Effect of Alloying Elements on the Sr Modification of Al-Si Cast Alloys
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metals
Article
Effect of Alloying Elements on the Sr Modification of Al-Si
Cast Alloys
Elisa Fracchia 1, * , Federico Simone Gobber 1 and Mario Rosso 2
1 Department of Applied Science and Technology (DISAT), Polytechnic of Turin, Viale T. Michel 5,
15121 Alessandria, Italy; federico.gobber@polito.it
2 Department of Applied Science and Technology (DISAT), INSTM c/o Polytechnic of Turin, Viale T. Michel 5,
15121 Alessandria, Italy; mario.rosso@formerfaculties.polito.it
* Correspondence: elisa.fracchia@polito.it; Tel.: +39-0131-2293-58
Abstract: Strontium-based modifier alloys are commonly adopted to modify the eutectic silicon
in aluminum-silicon casting alloys by changing the silicon shape from an acicular to a spherical
form. Usually, the modifier alloy necessary to properly change the silicon shape depends on the
silicon content, but the alloying elements’ content may have an influence. The AlSr10 master alloy’s
modifying effect was studied on four Al-Si alloys through the characterization of microstructural
and mechanical properties (micro-hardness and impact tests). The experimental results obtained
on gravity cast samples highlighted the interdependence in the modification of silicon between the
Si content and the alloying elements. After modification, a higher microstructural homogeneity
characterized by a reduction of up to 22.8% in the size of intermetallics was observed, with a
generalized reduction in secondary dendritic arm spacing. The presence of iron-based polygonal-
shaped intermetallics negatively affects Sr modification; coarser silicon particles tend to grow close
to α-Fe. The presence of casting defects such as bifilm reduces Sr modification’s beneficial effects,
and little increase in absorbed impact energy is observed in this work.
Keywords: Sr modification; EN AC 45300; EN AC43500; EN AC 48000; EN AC 42100; microstructures;
Citation: Fracchia, E.; Gobber, F.S.; impact toughness
Rosso, M. Effect of Alloying Elements
on the Sr Modification of Al-Si Cast
Alloys. Metals 2021, 11, 342. https://
doi.org/10.3390/met11020342
1. Introduction
Academic Editor: Gunter Gerbeth
In recent years, aluminum-silicon casting alloys have been given high consideration
Received: 11 January 2021 thanks to their excellent mechanical properties and generally good corrosion resistance [1,2].
Accepted: 13 February 2021 Moreover, these alloys are characterized by high fluidity, good weldability, and low thermal
Published: 19 February 2021 expansion coefficient [3].
Several fields of application adopt aluminum alloys or aluminum composites [4],
Publisher’s Note: MDPI stays neutral especially the automotive industry [5,6], where the light weight of aluminum alloys allows
with regard to jurisdictional claims in the reduction in cars’ weights and, consequently, greenhouse gas emissions [7]. From this
published maps and institutional affil- point of view, it is not surprising that a great deal of publications consider aluminum-
iations. silicon alloys and aluminum composites or functionally graded materials for automotive
applications, especially adopted to manufacture powertrain components such as pistons by
traditional techniques [8,9] or by powder metallurgy [10,11], and process-related issues [12].
Thanks to alloying elements such as Mg and Cu, aluminum-silicon alloys are susceptible
Copyright: © 2021 by the authors. to heat treatment to achieve better mechanical properties. Heat treatment usually involves
Licensee MDPI, Basel, Switzerland. a first step where solubilization takes place, as the alloying elements solubilize into the
This article is an open access article α-aluminum matrix, and a second step of artificial aging, where intermetallic phases
distributed under the terms and nucleate into the α-aluminum matrix, thus increasing mechanical properties [13]. During
conditions of the Creative Commons heat treatment, silicon shape modification takes place, and the eutectic silicon morphology
Attribution (CC BY) license (https:// changes from acicular to spherical; the magnitude of this transformation depends on the
creativecommons.org/licenses/by/ heat treatment parameters (times and temperatures) [14].
4.0/).
Metals 2021, 11, 342. https://doi.org/10.3390/met11020342 https://www.mdpi.com/journal/metals
Metals 2021, 11, 342 2 of 19
On the other hand, several chemical elements can promote the modification of eutectic
silicon [15] instead of heat treatment: Sr, Na, K, Rb, Ca, Ce, La, Ba, and Yb. In [16], the
authors investigated the effect of Ba, Ca, Y, and Yb on 356.0 alloy, observing that these
elements modify the eutectic silicon to different degrees. Ba and Ca lead to a good or very
good modification of Si into a fine fibrous silicon structure, especially Ba, which led to the
best modification among the elements analyzed for the specific alloy composition. Y and
Yb caused a plate-like structure, and the same result was underlined in [17] for alloy A357
(EN AC 42100). When aluminum-silicon alloys were modified by Yb and Sr, the nucleation
temperature of the eutectic silicon decreased, its shape changed from a coarse plate-like
to a fine fibrous structure [18]. As regards Eu [19], research has evidenced a very good
modification already with 0.05% w.t. Eu in Al-5% Si alloy. Addition of 1% w.t. Ce [20] led
to a partial eutectic modification, while the addition of 1% w.t. Ce and 0.04% w.t. Sr can
modify the eutectic structure.
In [21,22], the authors studied the relation between the increase in Sc levels and
the refining and modification effects in Al-Mg2 Si alloys and found that the silicon shape
evolves from a rough plate-like shape to a thin coral-like and fine fibrous morphology.
Al-Si alloys are commonly modified in industrial practice by employing the master
alloy Al-Sr10 [23,24]. Experimental evidence demonstrated that Sr modification’s effective-
ness depends on both the Sr quantity and the metal quality. High metal quality (minor
oxide into the melt) requires a low Sr amount [25]. As proposed in [26], the eutectic phase
and the intermetallic phase β-Al5FeSi nucleate on the same nuclei; it is conceivable that Fe
affects the eutectic phase. In fact, in [27], it was demonstrated that the number of eutectic
grains in Sr-modified alloys containing 10% w.t. silicon is lower when the amount of Fe
is higher. As regards mechanical properties, in [28], the authors investigated the effect of
refining the Sr-modified A356.2 alloy on the mechanical properties, finding an increase
in impact-released energies with the decrease in the grain size after Sr addition. In [29],
Sr addition was investigated in the alloy EN AC 46000, and it was found that Sr causes
a substantial improvement in tensile properties compared to the as-cast properties. The
addition of 276 ppm of Sr increases ultimate tensile strength from 300 to 350 MPa and
causes an increase in elongation at rupture from 3% to 6%. Furthermore, a decrease in
eutectic temperature nucleation was observed and outlined in alloy A319 [30]. The same
behavior was outlined in [31] for high-purity hypoeutectic alloys.
In this work, four aluminum-silicon casting alloys were analyzed. Samples were
studied in the unmodified condition or after the use of the master alloy AlSr10. Alloys were
cast by gravity casting, and samples were prepared and analyzed in terms of microstructure
(silicon shape and intermetallic fraction) and mechanical properties (micro-hardness and
impact tests) to find a relationship between the modification and all of these parameters.
Intermetallic phase evolutions after the Sr adoption were also investigated to highlight how
the various intermetallic phases are affected by Sr for each alloy. Based on the literature,
the main intermetallics found in the alloys are reported in Table 1. The goal of the work is
to highlight relationships between eutectic characteristics and Sr addition.
Table 1. Intermetallic phases expected in the alloys studied from literature [32–35].
Alloy Intermetallic Phases Alloy Intermetallic Phases
Alloy 1—EN AC 45300 θ-Al2 Cu; β-Mg2 Si; β-Al5 FeSi; Alloy 3—EN AC 43500
α-Al8 Fe2 Si; β-Al5 FeSi; β-Mg2 Si;
(alloy 1) α-Al15 (Fe,Mn)3 Si2 (alloy 3)
β-Mg2 Si; ε-Al3 Ni; θ-Al2 Cu;
β-Mg2 Si; α-Al8 Fe2 Si;
Alloy 2—EN AC 42100 Alloy 4—EN AC 48000 α-Al15 (Fe,Mn)3 Si2 ; β-Al5 FeSi; α-Al8 Fe2 Si;
β-Al5 FeSi; α-Al15 (Fe,Mn)3 Si2 ;
(alloy 2) (alloy 4) Al2 CuMg; Al9 FeMg3 Si5 ;
π-Al8 Mg3 FeSi6 ; Al9 FeMg3 Si5
π-Al8 Mg3 FeSi6 ; Q-Al5 Cu2 Mg8 Si6 ;
Metals 2021, 11, 342 3 of 19
2. Materials and Methods
The effect of AlSr master alloy addition to four aluminum-silicon alloys with different
silicon contents was studied in this work. Three alloys belong to the hypoeutectic alloys
group—EN AC 42100 (AlSi7Mg0.3), EN AC 45300 (AlSi5Cu1Mg), and EN AC 43500
(AlSi10MnMg)—while the fourth alloy belongs to the eutectic system, EN AC 48000
(AlSi12CuNiMg). Compositional details for the alloys, from now on named alloy 1, alloy 2,
alloy 3, and alloy 4, are shown in Table 2. The composition data refer to standard 1706 and
technical datasheets.
Table 2. Alloys’ composition from regulation EN 1706 and technical datasheets.
EN AB/AC 45300-AlSi5Cu1Mg (Alloy 1)
Elements Si Fe Cu Mn Mg Ni Zn Pb Sn Ti Al
Min. 4.5 - 1.0 - 0.40 - - - - -
Res.
Max. 5.5 0.55 1.50 0.55 0.65 0.25 0.15 0.15 0.05 0.20
Mechanical properties A% = 1 ÷ 2% Rm = 145 ÷ 175 MPa Rp0.2 = 125 ÷ 145 MPa
EN AB/AC 42100-AlSi7Mg0.3 (Alloy 2)
Elements Si Fe Cu Mn Mg Zn Ti Al
Min. 6.5 - - - 0.30 - 0.10
Res.
Max. 7.5 0.15 0.03 0.10 0.45 0.07 0.18
Mechanical properties A% = 2 ÷ 6% Rm = 140 ÷ 220 MPa Rp0.2 = 80 ÷ 140 MPa
EN AB/AC 43500-AlSi10MnMg (Alloy 3)
Elements Si Fe Cu Mn Mg Zn Ti Al
Min. 9.0 - - 0.40 0.15 - -
Res.
Max. 11.50 0.20 0.03 0.80 0.60 0.07 0.15
Mechanical properties A% = 5% Rm = 250 MPa Rp0.2 = 120 MPa
EN AB/AC 48000-AlSi12CuNiMg (Alloy 4)
Elements Si Fe Cu Mn Mg Ni Zn Ti Al
Min. 10.50 - 0.80 - 0.90 0.70 - -
Res.
Max. 13.50 0.60 1.50 0.35 1.50 1.30 0.35 0.20
Mechanical properties A% = 2 ÷ 6% (T5) Rm = 200 (T5) MPa Rp0.2 =185 (T5) MPa
All of these alloys can be reinforced by precipitation hardening because they contain
a certain Mg tenor (reinforcing by nucleation of Mg2 Si intermetallics); alloys contain-
ing Cu can additionally form Al2 Cu and AlCuMg phases [36], and Ni-containing alloys
can form Al3 Ni and other complex intermetallic phases [37]. The presence of Fe and
Mn leads to obtaining Fe-based intermetallic phases such as Al5 FeSi, Al8 FeMg3 Si6 , and
Al15 (FeMn)3 Si2 [38], as reported in Table 1.
Each alloy was separately melted in a graphite crucible, and 250 ppm AlSr10 master
alloy was added for the modification 10 minutes before casting. Casting temperatures
were set to 710 ◦ C for alloy 1 and alloy 2 and 730 ◦ C for alloy 3 and 4. No drossing
agents and fluxing agents were used. Castings obtained (eight in total: four modified and
four unmodified) were cut into smaller samples of 25 mm × 25 mm × 10 mm and then
polished using SiC papers from 180 to 2400 grit, diamond paste, and colloidal silica for
microstructural analysis and micro-hardness measurements.
After polishing, the microstructures were investigated using an optical microscope
(optical microscope LEICA MEF4M, Leica Microsystems, Heerbrugg, Switzerland), and the
eutectic area percentage, eutectic particles’ shape, and particles’ dimensions were measured
at three different parts of each sample using an image analysis software (LEICA QWin,
version 3.5, Leica Microsystems, Heerbrugg, Switzerland). It is important to note that
the measured morphological features (eutectic average particle size, percentage of the
occupied area, and circularity) included intermetallic phases and silicon. Intermetallic
Metals 2021, 11, 342 4 of 19
Metals 2021, 11, x FOR PEER REVIEW 4 of 19
phaseareas
phase areaswere
werethen
thenmeasured
measuredusing
usingananimage
imageanalysis
analysisalgorithm
algorithmwith
withthe
theopen-source
open-source
software ImageJ.
software ImageJ.
Theeutectic
The eutecticarea
areapercentage,
percentage,which
whichisisthetheeutectic
eutecticpercentage
percentagecalculated
calculatedin
inaaspecific
specific
area, was measured for each alloy using three micrographs at magnification 20X. The The
area, was measured for each alloy using three micrographs at magnification 20X. eu-
eutectic
tectic shape,
shape, particle
particle dimensions,
dimensions, andandintermetallic
intermetallicdimensions
dimensionswereweremeasured
measured using
using
threeimages
three imagesatat50X
50Xmagnification
magnificationforforeach
eachsample,
sample,andandfinally,
finally, secondary
secondary dendrite
dendrite arm
arm
spacing (SDAS) was measured as reported in ref. [39] by measuring thirty
spacing (SDAS) was measured as reported in ref. [39] by measuring thirty dendrites for dendrites for
each sample, employing three images at 10X magnification. Details about
each sample, employing three images at 10X magnification. Details about the SDAS meas- the SDAS
measurement
urement and analysis
and image image analysis
are shownareinshown
Figurein 1. Figure
Alloys' 1. Alloys’ micro-hardness
micro-hardness was
was calculated
calculated as an average value from ten indentations for each specimen (parameters: 15 s,
as an average value from ten indentations for each specimen (parameters: 15 s, 500 gf, HV
500 gf, HV 0.5) using a LEICA VMHT tester, Leica Microsystems, Heerbrugg, Switzerland.
0.5) using a LEICA VMHT tester, Leica Microsystems, Heerbrugg, Switzerland. Micro-
Micro-hardness was adopted instead of macro-hardness because smaller indentations may
hardness was adopted instead of macro-hardness because smaller indentations may per-
permit observing the AlSr10 effect in eutectic regions.
mit observing the AlSr10 effect in eutectic regions.
Figure
Figure1.1.Secondary
Secondarydendrite
dendritearm spacing
arm (SDAS)
spacing measurement
(SDAS) measurement andand
image analysis.
image Left Left
analysis. image: SDASSDAS
image: measurement pro-
measurement
cedure
procedure as in [39]. Center and right images: image analysis details on the eutectic phase (average size, circularity, %area
as in [39]. Center and right images: image analysis details on the eutectic phase (average size, circularity, % area
occupied). Optical images at 50X magnification.
occupied). Optical images at 50X magnification.
Since
Sinceimpact
impacttests
testscan
canestimate
estimatethe
theductility
ductilityof
ofalloys
alloysunder
underconditions
conditionsof ofrapid
rapidload-
load-
ing
ing[23],
[23],these
thesetests
testswere
wereconducted
conductedto toassess
assessthe
theeffectiveness
effectivenessof ofthe
themodifications.
modifications.Tests
Tests
were
were performed using three
performed using threespecimens
specimensfor foreach
eachtype
type of of casting
casting (modified
(modified andand unmodi-
unmodified),
fied), milled at a size of 10 mm × 10 mm × 55 mm, following the
milled at a size of 10 mm × 10 mm × 55 mm, following the ASTM E29-16b standardASTM E29-16b standard
TG5113E,
TG5113E,ZwickZwickRoell,
Roell,Genova,
Genova,Italy.
Italy. Expected
Expectedabsorbed
absorbedenergies
energiesare arelow,
low, as
as noticeable
noticeable
from
from previous work where the energy absorbed for aluminum composite samples (alloys
previous work where the energy absorbed for aluminum composite samples (alloys
EN
ENACAC48000
48000andandENENAC AC42100)
42100)was,
was,atatmaximum,
maximum,20 20JJ[40].
[40].Moreover,
Moreover,other
otherresearch
research inin
the
theliterature
literaturereported
reportedabsorbed
absorbedenergy
energyof ofalmost
almost15 15JJinin[28],
[28],aamaximum
maximumof of77JJfor
for alloy
alloy
356.0
356.0inin[41],
[41],and
andananaverage
averagevalue
valueofof77JJfor
forvarious
various alloys
alloys AlSi10
AlSi10 [42]
[42].
3.3.Results
Resultsand
andDiscussion
Discussion
3.1. Microstructures andIntermetallic
3.1. Microstructures and IntermetallicPhases
Phases
Microstructuresininunmodified
Microstructures unmodifiedalloys
alloys depend
depend onon
thethe intermetallic
intermetallic phases,
phases, whichwhich
de-
depend on the alloying elements. As previously mentioned, the alloys studied
pend on the alloying elements. As previously mentioned, the alloys studied in the present in the
present
work workacontain
contain certain aamount
certainof
amount
Fe, Mn,ofand
Fe,Mg.
Mn,Additionally,
and Mg. Additionally, alloy
alloy 1 and alloy1 4and alloy
contain
4 contain
Cu and Ni Cu and Ni (as
(as reported reported
in Table in Table
2). The 2). The microstructures
microstructures of the
of the unmodified andunmodified
modified
and modified alloys are reported in Figure 2. Some intermetallic phases are detectable
alloys are reported in Figure 2. Some intermetallic phases are detectable by optical micros- by
optical microscopy. The amount of intermetallic phases in the alloys and their influence
copy. The amount of intermetallic phases in the alloys and their influence on mechanical
on mechanical properties depend mainly on the alloys’ compositions. Some of these
properties depend mainly on the alloys’ compositions. Some of these intermetallic phases
intermetallic phases are known to be detrimental [43] as β-Al5 FeSi, increasing the iron
are known to be detrimental [43] as β-Al5FeSi, increasing the iron content, causes elonga-
content, causes elongation reduction. Fe-β intermetallic phases were profusely found in
tion reduction. Fe-β intermetallic phases were profusely found in alloys 1 and 3. Cu sub-
alloys 1 and 3. Cu substantially influences both strength and hardness, already in the
stantially influences both strength and hardness, already in the as-cast state [44].
as-cast state [44].
Metals 2021, 11, 342 5 of 19
Metals 2021, 11, x FOR PEER REVIEW 5 of 19
Figure
Figure 2.
2. Optical
Optical microscope
microscope images
images at
at 20X magnification. Intermetallic
20X magnification. Intermetallic phases
phases that
that nucleated
nucleated in
in the
the alloys.
alloys. The
The arrows
arrows
indicate
indicate some intermetallic phases. A: Eutectic silicon; B: α-aluminum matrix; C: acicular Fe intermetallic; D: Cu-based
some intermetallic phases. A: Eutectic silicon; B: α-aluminum matrix; C: acicular Fe intermetallic; D: Cu-based
phase; E: Chinese script; F: Q-phase; G: Mg2Si.
phase; E: Chinese script; F: Q-phase; G: Mg2 Si.
3.2. Micro-Hardness
3.2. Micro-Hardness
Micro-hardness measurements were carried out in the middle of the samples, and
Micro-hardness measurements were carried out in the middle of the samples, and
the results are reported in Figure 3. Micro-hardness varies slightly after adding Al-Sr; for
the results are reported in Figure 3. Micro-hardness varies slightly after adding Al-Sr;
alloys 1 to 3, a subtle increase in micro-hardness was observed after Al-Sr addition, while
for alloys 1 to 3, a subtle increase in micro-hardness was observed after Al-Sr addition,
micro-hardness decreased for alloy 4 after Al-Sr addition. Such differences in micro-hard-
while micro-hardness decreased for alloy 4 after Al-Sr addition. Such differences in micro-
ness are relatively narrow, and a neat, systematic trend was not observed. Positively, in-
hardness are relatively narrow, and a neat, systematic trend was not observed. Positively,
termetallic phases affect the alloys' hardness; particularly, alloys with higher Cu, Fe, Ni,
intermetallic phases affect the alloys’ hardness; particularly, alloys with higher Cu, Fe, Ni,
and Mg contents can form harder intermetallic phases than other alloys. Based on the al-
and Mg contents can form harder intermetallic phases than other alloys. Based on the
loying elements shown in Table 2 and intermetallic phases listed in Table 1, alloy 1 and
alloying elements shown in Table 2 and intermetallic phases listed in Table 1, alloy 1 and
alloy
alloy 44 are
are expected
expected to
to be
be the
the hardest ones because
hardest ones because they
they contain
contain strengthening
strengthening elements
elements
such
such as Cu, Mg, Ni, Fe, and Mn; moreover, alloy 4 is the hardest because
as Cu, Mg, Ni, Fe, and Mn; moreover, alloy 4 is the hardest because it
it contains
contains the
the
highest quantity of silicon. For the same reasons, alloy 3 is harder than alloy 2 because it
contains a higher Si amount.
Metals 2021, 11, x FOR PEER REVIEW 6 of 19
Metals 2021, 11, 342 6 of 19
A larger dispersion in the measured micro-hardness was observed for the hardest
alloys (alloys 1 and 4); such variability is due to the significant difference between the
micro-hardness measured near the hard intermetallics and that measured in the soft α-Al
highest quantity of silicon. For the same reasons, alloy 3 is harder than alloy 2 because it
matrix (maximum and minimum values measured were 100 and 72 HV0.5, respectively,
contains a higher Si amount.
in alloy 1 and 115 and 81 HV0.5, respectively, in alloy 4).
Micro-hardnessmeasurements
Figure 3. Micro-hardness measurementsforfor allall
thethe alloys,
alloys, unmodified
unmodified (left(left
side)side) and modified
and modified (right(right
side). side).
ValuesValues re-
reported
are average
ported values. values.
are average The table reports
The table all micro-hardness
reports measurements.
all micro-hardness Alloy 1: 86-93
measurements. AlloyHV0.5 = α-Al;
1: 86-93 HV0.5from 93 to from
= α-Al; 100 HV0.5
93 to
= eutectic;
100 HV0.5 up to 100 HV0.5
= eutectic; up to =100
intermetallic phases. Alloy
HV0.5 = intermetallic 2: 65–70
phases. HV0.5
Alloy 2: =65–70
α-Al;HV0.5
up to 70 HV0.5up= eutectic.
= α-Al; to 70 HV0.5Alloy 3: 70–80
= eutectic.
HV0.53:= 70–80
Alloy α-Al; HV0.5
from 80= to 83 HV0.5
α-Al; from 80 = eutectic silicon;
to 83 HV0.5 up to 83
= eutectic HV0.5up
silicon; = intermetallic
to 83 HV0.5 =phases. Alloy 4:
intermetallic 96–100Alloy
phases. HV0.5 = α-Al;
4: 96–100
from 100 to 110 HV0.5 = eutectic silicon; up to 110 HV0.5 = intermetallic phases.
HV0.5 = α-Al; from 100 to 110 HV0.5 = eutectic silicon; up to 110 HV0.5 = intermetallic phases.
Micro-hardness
A larger dispersion appears
in thedependent
measuredon alloying elements.
micro-hardness This dependence
was observed is out-
for the hardest
lined
alloysin(alloys
Figure14.and
Only4);Si, Fe, variability
such Mg, and Cuiswere
due toconsidered, because
the significant these elements
difference between affect
the
the alloys' micro-hardness.
micro-hardness measured near In Figure 4A,intermetallics
the hard a parabolic trendand between Si content
that measured in theand micro-
soft α-Al
hardness values is and
matrix (maximum noticeable.
minimum Despite
valuesthe increasewere
measured in Si 100
tenor,
andalloy 2 and respectively,
72 HV0.5, alloy 3 present in
less
alloymicro-hardness
1 and 115 and 81 than alloyrespectively,
HV0.5, 1. This behavior was4).due to their low amount of alloying
in alloy
elements. In fact, in appears
Micro-hardness Figure 4B–D, increases
dependent in micro-hardness
on alloying elements. This bydependence
parabolic trends with
is outlined
Mg addition,
in Figure Fe addition,
4. Only andand
Si, Fe, Mg, Cu addition, respectively,because
Cu were considered, in modified
theseorelements
unmodified alloys
affect the
are visible.
alloys’ The Fe effect In
micro-hardness. could be well
Figure 4A, approximated
a parabolic trend by abetween
linear trend too. Furthermore,
Si content and micro-
trends
hardness in values
Figure is
4E–H furtherDespite
noticeable. clarify the
the impact
increaseofinFe, Cu, and
Si tenor, alloyMg2 onandmicro-hardness.
alloy 3 present
less micro-hardness
Alloy 2 and alloy 3 havethanlower
alloyMg
1. This behavior
and Fe contents,was due these
while to their low amount
elements cause anof alloying
increase
elements.
in In fact, in Figure 4B–D, increases in micro-hardness by parabolic trends with
micro-hardness.
Mg addition,
Figure 4D Feclearly
addition,
shownandthat
Cu addition, respectively,
the increase in modified
in micro-hardness or unmodified
is related alloys
to the presence
are visible. The Fe effect could be well approximated by a linear trend
of Cu. The higher the average Cu amount, the higher the micro-hardness, as observed for too. Furthermore,
trends
all in alloying
of the Figure 4E–H further
elements clarify The
analyzed. the impact
trends inofFigure
Fe, Cu,4E,GandareMgsimilar:
on micro-hardness.
it seems that
Alloy
the Fe2amount
and alloy 3 have
almost lower
does notMg andthe
affect Fe parabolic
contents, while
trend;these elements
in fact, cause an
the R2 values areincrease
similar
in each
in micro-hardness.
case. On the other hand, trends in Figure 4B,F are similar, but the R2 value changes
in both the alloy states (unmodified and modified); this makes the effect of Fe on micro-
hardness values more evident.
Studying the evolution in micro-hardness as a function of the alloying elements may
permit to assume, in advance, the micro-hardness attended for a specific alloy in the as-
Metals 2021, 11, x FOR PEER REVIEW 7 of 19
Metals 2021, 11, 342 7 of 19
cast condition. In respect to the micro-hardness, all samples, modified and unmodified,
have the same pattern.
Figure
Figure4.
4.Correlation
Correlationbetween
betweenmicro-hardness
micro-hardness and
and average
average elements
elements contained
contained in
in alloys.
alloys. (A).
(A):Relation
Relationbetween
betweenaverage
averageSi
Si
content
content and micro-hardness. (B): Relation between average Mg content and micro-hardness. (C): Relation betweenaverage
and micro-hardness. (B). Relation between average Mg content and micro-hardness. (C). Relation between average
Fe content and micro-hardness. (D). Relation between average Cu content and micro-hardness. (E). Relation between av-
Fe content and micro-hardness. (D): Relation between average Cu content and micro-hardness. (E): Relation between
erage Fe + Mg + Cu content and micro-hardness. (F). Relation between average Fe + Mg content and micro-hardness. (G).
average Fe + Mg + Cu content and micro-hardness. (F): Relation between average Fe + Mg content and micro-hardness.
Relation between average Mg + Cu content and micro-hardness. (H). Relation between average Fe + Cu content and micro-
(G): Relation between average Mg + Cu content and micro-hardness. (H): Relation between average Fe + Cu content and
hardness.
micro-hardness.
3.3. Secondary Dendrite Arm Spacing (SDAS)
Figure 4D clearly shown that the increase in micro-hardness is related to the presence
SDAS
of Cu. Themeasurements were Cu
higher the average carried out in
amount, theallhigher
samplesthe to observe the difference
micro-hardness, be-
as observed
tween
for allthe modified
of the alloyingand unmodified
elements specimens,
analyzed. following
The trends the procedure
in Figure reporteditin
4E,G are similar: Fig-
seems
ure
that1.the
AsFenoticeable from Table
amount almost does3,not
theaffect
SDASthe
decreased
parabolicintrend;
all alloys afterthe
in fact, theR2addition of
values are
AlSr10. Remarkably, the most significant decrease was noticed for those compositions
similar in each case. On the other hand, trends in Figure 4B,F are similar, but the R value 2
with lower
changes silicon
in both thecontent (alloy(unmodified
alloy states 1), while a minor decreasethis
and modified); wasmakes
observed in alloy
the effect 4. on
of Fe In
detail, SDAS in alloy
micro-hardness values1 was
morereduced
evident. by 34%, in alloys 2 and 4 almost by 16%, and in alloy
Studying the evolution in micro-hardness as a function of the alloying elements may
permit to assume, in advance, the micro-hardness attended for a specific alloy in the as-cast
condition. In respect to the micro-hardness, all samples, modified and unmodified, have
the same pattern.
Metals 2021, 11, 342 8 of 19
3.3. Secondary Dendrite Arm Spacing (SDAS)
SDAS measurements were carried out in all samples to observe the difference between
the modified and unmodified specimens, following the procedure reported in Figure 1.
As noticeable from Table 3, the SDAS decreased in all alloys after the addition of AlSr10.
Remarkably, the most significant decrease was noticed for those compositions with lower
silicon content (alloy 1), while a minor decrease was observed in alloy 4. In detail, SDAS in
alloy 1 was reduced by 34%, in alloys 2 and 4 almost by 16%, and in alloy 3 by approximately
8%. These average results suggest that Al-Sr reduces SDAS significantly for alloys with
lower silicon contents.
Table 3. SDAS values measured.
Alloy 1 Alloy 2
SDAS Values
Unmodified Modified Unmodified Modified
SDAS (µm) 27.48 ± 4.15 18.09 ± 4.15 25.03 ± 3.14 20.80 ± 5.14
Alloy 3 Alloy 4
SDAS (µm) 15.02 ±2.68 13.77 ±2.18 19.51 ±2.07 16.06 ±1.08
However, the casting removal from the mold may affect SDAS comparison in alloys
having similar silicon contents because delays in the casting extraction can slow the cooling
rate down, thus increasing the measured SDAS.
3.4. Eutectic Modification
The eutectic region includes the eutectic silicon and the intermetallic phases. The Sr
modification mainly influences the average particle size of the silicon, the silicon shape,
and the area occupied by the eutectic phase into the alloys’ bulk (percentage of the eutectic
silicon in a specific measured area). The eutectic size was strongly reduced after AlSr
addition, as visible in Table 4 and Figure 5.
Table 4. Image analysis results for the characterization of the eutectic area.
Alloy 1 Alloy 2
Image Analysis
Unmodified Modified Unmodified Modified
Average size (µm) 17.46 10.41 32.67 9.92
Circularity 0.726 0.72 0.476 0.756
Solidity 0.825 0.829 0.729 0.834
% Eutectic phase 30.3 ± 0.52 48.4 ± 2.94 21.9 ± 0.9 35.7 ± 0.85
Alloy 3 Alloy 4
Image Analysis
Unmodified Modified Unmodified Modified
Average size (µm) 22.06 3.09 14.53 2.16
Circularity 0.75 0.675 0.822 0.806
Solidity 0.845 0.796 0.859 0.884
% Eutectic phase 48.1 ± 2.64 52.3 ± 1.92 52.9 ± 4.64 53.7 ± 0.71
Metals 2021, 11, 342 9 of 19
Metals 2021, 11, x FOR PEER REVIEW 9 of 19
Figure
Figure 5.
5. (A):
(A):Graphical
Graphicalrepresentation
representationofofthe
the average
average particle
particle area;
area; (B):
(B): graphical
graphical representation
representation of
of the
the percentage
percentage of
of
eutectic
eutectic phase; (C): the relation between average Si content and SDAS; (D): the relation between average Fe ++ Mg
phase; (C): the relation between average Si content and SDAS; (D): the relation between average Fe Mg ++ Cu
Cu
content
contentand
andSDAS;
SDAS; (E):
(E): the
the relation
relation between
between average
average Si
Si ++Fe
Fe++MgMg++Cu Cucontent
contentand
andSDAS.
SDAS.
The
The measurement
eutectic phase’s ofsize
circularity evaluates
is different amongthe theparticle's
unmodified perimeter's
alloys, but smoothness: the
such difference
smoother a particle becomes, the higher the circularity value. From Table
is reduced after modification with AlSr. Alloys with less silicon (alloys 1 and 2) have a 4, it seems that
the best results
eutectic size ofwere obtained for10
approximately alloys
µm, 3and
andalloys
4. Thewith
evaluation of solidity
more silicon instead
(alloys is useful
3 and 4) are
to characterize
closer to 2.5 µm;theall
regularity andfrom
the results roughness of a particle
the image analysisasareit represents
reported inthe ratio3between
Table for each
the areaFigure
alloy. of the5measured
shows theparticle and thefound
relationships perimeter of theSDAS
between imaginary convex elements.
and alloying hull around In
it. As regards
particular, in solidity,
Figure 5A, as the particle becomes
a parabolic morethe
trend in both solid, the solidity
unmodified andvalue approaches
modified alloys
one.
is observed. Figure 5B shows the relation between SDAS and the average content of the
alloying elements
A parabolic Fe, Mg, and
relationship Cu: insilicon
between this graph,
contentit and
is noticeable thatwas
particle size thefoundR2 parameter
(Figure
resulted
6A). Alloysvery low; athere
having loweris content
not a clear trend between
of alloying elements thehave
alloying
largerelements
eutectic and the SDAS
particle sizes.
measured.
This behaviorFinally,
is dueintoFigure 5C, presence
the large all the main alloyingelements
of alloying elementsfavoring
(Si, Fe, Mg, and Cu)ofwere
nucleation in-
considered: into
termetallics for unmodified
the eutecticcompositions, an excellent
area, thus limiting parabolic
the growth approximation
of silicon. On the other having
hand,an
R 2 value of 0.999 is observed. This result indicates that there seems to exist a mathematical
in the modified alloys, a linear correlation between the reduction in eutectic particle size
relation
and between
the Si contentSDAS andisaverage
increase alloying
noticeable: element,
the greater theasSiincontent,
Equation (1):lower the Si size.
the
Figure 6A–D display the eutectic particle size evolution related to Si, Fe, and Mg contents.
No clear trend was observed 6.923 x2 Fe
y = −between + 31.71
or Mgx content
+ 11.103and the reduction in eutectic (1)
silicon particle size. Essentially, the higher the Fe and Mg content, the higher the interme-
where y is the SDAS and x is the average sum of the elements Si, Fe, Mg, and Cu.
tallic phases that occur into the eutectic region; consequently, it seems that the modifica-
The measurement of circularity evaluates the particle’s perimeter’s smoothness: the
tion acts to reduce some intermetallic phases’ size too, resulting in smaller average eutectic
smoother a particle becomes, the higher the circularity value. From Table 4, it seems that
size, even though from the literature [45], it seems that the increase in Mg harms Sr mod-
the best results were obtained for alloys 3 and 4. The evaluation of solidity instead is useful
Metals 2021, 11, 342 10 of 19
Metals 2021, 11, x FOR PEER REVIEW 10 of 19
to characterize the regularity and roughness of a particle as it represents the ratio between
the area of the measured particle and the perimeter of the imaginary convex hull around it.
As regards solidity, as the particle becomes more solid, the solidity value approaches one.
A parabolic
ification. relationship
According to otherbetween
researchsilicon content
from the and particle
literature [34], asize was found
reduced effect(Figure 6A).
of Sr mod-
Alloys having a lower content of alloying elements have larger eutectic
ification was observed in alloys with higher Mg content. The reduction in the area occu- particle sizes.
This behavior
pied is duephase
by the eutectic to thewaslarge presence
reduced to aoflesser
alloying
extentelements favoring in
(in magnitude) nucleation
alloy 1 and of
intermetallics
alloy 4, which into the eutectic
are richer in Mgarea,
thanthus limiting
the other the growth
alloys. of silicon.
Furthermore, On the et
Shabestari other hand,
al. in [41]
in the modifiedthat
demonstrated alloys,
if Mna islinear correlation
present, Sr has abetween
negligible theeffect
reduction in eutectic
on modifying theparticle
form ofsizethe
and the Si
primary content
β-Fe phase. increase
In this is noticeable:
respect, a largethe greaterofthe
amount β-FeSi phase
content,wasthefound
lowerinthe Si size.
modified
Figure16A–D
alloys and 3,display the eutectic
compositions whichparticle size
are rich inevolution
Mn. related to Si, Fe, and Mg contents.
Figure 6.
Figure 6. The
The relationships
relationshipsbetween
betweenparticle
particlesize
sizeand
and
Si,Si,
Mg,Mg,
andand
Fe Fe contents.
contents. (A):(A). Relation
Relation between
between average
average Si content
Si content and
and eutectic particles’ size.; (B). Relation between average Mg content and eutectic particles’ size. (C). Relation between
eutectic particles’ size.; (B): Relation between average Mg content and eutectic particles’ size. (C): Relation between average
average Fe content and eutectic particles’ size. (D). Relation between average Mg + Fe content and eutectic particles’ size.
Fe content and eutectic particles’ size. (D): Relation between average Mg + Fe content and eutectic particles’ size.
In
No[46],
clearthe authors
trend was underline
observed betweenthat despite Fe orSrMg
addition,
contentα-Feandintermetallics
the reduction do not dis-
in eutectic
solve into the Al matrix, while β-Fe intermetallics can be affected by
silicon particle size. Essentially, the higher the Fe and Mg content, the higher the intermetal- Sr addition [47]. This
behavior can be highlighted by measuring the main intermetallic
lic phases that occur into the eutectic region; consequently, it seems that the modification phases, easily discerni-
ble
actsunder an optical
to reduce microscope. phases’
some intermetallic The evolution
size too,ofresulting
differentinintermetallic
smaller averagephases was stud-
eutectic size,
ied
eveninthough
each alloy.
from Mainly,
the literatureα-Fe [45],
phases, β-Fe that
it seems phases, and Fe-Mg
the increase in Mgintermetallics were ana-
harms Sr modification.
lyzed through
According the image
to other analysis,
research fromfinding a relation
the literature between
[34], a reducedSr addition
effect ofand intermetallic
Sr modification
modification. Figure 7 shows the results regarding the main
was observed in alloys with higher Mg content. The reduction in the area occupied intermetallic phases detected
by the
in
eutectic phase was reduced to a lesser extent (in magnitude) in alloy 1 and alloy 4, Figure
each alloy and their evolution (in terms of area occupied) after Sr addition. From which
6,
areitricher
is arguable
in Mg thanthat the
β-Feotherphases decrease
alloys. after Sr Shabestari
Furthermore, addition; the area
et al. occupied
in [41] by β-Fe
demonstrated
intermetallic phases decreases
that if Mn is present, further ineffect
Sr has a negligible alloyson1 and 3, where
modifying thea form
high amount of β-Feβ-Fe
of the primary was
detected—from
phase. In this respect, 33.7 toa5.4large µmamount
2 in alloy 1 and from 15.9 to 2.8 µm2 in alloy 3.
of β-Fe phase was found in modified alloys 1 and 3,
Furthermore,
compositions which a decrease
are rich in inMn.
the average occupied area of α-Fe in alloy 1 after Sr addi-
tion, Infrom 55.9 to 39.7 µm 2 , was observed,
[46], the authors underline that despite with similar standardα-Fe
Sr addition, deviations. Alloy 4do
intermetallics shows
not
adissolve
sharp increase
into thein Alα-Fe afterwhile
matrix, Sr addition, from 28.2 tocan
β-Fe intermetallics almost 80 µm .by
be affected 2 Looking at the[47].
Sr addition av-
erage area results
This behavior can (Figure 7), it may
be highlighted bybemeasuring
argued that theSrmain
acts intermetallic
on β-Fe phases, causing
phases, its
easily
discernible under
fragmentation in allanthe
optical
alloys. microscope. The evolution
As for Mg-containing of different
phases, intermetallic
they decrease phases
their occupied
was
area,studied
apart fromin each
alloyalloy.
4, which Mainly, phases,
α-Fe both
increases Mg2β-Fe
Si andphases, and Fe-Mg
α-Fe phases. Alloyintermetallics
3 shows sim-
were analyzed
ilar values through
in terms the image
of average analysis,phase
intermetallic finding a relation between Sr addition and
area.
intermetallic modification.
Intermetallic behaviors Figurecould be 7 shows
relatedthe results
to the regarding
eutectic thesize.
particles’ main intermetallic
For alloy 1, eu-
phases
tectic detected
particle sizeindecreased
each alloy byand 40.4%,their
andevolution (in terms of
all the intermetallic area occupied)
phases decreased after Sr
in aver-
age area; this indicates that the eutectic particle size decrease was mainly due to the inter-
metallic area reduction rather than Si reduction. Alloy 2 eutectic particle sizes decreased
by about 70%, while an increase in the α-Fe area of about 50% was observed. This evidenceMetals 2021, 11, x FOR PEER REVIEW 11 of 19
Metals 2021, 11, 342 11 of 19
indicates that the average eutectic area's substantial decrease was mainly due to silicon
reduction and Fe-Mg intermetallic reduction.
As regards alloy 3, the average eutectic particle area shrank by 86% after Sr addition.
addition.atFrom
Looking Figure 6, it is area
the intermetallic arguable that β-Fe
evolution, phases decrease
it appears that the after Sr addition;
eutectic the area
area decrease is
occupied by β-Fe intermetallic phases decreases
attributable to both silicon and intermetallic reduction.further in alloys
Finally, in 1 and
alloy 3,
4, where
an a high
increase in
amount
α-Fe andof was detected—from
β-Fe phases
Mg2Si was detected.33.7
The to 5.4 µm2 in
reduction in the
alloy 1 and from
average 15.9particles’
eutectic to 2.8 µm 2 in
area
alloy 3. 85%) is mostly attributable to a decrease in Si size.
(almost
Figure
Figure 7.
7. Intermetallics
Intermetallics measured
measured for each alloy.
for each alloy. Intermetallic
Intermetallicmeasurements
measurementsare
areevidenced
evidencedininthe
themicrographs
micrographsbybydifferent
differ-
ent colors—the table shows the area occupied for each intermetallic phase. Data for the α-Fe and β-Fe in the unmodified
colors—the table shows the area occupied for each intermetallic phase. Data for the α-Fe and β-Fe in the unmodified
condition for alloys 1 and 3 refer to previous work [32].
condition for alloys 1 and 3 refer to previous work [32].
3.5. Impact Toughnessa decrease in the average occupied area of α-Fe in alloy 1 after Sr
Furthermore,
Figurefrom
addition, 8 shows the39.7
55.9 to µm2 , was observed,
microstructures and the with
impact toughness
similar values
standard for alloysAlloy
deviations. in the4
unmodified
shows a sharp (bars on thein
increase left side)
α-Fe andSrmodified
after addition,conditions
from 28.2 (bars on the
to almost µm2 .side).
80right After
Looking at
modification,
the average area silicon particles
results (Figure in7),
alloy 1 still
it may behad an acicular
argued shape
that Sr acts on (Figure 8E), despite
β-Fe phases, causingthe
its
average particleinsize
fragmentation decreasing
all the alloys. As byforalmost 70% for the
Mg-containing unmodified
phases, sampletheir
they decrease (Figure 8A).
occupied
This
area,isapart
not surprising;
from alloyin4,fact, whichthe increases
measuredboth circularity
Mg2 Si reported
and α-Feinphases.
Figure 3Alloy
remains con-
3 shows
similar
stant. values
This in terms
behavior mayof be
average intermetallic
associated with the phase area. of large polyhedral Fe-based
presence
Intermetallic
intermetallics in thebehaviors could be
eutectic region. As related
previously to the eutectic
brought up,particles’ size.regions
the eutectic For alloy
in al-1,
eutectic
loys 1 andparticle size decreased
4 are composed by 40.4%,
of eutectic siliconandandall the intermetallic
various intermetallicphases
phases.decreased in
Since inter-
averagephases
metallic area; this indicates
typically havethat the eutectic
polyhedral shapes,particle
thosesize decrease
phases was mainly
negatively due to
impact tough-
the intermetallic
ness and fracture area reduction
behavior. Hence,rather thanthe
despite Si eutectic
reduction. Alloy 2 eutectic
modification, impact particle
energysizes
ab-
decreased by about 70%, while an increase in the α-Fe area of about
sorbed cannot increase significantly, as depicted in the bar chart of Figure 8: alloy 1 energy 50% was observed.
Thisfrom
rose evidence
5.43 ±indicates that± the
0.16 to 6.29 0.69average
J, and thateutectic area’s
of alloy substantial
4 increased fromdecrease wastomainly
2.83 ± 0.13 3.17 ±
due to silicon reduction and Fe-Mg intermetallic reduction.
0.22 J. As regards alloy 2, many bifilms were found in the modified specimens; these bi-
films As regardsreduce
probably alloy 3,thethefinal
average eutectic
absorbed particleAs
energies. area shrank in
reported by[25],
86% the
aftermelt
Sr addition.
quality
Looking at the intermetallic area evolution, it appears that the eutectic area decrease is
attributable to both silicon and intermetallic reduction. Finally, in alloy 4, an increase in
α-Fe and Mg2Si phases was detected. The reduction in the average eutectic particles’ area
(almost 85%) is mostly attributable to a decrease in Si size.Metals 2021, 11, 342 12 of 19
3.5. Impact Toughness
Figure 8 shows the microstructures and the impact toughness values for alloys in the
unmodified (bars on the left side) and modified conditions (bars on the right side). After
modification, silicon particles in alloy 1 still had an acicular shape (Figure 8E), despite the
average particle size decreasing by almost 70% for the unmodified sample (Figure 8A).
This is not surprising; in fact, the measured circularity reported in Figure 3 remains
constant. This behavior may be associated with the presence of large polyhedral Fe-based
intermetallics in the eutectic region. As previously brought up, the eutectic regions in
alloys 1 and 4 are composed of eutectic silicon and various intermetallic phases. Since
intermetallic phases typically have polyhedral shapes, those phases negatively impact
toughness and fracture behavior. Hence, despite the eutectic modification, impact energy
absorbed cannot increase significantly, as depicted in the bar chart of Figure 8: alloy 1
Metals 2021, 11, x FOR PEER REVIEW 12 of 19
energy rose from 5.43 ± 0.16 to 6.29 ± 0.69 J, and that of alloy 4 increased from 2.83 ± 0.13
to 3.17 ± 0.22 J. As regards alloy 2, many bifilms were found in the modified specimens;
these bifilms probably reduce the final absorbed energies. As reported in [25], the melt
affects
qualitythe effectiveness
affects of Sr modification.
the effectiveness In this sense,
of Sr modification. the
In this presence
sense, of bifilmof
the presence onbifilm
the frac-
on
ture surfacesurface
the fracture indicated a quality
indicated of melt
a quality of not
meltsufficient to obtain
not sufficient the the
to obtain bestbest
performances
performancesin
terms of modification.
in terms of modification.
Figure 8.
Figure 8. Impact
Impact test
test results
resultsand
andalloys’
alloys'microstructures
microstructures(optical
(opticalmicroscope).
microscope). Microstructures
Microstructures (A–D)
(A–D) refer
refer to the
to the unmodi-
unmodified
fied alloys, as on the bar chart's left side. Microstructures (E–H) refer to the modified alloys, as on the bar chart's right side.
alloys, as on the bar chart’s left side. Microstructures (E–H) refer to the modified alloys, as on the bar chart’s right side.
Image (A,E): alloy 1; image (B,F): alloy 2; image (C,G): alloy 3; image (D,H): alloy 4.
Image (A,E): alloy 1; image (B,F): alloy 2; image (C,G): alloy 3; image (D,H): alloy 4.
Figure 9 documents the relations between intermetallics and silicon shape observed.
In particular, Figure 9 refers to alloy 3. Some intermetallic phases seem to influence the
eutectic silicon dimensions and shapes. In alloy 3, the decrease in silicon length away from
intermetallic phases was noticeable, as observed in previous work too [32] (see Table 4).
Despite that, an uneven modification of the eutectic silicon from a plate-like to a spherical-
like shape was documented. As the eutectic spherical shape promotes ductile fracture [48],Metals 2021, 11, 342 13 of 19
Figure 9 documents the relations between intermetallics and silicon shape observed.
In particular, Figure 9 refers to alloy 3. Some intermetallic phases seem to influence the
eutectic silicon dimensions and shapes. In alloy 3, the decrease in silicon length away
from intermetallic phases was noticeable, as observed in previous work too [32] (see
Table 4). Despite that, an uneven modification of the eutectic silicon from a plate-like to a
spherical-like shape was documented. As the eutectic spherical shape promotes ductile
fracture [48], the uneven modification resulted in a slight increase in absorbed energy from
the unmodified compared to the modified samples. On the other hand, it seems that the
β-Fe acicular phases mainly detected in the modified alloy 3 (blue arrow in Figure 9) do
Metals 2021, 11, x FOR PEER REVIEW 13 (red
not affect the silicon particle shapes, while, on the contrary, Fe plate-like intermetallics of 19
arrow in Figure 9) strongly affect the silicon modification in their surrounding areas.
Figure 9. Effect of intermetallic
Figurephases on of
9. Effect silicon shape. Optical
intermetallic phases micrograph of alloy
on silicon shape. 3 modified.
Optical Redof
micrograph arrow
alloyindicates Fe Red
3 modified.
plate intermetallics; blue arrow indicates acicular Fe intermetallic. The uneven silicon shape is well visible near the inter-
arrow indicates Fe plate intermetallics; blue arrow indicates acicular Fe intermetallic. The uneven
metallics. silicon shape is well visible near the intermetallics.
Twosurface
Two surfacefractures
fracturesafter
afterimpact
impacttests
testsare
areshown
shownin inFigures
Figures10 10and
and11.
11.In
In particular,
particular,
Figure 10 highlights the presence of bifilms in the bulk of the specimens.
Figure 10 highlights the presence of bifilms in the bulk of the specimens. Alloy 4 resulted Alloy 4 resulted
in
in low absorbed energy because of the extensive presence of
low absorbed energy because of the extensive presence of oxide skins, while alloy 3 resultedoxide skins, while alloy 3
resultedand
cleaner cleaner
showed andhigher
showed higher
impact impact
energy energy absorbed.
absorbed. In Figure 11, In Figure
the SEM 11,micrographs
the SEM mi-
crographsthe
represent represent
different thefracture
different fracture behaviors
behaviors detected in detected
the tested in the tested specimens.
specimens. Par-
Particularly,
ticularly,
samples samples
from alloysfrom
3 andalloys 3 and 4toare
4 are shown shown the
highlight to highlight
different kindsthe different kinds
of fractures of frac-
detected.
tures detected.
Specimens realizedSpecimens
by alloyrealized by alloy
4 fractured 4 fractured
in a fragile manner, in aas fragile manner,
noticeable as noticeable
by the cleavage
by theand
facets cleavage facets
the rivers and theinrivers
detected Figure detected
11A. Byincontrast,
Figure 11A.samples By contrast,
realized samples
by alloy realized
3 show
aby
morealloy 3 show
ductile a more as
behavior, ductile behavior,
depicted as depicted
in Figure 11B, withina Figure 11B, with
large number a large number
of dimples on the
fracture
of dimples surface.
on the The fracture
fracture surfaces
surface. Theforfracture
alloy 1 are similar
surfaces fortoalloy
alloy14are while thosetoofalloy
similar alloy4
2while
are similar
those of to alloy 3.2 are similar to alloy 3.
On
Onalloy
alloy44(and,
(and,similarly,
similarly,on onalloy
alloy1), 1),cleavage
cleavagefacets
facetswereweredetected,
detected,while
whiledimples
dimples
related to the modified eutectic silicon were observed only to a limited
related to the modified eutectic silicon were observed only to a limited extent on the frac- extent on the fracture
surface [49]. Despite
ture surface the increase
[49]. Despite in average
the increase eutecticeutectic
in average size, circularity and solidity
size, circularity indicated
and solidity in-
an incomplete
dicated modification;
an incomplete the impact
modification; theenergy
impact increased slightly. On
energy increased alloy 3,On
slightly. thealloy
presence
3, the
of fewer intermetallic
presence phases positively
of fewer intermetallic affects the
phases positively impact
affects toughness.
the impact SiliconSilicon
toughness. particles
par-
break,
ticles forming dimples,dimples,
break, forming but the roundness of the silicon
but the roundness of theparticles
siliconcan be further
particles can improved.
be further
improved.
Figure 12 shows the impact specimens’ edge for each composition studied. Particu-
larly, cracks on the edge of the samples and absorbed energies were documented and re-
lated with the microstructural behaviors previously measured: SDAS, eutectic particle
size, and intermetallic phases (α-Fe, β-Fe, and Fe-Mg).
As noticeable in Figure 12, the edges of the impact test specimen of alloy 1 show
different features. In the unmodified alloy sample, many cracks were detected, while the
modified alloy showed a limited number of defects. These defects are related to the spec-Metals2021,
Metals 11,x342
2021,11, FOR PEER REVIEW 1414ofof19
19
Metals 2021, 11, x FOR PEER REVIEW 14 of 19
Figure
Figure10.
10.Macrographic
Macrographic images of of
images fractured surfaces
fractured in impact
surfaces specimens. (A): (A):
A brittle fracture in alloy 4 modified (im-
10. Macrographic images of fractured surfaces in in impact
impact specimens.
specimens. A brittle
(A): A brittle fracture
fracture in alloy
in alloy 4 modified
4 modified (im-
pact energy 3.17 J). (B): A ductile fracture in alloy 3 modified (impact energy 8.82 J).
(impact
pact energy
energy 3.173.17 J). (B):
J). (B): A ductile
A ductile fracture
fracture in alloy
in alloy 3 modified
3 modified (impact
(impact energy
energy 8.828.82
J). J).
Figure 11. SEM images of fracture surfaces in impact specimens. (A): A brittle fracture in alloy 4 modified (impact energy
Figure
Figure 11. SEM
SEM
11. A images
images of
of fracture
fracture surfaces
surfaces in
in impact
impact specimens.
specimens. (A): A
A brittle
(A):J). brittle fracture
fracture in
in alloy
alloy 44 modified
modified (impact
(impact energy
energy
3.17 J). (B): ductile fracture in alloy 3 modified (impact energy 8.82
3.17 J). (B): A ductile fracture in alloy 3 modified (impact energy 8.82 J).
3.17 J). (B): A ductile fracture in alloy 3 modified (impact energy 8.82 J).
In
In alloy
alloy121,1,after
afterSr Sr modification, an anSDAS reduction of of34%,
34%,aaparticle size reduction
Figure shows themodification,
impact specimens’ SDASedgereduction
for each composition particle sizeParticularly,
studied. reduction
of
of about
about 40%,
40%, reduction
reduction through
through the
the fragmentation
fragmentation process
process of
of β-Fe
β-Fe intermetallic,
intermetallic, and
andaa
cracks on the edge of the samples and absorbed energies were documented and related
slight
slight increase
increase in micro-hardness
in micro-hardness from 91.3
from 91.3 to to 96 HV0.5
96 HV0.5 (average
(average values)
values) were
were docu-
docu-
with the microstructural behaviors previously measured: SDAS, eutectic particle size, and
mented.
mented. SDAS
SDAS reduction
reduction affects
affects the alloy’s mechanical
mechanical resistance and the particle sizere-
resistance and the particle size
intermetallic phases (α-Fe, β-Fe, theandalloy’s
Fe-Mg). re-
duction
duction influences the rupture behavior by
byfostering the
theductile fracture
fracturewhile the
the1modi-
As influences
noticeable the rupture12,
in Figure behavior
the edges fostering
of the impact ductile
test specimen while
of alloy modi-
show
fication
fication occurred. In this sense, the
themodification, along
alongwith
withthe β-Fe
β-Fefragmentation, seems
differentoccurred.
features.InIn this sense,
the unmodified modification,
alloy sample, many the
cracks fragmentation,
were detected, seems
while
to
to limit the presence of cracks. Consequently, the average absorbed energy increases.
thelimit the presence
modified of cracks.
alloy showed Consequently,
a limited numberthe average These
of defects. absorbed energy
defects are increases.
related to the
As
As regards
regards alloy
alloy2,2,aa similar
similar trendtrend totoalloy
alloy11 waswas highlighted.
highlighted. SDAS SDAS decreased
decreasedby by
specimen’s microstructure.
16%,
16%, Inthe eutectic
thealloy
eutectic particle
particle size by
size by 70%, 70%, and the Fe-Mg intermetallic-occupied area was
1, after Sr modification, an and
SDAS the Fe-Mg intermetallic-occupied
reduction area was
of 34%, a particle size reduction
halved
halved after
after Sr
Sraddition.
addition.The The impact
impact specimen's edge resulted with few cracks
cracksin inthe un-
of about 40%, reduction through thespecimen's
fragmentation edgeprocess
resultedof with
β-Fefewintermetallic, the
and un-a
modified
modified composition,
composition, while no cracks
while no cracks were
were detected in the samples made with modified
slight increase in micro-hardness from 91.3 to detected
96 HV0.5in(average
the samples
values)made
were with modified
documented.
alloy.
SDASIn
alloy. In alloy
alloy3,3,decreases
reduction decreases
affects the in SDAS
inalloy’s (8%)
(8%)and
SDASmechanical andthe the eutectic
eutecticparticle
resistance thesize
particle
and size(of
(of86%)
particle 86%) were
wereno-
size reduction no-
ticed,
ticed, but
influences the
but the intermetallic
the rupture behavior
intermetallic phase-occupied
by fostering
phase-occupied areas
areas remained
theremained constant.
ductile fracture
constant. As
whileβ-Fe promotes
the promotes
As β-Fe modificationthe
the
fragile fracture,
occurred. In this its short
sense, fragmentation
the modification, probably
along affects
with the the
β-Fe
fragile fracture, its short fragmentation probably affects the absorbed energy, hindering absorbed energy,
fragmentation, hindering
seems to limit
its
theincrease.
its presenceCracks
increase. of cracks.
Cracks detected inin the
Consequently,
detected the modified
the average
modified and unmodified
andabsorbed
unmodified specimens
energy are
increases.
specimens are similar
similar in in
number and extent. Since this composition has few alloying
number and extent. Since this composition has few alloying elements, the intermetallic elements, the intermetallic
phases
phaseshave haveless
lessinfluence
influenceon onimpact
impactenergyenergyin inboth
bothcases.
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