Materials Science and Engineering A259 (1999) 43–52

The effect of grain refinement and silicon content on grainformation in hypoeutectic Al–Si alloys

Y.C. Lee *, A.K. Dahle, D.H. StJohn, J.E.C. HuttCRC for Alloy and Solidification Technology (CAST), Department of Mining, Minerals and Materials Engineering, The Uni6ersity of Queensland,

Brisbane, Qld 4072, Australia

Received 31 March 1998; received in revised form 10 August 1998

Abstract

The effect of increasing the amount of added grain refiner on grain size and morphology has been investigated for a range ofhypoeutectic Al–Si alloys. The results show a transition in grain size at a silicon concentration of about 3 wt% in unrefined alloys;the grain size decreasing with silicon content before the transition, and increasing beyond the transition point. A change inmorphology also occurs with increased silicon content. The addition of grain refiner leads to greater refinement for silicon contentsbelow the transition point than for those contents above the transition point, while the transition point seems to remainunchanged. The slope of the grain size versus silicon content curve after the transition seems to be unaffected by the degree ofgrain refinement. The results are related to the competitive processes of nucleation and constitutional effects during growth andtheir impact on nucleation kinetics. © 1999 Elsevier Science S.A. All rights reserved.

1. Introduction

The addition of grain refining master alloys to alu-minium alloys is common practice in many commercialfoundries. Aluminium master alloy additions contain-ing titanium or different combinations of titanium andboron contents are the most common and were initiallydeveloped for wrought alloys.

The equiaxed grain structure is determined by twointerlinked factors. The first is the existence of sub-strates in the melt that can act as nucleation sites and,second, the growth of the nucleated crystals affects thefurther nucleation of crystals in the remaining part ofthe solidifying volume. Growth of the solid/liquid inter-face is controlled by the undercooling existing at theinterface, viz. thermal undercooling (DTt), constitu-tional undercooling (DTc) and capillarity undercooling(DTr) [1]. If the solute content is increased the constitu-tional undercooling at the interface is increased as moresolute is partitioned.

Although many studies of grain size have been per-formed over the last five decades [2], these have mostlyconsidered the action and development of commercial

grain refiners and their operating mechanisms inwrought alloy compositions. These studies have shownthat to achieve a small grain structure it is necessary toprovide numerous substrates in the melt that are acti-vated at a low undercooling. It is also necessary toprovide undercooling so that the nuclei can survive andform equiaxed crystals [3]. Studies have shown thatundercooling provided by the casting process throughfaster heat extraction rates also decreases the grain size[4].

The effect of alloy composition seems to be quitecomplex. It has been documented in two alloy systems,e.g. Al–Si [4–9] and Pb–Sb [5], that the grain size firstdecreases with increasing alloy concentration and then,after reaching a minimum, the grain size increases withfurther alloy additions. The minimum is sometimesclose to the maximum solid solubility limit and someinvestigators have therefore related the minimum ingrain size to a maximum solidification range, and there-fore maximum solidification time, proposing that analloy with a large solidification range allows longer timefor nucleation. However, Backerud and Johnsson [7]recently suggested that a transition from cellular,clover-leaf shaped crystals to dendrites with well devel-oped orthogonal branches is responsible for the transi-tion, i.e. a cellular–dendritic transition in the growth of* Corresponding author.

0921-5093/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.

PII S0921-5093(98)00884-3

Y.C. Lee et al. / Materials Science and Engineering A259 (1999) 43–5244

equiaxed crystals. They proposed that the transitionoccurs at a growth restriction factor of:20. Thegrowth restriction factor is equal to SimiC0,i(ki−1)where m is the slope of the liquidus line, C0 is compo-sition and k is the equilibrium distribution coefficientfor all elements i assuming full solid solubility andthat the liquidus and solidus lines are straight. How-ever, Spittle et al. [9] did not find any transition inAl–Zn alloys for Zn contents up to 50 wt% and thisindicated that the growth restriction factor alone doesnot correlate with the transition. This may, however,be related to the large solid solubility in the Al–Znsystem. Spittle et al. [9] suggested that other mecha-nisms responsible for the transition could be siliconinteracting with Ti, coating of the nucleants with sili-cides, removing excess Ti or modification of the com-position of the nucleants.

Several studies have shown the transition in grainsize in Al–Si alloys [4–9]. The transition is clearlyevident in unrefined alloys where a number of workershave observed the minimum to be at 3 wt% Si [4–8].Tøndel [6] also observed that the measured thermalundercooling correlated with the observed grain size.A small undercooling corresponded to a small grainsize.

The results from experiments using grain refiner ad-ditions suggest that the transition is moved to lowersilicon contents [4–7,9,10]. Abdel-Reihim et al. [5]found a minimum in grain size at about 0.5 wt% Si inalloys grain refined with 0.01 wt% Ti added asAlTi5B1 master alloy. The grain size increasedmarkedly above 3 wt% Si. It was also shown that themorphology changed from compact, globular, tohighly branched dendritic at the transition point. Inunrefined alloys the morphology changed markedly at1.6 wt% Si, i.e. close to the solid solubility limit.Comparing his results from unrefined alloys to alloysgrain refined with 0.01 and 0.05 wt% Ti added asAlTi5B1, reported by Johnsson [10], Tøndel [6] foundthat the transition was shifted to lower silicon con-tents with increased grain refinement; from :3 wt%Si without grain refinement to :0.5 wt% Si with 0.05wt% Ti added. Within the limitation of their experi-mental results, the results of Hoefs et al. [4] indicatedthat the transition point was displaced to lower siliconcontents somewhere between 0.5 and 3 wt% upon theaddition of 0.01 wt% Ti as AlTi5B1. The effect ofgrain refiner level is not clear from these results aseach researcher used different levels of grain refinerfor their experiments. It is possible that increasing theamount of grain refiner reduces the silicon content atthe transition point [10], but comparison with otherresults [4–6,9] suggests that there may be no effect.

Backerud and Johnsson [7] indicated that coolingrate does not affect the transition, whereas grain refin-ement displaced the transition to silicon contents of

:1 wt%. Hutt et al. [8] showed that mould preheat(i.e. cooling rate and temperature gradient) had aneffect on the transition point in Al–Si alloys. At alow mould preheat (150°C) the transition point oc-curred at a silicon content of 6 wt% and a grain sizeof 600 mm, whereas at a high mould preheat (500°C)the transition point occurred at 3 wt% Si and a grainsize of 1600 mm. It is also notable that when Hutt etal. [8] cast under conditions where a large number ofcrystals are produced at the wall the transition van-ished, producing a continuous decrease in grain sizewith increasing silicon content.

This paper reports the results of an investigation ofgrain refinement of hypoeutectic Al–Si alloys. Theaims of the study are to determine the effect of grainrefiner addition on the silicon content at which thegrain size transition occurs, and which of the pro-posed mechanisms (cellular–dendritic transition or achange in the nature of the nucleant particles) is re-sponsible for the occurrence of the grain size transi-tion.

2. Experimental procedure

Binary Al–Si alloys were made in an induction fur-nace by mixing commercial purity aluminium (99.7%)and commercial purity silicon (99.6%). The followingcompositions were produced: 1, 2, 3, 4, 5, and 8 wt%Si. Chemical analyses were performed by optical spec-troscopic measurements.

These binary alloys served as a base for the grainrefinement studies and were remelted in an electricalresistance furnace and held at a constant temperatureof 800°C. Grain refinement was achieved by addingAlTi5B1 master alloy rods. The contact time of thegrain refiner was 20 min and the melt was stirred for30 s, 5 min prior to sampling. In addition to the purealloy, the following five grain refinement levels wereinvestigated: 0.01, 0.03, 0.05, 0.10, and 0.15 wt% Ti.

The samples were solidified in the set-up for ther-mal analysis developed by Backerud et al. [11]. Asample of the melt was withdrawn in a preheatedcylindrical graphite crucible with an outer diameter of50 mm, a height of 50 mm and a uniform wall thick-ness of 10 mm. The crucible was placed on 10 mmthick fiberfrax felt and a fiberfrax lid was placed overthe open top face. The crucible was then allowed tocool in air giving a cooling rate of:0.7 K/s.

After solidification the samples were sectioned mid-way from the bottom of the cylinder. One section wasmacroetched and the other was anodised. Grain sizeswere measured by the lineal intercept method on theanodised samples in an optical microscope under po-larised light. The grain size measurements were carriedout according to the ASTM standard E112-88.

Y.C. Lee et al. / Materials Science and Engineering A259 (1999) 43–52 45

Fig. 1. (a) Grain size vs. titanium content for the six silicon concentrations investigated; 1, 2, 3, 4, 5, and 8 wt%. (b) Grain size vs. silicon contentfor six different levels of grain refinement by AlTi5B1 master alloy additions.

3. Experimental results

Six different levels of grain refinement additions byAlTi5B1 were investigated in order to study the effectof grain refinement on the grain size and growth mor-phology in Al–Si alloys.

Fig. 1 (a) plots grain size against Ti content where itcan be seen that the most marked effect of the grainrefiner was observed with the lowest addition level (0.01wt% Ti) for all the silicon contents. The degree ofreduction in grain size decreased with each increase ingrain refiner level and an insignificant reduction wasobserved when the grain refiner level increased from0.10 to 0.15 wt% Ti.

Up to a titanium content of about 0.05 wt% the grainsize decreases with increased level of grain refiner, withthe lowest level of grain refiner addition (0.01 wt% Ti)having the largest effect. Increasing the grain refine-ment level above 0.05 wt% Ti does not produce asignificant further reduction in the grain size.

Fig. 1 (b) shows the results of grain size measure-ments at various silicon contents for the six levels of

grain refinement. In the unrefined alloy the grain size isobserved to decrease continuously up to a silicon con-tent of 3 wt% with a grain size of 678 mm. A transitionin behaviour is observed at this point, whereafter grainsize is observed to increase with increasing silicon con-tent. With grain refiner addition this effect is less pro-nounced, although still discernible. The transition pointis identified as the silicon content where the smallestgrain size is observed. From Fig. 1 (b) the transitionpoint appears to be between 2 and 3 wt% Si. For theunrefined alloy the transition point is at 3 wt% Si, andwhen 0.01 wt% Ti is added the transition point remainsunchanged. The addition of grain refiner is observed tobe more efficient at lower silicon contents.

With the exception of the unrefined alloy, the alloyswith low silicon contents (1–3 wt%) have smaller grainsizes than those with higher silicon contents. In thegrain refined alloys there is not a significant change ingrain size before the transition point. After the transi-tion point the grain size increased with increasing sili-con content, independent of grain refiner content. It isinteresting to note that the slopes of the curves after the

Y.C. Lee et al. / Materials Science and Engineering A259 (1999) 43–5246

Fig. 2. Micrographs of A1–3% Si with increasing levels of grain refinement. (a) unrefined; (b) 0.01 wt% Ti; (c) 0.03 wt% Ti; (d) 0.05 wt% Ti; (e)0.1 wt% Ti; (f) 0.15 wt% Ti.

transition point are approximately constant and inde-pendent of grain refiner level.

Fig. 2 shows micrographs of the 3% Si samples wherethe continuing decrease in grain size can be easilyobserved as well as a change in grain morphology to amore globular form as the grain size decreases.

Fig. 3 and Fig. 4 show micrographs for all siliconconcentrations for the unrefined alloys and alloys witha high level of grain refinement (0.15 wt% Ti). Thechanges in grain size reported in Fig. 1 (a) are clearlyvisible, with an increase in grain size occurring with

increasing silicon content above 3 wt% Si. There arealso changes in the growth morphology of the primaryequiaxed crystals. In Fig. 3 it can be observed that thelower silicon concentrations (1, 2, and 3 wt% Si) displaymore compact grains, and above 4 wt% Si the grain sizeincreases and dendrite tips become more visible withincreasing silicon content. At 5 wt% Si the grains arestill compact, and at 8 wt% Si well-developed dendriteswith secondary arms are clearly visible. These morpho-logical transitions are also clearly evident in the wellgrain refined samples shown in Fig. 4. Up to 3 wt% Si

Y.C. Lee et al. / Materials Science and Engineering A259 (1999) 43–52 47

Fig. 3. Micrographs showing the change in morphology with increasing silicon content for alloys with 0 wt% Ti. (a) 1 wt% Si; (b) 2 wt% Si; (c)3wt% Si; (d) 4 wt% Si; (e) 5 wt% Si; (f) 8 wt% Si.

the grains are very globular. Secondary dendrite armsbecome visible at 4 wt% Si (Fig. 4(d)) and at the twohigher silicon contents (5 and 8 wt% in Fig. 4 (e) and(f), respectively) they are very well developed. Theseobservations are the consequence of increasing volumefraction of eutectic with silicon content (more clearlydelineated dendrites) and the addition of grain refiner(small globular grains at low silicon contents).

4. Discussion

When the results for unrefined alloys are comparedwith those of the literature [4–9], Fig. 5, it is clearlyobserved that as the silicon content is increased, thegrain size initially decreases, reaches a minimum at 3wt% Si, and then increases. The decrease in grain size islarger than the increase observed after the minimum.

Y.C. Lee et al. / Materials Science and Engineering A259 (1999) 43–5248

Fig. 4. Micrographs showing the effect of silicon content on growth morphology in samples grain refined with 0.15 wt% Ti. (a) 1 wt% Si; (b)2wt% Si; (c)3 wt% Si; (d)4 wt% Si; (e)5 wt% Si; (f)8 wt% Si.

Adding grain refiner to the alloy results in the ex-pected reduction in grain size, Fig. 1 (a). It is interestingto note that the addition of 0.01 wt% Ti has thegreatest impact on grain size. Further additions of grainrefiner continue the observed trend, but the reduction ingrain size is less than for the lowest addition level (0.01wt% Ti). This is consistent with a cube root relationshipbetween the amount of nucleants added and the result-ing change in grain size. The 15-fold increase in grainrefiner addition between the lowest (0.01 wt% Ti) andhighest level (0.15 wt% Ti) added in this study will

result in a decrease in grain size of between two andthree times. Given the inherent error in grain sizemeasurements this appears to be validated by Fig. 1 (a)except for the 8% Si alloy. Curve-fitting of the results ofFig. 1 (a) gives an average power–law exponent of−0.32, which is very close to −1/3 for a cube rootrelationship. The effect of grain refiner is much greaterbefore the transition point than after, as can be seenfrom Fig. 1 (b). For silicon contents from 1 to 3% Sithe grain size is reduced to about 100–200 mm regard-less of silicon content.

Y.C. Lee et al. / Materials Science and Engineering A259 (1999) 43–52 49

Fig. 5. Comparison of grain size measurements on unrefined samples with increased silicon content from the current work and the literature.

Several investigators have shown curves illustratingthe effect of grain refinement on the transition point[4,6,7]. Fig. 6 compares all the results available forgrain refined alloys. It is observed that they all essen-tially fall within the same bounds. This is not unex-pected as similar experimental conditions and methodswere used by Tøndel [6], Backerud and Johnsson [7],Hutt et al. [8] and in this work. Fig. 6 suggests that thetransition point is not shifted as a result of grainrefinement but rather the grain size is reduced to arelatively constant level for silicon contents below 3%Si. Above 3% Si the grain size increases in all cases,

however some variation in absolute values is observed.The predominant variation is caused by Backerud andJohnsson’s results [7] where the grain size above 6% Siwas observed to increase with increasing grain refineraddition rather than decreasing as observed in thisstudy and other work [6,12]. It is difficult to find anexplanation for this difference.

Except for one set of data points (i.e. Ref. [10]), Fig.6 shows that the slope of grain size versus siliconcontent before the transition point is negligible and thegrain size does not increase until the silicon content isgreater than 3 wt%. At higher titanium levels the

Fig. 6. Comparison of grain size with increased silicon content of grain refined samples reported in the literature and the current work.

Y.C. Lee et al. / Materials Science and Engineering A259 (1999) 43–5250

Fig. 7. The expected distribution curve for undercooling required for activation of nucleants, (a) before the transition; and (b) after the transition.

change in grain size with further refiner addition, be-comes less significant as explained in the results section.However, the slope of the increase in grain size withsilicon content after the transition remains essentially thesame. This indicates that after the transition at 3% Si,silicon is having the predominant effect on grain sizeregardless of the amount of refiner added. Other factorsthat may influence the results are the decreasing solidifi-cation range and increasing superheat, and also latentheat, with increased silicon content. However, it is notclear how these factors could cause a transition at 3 wt%Si.

The morphological changes shown in Fig. 3 are inaccordance with the observations of Abdel-Reihim et al.[5], Backerud and Johnsson [7], and Hutt et al. [8].Backerud and Johnsson [7] suggested that these morpho-logical differences are the result of a cellular–dendritictransition in the growth of equiaxed crystals, similar tothat observed in columnar growth. However, this is notclearly supported by Figs. 3 and 4. For the grain refinedmaterial, Fig. 4, the grains are clearly globular with littleevidence of cells up to 3% Si, and at 4 and 5% themicrostructure appears more cellular while the dendriticnature is not present until 8% Si. However, the globulargrains are similar in size to the cells appearing in thehigher silicon content alloys and their size may thereforesimply be a function of grain size rather than a growthtransition. This is supported by Fig. 2 where increasedTi leads to an apparent transition in the oppositedirection (i.e. cellular to globular as the Ti contentincreases) because the grain size has become less than orequal to the cell size.

For the unrefined alloys, Fig. 3, it can be argued thatthere is a transition from cellular to dendritic grains withincreasing silicon content. However, closer examinationreveals that the differences are small. Comparison of Fig.3(a) and (f) shows that both microstructures exhibit adendritic morphology with the main differences being thevolume fraction of eutectic and a finer dendrite armspacing in the 8% Si alloy. Therefore, as noted in the

results section, the morphological differences are due todifferences in silicon content rather than differences ingrowth morphology.

The combined evidence provided by Figs. 2–4 showsthat the minimum in grain size is not associated with acellular–dendritic transition. Also, the appearance of amorphological transition is simply a function of differ-ences in grain size and volume fraction of eutectic.

If there is no transition in growth mode then why doesthe grain size increase beyond 3% Si? The degree ofconstitutional undercooling will increase as the siliconcontent is increased for a planar interface. If we assumethe casting conditions are constant and taking intoaccount the effect of thinner dendrites/dendrite arms andsmaller tip radii as the solute content increases, theconstitutional undercooling at the interface may continueto increase, change little, or decrease as silicon contentincreases. However, any of these possible changes areunlikely to result in a distinct transition in grain size. Thechange in grain size observed for Al–Zn alloys [9] is morelikely. Therefore, the transition in grain size may berelated to the number of activated nuclei. Fig. 1 (b) showsthat the smallest grain size and largest refinement can beobtained by adding grain refiner to the 1% Si alloy. Asthe silicon content is increased to 3% the grain size ismore or less constant indicating that the same numberof nucleants have been activated. After that the numberof nucleants activated is decreased.

In other work on grain refinement of aluminium [13]it was proposed that effective refinement occurs when thedistribution of nucleant potencies falls within the consti-tutionally undercooled zone, Fig. 7 (a). When thissituation has been achieved the grain size is not furtherdecreased by extra solute. Since the best refinementoccurs for the 1, 2, and 3% Si alloys, it suggests that thepotency distribution falls largely within the constitu-tional zone for each alloy. However, above 3% Si thegrain size begins to increase indicating that for a givennumber of nucleants fewer are activated. This means thatthe potency distribution has been changed or the consti-

Y.C. Lee et al. / Materials Science and Engineering A259 (1999) 43–52 51

tutional zone has been reduced so that many particleshave potencies outside the constitutional zone, Fig. 7(b).

Consideration of this mechanism explains the rela-tively flat slope for grain size versus silicon contentbefore 3% Si. Although adding 0.01% Ti refiner to 1%Si increases the growth restriction factor by a smallamount (5.9–8.4 for 1 wt% Si+0.01% Ti in solutioncompared with 17.7 for 3% Si) the level of refinementachieved is near the maximum observed. This is becausethe potency of the added nucleant particles is very high(i.e. low Si) and the level of solute is sufficient tomaximise nucleation (satisfying the requirements of Fig.7 (a)). As the silicon content increases from 1 to 3% Sithe nucleant potency begins to decrease but the growthrestriction factor also increases. Thus, approximatelythe same number of nucleant particles are activatedresulting in a relatively unchanged grain size.

When grain refiner is not added, the nucleants arenot effective (i.e. have a high nucleation undercoolingrequirement, DTn), and have a distribution whichspreads to high levels of undercooling similar to thatshown in Fig. 7 (b). Therefore as the silicon content isincreased the increased constitutional undercooling en-compasses more of the distribution hence decreasingthe grain size. However, as with the grain refined alloys,the nucleant potencies are being reduced as the siliconcontent increases. 3 wt% Si corresponds to the pointwhere the combination of reducing nucleant potenciesand changes in constitutional undercooling overlap giv-ing a minimum in grain size.

Spittle et al. [9] also suggested that nucleation couldplay a role in causing a change in grain size. Theyproposed that the nucleants may be affected by siliconinteracting with Ti and thus removing excess Ti, forma-tion of silicide coatings on the nucleants or modifica-tion of the composition of the nucleants. Since thetransition is observed in unrefined alloys as well refinedalloys, then the Ti-related causes are not relevant. Onesimpler reason for the change is that silicon is impedingthe nucleation of a-aluminium crystals as silicon hasvery low solubility in a-aluminium. Adding silicon islikely to affect interface stability by changing the solid/liquid surface tension and the entropy of fusion, i.e. theGibbs–Thompson coefficient. However, no quantita-tive information is available. Whatever the mechanism,as the amount of silicon increases the activation energyfor nucleation of nearly pure aluminium would in-crease, effectively reducing the apparent potency of thenucleant particles. This would explain why the sameeffect is observed when TiB2 is not present as well aswhen it is.

Based on the results reported in this work it seemstherefore appropriate to consider a division of the grainformation process into two regimes. Before the transi-

tion point it is a nucleation dependent effect (confirmedby the large decrease effected by grain refinement),where the increased constitutional undercooling pro-motes a reduction of grain size with increased siliconcontent. After the transition, the loss of potency of thenucleants relative to the constitutional effects becauseof higher levels of silicon in the liquid, begins to affectthe grain size leading to an increase in grain size. Thereason a loss in nucleant potency is not observed below3% Si is that although the distribution of potencies iscovering a larger range of undercooling with increasingsilicon content, this distribution still falls within thezone of constitutional undercooling. Above 3% Si thedistribution extends beyond the constitutional under-cooling leading to an increase in grain size. To confirmthe mechanisms suggested in this work it is necessary toquantify:1. the effect of silicon content on the constitutional

profiles at the solid–liquid interface; and2. the distribution of nucleant potencies and how they

are affected by silicon concentration for TiB2 grainrefiners, as well as the nucleants present in unrefinedalloys.

5. Conclusions

The effect of grain refiner additions, by AlTi5B1master alloy, and silicon content on grain size has beeninvestigated in hypoeutectic Al–Si alloys. Samples weresolidified in preheated graphite crucibles which gave acooling rate of 0.7 K s−1. The conclusions can besummarised as follows:� Without grain refiner additions, a minimum in grain

size was observed at 3 wt% Si. There is not a cleartransition in growth morphology from cellular todendritic equiaxed grains near the silicon contentcorresponding to the minimum in grain size. Themorphological transition seems rather to be a func-tion of the grain size;

� Addition of grain refiner was found to decrease grainsize for all silicon contents investigated. The firstgrain refinement addition (0.01 wt% Ti) gave thelargest decrease in grain size. Addition of furthergrain refiner (0.03, 0.05, 0.10, 0.15 wt% Ti) had asmaller effect on further grain refinement. Grain sizefollows a cube root relationship with the grainrefiner content;

� Addition of grain refiner was more efficient beforethan after the transition point. The grain size wasreduced to a more or less constant level for siliconcontents B3 wt% Si. The transition point remainedconstant at about 3 wt% Si regardless of the amountof grain refiner added;

� The slope of grain size versus silicon content afterthe transition point was essentially the same for all

Y.C. Lee et al. / Materials Science and Engineering A259 (1999) 43–5252

levels of added grain refiner indicating that siliconcontent is controlling the amount of refinementachieved above the transition.

These results indicate that the grain size is controlledby a combination of nucleant potency and constitu-tional conditions at the growing crystal interface.

Acknowledgements

Financial support from the CRC for Alloy and So-lidification Technology (CAST) is gratefully acknowl-edged. CAST was established under the AustralianGovernment’s Co-operative Research Centres Scheme.

References

[1] W. Kurz, D.J. Fisher, Fundamentals of Solidification, Trans-Tech Publications, Switzerland, 1989.

[2] D.G. McCartney, Int. Mat. Rev. 34 (1989) 247–260.[3] J. Hutt, D.H. StJohn, Int. J. Cast Met. Res. 11 (1998) 13–22.[4] P. Hoefs, W. Reif, W. Schneider, Giesserei 81 (12) (1994) 398–

406.[5] M. Abdel-Reihim, N. Hess, W. Reif, M.E.J. Birch, J. Mat. Sci.

22 (1987) 213–218.[6] P.A. Tøndel, PhD thesis, Norwegian Institute of Technology,

Trondheim, Norway, 1994.[7] L. Backerud, M. Johnsson, in: W. Hale (Ed), Light Metals 1996,

The Minerals, Metals and Materials Society, Warrendale, PA,1996, pp. 679–685.

[8] J. Hutt, D.H. StJohn, L. Hogan, A.K. Dahle, Mat. Sci. Tech.(1998), in press.

[9] J.A. Spittle, J.M. Keeble, M. Al-Meshhedani, in: R. Huglen(Ed.), Light Metals 1997, The Minerals, Metals and MaterialsSociety, Warrendale, PA, 1997, pp. 795–800.

[10] M. Johnsson, PhD thesis, University of Stockholm, Sweden,1993.

[11] L. Backerud, E. Krol, J. Tamminen, Solidification Characteris-tics of Aluminium Alloys: Wrought Alloys, vol. 1, Universitets-forlaget AS, Oslo, Norway, 1986.

[12] G.K. Sigworth, M.M Guzowski, AFS Trans. 93 (1985) 907–912.[13] M. Easton, D.H. StJohn, Met. Trans. A (1998) in press.

.