composite and AM50A in T4 condition.

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the primary magnesium grains and at the grain boundaries suggests that both particle capture andpushing occurred during the solidification of the composite. This interpretation is evidently supportedby the SEM observation on the heat-treated composite specimen as indicated in Figure 8. It appears thatSiC particulates are situated inside the primary magnesium grains. This observation implies thatheterogeneous nucleation may occur during the solidification of the composite. On the other hand,significant particle pushing effect is suggested by the appearance of many SiC particulates around thegrains indicating that the growth rate of the primary phase may be reduced when the compositesolidifies. This is because SiC particulates present around the growing primary magnesium crystalscould act as diffusion barriers to their growth. Consequently, the restricted growth of the primary phasewould allow the melt to have sufficient time to create more nuclei. Therefore, it can be deduced fromthe experimental evidence that, for the AM50A/SiCp composite, a fine grain size in the resultingsolidified microstructure is attributable to both the heterogeneous nucleation and the restricted primarycrystal growth.

Conclusions

The observation of no apparent supercooling on the cooling curve of the SiC particulate-reinforcedAM50A/SiC composite implies a significant refinement of the matrix grain structure. This implicationis evidenced by both optical and SEM microstructural study, and the grain size measurement. Themicrostructural analysis suggests occurrence of both particle capture and pushing during the solidifi-cation of the composite. The investigation of grain refinement mechanisms indicates that the coupledeffect of the heterogeneous nucleation of the primary magnesium phase on SiC particulates and therestricted growth of magnesium crystals may be responsible for the final formation of small grains inthe AM50A/SiCp composite.

Figure 8. SEM micrograph showing SiC particle distribution and grain structure of AM50A/SiCp composite in T4 condition, A:captured and B: pushed particulates.

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Acknowledgments

The author would like to express his appreciations to the Institute of Magnesium Technology (ITM) forsupporting this work, to Mr. P. Vermette for his assistance in some of the experiments, and acknowl-edge the Natural Sciences and Engineering Research Council of Canada for financial support in theform of an Industrial Research Fellowship.

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