on grain coarsening and refining of the mg-3al alloy by sm377254/uq377254_oa.pdf · grain refining...
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On grain coarsening and refining of the Mg-3Al alloy by Sm
Xiaoyu Hu, Penghuai Fu, David StJohn, Liming Peng, Ming Sun, Mingxing Zhang
PII: S0925-8388(15)31750-3
DOI: 10.1016/j.jallcom.2015.11.193
Reference: JALCOM 36048
To appear in: Journal of Alloys and Compounds
Received Date: 23 August 2015
Revised Date: 21 November 2015
Accepted Date: 25 November 2015
Please cite this article as: X. Hu, P. Fu, D. StJohn, L. Peng, M. Sun, M. Zhang, On grain coarseningand refining of the Mg-3Al alloy by Sm, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.11.193.
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http://dx.doi.org/10.1016/j.jallcom.2015.11.193
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On grain coarsening and refining of the Mg-3Al alloy by
Sm
Xiaoyu Hua,b
, Penghuai Fua, David StJohn
b,*, Liming Peng
a,*, Ming Sun
a, Mingxing
Zhangb
a. National Engineering Research Center of Light Alloy Net Forming and State Key
Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, 800
Dongchuan Road, Shanghai 200240, China
b. Centre for Advanced Materials Processing and Manufacturing, School of
Mechanical and Materials Engineering, The University of Queensland, St Lucia,
Queensland, 4072, Australia1
Abstract: The effect of Sm on the microstructure of a Mg-3Al alloy has been
investigated where it was found that the size of the primary α-Mg grains was
significantly coarsened by the addition of 0.7 wt. % Sm, but dramatically refined by
2.1 wt. % Sm. It is proposed that the coarsening effect of Sm is due to a decrease in
the nucleation potency of the native Al-Fe-C-O particles when they transform to the
lower potency Al-Fe-Sm-C-O particles. The grain refinement observed when 2.1 wt. %
Sm is added, is caused by the formation of potent Al2Sm nucleant particles prior to
the solidification of α-Mg. The “Interdependence Theory” was then applied to
describe the nucleant selection process in the Mg-3Al-2.1Sm alloy.
Key words: Solidification; Metals and alloys; Rare earth compounds; Nucleation
* David StJohn, Tel.: +61 7 3365 3641, fax: +61 7 3365 3888. E-mail address:
d.stjohn@uq.edu.au; Corresponding author. Liming Peng, Tel. +86 21 54742911, fax:
+86 2134202794. E-mail address: plm616@sjtu.edu.cn.
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1. Introduction
Grain refining is one of the most important methods for improving the mechanical
properties of metallic materials [1, 2]. Mg-Al alloys are the most common and
economical Mg alloys, but it is difficult to achieve grain refinement in cast Mg-Al
alloys [3, 4]. It is now understood that two key factors promote grain refinement [5]:
Solute elements and heterogeneous nucleant particles.
The addition of solute elements with a high growth restriction factor, Q, provides
rapid development of constitutional supercooling, which restricts the growth of
existing grains allowing additional nucleation events to occur. For example, Park et al.
[6] recently reported that a Mg-8Al-2Zn alloy was significantly refined by 2 ~ 6 % Sn
(all compositions are in wt. % hereinafter). Sevik et al. [7] reported that 2 % Sr
refined a Mg-6Al-0.3Mn-0.3Ti-1Sn alloy from ~ 50 μm to 30 μm significantly
improving the mechanical properties. Choi et al [8] showed that 0.02 % Ti refined the
microstructure of a Mg-6Al-1Zn alloy and improved the corrosion properties. All
these solute elements have sufficiently high values of Q to rapidly form constitutional
supercooling in a Mg melt [9].
In most Al-containing Mg alloys, carbon-based particles, such as Al4C3, Al2CO and
Al2MgC2, are believed to be the main heterogeneous nuclei for the α-Mg phase
during solidification [10-12]. Hence, much research effort has been applied to
identify carbon-based particles which can inoculate the melt in a cost-effective and
reliable manner [13-16]. Recently, Liu et al. added a new Mg-50%Al4C3 master alloy
into an AZ91 alloy and achieved remarkable grain refinement [17]. Suresh et al [18]
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found that an addition of 0.4 % charcoal reduced the grain size of an AZ91 alloy from
100 μm to 14 μm. Other particles have also been reported to refine Mg-Al alloys. For
example, Wang et al [19] found that an addition of Al-4.99Zr-1.1B master alloy which
contains ZrB2 particles, reduced the grain size of an AZ31 alloy from 170 μm to 45 μm.
Fu et al. [20] reported that adding ZnO particles to Mg produced grain refinement,
although the efficiency was affected by reaction between Mg and ZnO reducing the
ZnO to Zn in solution. However, problems of pollution and/or poor dispersion of
particles make most of the above methods inappropriate for industrial application.
Fe or Mn additions can also reduce the grain size of Mg-Al alloys by generating
compounds such as Al8(Mn,Fe)5 [13]. Crystallographic studies by Qiu et al. [21] have
shown that Al8Mn5 and Al8(Mn,Fe)5 are both potential nucleant particles. Cao et al.
[22] suggested that native Al-Fe-C-O particles naturally present in an alloy can also
act as active nucleant particles. Superheating and Elfinal methods [13] have also
been applied to refine Mg-Al alloys, but have proved to be inappropriate for
industrial application because they are not reliable or safe and react with the steel
crucibles. Achieving reliable and safe refinement of the microstructure of Mg-Al
alloys is still an unsolved issue [23].
Recently, Qiu et al. [24] concluded that Al2RE compounds (RE = Rare Earth elements,
such as Gd, Sm, Y, Dy, Nd or Ce), which have crystallographic misfits with the Mg
matrix of less than 2%, are located in a ‘fully wetting zone’ and all have the potential
to act as effective grain refiners for cast Mg alloys. This was confirmed by adding 1 %
Al to a Mg-10Y alloy reducing the grain size from ~180 to ~36 μm [25]. Similar effects
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were also found by Dai et al. [26], who reported that the Mg-10Gd-1Al alloy had a
fine grain size of ~37 μm. Recently, Wang et al [27] found that the grain size of a
Mg-6Sm alloy was reduced by 3 % Al from ~ 1830 μm to ~ 30 μm. Jiang et al. [28]
reported that the grain size of a Mg-6Ce alloy was significantly reduced by adding 3 ~
5 % Al.
Since there is still no commercially efficient method for producing grain refinement
in Mg-Al alloys, Al2RE based refiners for the Mg-Al system should be considered.
However, when REs were added to Mg-Al alloys to generate Al2RE phases, different
results were obtained by researchers. For example, while Yu et al. [29] reported that
0.4 % Ce significantly refined an AZ31 alloy, Pan et al. [30] pointed out that the range
of 0.5 ~ 1.5 % Ce significantly coarsened the AZ31 alloy. Chaubey et al [31]
investigated Mg-10Al-xCe alloys and claimed that 1 % Ce had a refining effect. Tong
et al. [32] reported that 1 % Nd slightly refined the AZ91 alloy, but Li et al. [33] found
that adding a Nd-containing mischmetal coarsened the grain size which increased
further as more mischmetal was added. Coarsening also occurred when Mg-Al-Y
alloys were investigated [34]. Li et al [35] reported that an addition of 1 ~ 3 % Gd to
an AZ31 alloy produced significant grain refinement. Liu et al [36] found that a
combined addition of 1 % Gd and Ca reduced the grain size of a Mg-7Al-1Si alloy, but
the grain size became larger after more Gd-Ca was added. It seems that the
experimental results from different researchers often contradict each other. In the
present paper, we investigate the effects of Sm on the microstructure and grain
refinement of the Mg-3Al alloy. Unlike the results reported above from the literature
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where either refining or coarsening of the grain size by the addition of REs to Mg-Al
alloys, the present study observed both coarsening and refining depending on the
amount of RE addition. This study was designed to reveal the mechanisms
responsible for the observed change from grain coarsening to grain refining by the
addition of Sm.
2. Experimental details
Each ~ 3kg batch of Mg-3Al-xSm alloy (x = 0, 0.7, 1.4, 2.1, wt.%) was prepared by
melting pure Mg (99.7 %), pure Al (99.9 %) and Mg-25%Sm master alloy at 1003 K in
a steel crucible heated by an electric resistance furnace under a protective gas
mixture of CO2 and SF6. The melt was stirred for 5 min to ensure complete mixing of
the elements and then held for 10 min before sampling. A sand mould, which
provided a cooling rate of ~ 5 K/s, was used. A K-type thermocouple was placed at
the center of the mould before casting in order to obtain the cooling curves. The tip
of thermocouple was located at 10 mm from the bottom of the mould. Once the
alloy melt was cast into the mould, the cooling curves were recorded by a computer.
For each test, two identical conical samples (Φ40mm-Φ20mm×h50mm) were
obtained to ensure reproducibility. Chemical analysis by ICP-AES (Inductively Coupled
Plasma-Atomic Emission Spectroscopy) revealed that the actual compositions only
deviated slightly from the nominal compositions (Table 1). The specimens for
microstructural observation were obtained by cutting along the radial direction at ~
10 mm from the bottom of the conical samples, polished with grinding and polishing
papers and MgO solution until no significant scratches could be observed, and then
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etched in a picric ethanol solution (30 g picric acid, 40 ml water, 60 ml acetic acid and
360 ml ethanol) for ~ 6 second. Optical microscopy (OM) and scanning electron
microscopy (SEM) were used to observe the microstructure. The accelerating voltage
of the SEM was set as 20 KV. The grain size of each alloy was measured according to
ASTM 112-96. Energy dispersive X-ray (EDS) and Electron backscatter diffraction
(EBSD) analyses were performed to determine the compositions of the different
phases present in the microstructure. The size distribution of the Al2Sm particles in
the Mg-3Al-2.1Sm alloy was measured on the SEM and OM images using image
analysis software (Image-Pro Plus) [26-27]. More than 15 images of ~ 0.6mm2 in area
were analyzed. The “Interdependence Theory” proposed by StJohn et al. [37] was
then used to describe the nucleation selection process. For this purpose, the
characteristics of the Al2Sm particles, such as the relationship between nucleation
undercooling (ΔTn), the average particle spacing (SΩ) and the particle size distribution,
were calculated according to the methods provided in [37].
3. Results
3.1 Microstructures
The microstructure of the Mg-3Al alloy contains α-Mg and a small amount of
β-Mg17Al12 (Fig. 1(a)). When 0.7 % or 1.4 % Sm was added to the Mg-3Al alloy, an
additional phase Al11Sm3 [38, 39] was observed (Fig. 1(b-c)). Most of the Al11Sm3
particles have an irregular and needle-like morphology and are located at the grain
boundaries. When 2.1 % Sm is added, both Al11Sm3 and Al2Sm phases [38, 39] are
formed and most of the Al2Sm particles have a blocky morphology and many of them
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are located in the center of the grains (Fig. 1(d)). These two phases were identified
by EDS on at least ten locations as shown in Fig. 1(e): the ratio of Al to Sm at location
“1” marked in Fig. 1(e) is 2.2:1 which is very close to 2:1, and at location “2” the ratio
is 11.9:3 which is very close to 11:3. The presence of Mg in the EDS results is due to
the influence of the α-Mg matrix since the Al:Sm ratio remains relatively constant
while the proportion of Mg varies from 10% to 80%.
Besides α-Mg and Mg17Al12, Al-Fe-C-O particles, which are reported to be
heterogeneous nucleant particles naturally present in Mg-Al alloys [22], were also
found in the Mg-3Al alloy as in Fig. 1(a). Fig. 2(a) shows the EDS line-scanning results
of a typical native Al-Fe-C-O particle. Although it is difficult to identify the exact
chemical composition because of the small size of the particles, these particles were
found to be rich in Al, Fe, C and O compared to the adjacent α-Mg matrix. Similar
particles were also observed in all of the Sm-bearing alloys (Figs. 2(b-c)). However,
the EDS line-scanning results (Fig. 2(b)) show that the particles in the Sm-bearing
alloys contain not only Al, Fe, C and O, but also a high level of Sm. These particles are
uniformly distributed throughout the microstructure. At least 15 particles were
analyzed by EDS to confirm the existence of Sm.
3.2 Grain size
The relationship between the grain size of the Mg-3Al-xSm alloys and the Sm content
is shown in Fig. 3. Figs.3(a-d) are the OM images that show the changing trend of
grain size when the Sm content varies from 0 to 2.1 %. Fig. 3(d) has a higher
magnification clearly showing that many of the Al2Sm particles are at the center of
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the grains. Fig. 3(e) presents a plot of grain size versus Sm content. The grain size of
the Mg-3Al alloy is ~250 μm. With 0.7 % Sm the grain size coarsens to ~1000 μm,
while the addition of 2.1 % Sm refines it to ~70 μm (Figs. 3(a-d)). It is interesting to
note that the relationship between Sm content and grain size is non-monotonic, i.e.
small amounts of Sm (0.7 % and 1.4 %) coarsens the Mg-3Al alloy grain size while a
larger amount of Sm (2.1 %) dramatically refines it. This relationship differs from the
results in the literature [29-34], where RE addition was reported to either refine or
coarsen the grain size of the Mg-Al alloys but not both.
4. Discussion
4.1 The grain coarsening effect of 0.7 % or 1.4 % Sm
As mentioned in the introduction, it is well accepted that the two key factors
promoting grain refinement are (1) solutes that cause rapid development of
constitutional supercooling (i.e. solutes with a high growth restriction factor, Q) and
(2) heterogeneous nucleant particles of high nucleation potency [5, 37, 40].
Correspondingly the grain size (dgs) can be expressed as [40]:
dgs = a + b/Q Eq. 1
where the slope b is related to the potency of the nucleant particles and a
corresponds to the maximum number of activatable nuclei. For a binary alloy Q can
be expressed as [41]:
Q=m C0 (k-1) Eq. 2
where m is the slope of the liquidus; k is the partition coefficient; C0 is the
concentration of the solute in the alloy. If there is more than one solute element
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then the total value of Q can be estimated by summing the Q values provided by
each element [42]:
Qtotal=ΣmiCi(ki-1)i Eq. 3
The Q value for the Mg-3Al-0.7Sm alloy is calculated to be ~ 14 K, which is slightly
higher than for the Mg-3Al alloy at ~ 13 K. The increase of Q caused by the addition
of 0.7 % Sm would only generate a small degree of grain refinement according to Eq.
1, but coarsening would not be expected. However, the grain size of the
Mg-3Al-0.7Sm alloy is much larger than the Mg-3Al alloy. Hence, a in Eq. 1
dramatically increases and b is not a linear relationship indicating that the potency of
the heterogeneous nucleant particles present in the Mg-3Al alloy has been reduced
when Sm is added to the binary alloy.
According to the literature, the native Al-Fe-C-O particles can act as heterogeneous
nucleant particles in Mg-Al alloys [10-12]. Although Al-C-O (or Al4C3) particles are
usually considered to be the native heterogeneous nucleant particle, Al-Fe-C-O
particles will form when steel crucibles are used [11-12]. In the present study,
Al-Fe-C-O particles were found in the Mg-3Al alloy as shown in Fig. 1(a) and Fig. 2(a).
The EDS line-scanning results show that these particles are rich in Al, Fe, C and O,
which agrees well with the literature [11-12]. However, in the Sm-bearing alloys,
almost all of the similar particles contain not only Al, Fe and C but also a high level of
Sm (Fig. 3(b)). It seems that the Al-Fe-C-O particles in the Mg-3Al alloy have
transformed to Al-Fe-Sm-C-O particles after Sm addition. Despite the crystal type and
parameters of Al-Fe-Sm-C-O being unknown, it is reasonable to conclude that Sm
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combines with the Al-Fe-C-O particles producing Al-Fe-Sm-C-O particles that have a
lower nucleation potency than the native nuclei in the Mg-Al alloy, resulting in
coarsening of the grain size of the Mg-3Al-0.7Sm and Mg-3Al-1.4Sm alloys. Note that
the grain size of the Mg-3Al-1.4Sm alloy is slightly finer than that of the
Mg-3Al-0.7Sm alloy. According to Eq. 1 and assuming the number of Al-Fe-Sm-C-O
particles is similar, this decrease in grain size could be due to the slightly higher
Q-value of the Mg-3Al-1.4Sm alloy (~ 15 K) than that of the Mg-3Al-0.7Sm alloy (~ 14
K).
4.2 The grain refining effect of 2.1 % Sm
When 2.1 % Sm is added to the Mg-3Al alloy, Al2Sm particles solidify prior to α-Mg
during solidification and the alloy is significantly refined. The solidification of Al2Sm is
confirmed by the solidification cooling curves of the Mg-3Al-0.7Sm and
Mg-3Al-2.1Sm alloys (Fig.4). The peak of the first derivative of the cooling curve at
~955.5 K in the Mg-3Al-2.1Sm alloy (Fig. 4(b)) indicates that the solidification of the
Al2Sm phase occurs prior to the nucleation and solidification of the α-Mg grains. In
contrast, there is no similar peak present in the derivative curve of the Mg-3Al-0.7Sm
alloy (Fig. 4(a)), indicating that Al2Sm does not solidify prior to α-Mg.
According to the Edge-to-Edge Matching (E2EM) model [24], the Al2Sm compounds
have the potential to become heterogeneous nuclei for the α-Mg phase and the
orientation relationship (OR) between Al2Sm and α-Mg can be described as follows:
[112]Al2Sm|| [21— —— —— —
1— —— —— —
0]α-Mg,
(11———————
0) Al2Sm || (01—— —————
10)α-Mg, (1—————— —
1———————
1)Al2Sm|| (0001)α-Mg
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In this case, the misfit between the Al2Sm particles and the α-Mg matrix is ~ 0.45 %,
which is considerably lower than 2% indicating that Al2Sm has the potential to be a
potent heterogeneous nucleant for α-Mg. In the Mg-3Al-2.1Sm alloy, most of the
Al2Sm particles are at the center of the grains (Fig. 2(d)), indicating they are
heterogeneous nuclei for α-Mg. EBSD analysis (Fig. 5) confirms that the OR between
Al2Sm and α-Mg presented above can be reproducibly observed in the Mg-3Al-2.1Sm
alloy. Fig. 5(a) is an SEM image of the Mg-3Al-2.1Sm alloy in which three phases were
observed: α-Mg, polygonal Al2Sm and needle-like Al11Sm3 [25]. Figs. 5(b-c) show the
Kikuchi band patterns collected from the Al2Sm particle and its nearby α-Mg,
respectively. It is clear that the [21—— —— —— —
1—— —— —— —
]Al2Sm Kikuchi pole is very close to the [12—————— —
10]α-Mg
pole, and the (111)Al2Sm band is almost parallel to the (0001) α-Mg band. These results
agree well with the literature [24-25], indicating that the grain refining effect of 2.1 %
Sm is due to Al2Sm acting as the nuclei for α-Mg.
4.3 The nucleant selection process in the Mg-3Al-2.1Sm alloy
As mentioned above, a small crystallographic misfit between particles and the matrix
is an important prerequisite for particles being potent nucleants. Besides this, the
final grain size of an alloy is also determined by how many potent nucleant particles
will become active nuclei. In alloys where master alloys are added, typically in
aluminum alloys, grains nucleate on a very small proportion (1%-2%) of the added
particles while the vast majority (≥98%) do not contribute to grain refinement.
Recently, StJohn et al [37] explained this phenomenon through two aspects. Firstly,
the nucleation undercooling (ΔTn, which represents the undercooling needed to
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activate nucleation) is inversely correlated with the particle size (Ω) [43]. This analysis
shows that most of the particles were of much lower potency compared to the
largest particles. Hence, ΔTCS (the amount of constitutional supercooling) that is
needed to trigger nucleation will be less for particles with the largest size. Secondly, a
Nucleation-Free Zone (NFZ) will immediately form around each new growing grain.
Since the constitutional supercooling within NFZ is always lower than at the end of
the NFZ zone, nucleation on all the potent nucleant particles existing within NFZ will
be suppressed. Thus, once a new grain nucleates on the largest particle, all the other
particles around it will not be able to be activated because of NFZ. That is to say, the
α-Mg grains will only nucleate on the largest particles.
In the present study, the total number density of Al2Sm particles (Nv) in the
Mg-3Al-2.1Sm alloy is 8.06×106 cm
-3. By comparing the number density of Al2Sm
particles with the number of grains calculated from the measured grain size of the
Mg-3Al-2.1Sm alloy, the proportion of the effective nuclei can be calculated to be ~
40%. This proportion is much higher than the 1 ~ 2% reported for the situation where
heterogeneous nucleant particles are added to other alloys by master alloy additions
[4]. Fig. 6 presents the distribution characteristics of the Al2Sm particles in the same
Mg-3Al-2.1Sm sample. Fig. 6(a) shows the distribution NΩ of the number of potent
nucleant particles for each value of particle size Ω. It can be seen that the majority of
particles have a size of 8 ~ 9 μm. As the particle size increases or decreases from 8 ~
9 μm, the number densities of these particles decrease. Fig. 6(b) shows the
relationship between nucleation undercooling ΔTn and Ω according to the
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relationship given by the following equation [43]:
∆�� =��
∆��∙
Eq.4
where σ is the Solid-Liquid interfacial energy and ΔSV is the entropy of fusion. All the
related data can be found in [24]. Fig. 6(c) represents the relationship between the
average particle spacing SΩ and Ω, which shows an opposite trend to that in Fig. 6(a).
This means that as the number density increases for particles of a particular size, the
average particle spacing SΩ becomes smaller. By bringing together the information in
Figs. 6(a-c), Fig. 6(d) plots ΔTn against SΩ.
According to the Interdependence theory [37], the grain size dgs of a cast alloy
consists of two parts:
dgs = xnfz + xSΩ Eq. 5
where xnfz represents the length of NFZ where the amount of constitutional
supercooling is always lower than ΔTn for the most potent nucleant particles, and xSΩ
represents the average particle spacing (SΩ) of operative particles. This relationship is
illustrated in Fig. 7, where TE represents the equilibrium liquidus temperature and TA
represents the actual temperature of the melt. Note that SΩ is just one component of
dgs. Hence, SΩ will be smaller than the grain size dgs. In the present study, the average
spacing SΩ between the largest particles is about 120 μm. Assuming that only the
largest particles are operative, the grain size was predicted to be larger than 120 μm.
However, the grain size of the Mg-3Al-2.1Sm alloy is ~ 70 μm which is about half of
the predicted SΩ between the largest particles. This indicates that nucleation occurs
on more particles than all of the largest particles present in the alloy. However, the
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three largest particle sizes have values of ΔTn within 0.05 K of each other (Fig. 6(d)).
Since there will be a temperature gradient in the melt due to heat extraction, any of
the particles in this size range could be activated. Hence, as shown in Fig. 7, instead
of the average spacing SΩ between the largest particles being 120 μm, the average SΩ
between any of the particles whose ΔTn is less than 0.2 K is ~ 60 μm which was
calculated by adding the particle densities for the three largest particle sizes and
converting this number to grain size. According to Fig. 4b the liquidus temperature
(TE) of this alloy is estimated to be ~ 916 K. When the amount of constitutional
supercooling (ΔTCS) reaches a value between 0.05 K and 0.2 K below 916 K, a particle
whose ΔTn is lower than 0.2 K will be activated. Those smaller particles whose ΔTn is
greater than 0.2 K are unlikely to be activated because they are further away from
the end of NFZ and more constitutional supercooling will be required to activate
nucleation. From this analysis the probability of the grain size being about 70 µm due
to a higher proportion of effective nuclei is a reasonable expectation. In addition, the
low value of ΔTn of less than 0.2 K would result in a small NFZ zone which also
promotes a fine grain size. Recalling that the length of NFZ and the length of SΩ (in
this case ~60 μm) together define the grain size, the analysis to create Fig. 7 implies
that NFZ would be about 10 µm in length.
An alternative explanation may be related to the fact that the Mg-Al-Sm alloy system
is a typical peritectic system [34]. The peritectic reaction during solidification is
Al2Sm+Liquid→Mg, which may also contribute to the observed grain refinement. As
Al2Sm is one of the reactants to generate α-Mg, the chemical reaction is favorable to
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the nucleation of α-Mg on the surface of Al2Sm [26, 40, 41-44]. However, in normal
cooling situations the classic peritectic reaction hardly ever occurs [45]. Hence, more
work is still needed to understand this deeply.
4.4 Implications for achieving refinement during the solidification of Mg-Al-RE alloys
As mentioned in the Introduction, previous research observed either refining or
coarsening of the grain size in a range of Mg-Al-RE systems. Considering the results of
this study, a review of this research shows that refining occurs when Al2RE is formed,
or is expected to form, before the solidification of α-Mg due to higher concentrations
of RE elements [25-26, 34]. Coarsening is observed in alloys of low RE content in
AZ31 where Al2RE is unlikely to form. In this case coarsening is caused by the
combination of the RE element with the Al-Fe-C-O phase forming an Al-Fe-RE-C-O
phase dramatically reducing the nucleation potency of this phase. It is proposed that
the findings of this work can be generalized to other Mg-Al-RE systems because most
of the Al2RE phases have very small crystallographic misfits with α-Mg as shown in
Table 2 [24]. Further, the coarsening behavior may be influenced by the impurity
level, especially the Fe and C contents, and possible differences in the affinity
between different REs and Fe or C. The only contradictory evidence is that reported
by Yu et al. [29] where a 0.4 % Ce alloy refined the AZ31 alloy. However, Pan et al. [30]
found that for a range of 0.5 to 1.5 % Ce contents coarsening occurred in an AZ31
alloy. The results given in [30] are more credible since both authors claimed that no
Al2Ce was found in the AZ31Ce alloys. However, a definitive conclusion cannot be
made because the actual chemical compositions of the nominal alloys were not
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provided in [29] and [30].
5. Conclusions
The addition of Sm to a Mg-3Al alloy has either a coarsening or refining effect on the
grain size depending on the addition amount. The addition of a small amount of Sm
(0.7 and 1.4 % Sm) leads to significant grain coarsening, which is proposed to be due
to a reduction in the nucleation potency of the Al-Fe-C-O particles when they
transform to Al-Fe-Sm-C-O particles. On the other hand, 2.1 % Sm refines the Mg-3Al
alloy because Al2Sm particles solidify prior to α-Mg and, due to a favorable
orientation relationship, are potent heterogeneous nucleation sites for α-Mg during
the solidification of the Mg-3Al-2.1Sm alloy. An analysis of the distribution of Al2Sm
particle sizes showed that there is a sufficiently high number density of large
particles with lower than 0.2 K nucleation undercooling (i.e. ~ 40% of the total
number of particles) to produce the measured grain size. It is proposed that these
findings can be generalized to all Mg-Al-RE alloys of a similar range of compositions.
Acknowledgements
This work is supported by National Natural Science Foundation of China (No.
51201103), SJTU Special Funds for Science and Technology Innovation (No.
13X100030018), the Program for Outstanding Academic Leader of Shanghai
(14XD1425000) and the Australian Research Council (ARC) Discovery Grant
(DP120101672). Dr. Dong Qiu from RMIT is gratefully acknowledged for the support
for the helpful discussion.
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Table 1 Actual compositions of each alloy according to the ICP-AES results.
Nominal
compositions
Al (wt.%) Sm(wt.%) Fe(wt.%) Mg(wt.%)
Mg-3Al 2.93
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Fig. 1 SEM images of (a) Mg-3Al, (b) Mg-3Al-0.7Sm, (c) Mg-3Al-1.4Sm, and (d) Mg-3Al-2.1Sm alloys;
(e) shows the EDS results for location 1 and 2 in Mg-3Al-2.1Sm alloy microstructure.
Al-Fe-C-O
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Fig.2 SEM images of Mg-3Al-xSm alloy: (a) Fe- rich particle (Al-Fe-C-O) in Mg-3Al alloy and (b) Sm-Fe-
rich particle (Al-Fe-Sm-C-O) in Mg-3Al-0.7Sm alloy; (c) Sm-Fe- rich particle in Mg-3Al-2.1Sm alloy.
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0.0 0.7 1.4 2.10
200
400
600
800
1000
1200
Gra
in s
ize
(µm
)
Sm content (wt %)
Grain size(e)
Fig. 3 OM images showing the grain size of (a) Mg-3Al, (b) Mg-3Al-0.7Sm, (c) Mg-3Al-1.4Sm, and (d)
Mg-3Al-2.1Sm alloys. Note that (d) has a higher magnification to show that most Al2Sm particles are
located in the central region of the grains. (e) the grain size plotted against Sm content for all alloys.
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Fig.4 Solidification cooling curves and the first derivative curves of (a) Mg-3Al-0.7Sm alloy and (b)
Mg-3Al-2.1Sm alloy.
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Fig. 5 (a) SEM image of Mg-3Al-2.1Sm alloy; the corresponding EBSD Kikuchi band patterns of (b) Mg
matrix obtained at the pentagon marked in (a), and (c) its nearby Al2Sm particle obtained at the
square in (a).
0 2 4 6 8 10 12
2.0x105
4.0x105
6.0x105
8.0x105
1.0x106
1.2x106
1.4x106
1.6x106
1.8x106
2.0x106
2.2x106
Num
ber
of p
artic
les
(cm
-3)
Ω (µm)
Number of particles(a)
0 2 4 6 8 10 120.0
0.2
0.4
0.6
0.8
1.0
1.2
∆Tn
(K)
Ω (µm)
Ω-∆Tn(b)
0 2 4 6 8 10 12
60
80
100
120
140
160
180
200
SΩΩ ΩΩ ( µµ µµ
m)
Ω (µm)
d-SΩ(c)
0.0 0.2 0.4 0.6 0.8 1.0 1.2
60
80
100
120
140
160
180
200
SΩΩ ΩΩ (
µµ µµm)
∆∆∆∆Tn (K)
∆Τn-SΩ(d)
Fig. 6 Characteristics of Al2Sm particles in Mg-3Al-2.1Sm alloy. (a) the distribution NΩ of the number
of potent particles for each value of diameter Ω. (b) The relationship between nucleation
undercooling ΔTn and d. (c) Relationship between the average particle spacing SΩ and Ω. (d) ΔTn
plotted against SΩ.
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Fig. 7 A graph that illustrates the interrelationship between the development of constitutional
supercooling (∆∆∆∆TCS) between the equilibrium liquidus temperature TE and the actual temperature of the melt TA, and the distribution of particles for the range of particle sizes converted to their
nucleation undercooling (∆∆∆∆Tn-SΩ) that together establish the grain size of Mg-3Al-2.1Sm alloy.
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Highlights
(1) The Mg-3Al alloy was significantly coarsened by the addition of 0.7 wt. % Sm, but
dramatically refined by 2.1 wt. % Sm.
(2) Sm coarsened the Mg-3Al alloy by inhibiting the Al-Fe-C-O particles nucleant potency.
(3) 2.1% Sm significantly refined the Mg-3Al alloy because of the solidification of Al2Sm
particles prior to the solidification of α-Mg.
(4) “Interdependence Theory” was applied to describe the nucleant selection process in the
Mg-3Al-2.1Sm alloy.
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