Effect of chilling and cerium addition onmicrostructure and cooling curve parametersof Al–14%Si alloy

V. Vijayan and K. Narayan Prabhu*

Al–14%Si alloys, with and without cerium, were cast at varying cooling rates by solidifying them in

a crucible and against chills. The effect of melt treatment and chilling on microstructure and

cooling curve parameters of the alloy was assessed. Ce treated alloys solidified in clay graphite

crucible at a slow cooling rate showed refinement of primary silicon and the formation of Al–Si–Ce

ternary intermetallic compound. The addition of Ce to the alloy solidified against chills resulted in

simultaneous refinement and modification of primary and eutectic silicon. Nucleation tempera-

tures of both primary and eutectic silicon decreased on addition of cerium. The formation of the

intermetallic compound decreased with increase in cooling rate, leading to the modification of the

eutectic silicon. The increase in the degree of modification of the eutectic Si was associated with

the decrease in the volume fraction of the intermetallic compound formed.

On a coule des alliages d’Al–14%Si, avec ou sans cerium, a des vitesses variables de

refroidissement en les faisant solidifier dans un creuset ou contre des refroidisseurs. On a evalue

l’effet du traitement du bain et du refroidissement sur la microstructure et sur les parametres de la

courbe de refroidissement de l’alliage. Les alliages traites au Ce et solidifies dans un creuset

d’argile et graphite a une faible vitesse de refroidissement montraient un raffinement du silicium

primaire et la formation d’un compose intermetallique ternaire d’Al–Si–Ce. L’addition de Ce aux

alliages solidifies contre les refroidisseurs avait pour resultat un raffinement et une modification

simultanes du silicium primaire et eutectique. Les temperatures de nucleation tant du silicium

primaire qu’eutectique diminuaient avec l’addition de cerium. La formation du compose

intermetallique diminuait avec l’augmentation de la vitesse de refroidissement, menant a la

modification du silicium eutectique. L’augmentation du degre de modification du Si eutectique

etait associee a la diminution de la fraction volumique du compose intermetallique forme.

Keywords: Chilling, Cerium, Eutectic silicon, Modification, Primary silicon, Refinement, Thermal analysis

List of symbolsTN (PSC) Primary silicon nucleation temperature

Tmin(Eutectic) Al–Si eutectic minimum temperature

TG(Eutectic) Al–Si eutectic growth temperature

DTG The eutectic growth temperature differencebetween Ce added alloys and base alloy

DTrecalescence The difference between eutectic growthtemperature and the minimum tempera-ture in each alloy

IntroductionAl–Si alloys with Si content greater than 13 wt-% arecategorised as hypereutectic Al–Si alloys. Owing to the lowcoefficient of thermal expansion and high wear resistanceproperties, hypereutectic Al–Si alloys are generally usedfor internal combustion engine parts, especially as pistons.The other applications of hypereutectic Al–Si alloysinclude high performance automobile engine parts such

Department of Metallurgical and Materials Engineering, National Instituteof Technology Karnataka, Surathkal, Srinivasnagar, Mangalore 575025,India

*Corresponding author, email prabhukn_2002@yahoo.co.in

66

� 2015 Canadian Institute of Mining, Metallurgy and PetroleumPublished by Maney on behalf of the InstituteReceived 1 March 2014; accepted 12 June 2014DOI 10.1179/1879139514Y.0000000151 Canadian Metallurgical Quarterly 2015 VOL 54 NO 1

as connecting rods, rocker arms, cylinder sleeves andpiston rings. The cast microstructure of hypereutectic alloygenerally consists of coarse and segregated primary siliconcrystals along with unmodified eutectic silicon. Generally,primary silicon is refined by phosphorous treatment, butthe treatment does not have any effect on the eutecticsilicon. To achieve further improvement in the mechanicalproperties of hypereutectic Al–Si alloys it is important tosimultaneously modify eutectic silicon along with therefinement of primary silicon.1–3 Among the alternatives tophosphorous, few rare-earth elements have recently gainedattention due to their ability in modifying eutectic silicon.

Kowata et al.4 investigated the effect of rare earth(45%Ce, 31%La, 15%Nd, 5%Pr) addition on therefinement of primary silicon crystals in a hypereutecticAl–20 wt-%Si alloy and concluded that the primarysilicon crystals were refined with the addition of rareearth elements to Al–Si melt. Chang et al.5 studied theeffect of RE addition to Al–21 wt-%Si alloy in a wedgeshaped cast iron mould and reported that the REaddition bought simultaneous refinement of bothprimary and eutectic silicon. The results also showed12–17uC and 2–7uC depression in the nucleationtemperatures of primary silicon and eutectic silicon.Ouyang et al.6 investigated the effect of La addition onAl–18%Si alloy solidified on a preheated metal mould.The La master alloy was added in combination with P.The simultaneous modification of primary and eutecticsilicon was achieved on combined La and P addition.The morphology of eutectic silicon changed from longneedle-like structure to short rod-like structure. Dai andLiu7 studied the combined and individual effect of P, Band Ce on Al–30%Si and found that Ce has moderateeffect on primary and eutectic silicon. The alloy wassolidified in preheated (473 K) permanent mould ofdimension W35675 mm. They also found that theaddition of Ce along with B had good modification

effect on eutectic Si due to the large undercooling effect.Xing et al.8 found that optimal addition of rare earthelement (Er) to Al–17%Si and Al–25%Si alloys yielded amodified microstructure. The melt at 800uC was pouredto a metal mould preheated to 200uC. The size ofthe primary silicon decreased with Er addition and themechanical properties was found to be highest at theoptimal concentration of Er addition. Chen et al.9

studied the complex modification of P and RE on Al–20Si–1?6Cu–0?7Mn–0?6Mg alloys solidified in a perma-nent mould preheated at 250uC. They reported that theaddition of RE along with P resulted in the refinementand modification of primary and eutectic Si. Theprimary silicon was refined to 23?3 from 64?4 mm andeutectic silicon was modified to fine fibrous or lamellarform with an average size of 5?3 mm. The tensile strengthand elongation of the alloys also improved by 20 and40% respectively, owing to the refinement and modifica-tion achieved. Kores et al.10 studied the effect of Ceaddition on cast iron mould solidified Al–17%Si alloyand reported that the addition of 1% Ce resulted in therefinement of primary and eutectic silicon. The primarysilicon nucleation temperature decreased from 686 to591?9uC on Ce addition. However, this was contra-dictory to the results obtained by Wesis and Loper.11 Intheir studies, they reported that cerium did not refineprimary silicon but it moderately affected the eutecticsilicon. Recently, Li et al.12 reported that the Ce couldsignificantly refine and modify primary and eutecticsilicon. They studied the effect of Ce addition on themicrostructure and tensile properties of Al–20%Si alloy.The alloy was solidified in a 200uC preheated permanentsteel mould of 20 mm inner diameter and 50 mm length.The addition of Ce refined the primary silicon size from94 to 33 mm and transformed eutectic silicon to finefibrous form. The addition also significantly improvedthe tensile strength and elongation.

From the literature, it is clear that the Ce hassignificant effect on the microstructure of hypereutecticAl–Si alloys. The existing literature on Ce modificationof Al–Si alloy is scant and the reported results arecontradictory. One of the reasons for contradictioncould be the varying solidification conditions underwhich experiments were carried out by several research-ers. The effects of addition of Ce on the thermalparameter and the degree of modification have not beenreported. In the present investigation, attempt was madeto study the effect of Ce treatment and cooling rate oncooling curve analysis parameter and evolution ofmicrostructure in Al–14%Si alloy.

Experimental detailsAl–14%Si–2?6%Cu–0?8%Mg–0?3%Fe alloy was used inthe present study. About 350¡50 g of the alloy samplewas melted in a clay graphite crucible using an electricalresistance furnace. Cerium [Alfa Aesar, Cerium ingot,99% pure (REO)] was added to the melt in varyingquantities (0?5 wt-%, 1 wt-%, 1?5 wt-% and 2 wt-%) at750uC. After the addition of Ce, the liquid metal wasmaintained at 750uC for 30 min. The holding time was30 min for all the experiments. For slow solidification,

1 Schematic sketch of solidification set-up

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Canadian Metallurgical Quarterly 2015 VOL 54 NO 1 67

the crucible with the melt was removed from the furnaceand cooled to room temperature under near equilibriumconditions. For chill solidification, the melt was quicklypoured into the stainless tube of 50 mm diameter with achill at the bottom. Stainless steel tube was selected as it haslow thermal conductivity (16 W m21 K21). Copper, brassand cast iron chills with varying thermal conductivities

were selected to obtain different cooling rates. The chillroughness before experiment was set at 0.5¡0.05 mm. Aschematic sketch of the experimental setup is shown inFig. 1. The temperature data from the castings wasrecorded using computerised data acquisition system. AK-type thermocouple was inserted at the centre of thecrucible to record the cooling behaviour of the alloy in the

2 Micrographs of crucible cooled Al–14%Si alloy with varying Ce content: a untreated; b 0?5 wt-%Ce; c 1?0 wt-%Ce (1:

primary silicon; 2: eutectic silicon; 3: Ce phase); d 1?5 wt-%Ce; e 2?0 wt-%Ce

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range of 750–400uC during solidification. The thermocou-ple was connected to a high speed data acquisition system(NI USB 9162) interfaced with a PC. The temperature wasrecorded with a time step of 0?1 s and the accuracy ofthermocouple used was ¡0?5uC. The experimental set-upwas covered using an insulation blanket to maintainconstant cooling conditions for all experiments.

For microstructure evaluation, samples were preparedfrom castings and then polished for metallographicobservations. The microstructures of specimens werethen examined under a JEOL JSM-6380LA scanningelectron microscope. Quantitative measurements ofSi particle characteristics were carried out usingAxio Vision image analysis software. The eutectic Si

3 Micrographs of Al–14%Si alloy solidified on different chills: a untreated cast iron chill; b 1?5 wt-%Ce cast iron chill; c

untreated brass chill; d 1 wt-%Ce brass chill; e untreated copper chill; f 0?5 wt-%Ce copper chill

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Canadian Metallurgical Quarterly 2015 VOL 54 NO 1 69

characteristics like area, length, perimeter, aspect ratio(length/perimeter) and roundness factor (P2/pA, where,P is perimeter and A is the area of silicon particle) arequantitatively measured at different locations of Ceadded alloys for all solidifying conditions. The volumefraction of Ce intermetallics was also measured usingmultiphase analysis module.

Results and discussion

Microstructural characterisationFigure 2 shows the micrograph of untreated Al–14%Sialloy solidified in a clay graphite crucible at slow coolingrate. The microstructure consists of irregular shapedprimary silicon and long needle-like eutectic silicon in aeutectic matrix. During solidification, the silicon parti-cles grow into a wide variety of shapes depending on thecooling rate and under-cooling achieved. The morphol-ogy of the primary silicon particles depend on theirnucleation and growth behaviour during solidification.Figure 2b–e shows the effect of Ce addition on themicrostructure of the alloy. The addition of Cetransformed the morphology of primary silicon fromirregular shape to polyhedral shapes. The size ofprimary silicon decreased with Ce addition and mini-mum size was at 1?5 wt-%Ce addition. The microstruc-tures also showed the presence of an intermetalliccompound with addition of Ce. With increasing Ceaddition, morphology of the intermetallic transformedfrom needle-like shapes (0?5 wt-%Ce) to block-likeshapes (1?0 wt-%Ce). Figure 3 shows the microstructureof the alloy solidified against cast iron, brass and copperchills. The microstructures showed that the size of theprimary silicon decreased with an increase in the thermalconductivity of the chill. The average sizes of theprimary silicon solidified in crucible and against castiron, brass and copper chills were found to be 137¡39and 50¡9, 22¡3, 12¡2 mm respectively. The alloysolidified against copper chill showed finer and well

distributed primary silicon. The size of primary siliconsolidified against copper chill was 76% finer than theprimary silicon solidified against cast iron chill.

The spherical nature of the eutectic Si particle is assessedby the roundness factor and the aspect ratio (Table 1). Aroundness factor and aspect ratio of 1 indicates that theparticle is perfect sphere. The untreated alloy displays thelargest particle size (length, area and perimeter) and aroundness value of 7, indicates that the silicon particle iscoarse and acicular nature. The addition of Ce to cruciblecooled alloy did not show much effect on the acicularnature of the silicon particle even though the particlelength decreased by 38%. On the other hand, the untreatedalloy solidified against chills showed a significant decreasein the particle characteristics compared to crucible cooledalloys, but the acicular nature of the eutectic Si was leastaffected and is seen in Fig 3a, c and d. With the addition ofCe to chilled alloys, the particle parameters decreasedsignificantly. The length of the eutectic particle decreasedfrom 19 to 2 mm on 1?5% Ce addition to cast iron chilledalloys. Similar results were obtained for alloys solidifiedagainst copper and brass chills as well. Similarly, the aspectratio of the particle was found to decrease with Ceaddition. This indicates that the acicular Si particles aretransformed to fine fibrous form with Ce addition.

The roundness factor approached unity with varyingCe content depending on the thermal conductivity of thechill. For example, addition of 1?5% Ce to the alloysolidified against cast iron chill reduced the roundnessfactor to 1?4 from 4?3. This indicates that the eutectic Siparticle has become more spherical with addition of Cecompared to Si in the untreated alloy. Figure 3a–ecompares the microstructures of unmodified and Cemodified alloy samples for different chill conditionsrespectively. The quantity of Ce required for modifica-tion varied with the thermal conductivity of the chillused. In the case of cast iron chill, the alloy with 1?5 wt-%Ce showed the highest degree of eutectic modification,whereas, for brass and copper chills the maximum

Table 1 Eutectic silicon particle characterstics of Al–14%Si alloy solidified under different cooling conditions

Ce/wt-% Length/mm Aspect ratio Area/mm2 Perimeter/mm Roundness factor

0 129¡40 22¡15 827¡350 270¡83 7¡20.5 92¡50 13¡10 490¡180 200¡100 6.5¡3

Crucible 1 80¡15 15¡5 301¡65 170¡35 8¡21.5 86¡18 16¡5 319¡90 182¡42 8.3¡22 95¡25 16¡15 406¡100 210¡50 9¡40 19¡7 7¡3 52¡18 44¡16 5¡1

Cast iron 0.5 11¡6 6¡3 23¡10 26¡13 2.4¡11 5¡2 3¡2 9¡3 13¡5 1.7¡0.51.5 3¡1 2¡1.1 5.3¡2.5 9¡3 1.4¡0.52 2¡1 1.2¡0.5 3¡0.5 8.6¡2.5 1.8¡1.00 10¡2 7¡2.5 12¡3 23¡6 3.5¡10.5 3.2¡1.5 2.6¡1.5 3.2¡1 9¡3.5 2¡1

Brass 1 2.3¡0.5 1.5¡0.5 3¡1 8¡2 1.7¡0.51.5 2.2¡0.5 1.3¡0.5 2.5¡1 8¡2 1.7¡0.22 1.8¡0.2 1.1¡0.2 1.9¡0.3 6.7¡1 1.8¡0.20 7.5¡2.5 5.6¡2.2 7¡2 18¡6 4¡10.5 2¡0.5 1.2¡0.2 3.2¡1.0 7.5¡1 1.36¡0.1

Copper 1 1.5¡0.3 1.1¡0.2 1.8¡0.4 5.8¡0.7 1.45¡0.21.5 1.3¡0.2 1¡1 1.4¡0.3 5.2¡0.7 1.47¡0.22 1.6¡0.2 1.1¡0.2 2¡0.8 6.2¡2 1.5¡0.5

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modification was observed at 1?0 wt-%Ce and 0?5 wt-%Ce respectively.

To assess the effect of chilling on degree of siliconmodification, the silicon particle parameter (Roundnessfactor) was visually compared with AFS standard chartsfor silicon modification level shown in Fig. 4. The degreeof Si modification was found to vary exponentially with

roundness factor and is shown in Fig. 5. The equationwas used to find out the Si modification level for varyingCe addition at different solidifying conditions. Figure 6shows the combined effect of chilling and Ce treatmenton the Si modification level. As can be seen, themodification levels more or less remain the same forcrucible cooled alloys even after the addition of Ce. The

4 Representative Si structure morphologies corresponding to AFS chart13

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alloys solidified against chills showed significant increasein degree of silicon modification with the addition of Ce.

Thermal analysisFigure 7 shows the cooling and first derivative curves ofcrucible cooled Al–14%Si alloy without any addition.The primary and eutectic nucleation temperatures ofuntreated alloy were found to be 612 and 575uCrespectively. Figure 8 and Table 2 show the effect ofCe on cooling curves of the alloy under differentsolidifying conditions. The primary silicon nucleationtemperature decreased with the addition of Ce for allconditions. Copper chilled alloy showed a minimumtemperature of 582uC on 2% Ce addition. A similar kindof decrease in nucleation temperature of primary siliconwas reported by Kores et al.10 They studied the effect of

Ce addition on Al–17% Si alloy solidified in cast ironmould and reported that, the primary silicon nucleationtemperature decreased from 686 to 591?9uC on 1% Ceaddition to the melt. The reason for this kind of decreasein primary silicon nucleation temperature with Ceaddition is due to the adsorption of Ce atom to theinterface of primary silicon. The fraction solid calculatedfor Ce added alloys before eutectic nucleation supportsthis theory. The fraction solid up to the nucleation ofeutectic silicon is given in Table 3. The results indicatethat the fraction solid formed decreases with theaddition of Ce. This implies that the formation ofprimary silicon is suppressed due to the addition of Ceand as a result, the nucleation temperature is decreased.

The effect of Ce addition on eutectic silicon nucleationand growth temperature is shown in Table 2. The resultindicated that the thermal analysis parameters decreasedon addition of Ce. In the case of crucible cooled alloy,Tmin decreased from 575?0uC (for 0 wt-%Ce) to 573?5uC(2 wt-%Ce) and this corresponds to a moderate mod-ification. In the case of copper chilled alloy, the Tmin

decreased to 564?2¡1?2uC on 2% Ce addition, dueto the complete eutectic Si modification. DTG andDTrecalescence are used to quantify the effect of Ceaddition on thermal parameters for varying solidifyingconditions. Figure 9 shows the variation of DTG andDTrecalescence with Ce. According to the results, DTG

values increased with the addition of Ce and thisindicates that the growth temperature decreased withthe addition. The chilled alloys showed significantdecrease in growth temperatures compared to thecrucible cooled alloys. The recalescence undercoolingof eutectic reaction is determined by the differencebetween eutectic growth temperature and minimumtemperature. The effect of Ce addition on recalescenseundercooling for different solidifying conditions is

5 Quantitative relationship between roundness factor of silicon and AFS modification level

6 Si modification level vs cerium concentration

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shown in Fig. 9b. The undercooling increased with theaddition of Ce up to a particular concentrationdepending on the chill used and then remained constanton excess addition of Ce. The trend remained same forall cooling conditions. The Ce content for eutectic Simodification of alloys solidified against copper, brassand copper chills were found to be 1?5, 1?0 and 0?5 wt-%Ce respectively.

The reason for higher growth temperature in cruciblecooled alloys in spite of Ce treatment is due to theformation of large Ce ternary compounds as shown inFig. 2. The presence of Ce bearing compound has beenconfirmed by X-ray diffraction and is shown in Fig. 10.According to Grobner et al.14 in an Al–Ce–Si system forlow concentration of Ce only two phases c1 [Ce(Si1–x

Alx)2] and c2 [AlCeSi2] are thermodynamically stable.

Hence in the present study either of these two phasesmight have been formed during solidification. Theformation of Ce intermetallics might have led to the

7 Cooling and first derivative curve for Al–14%Si alloy

Table 2 Effect of Ce addition on solidification parameters of Al–14% Si alloy for different solidifying conditions

Ce/wt-% TN(Primary Si)/uC Tmin(Eutectic)/uC TG(Eutectic)/uC DTRecalescence/uC DTG/uC

0 612.7¡0.6 575.0¡0.4 575.64¡0.6 0.6 0.00.5 591.2¡0.1 574.6¡0.2 575.6¡0.1 1.0 0.0

Crucible 1 590.5¡0.1 574¡0.4 575.2¡0.8 1.2 0.41.5 589¡0.5 573¡0.5 575¡0.2 2.0 0.62 589.7¡0.6 573.5¡0.8 574.9¡0.4 1.4 0.70 603.5¡2 574.6¡0.4 575¡0.1 0.4 0.60.5 598¡3 572.2¡0.4 573.2¡0.1 1.0 2.4

Cast iron 1 592¡3 568.3¡0.3 569.5¡0.3 1.2 6.11.5 587¡1.5 567.3¡0.5 568.9¡0.4 1.6 6.72 587¡0.5 566.2¡0.4 567.5¡0.4 1.3 8.10 598.1¡1.2 572.8¡0.2 573.3¡0.1 0.5 2.30.5 590¡2 571.7¡0.5 572.4¡0.5 0.7 3.2

Brass 1 587¡1.5 569.3¡0.7 570.5¡0.2 1.2 5.11.5 585¡1 567.8¡1 569¡1.5 1.2 6.62 586¡2 565.6¡0.9 566.6¡0.2 1.0 9.00 595¡2 573¡0.5 573.3¡0.5 0.3 2.30.5 590.0¡1.5 571.9¡0.4 572.5¡0.2 0.7 3.1

Copper 1 585.9¡2 570.3¡0.8 571.0¡1 0.7 4.61.5 583.4¡1 566.4¡1 567.1¡0.7 0.7 8.52 582.8¡0.7 564.1¡0.9 564.8¡1.5 0.6 12.2

Table 3 Effect of Ce addition on fraction solid formed upto eutectic solidification for Al–14%Si alloysolidified on different chills

Ce/wt-%

Fraction solid at eutectic nucleation

Cast iron Brass Copper

Without addition 0.16 0.12 0.130.5 0.14 0.10 0.031.0 0.13 0.08 0.051.5 0.11 0.04 0.062.0 0.14 0.07 0.09

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8 Cooling curves of Al–14%Si alloys with varying Ce content solidified on a crucible cooled; b cast iron; c brass and;

d copper chills

9 Variation of eutectic solidification charcterstics with addition of Ce: a growth temperature differnce (DTG); b recales-

cence undercooling (DTrecalescence)

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reduction of Ce atoms in eutectic matrix to poison thegrowth of eutectic Si. In the case of chilled alloys,the degree of eutectic Si modification increased due tothe presence of Ce atoms in eutectic matrix. Accordingto Lu and Hellawell,15 the best modifying effect isachieved when the ratio of atomic radius of themodifying agent and atomic radius of Si is closer to1?65. The radii ratio (Rce/RSi) for cerium and silicon is1?36 and is near to 1?65.

Figure 11 shows the volume fraction analysis of Ceintermetallics formed during solidification of Ce treatedAl–14%Si alloy. The results indicate that the Ceintermetallics formed increased with the Ce addition.This is due to the high chemical affinity of Ce in theformation of ternary intermetallics. Under slow solidify-ing conditions, the Ce atoms from near surroundingsdiffuse and cluster together to form large block-likeparticles, making it unavailable for eutectic modifica-tion. Figure 11 reveals that the volume fraction of Ceintermetallic was higher in the case of crucible cooledalloy and it decreases with increasing cooling rates. In thecase of chilled alloys, the movement of Ce atom ishindered by the fast moving solidification front and hence

smaller and thinner intermetallics are formed as shown inFig. 3. Therefore, the presence of the Ce atoms in eutecticmatrix will lead to the modification of eutectic Si.

For online prediction of eutectic silicon modification,it is necessary to correlate the thermal characteristicswith the degree of Si modification. In the present study,DTG was correlated with the corresponding degree ofmodification estimated using quantitative metallo-graphic technique. DTG is the difference in growthtemperature between Ce treated alloys and untreatedcrucible cooled alloy. The untreated crucible cooledalloy was found to contain eutectic Si with modificationlevel 1. DTG versus modification level for Al–14% Sialloy is shown in Fig. 12. It is clear that the modificationlevel converges to 1 (extremely coarse) when DTG

approaches zero. For DTG values greater than 7, themodification level remains constant at 5. The effect ofchilling on modification level can be directly found fromthe curve by determining the growth temperaturedifference between the treated alloy and the untreatedalloy. For example, the DTG for untreated alloysolidified against copper is 2?3uC and the correspondingdegree of modification is 2 (Coarse). The alloy with DTG

values equal to 5uC showed a completely modifiedmicrostructure.

ConclusionDuring solidification of hypereutectic Al–Si alloys (Al–14%Si) in clay graphite crucible and against differentchills (cast iron, brass and copper), the effect of Ce melttreatment on microstructure, and thermal analysis para-meters was studied. Based on the results, the followingconclusions were drawn:1. Ce additions to Al–14%Si alloy solidified at slow

cooling rate in a clay graphite crucible resulted inthe transformation of primary silicon from irregu-lar shape to polyhedral shape. The eutectic siliconwas found to be modified on Ce addition. Themicrostructure showed the presence of blockyshaped Al–Si–Ce ternary intermetallics formeddue to the addition of Ce.

10 X-ray diffraction pattern of Ce added Al–14% Si alloys

11 Volume fraction of Al–Si–Ce intermetallic formed ver-

sus Ce addition (wt-%)

12 Growth temperature difference vs modification level

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2. Simultaneous refinement and modification of pri-mary and eutectic silicon was obtained with additionof Ce to alloys solidified against chills. The degree ofeutectic modification increased with increase in Cecontent. The microstructure showed high degree ofeutectic modification at 1?5, 1?0 and 0?5 wt-%cerium additions to the alloy solidified against castiron, brass and copper chills respectively.

3. The roundness factor particle characteristic wasused to assess the AFS Si modification level. In thecase of chilled alloys, the degree of Si modificationincreased with cerium addition.

4. An analysis of volume fraction results indicatedthat the percentage of Al–Si–Ce intermetallicsformed decreased with an increase in cooling rate.The morphology of the intermetallic formedtransformed from blocky shape to needle shape athigher cooling rates. The lower the volume fractionof intermetallics formed, the higher the degree ofeutectic Si modification achieved.

5. Cooling curve analysis results showed that thenucleation temperatures of primary and eutectic Sidecreased marginally on Ce addition to slowlycooled alloys in a clay graphite crucible anddecreased significantly for chilled alloys. DTG

(growth temperature difference) was correlatedwith the degree of eutectic Si modification.

Acknowledgements

V. Vijayan thanks National Institute of TechnologyKarnataka, (NITK) Surathkal, Mangalore, India for theResearch Scholarship. The authors acknowledge the

help received from Mr Sathish and Mr Dinesha,Technicians at the Department of Metallurgical andMaterials Engineering, National Institute of TechnologyKarnataka (NITK), Surathkal, India, during the castingprocess. The authors also thank Ms Rashmi Banjan,SEM operator, National Institute of TechnologyKarnataka (NITK), Surathkal, India, for her assistanceduring scanning electron microscopy.

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