pressure effect on al-si alloy 413 and colour metallography

5/22/2018 Pressure Effect on Al-Si Alloy 413 and Colour Metallography

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Color Metallography and Electron MicroscopyTechniques Applied to the Characterizationof 413.0 Aluminum Alloys

George Vander Voort,1 Juan Asensio-Lozano,2,* and Beatriz Surez-Pea3

1Consultant, Struers Inc., 2887 N. Southern Hills Drive, Wadsworth, IL 60083-9293, USA2Materials Science and Metallurgical Engineering Department, The School of Mines, The University of Oviedo,

Oviedo, Asturias 33004, Spain3Materials Science and Metallurgical Engineering Department, The University of Oviedo, Gijn Polytechnic School

of Engineering, Viesques Campus, Gijn, Asturias 33203, Spain

Abstract: The influence on alloy 413.0 of the refinement and modification of its microstructure was analyzed

by means of several microscopy techniques, as well as the effect of the application of high pressure during

solidification. For each treatment and solidification pressure condition employed, the most suitable microscopy

techniques for identifying and characterizing the phases present were investigated. Color metallography and

electron microscopy techniques were applied to the qualitative microstructural analysis. Volume fraction and

grain size of the primarya-Al were characterized by quantitative metallographic techniques. The results showthat the effect caused by applying high pressure during solidification of the alloy is more pronounced than that

caused by modification and refinement of the microstructure when it solidifies at atmospheric pressure.

Furthermore, it has been shown that, for AlSi alloy characterization, when aiming to characterize the primary

a-Al phase, optical color metallography observed under crossed polarized light plus a sensitive tint filter is themost suitable technique. When the goal is to characterize the eutectic Si, the use of optical color metallography

or electron microscopy is equally valid. The characterization of iron-rich intermetallic compounds should

preferably be performed by means of backscattered electron imaging.

Key words: optical color microscopy, scanning electron microscopy, quantitative metallographic characteriza-

tion, cast aluminum alloys, grain refining, eutectic modification, high pressure die casting

INTRODUCTION

The 413.0~Al12Si!eutectic alloy exhibits excellent fluidityand a low tendency to develop microshrinkage, resulting insound components of high compactness ~Liao & Sun, 2003!.As constituents of the eutectic alloy differ notably in termsof their melting temperatures and their weight ratios, it isreferred to as an abnormal eutectic. These two aspects,together with physical and chemical heterogeneities occur-ring during solidification, determine the resulting micro-structure~Abbaschian et al., 2009!. Therefore, owing to therelationships between microstructure parameters and prop-erties, mechanical properties of the alloy can also be esti-mated~Vander Voort & Asensio-Lozano, 2009!.

In this particular eutectic system, the first constituentto solidify is the one with the higher melting point ~Si!,which will then be followed by the other constituent ~ a-Al!in the amount necessary for the residual liquid to maintainthe appropriate composition ~12.6 wt% Si!. The growth rateof the Si particles in the eutectic is faster than that ofa-Albecause of the greater driving force for solidification of thisphase ~this will be proportional to the temperature differ-ence between the solidification temperature of the constitu-ent phases in the eutectic and the eutectic temperature:14145778378C for silicon, whereas it will be proportional

to 660577838C for eutectic aluminum! ~Massalsky, 1986;Gruzleski, 2000a!. Consequently, the timetemperature

transformation curve for Si will be much closer to the timeorigin than that of Al. This results in rapid nucleation of theSi, which will develop quickly before being enveloped by ana-Al amount such that the solid reaches the eutectic com-position. As a result of nonequilibrium cooling and slightvariations in composition, the presence of polyhedral parti-cles of Si is often observed ~Nogita & Dahle, 2001!.

The commercial importance of near-eutectic AlSi al-loys has been a topic of many research projects. The eutecticsilicon in unmodified AlSi alloys is present as a flake-likemorphology and it has a deleterious effect on the ductility.Small additions of strontium lead a morphological transi-

tion from coarse flakes to fine fibers ~Nogita et al., 2006!.The benefits of modification with strontium are more im-portant in sand castings, although certain benefits are alsoobserved when the cooling rates increase ~Lim, 2009!. More-over, most of commercial eutectic AlSi alloys solidify witha large fraction of primary a-Al in their microstructures.Additions of titanium and boron have been used to try torefine thea-Al grains. The weight ratio needed to form thenucleating TiB2 particles is Ti:B 2.2:1, although most ofthe grain refiners present higher ratios of Ti:B, such as3Ti1B, 5Ti1B, and 5Ti0.2B. The Al5TiB is an efficientgrain refiner in pure aluminum and in alloys with a low Sicontent. However, when the content in silicon is higher than

Received August 2, 2012; accepted March 8, 2013

*Corresponding author. E-mail: [email protected]

Microsc. Microanal. 19, 10191026, 2013doi:10.1017/S1431927613000585 Microscopy AN D

Microanalysis MICROSCOPY SOCIETY OF AMERICA 2013


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or equal to 7 wt%, the refining power of this master alloy is

less than that of master alloys with a lower Ti:B ratio~Sritharan & Li, 1997; Murty et al., 2002!. On the otherhand, it is clear that improving the quality of the aluminumalloys requires a more in-depth understanding of the micro-structure development. Conventional etching techniques tra-ditionally used in microscopy observation does not allow ananalysis of the microstructural evolution of the phasespresent or the mechanisms of segregation and nucleationgoverning solidification of Al12Si alloys.

The present work analyzes the influence of the modifi-cation and refinement of the alloy 413.0 microstructure, aswell as the effect of the application of high pressure duringsolidification. For each treatment and solidification pressure

condition employed, the most suitable microscopy tech-niques for identifying and characterizing the phases presentwere investigated. Color metallography techniques ~VanderVoort, 2004a! employing light optical microscopy ~LOM!

~Vander Voort, 2004b! and scanning electron microscopy~SEM!were used as analytical tools.

MATE RIAL S AND EXPERIMENT ALPRO CE DURE S

Figure 1 describes the experimental procedure followed for

the different samples at different stages of the manufactur-ing process. Ingots of the 413.0 alloy composition fromAlcoa~Avils, Spain! were melted in an induction furnace.The melting temperature was in the range of 7407708C.Then, the liquid metal was degassed first with nitrogen andthereafter with argon; the degassing rate was;22 L/min for7 min. At this stage, a small amount of liquid was poured at;7208C into a mold ~gray iron, preheated to 3508C!.

The composition of the commercial alloy in the firststage of this study, coded Alloy 1, is shown in Table 1. Asecond cast was additionally obtained, coded Alloy 2~Table 2!, by adding 0.05 wt% Sr ~Al10Sr! and 0.05 wt%

Ti~Al3TiB! to the melt in a treatment ladle, as modifierand refiner of the microstructure, respectively. In thissecond stage, hot metal was also extracted to solidify underthe aforementioned atmospheric conditions.

Both heats, base alloy and grain-refined alloy, were firsttransferred to a holding furnace. Then, predeterminedweights of hot metal of both heats were transferred to thecold chamber metal injection molding machine, where solid-ification took place at 34 MPa. Components manufacturedfrom both alloys were subsequently set aside. Metallographicsamples were then obtained of the two alloys solidified un-der the aforementioned conditions.

Etching of the samples was performed at room tem-

perature, using in some cases an aqueous solution of HFat 0.5 vol% ~Warmuzek, 2004!, and in others the re-agent developed by Weck and Leistner ~1986!for aluminumalloys. This consists of a solution of potassium permanga-

Figure 1. Experimental procedure followed for the different sam-

ples at different stages of the manufacturing process.

Table 1. Chemical Composition* of the 413.0 Commercial Alloy~Alloy 1!.

Si Fe Cu Mn Mg Ni Zn Pb Ti Cr

12.90 0.82 0.04 0.20 0.01 0.006 0.03 0.02 0.008 0.013

All values expressed in wt%.

*Other products may pertain to the composition but are not listed, for example, Ag, Zr, Sb, Co, V, Be, B, Ca, etc.

Table 2. Alloy Identification Based on the Basic Chemistry in Ti and Sr Content. Quantitative Results for the Volume Fraction~VV!andMean Grain Size~ NA

a!ofa-Al in the Two Different Solidification Conditions Studied with Indication of the 95% Confident Limits of the

Determinations.

Solidification Pressure

0.1 MPa 34 MPa

Alloy Identification

Ti

~wt%!

Sr

~wt%!

VV~ a-Al! 6 CL95%~vol%!

NAa

6 CL95%~ mm2!

VV~ a-Al! 6 CL95%~vol%!

NAa

6 CL95%~ mm2!

Alloy 1~commercial alloy! 21.87 6 2.16 11,296 6 823 46.23 6 4.14 553 6 35

Alloy 2~refined and modified! 0.05 0.05 31.71 6 3.14 1,302 6 141 53.48 6 5.19 3076 23

1020 George Vander Voort et al.


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nate, sodium hydroxide, and water in the followingproportions:

Distilled water~100 mL!KMnO4~4 g!NaOH~1 g!

In some cases, several cycles of final polishing and etching

were needed to obtain a suitable contrast of the microcon-stituents ~Vander Voort, 1981!. The equipment used forobservations was a Nikon Epiphot~Nikon Instruments Inc.,Tokyo, Japan! inverted metallographic microscope con-nected to a Kappa ImageBase image analyzer ~Kappa optron-ics GmbH, Gleichen, Germany! for LOM, and a JEOL JSM6100 ~JEOL Ltd., Tokyo, Japan!for SEM.

The volume fraction and grain size of the primarya-Alwere characterized by quantitative metallographic techniques~Vander Voort, 1999; Muirhead et al., 2000; Roebuck, 2000;Higginson & Sellars, 2003!. A manual point counting tech-nique was used to obtain the volume fraction, whereas thegrain size was characterized by means of the semiautomatic

areal analysis technique ~ASTM E138297, 2010!. In orderto minimize statistical errors in the determinations, 250grains were counted for the volume fractions or, failing that,25 micrographs were assessed ~Vander Voort, 1994, 1996!.To determine the grain size of the a-Al, 400 grains wereevaluated or, failing that, 25 micrographs were assessed. Inboth cases, a relative error of less than 0.05 was sought.

RESULTS

Figures 2 and 3 show the microstructure of Alloy 1 solidi-fied at atmospheric pressure in a preheated chill cast mold.Observation under LOM was performed after etching withthe HF reagent in Figures 2a and 3a for black and white~B&W!imaging and using Wecks reagent in Figures 2b and3b for color imaging. Bright field ~BF! illumination wasemployed in Figures 2 and 3a, and polarized light plus thesensitive tint filter ~XPST! in Figure 3b ~Chapon & Szkli-niarz, 2001!. All the aforementioned micrographs confirmthe presence of primary aluminum embedded in an AlSi

Figure 2. Light optical microscopy microstructure of Alloy 1 so-lidified at atmospheric pressure in a preheated chill cast mold.

Observation performed after etching with: ~a! HF reagent and

~b!Wecks reagent. Bright field illumination was employed.

Figure 3. Microstructure of Alloy 1 solidified at atmospheric pres-sure in a preheated chill cast mold. Observation was performed:

~a!after etching with HF and bright field illumination and~b!af-

ter etching with Wecks reagent and polarized light plus sensitive

tint to filter.

Color LOM and SEM Applied to Al12Si Alloys 1021


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eutectic matrix. Furthermore, lamellae of the intermetalliccompoundb-AlFeSi can be observed in Figure 3b, appear-ing as needles on the polished plane of the micrograph overa yellow background. In Figure 2b, the color etching tech-nique reveals the presence of microsegregation in the pri-marya-Al, as evidenced by a darker orange coloring in thecenter and yellow coloring at the edges. This is observed inboth the primary and secondary dendrite arms. However,this technique does not allow us to identify a net boundarybetween the primary and the eutectic a-Al, and therefore

impedes the quantitative characterization of the volume frac-tions and grain size of this primary phase. Moreover, a lesspronounced staining found between the center and bordersofa-Al lamella in the eutectic suggests a less pronouncedsegregation compared with that found in primarya-Al. Thetechnique is appropriate for the unambiguous identificationof cuboidal and acicular Si via their gray coloring.

Study of the sample under LOM employing XPST~Fig. 3b! allows us to distinguish the interface between theAlSi eutectic phase and thea-Al dendrites. This techniqueis ideal for defining the outline of the a-Al dendrites,allowing precise quantitative characterization of this phase.

The technique also allows characterization of the intermetal-lic b-AlFeSi, which, as already mentioned, is seen on themicrograph as dark needles over a pale yellow background.This technique likewise enables us to appreciate the segrega-tion of the primarya-Al and its absence in the a-Al of theAlSi eutectic phase.

Figure 4 is a SEM-backscattered electron image ~BEI!micrograph of Alloy 1 and shows the appearance and out-line, both of the eutectic Si and the b-AlFeSi needles. Thistechnique allows us to differentiate both types of needles, asthose rich in Fe show a brighter contrast.

In industrial practice, modification and refinement ofthe microstructure is used to improve the mechanical prop-

erties of alloys of this type ~Asensio-Lozano et al., 2007!. Srbrings about changes in the morphology of the eutectic Si,which becomes globular ~Asensio-Lozano et al., 2005!. Italso leads to a decrease in the size of the Si cuboids and anincrease in the volume fraction of thea-Al phase~Sritharan& Li, 1997; Hengcheng et al., 2002!. Ti causes an increase inthe volume fraction and refinement of thea-Al phase~Liu

et al., 2004!. The results ~Table 2! show that there is asignificant decrease in the grain size of the primary a-Alphase in Alloy 2, solidified at atmospheric pressure in apreheated chill cast mold, when its microstructure is modi-fied with 0.05% Sr and refined with 0.05% Ti ~Asensio-Lozano et al., 2006!. The micrographs in Figure 5 correspondto the BF micrograph of Alloy 2 ~refined and modified!solidified at atmospheric pressure after the color etchingtechnique under LOM and reveal the segregation of theprimarya-Al. They also reveal two different sized popula-tions ofb-AlFeSi needles, which are made evident becausethe coarse fraction serves as the heterogeneous nucleationsites for primary a-Al dendrites, whereas a smaller sized

Figure 4. Backscattered electron image micrograph of Alloy 1

solidified at atmospheric pressure after etching with HF reagent

showing the appearance of the eutectic Si and the b-AlFeSi nee-dles. Those rich in Fe show brighter. Scale bar 10mm.

Figure 5. Bright field color etching technique under light optical

microscopy reveals the segregation in primarya-Al: ~a!two differ-

ent sized populations ofb-AlFeSi needles are observed and ~b! het-erogeneity occurs with modification of the eutectic Si.



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fraction appears well dispersed within the eutectic phase.The different degrees of modification of the eutectic phasecan be appreciated by the existence of regions of differentfineness of eutectic silicon ~Fig. 5b!. Figure 6 is an SEMmicrograph of Alloy 2, solidified at atmospheric pressure,after etching with HF. This figure shows the fragility of thesmaller b-AlFeSi needles, which appear fragmented at sev-eral points, and the power of heterogeneous nucleation thatthe larger needles exhibit over other phases of the system.

The high pressures required for pressure molding ofthese alloys can modify the information that emerges fromthe equilibrium diagrams. The results ~Fig. 7a and Table 2!show the variation in the microstructure of the alloy afterbeing subjected to forming via the injection molding tech-nique. The very high cooling conditions and high pressureexerted during solidification facilitate the development of amore refined uniformly dispersed a-Al phase comparedwith atmospheric solidification ~Fig. 2!. In contrast, Fig-ure 7b and the results in Table 2 show that the combinedeffect of Sr and Ti, when the alloy is solidified at highpressure, provides finer grains and a higher degree of homo-

geneity in the distribution of the phases present.The SEM micrograph in Figure 8 corresponds to Al-

loy 2 solidified at high pressure. It shows in detail theirregular outline of the eutectic Si as a result of a zig-zaggrowth mechanism leading to frequent changes of directionwithin individual particles during growth. Fe-based com-pounds are identified, once more, using BEI ~Gruzleski,2000b; Gndza et al., 2004!.

D ISCUSSION

The eutectic alloy has a Si content of approximately 12 wt%and its microstructure is formed by the eutectic of a-Al

with silicon. However, certain factors, such as the contentsof Fe, Mn, and Mg, and the solidification rate, may signifi-cantly alter the microstructure expected from the equilib-rium phase diagram, with the possible appearance of primarya-Al dendrites and silicon cuboids. Iron is one of the mostimportant impurities in aluminum alloys and is alwayspresent in die cast commercial alloys. Eutectic AlSi alloys

for pressure die casting include a certain percentage of ironadded to avoid the risk of welding to the dies.

In addition, the eutectic AlFeSi composition occursfor Fe contents of 0.8 wt%. It is not unusual to findrecommendations for the Fe content to range from 0.8 to1.1 wt% for these alloys. However, Fe can promote theformation of brittle plates ofb-AlFeSi or the other complexintermetallics in the presence of Mn. The Mg level is usuallyspecified as below 0.10 wt% to avoid the formation ofMg2Si, which results in deterioration of the tensile strength.~Gruzleski & Closset, 1999; Shankar & Apelian, 2002!.

In those structures adequately modified by the preciseaddition of Sr, the eutectic silicon is found in the form of


solidified at atmospheric pressure after etching with HF reagent

showing the fragility of the b-AlFeSi needles and the power ofheterogeneous nucleation that large needles exhibit. Scale bar 10mm.

Figure 7. Color etched microstructure~bright field!observed un-

der light optical microscopy reveals that the very high coolingconditions and high pressure exerted during solidification facili-

tates the development of a refined uniformly disperseda-Al phasein:~a!Alloy 1 and~b!Alloy 2.



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fine fibers. Refining of the a-Al phase grains by means ofthe addition of elements such as Ti and B is a commonindustrial practice. In the present work, the influence onalloy 413.0 of the modification and refinement of its micro-structure was analyzed by means of several microscopytechniques, as well as the effect of the application of highpressure during solidification. For each treatment and solid-ification pressure condition employed, the most suitablemicroscopy techniques for identifying and characterizing

the phases present were investigated. The most novel methodused was color etching, based on a reagent developed byWeck and Leistner ~1986! for LOM observation of alumi-num alloys.

Microstructure of Alloy 413.0 Solidified atAtmospheric Pressure

Color metallography applied to structures of Alloy 1 solidi-fied at atmospheric pressure in chill cast molds ~Figs. 2b, 3b!shows that the solidification of this quasi-eutectic alloy doesnot begin with the sequential nucleation of the eutectic Sianda-Al. Gruzleski~2000a!suggests the possibility that the

primarya-Al crystals are the first to nucleate~Fig. 9!, doingso below the eutectic temperature with a certain degree ofundercooling ~see point 2!. As these nuclei grow, the Sicontent in the fluid surrounding them increases, approach-ing the coupled zone ~shaded area in Fig. 9!. If thermo-dynamic and kinetic conditions are appropriate, the eutecticphase will eventually nucleate from the Si-enriched liquid~see point 3! in the area known as the coupled zone.Thus, when solidification is completed, the alloy has a pri-marya-Al alloy and a eutectic phase made up of Si polyhe-dral particles and needles surrounded bya-Al ~Figs. 2, 3!.During the solidification process, segregation phenomenaoccur in thea-Al dendrites, generating differences between

the electrochemical potential between periphery and core~Osorio et al., 2007!. The use of color metallography tech-niques has allowed us to highlight this phenomenon ofsegregation ~Figs. 2b, 3b! through differences in coloringevidencing the rejection of solute from the growing solid tothe liquid interface, resulting in dendrite microsegregation.It is also possible to appreciate that the eutectic phase doesnot exhibit microsegregation, as its solidification takes placeat constant temperature in a succession of liquid homogeni-zation microequilibrium stages. Needles of the intermetallic

b-AlFeSi compound can be observed accompanying the afore-mentioned phases, the presence of which can be appreciatedin the micrographs obtained by the color metallographic tech-

nique ~Figs. 2b, 3b!. These can also be distinguished usingthe BEI technique~Fig. 4!, the B&W technique~Figs. 2a, 3a!being suitable only for their qualitative evaluation. The BEImicrographs allow us to differentiate the needles of the Fe-rich compound from those of the eutectic Si and reveal theirsequence of formation during solidification~Fig. 4!. In fact,eutectic Si needles can be seen nucleated on lamellae or rodsof intermetallicb-AlFeSi, indicating that these Fe compoundswere formed before the eutectic phase.

The results in Table 2 show that the size of the primarya-Al in Alloy 2 has been reduced by about 88% comparedwith the commercial alloy with no modification or refine-

ment ~Alloy 1!. Furthermore, the volume fraction ofa-Alhas increased by 10%, underlining the a-forming nature ofthe Ti. The colorings adopted by the microconstituents afteretching also highlight the segregation of dendrites in Alloy 2~Fig. 5!. These colorings show that optical color metallogra-phy facilitates the identification of poorly modified regionsin the eutectic phase, which present a darker tone~Fig. 5b!.

Detailed analysis of the Fe-rich intermetallic com-pound in Alloy 2 reveals the existence of two populationsfor the needle size of this phase, which constitutes a distin-guishing feature with respect to Alloy 1. On the one hand,thick needles grew in the liquid phase, which can act asnucleating agents of the primarya-Al and the eutectic Si


solidified at high pressure after etching with HF reagent showing

the irregular outline of the eutectic Si originated in its zig-zaggrowth mechanism. Scale bar10mm.

Figure 9. Solidification route for Al12Si alloy after Gruzleski

~2000a!.



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phases ~Figs. 5a, 6!. Some are seen trapped between den-drites of the latter phase, denoting their proeutectic nature.On the other, there are small amounts of thin needles ofthis intermetallic compound, of similar coloring, intermin-gled with the eutectic phase ~Figs. 5, 6!. Precipitation ofprimaryb-phase needles in the liquid is a result of oversat-uration of Fe in the presence of the Sr added to the mixture.Sr causes generalized undercooling of the eutectic phaseand a shift in its composition towards higher Si contents~Tenekedjiev et al., 1995!. This makes the alloy slightlyhypoeutectic and favors the formation of primarya-Al onthe pre-existing Fe needles ~Narayan et al., 1994!. Owing tothe low solubility of Fe in Al, the a-Al dendrites continue to

grow, ejecting Fe into the residual liquid. On reaching theeutectic temperature, the Fe needles precipitate once again.As this occurs at lower temperatures, these needles aresmaller and more abundant than those formed previously.

Effect of Solidification at High Pressure on theMicrostructure of Alloy 413.0

Previous research ~Suarez-Pea et al., 2007! concluded thatan increase in pressure during solidification leads to anincrease in the temperature of solidification and the shiftingof the eutectic point toward higher Si contents ~Fig. 10!. Asoccurred in the samples solidified at atmospheric pressure

in the presence of Sr ~Alloy 2!, this results in hypoeutecticsolidification. It can be seen ~Table 2! that a more thantwofold increase in the volume fraction of primarya-Al isproduced in Alloy 1, allowing us to estimate the eutecticcomposition at 18 wt% Si under the action of pressure.

Comparing the volume fraction of the a-Al phase inAlloy 1 solidified at high pressure with that of Alloy 2solidified at atmospheric pressure ~Table 2!, we find that theformer is 15% higher. Furthermore, after solidification at apressure of 34 MPa, the grains of the a-Al phase exhibit analmost equiaxial morphology~Fig. 7a!, with sizes 57% smallerthan those obtained when the alloy is cooled at atmosphericpressure after modification and refinement ~Fig. 5!. In con-

clusion, the effect of pressure on the a-Al phase is morepronounced than that of modification and refinement insamples solidified at atmospheric pressure.

When these alloys solidify at a high pressure, modifica-tion and refinement do not bring about significant changesin the morphology of thea-Al phase, which remains equi-axial. However, an increase of around 7% in the volume

fraction of this phase can be appreciated together with adecrease in grain size of 45% ~Fig. 7, Table 2!. This showsthe complementary effect of pressure jointly with modifica-tion and refining on the microstructural changes of thea-Al primary phase. Changes in the morphology and size ofthe eutectic Si can also be observed with a globular morphol-ogy~point-like eutectic!, although these globules are not asfine as those found in the modified and refined alloy ~Al-loy 2!cooled at atmospheric pressure~Figs. 5, 7b!.

Electron microscopy~Fig. 8! confirms the tendency ofthe Fe-rich intermetallic compound to develop in acicularform; in particular, its preferential development in the eu-tectic region of Alloy 2 solidified at high pressure.

CONCLUSIONS

The present investigation analyzes the influence on 413.0alloy of the refinement and modification of its microstruc-ture, as well as the effect of the application of high pressureduring solidification. In addition, it reports a novel tech-nique for the qualitative and quantitative characterizationof the phases present. The methodology employed, whichcombines the use of color metallography and electron micros-copy, provides a valuable tool for researching and develop-ing AlSi-based alloys.

In summary, the conclusions of the study are summa-rized as follows:

1. Within the limit of the experimental parameters beingused in the present study, it can be stated that the effectcaused by applying high pressure during solidification ofeutectic AlSi alloys is more pronounced than that causedby modification and refinement of the microstructurewhen it solidifies at atmospheric pressure.

2. When aiming to characterize the primary a-Al phase,optical color metallography observed under crossed XPSTis the most suitable technique.

3. When the goal is to characterize the eutectic Si, the use ofoptical color metallography or electron microscopy isequally valid.

4. The characterization of iron-rich intermetallic com-pounds should preferably be performed by means ofbackscattered electron imaging.

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Figure 10. Modification of equilibrium AlSi phase diagram ~p1 atm!by the effect of high pressure applied during solidification;

and simplified microstructure evolution for Al12Si from the

liquid to the solid state.



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