RARE EARTH CONCENTRATION IN THE PRIMARY SiCRYSTAL IN RARE EARTH ADDED Al-21wt.%Si ALLOY

J.Y. Chang, G.H. Kim*, I.G. Moon and C.S. Choi**Alloy Design Center and *Ceramic Processing Center, Korea Institute of Science and Technology,

P.O. Box 131, Cheongryang, Seoul 130–650, Korea**Department of Metallurgical Engineering, Yonsei University,

Seodaemun-gu, Shinchon-dong 134, Seoul 120–749, Korea

(Received December 8, 1997)(Accepted March 26, 1998)

Introduction

Al-Si alloys containing more than about 12wt.%Si exhibit a hypereutectic microstructure, normallyconsisting of a primary silicon phase in an eutectic matrix [1]. The primary silicon in normalhypereutectic alloys is usually very coarse and thus leads to poor properties to these alloys. Therefore,alloys with a predominantly coarse primary silicon crystal must be modified to ensure adequatemechanical strength and ductility. Further improvement of mechanical properties of these alloys can beachieved by the modification of eutectic microstructure. Therefore, development of a modifier or refinerthat can produce both fine primary and eutectic Si is a major factor which can lead to significantenhancement of mechanical properties in hypereutectic Al-Si alloys.

Refinement of primary silicon is usually achieved by the addition of phosphor to the melt. On theother hand, it is reported that the rare earth (RE) elements are capable of modifying the eutecticstructure of cast Al-Si alloys. [2, 3] Weiss and Loper reported that cerium did not refine the primarysilicon as phosphor, but that it was moderately effective in modifying the eutectic silicon. [4] Recently,Kowata et al. investigated the effect of rare earth addition on the refinement of primary silicon crystalsin a hypereutectic Al-20wt.%Si alloy and concluded that primary silicon crystals were refined with rareearth elements. [5] The mechanism of modification by rare earth elements is not yet clearly defined,although numerous studies have been carried out and many hypotheses have been put forward [6, 7].

According to the literature, phosphor acts as a heterogeneous nucleation site of Si crystal by formingAlP intermetallic particles at high temperature, i.e., above liquidus temperature of Al-Si alloy. [8, 9]Unlike phosphor, RE was not known to form a stable compound with Al that can act as a nucleationsite at high temperature. [3] Therefore, the role of RE as a refiner should be considered by examiningthe behavior of RE as a solute in the melt. The distribution of RE within the primary Si and in the matrixof the alloy will provide a clue to the role of RE on the modification of primary Si during solidification.

Accurate assessment of the mechanism of refinement will be allowed only by the reliable experi-mental data on the concentration of RE elements in and around primary Si in Al-Si alloys. Conventionalmethod for the accurate measurements of solute concentration in the primary crystal is Electron ProbeMicroanalysis (EPMA). However, the spatial resolution of this method is limited to 1mm due to beamspreading and X-ray fluorescence into the specimen. This implies that the microstructural inhomoge-neity of less than 1mm will be ignored and may result in inaccurate measurements. This becomes a

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serious problem when the measurement is made close to the interface between primary Si and thematrix. Improvement of spatial resolution down to nanometer range is possible through convergentbeam electron diffraction (CBED) in the transmission electron microscope (TEM). In this method, thelattice parameter of Si is measured with high accuracy and then converted to the solute concentrationusing the linear relationship between the lattice parameter and solute content, generally known as theVegard’s law.

The purpose of present study is to investigate the distribution of RE elements in the primary Si.Concentration of RE in the primary Si is measured by EPMA method. Lattice parameters of primarySi crystals are determined using relative positions of the high order Laue zone (HOLZ) lines recordedfrom the CBED patterns. Measured lattice parameters are then converted to the RE concentrations tocompare with EPMA measurements for the accurate determination of solute concentration profile in theprimary Si.

Experimental Procedures

Al-21wt.%Si-2wt.%R.E alloy was prepared by melting Al-23%Si mother alloy, pure Al (99.9wt%) andrare earth metal at 850°C in a graphite crucible in a resistant heating furnace. Binary Al-21wt%Si alloy,as a base alloy, was also prepared to compare the morphology of primary Si crystals. The melt wascovered with flux and degassed with chlorine. Rare earth was added as a misch metal after completemelting of the Al-Si alloy and held for 45 minutes at 850°C. The cooling rate of the ingot was measuredto 45°C/sec in the mold preheated to 200°C. The chemical compositions of the alloys studied and mischmetal used are given in Table 1 and Table 2.

General microstructure of the specimens was observed using an optical microscope (OlympusAHMT-3) and Keller’s reagent was used for the etching of the polished specimen. Variations in REconcentration in primary Si crystals were measured using electron probe microanalyzer (EPMA,CAMECA SX51) operated at an acceleration voltage of 20kV, and probe current of 13 1028 amp. ForTEM analysis, specimens were cut into thin slices using a diamond cutter followed by mechanicalgrinding down to 80mm in thickness. The discs were punched out from the ground specimens and finalthinning was done by Ar ion beam milling (Technorg-Linda, IV3, Budapest, Hungary) on both sides ofthe discs with an ion beam energy of 10keV, an incident beam angle of 3° and specimen current of 3mA.Final perforation was carried out by applying 1 kV retarding potential on the specimen to ensure large,electron transparent area. The microstructures of the specimens were observed using a transmissionelectron microscope (Philips CM30 TEM) operated at 100kV. HOLZ lines of primary Si crystals were

TABLE 1Chemical Compositions of the Alloys Studied (wt%)

Alloy Al Si RE

base alloy bal. 21 02RE alloy bal. 21 1.88

TABLE 2Chemical Compositions of the Misch Metal Used (wt%)

element Ce La Nd Pr Fe Pa

wt% 51.90 25.09 14.25 5.33 1.43 0.12

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recorded from the transmitted disc of the CBED pattern at,111. zone axis orientation, usingapproximately 20nm electron probe.

Results and Discussion

The effect of RE addition on the modification of microstructure in Al-21wt%Si alloy is clearly shownin the optical micrographs given in Fig. 1. Primary Si crystals in unmodified alloy exhibit coarse plateletand star-shaped morphologies as shown in Fig. 1(a). It has been known that the star-shaped crystalsgrow from a twinned decahedral nucleus at moderate cooling rate, which is an assembly of five siliconcrystals in twinned orientation. [10] When rare earth elements were added, morphology of the primarySi drastically changed to fine polyhedral shape shown in Fig. 1(b), which illustrates the significantmodification effect of RE elements.

Overall distribution of RE in the alloy can easily be observed by back scattered electron (BSE)imaging in the scanning electron microscope. Due to large atomic number difference, compositionalimaging is possible with BSE imaging, which gives rise to brighter contrast of RE-rich particle in thesample. The BSE image of the sample containing 2wt% RE is given in Fig. 2. RE elements are enrichedin some regions between the primary Si crystals as well as discrete particles in the matrix. Based on thecompositional analysis with a wavelength dispersive spectroscopy (WDS), RE rich particle denoted asarrow1, randomly distributed in the matrix can be interpreted as a ternary intermetallic compoundAl2Si2RE. Sharan et al. have showed that the RE elements can easily form an intermetallic compoundwith Al and Si. [11]. It should be noted that the formation of this intermetallic compound begins belowthe solidus temperature of Al-Si alloy, and therefore does not contribute to the heterogeneous nucleationof primary Si.

In addition to RE rich particle, highly concentrated RE zone (arrow2 in Fig. 2) are also foundbetween primary Si crystals. No intermetallic compound is found and therefore, high concentration ofRE in these zones is directly related to the solidification process of primary Si. The concentration profileof RE in the zone between two primary Si crystals was analyzed with EPMA. Fig. 3 shows the resultof EPMA measurements and the compositional variation of RE elements near primary Si crystal isplotted. Measurements were made at an interval of 2mm, starting from one Si crystal, crossing thematrix region and finishing at another adjacent primary Si crystal. Highest concentration of 5.26wt%Ceand 2.38wt%La (RE) was recorded at the matrix region between two primary crystals. On the otherhand, concentration below 0.2wt%RE was measured inside the primary Si crystal. The accumulation ofRE elements may have originated in the rejection of RE solute from the primary Si to the melt during

Figure 1. Optical micrographs of Al-Si alloys, (a) unmodified base alloy (Al-21wt%Si) and (b) modified alloy (Al-21wt%Si-2%RE) showing the morphological change of primary Si by RE addition.

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solidification. It is also shown that a small amount of RE solute may remain in growing Si crystals. Theaccurate value of maximum solubility of RE in Si is not known and many considered this to benegligible. [12] Therefore, the content of RE in primary Si measured by EPMA needs to be confirmedby other analytical approach such as lattice parameter measurements.

To confirm the validity of EPMA results, the lattice parameters of primary Si crystals were measuredusing a series of defect HOLZ lines, which were observed in the transmitted disc of,111. zone axisCBED pattern. Detailed experimental methods are given in the paper by Kim et al. [13]. Fig. 4(a) showsHOLZ lines in the transmitted disc of the,111. CBED pattern of the standard Si wafer. This patternwas kinematically simulated to find an accelerating voltage which minimizes the error from dynamicdiffraction effect at [111] zone axis. The acceleration voltage for simulation was selected as 98.6kV.Shown in Fig. 4(b) is the,111. HOLZ pattern obtained from the simulation, which is very close tothe experimental pattern. Three lines A, B and C were drawn between the intersections of HOLZ lines,

Figure 2. Back scattered electron image of Al-Si alloy containing 2wt%RE alloy, showing the bright regions representing thesegregation of RE around primary Si crystals and in the matrix.

Figure 3. RE concentration profile across the region of matrix sandwiched by the two primary Si crystals in 2wt%RE alloy.Center point was chosen as the matrix between two Si crystals.

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{9 9 1}/{9 9 1}, {7 9 1}/ { 7 9 1} and {7 11 3}/{7 11 3}, respectively, as indicated in Fig. 4(b). Usingthe distance between the intersections of HOLZ lines reduces the error originating in the differentamount of shifts of the individual HOLZ line positions due to dynamic diffraction effect in the highlysymmetric zone axis orientation such as,111. employed in this study.

Fig. 5 shows the HOLZ patterns of Si obtained from the 0%RE alloy (a) and 2%RE alloy (b).Simulated patterns are also shown together with experimental patterns. The HOLZ pattern recordedfrom the 2%RE alloy was significantly different from that of 0%RE alloy. Lattice parameters weredetermined by first measuring the lengths A,B and C from the experimentally observed HOLZ patternsand then comparing C/A and C/B ratios with those obtained from the simulated patterns.

As shown in Fig. 6, lattice parameters of primary Si crystals in 2%RE were measured to be largerthan those of 0% RE specimen. In addition, the measured lattice parameters showed larger scatteringin the case of 2%RE alloys. In the case of the 2%RE alloy, the average lattice parameter of the primarySi crystal was 0.54379 nm from the C/A ratio and 0.54369 nm from the C/B ratio, respectively. Suchmeasurements indicate an increase of lattice parameter by 0.129% and 0.111% than that of standard Siwafer. On the other hand, the 0%RE alloy had similar C/A and C/B ratios and the average latticeparameter of the primary Si crystals was 0.54329nm or exhibited 0.037% lattice strain as compared tothe standard Si wafer.

Increase in lattice parameter of primary Si is explained by the existence of solutes such as Al andRE elements. To evaluate the concentration of RE elements in primary Si, the effect of Al has to beremoved. This can be done by comparing the lattice strain in 0%RE and 2%RE alloy. Since the latticestrain of Si measured from 0%RE is 0.037%, which is due to Al in Si, this value can be subtracted fromthe measured strain value of Si in 2%RE alloy. Therefore, by eliminating the effect of Al solute on thelattice strain, 0.074–0.092% increase of lattice parameter is attributed to the incorporation of REelements in primary Si.

Since no known formula exists between the lattice parameters and concentration of RE elements inSi, theoretical relationship suggested by Carruthers et al. is used. [14] The lattice strain caused bysubstitutional solute is expressed in the following equation.

«s 5 [1 1 n z (G3 2 1)]1/3 2 1 (1)

Figure 4. Reference [111] HOLZ patterns of standard Si wafer obtained from the (a) experiment and (b) simulation.

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where n is the atomic fraction of solute atoms andG is the ratio of Pauling covalent radius of solute tosilicon atom. [15] SubstitutingG 5 1.461 for RE in Si and rearranging the terms, Eq (2) is obtained.

at.%RE5 47.061{(«s 1 1}3 2 1} (2)

Substituting the values of lattice strains from 2%RE alloy (strain of 0.074–0.092%) to Eq. (2), theconcentration of RE in Si is estimated to be 0.105–0.13 at% which can be converted to 0.392–0.485wt%. The concentration of RE in Si measured by EPMA technique was approximately to 0.2wt% nearthe boundary and to 0.1wt% inside the Si crystal, and two techniques give a satisfactory result on REconcentration in Si, even though higher content of RE was found in CBED method. The higher valueof RE concentration by CBED technique may be attributed to the uncertainty in Pauling covalent radiusof RE solute to silicon atom or existence of interstitial RE species in Si crystals. The possible sourceof overestimation of RE solute content in Si by CBED as compared to EPMA result can be consideredas follows. First, RE was added in the form of a misch metal which contains several species of REelements as shown in Table 2. The ratio of Pauling covalent radius, however, was calculated from thatof Ce alone since only Ce was known to its Pauling covalent radius although both Ce and La weredetected by EPMA analysis in the primary Si. Trace amount of other RE elements in Si can exist andthis will lead to an error in estimating the lattice strain of Si. Second possible source of overestimationis that RE solute can be incorporated into Si as an interstitial solute rather than substitutional type. Itwas already shown that there is a very steep concentration gradient of RE solute surrounding the Si and

Figure 5. Experimental (left side) and simulated (right side) [111] HOLZ patterns of Si obtained from (a) 0% RE alloy (ao 50.54329nm) and (b) 2%RE alloy (ao 5 0.54369nm).

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furthermore, the cooling rate was moderately high enough to force RE solute into the interstitial sitesof Si lattice during solidification. This in turn, will cause large lattice strain of Si compared to that fromthe substitutional solute or RE. As a result of large lattice strain by interstitial solutes, overestimationof solute content can not be avoided.

The supersaturation of RE in primary Si and RE segregation around the primary Si is shown to existin hypereutectic Al-Si alloy containing 2wt%RE elements in this study. This is considered as animportant evidence to understand the refining effect of RE addition. In a binary hypereutectic Al-Sialloys, the growth of primary Si is achieved through the mechanism known as TPRE (twin planere-entrant) growth. [16] In this mechanism, preferential growth along,111. in Si produces coarse,star-shaped crystals related by twinning. In principle, it may be possible to produce fine, polyhedral Sicrystals by suppressing the anisotropic growth, either by poisoning the surface of Si nucleus withforeign solutes from the melt, or by changing the surface energy of Si through incorporation of solutesin the silicon lattice. Since it has been shown from this study that RE addition to Al-Si alloy providesboth solutes in primary Si and highly concentrated melt containing RE elements, the dominant refiningmechanism of RE addition in Al-Si alloy is still uncertain, and more study is needed to separate thesetwo effects on the refining of primary Si in hypereutectic Al-Si alloys.

Conclusions

1. A morphological change of the primary Si crystals from star like to polyhedral was observed when2wt%RE elements as a form of misch metal was added to hypereutectic Al-21wt%Si alloy.

2. Based on the results obtained from EPMA and CBED analyses, supersaturation of RE elements inprimary Si was found, containing approximately 0.2wt% RE.

3. Segregation of RE elements between primary Si crystals was observed and the concentration of REin the region was as high as 5.26wt%RE.

4. It is suggested that the TPRE growth mechanism of primary Si in hypereutectic Al-Si alloy iseffectively suppressed by RE addition, and modification of both solid-liquid interfacial energy andthe surface energy of solid Si is responsible for the refining effect of RE addition in this alloy.

Figure 6. Lattice parameters of Si crystals in Al-21wt%Si and Al-21wt%Si-2wt%RE alloy determined by using (a) C/A ratio and(b) C/B ratio of [111] HOLZ patterns. The numbers near the arrows indicate increase in the lattice parameter caused by dissolutionof solute in the primary Si.

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