Materials Transactions, Vol. 56, No. 7 (2015) pp. 1144 to 1146
© 2015 The Japan Institute of Metals and Materials
EXPRESS RAPID PUBLICATION
Effect of Sn on Thermal Conductivity of Mg-5Zn Based Alloys
H. Kang, J. Y. Suh, S. W. Kang and D. H. Bae+
Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 120-749, Korea
For the enhancement of high temperature performance of the Mg-Zn based alloys, Sn element has been alloyed. However, the effect of Sn
on the thermal conductivity of the Mg-Zn based alloys has been barely studied. In this study, pure Mg and Mg-5Zn alloys are alloyed with a 1, 3,
and 5 mass% of Sn, respectively, and the heat capacity, thermal diffusivity and thermal conductivity of the alloys are then evaluated in a
temperature range of 298 to 573 K. In the Mg-Sn binary alloys, the heat capacity and thermal diffusivity gradually decrease with increasing the
volume fraction of the Mg2Sn phase. In the Mg-Zn-Sn based alloys (ZT alloy), since the adding Sn element is mostly located at the MgZn2
phases, thermal diffusivity is not affected with an increment of Sn content. Furthermore, the thermal conductivity of the ZT55 alloy at elevated
temperatures is slightly higher than that of the ZT53 alloys. [doi:10.2320/matertrans.M2015123]
(Received March 24, 2015; Accepted May 1, 2015; Published June 12, 2015)
Keywords: thermal diffusivity, thermal conductivity, magnesium-zinc-tin, magnesium alloy, heat capacity
1.
Magnesium (Mg) as a light metal (1.74 g/cm3) has great
potential for applications in many engineering fields,
particularly in the automotive, aerospace, and electronics
fields.1) These applications require good mechanical properties at a service condition of 298 to 473 K. Hence, Mg is
commonly alloyed with aluminum (Al) and zinc (Zn) to
ensure the reliable mechanical properties and thus the Mg-Zn
and Mg-Al based casting alloys are largely used. Although
the Mg-Zn based alloys such as ZK50, and ZK60 alloys offer
excellent mechanical properties at room temperature, the
MgZn phases significantly decrease elevated temperature
performance.
To overcome the mechanical properties limitations of the
alloys at high temperatures, tin (Sn) has a great attention
because the Mg2Sn phase with high melting temperature of
about 1043 K provides barriers for dislocation slip and climb
at high temperatures.2) In recent decades, many researchers
have reported the effect of Sn on the mechanical properties
of Mg alloys at elevated temperatures.3­5) In contrast to the
tremendous work done on mechanical properties, information
on the thermophysical properties of Mg-Sn alloys is
relatively scarce. In general, the thermal conductivity of
Mg alloys significantly depends on alloying elements. Thus,
the thermal conductivity of the Mg-Zn based alloys sharply
decreases with increasing Zn content. In this paper, pure Mg
and the Mg-5 mass% Zn alloys are alloyed with 1, 3, and
5 mass% Sn, respectively. To evaluate the thermal characteristics, the thermal diffusivity and heat capacity of the alloys
are investigated at elevated temperatures.
2.
Experimental
Pure Mg and the Mg-5 mass% Zn (Mg-5Zn) were used in
these experiments. Pure Mg and the Mg-5Zn alloy were
melted at 993 K under a dynamic SF6 + CO2 atmosphere.
Additional element of Sn with 1, 3, and 5 mass% was added
into the Mg alloy melt. The melt was maintained for 30 min
to complete the dissolution of the alloying elements and then
+
Table 1
Introduction
Corresponding author, E-mail: [email protected]
Alloy
Chemical composition of the experimental alloys.
Composition (mass%)
Zn
Sn
Mg
Mg-1Sn
®
1
Bal.
Mg-3Sn
Mg-5Sn
®
®
3
5
Bal.
Bal.
ZT51
5
1
Bal.
ZT53
5
3
Bal.
ZT55
5
5
Bal.
poured into a permanent mold with a thickness of 12 mm.
The fabricated alloys are listed in Table 1.
The microstructure examination was carried out using
optical microscopy (OM, Nikon LU-100) and field emission
scanning electron microscopy (FESEM, JEOL JSM-7001).
Specimen density was estimated using a gas expansion
pycnometer (Ultrapycnometer 1000, Quantachrome Co. Ltd,
USA). Non-isothermal measurements were carried out using
differential scanning calorimetry (DSC; DSC 8000, Perkin
Elmer) in an argon atmosphere with different heating rates
(10 K/s) from 273 K to 573 K. All specimens were covered
by Al pans, a blank reference pan with the same composition
as the sample pan was used in all DSC run. Laser flash
analyzer (LFA 447, NETZSCH) was used to measure the
thermal diffusivity of the specimens. All specimens (diameter: º6 mm, thickness: ³2 mm) were polished up to 2000 grit
and then buffered.
3.
Results and Discussion
To clarify the effect of Sn on the thermal conductivity
of Mg-Zn alloys, it is essential to study the thermal
conductivity of the Mg-Sn binary systems so that the effect
of other alloying elements can be eliminated. Figure 1 shows
the optical microstructure of Mg-1Sn, Mg-3Sn, and Mg-5Sn
alloys. A typical dendritic structure with ¡-Mg matrix and a
large amount of the Mg2Sn intermetallics distributed along
dendritic boundaries was observed in all the alloys. More
intermetallics are precipitated with the increase in the Sn
concentration.
Effect of Sn on Thermal Conductivity of Mg-5Zn Based Alloys
(a)
(b)
(c)
Fig. 1 Optical microstructure of the (a) Mg-1Sn, (b) Mg-3Sn, and (c) Mg5Sn alloys.
(a)
1.1
Heat Capacity , Cp/ J/g/K
1.0
0.9
0.8
0.7
0.6
Mg-1Sn
Mg-3Sn
Mg-5Sn
0.5
0.4
300
350
400
450
500
550
600
Temperature, T/ K
2
100
100
D
90
k
Mg-1Sn
Mg-3Sn
Mg-5Sn
90
80
80
70
70
60
60
50
50
40
300
350
400
450
500
550
Thermal Conductivity, k/ W/m/K
Thermal Diffusivity, D/ mm /s
(b)
40
600
Temperature, T/ K
Fig. 2 Temperature dependences relating to the (a) heat capacity,
(b) thermal diffusivity and conductivity of the Mg-1, 3, and 5 Sn alloys.
To highlight the influence of Sn on the thermophysical
properties of the Mg-1, 3, and 5Sn alloys at elevated
temperatures, the temperature dependences relating to the
heat capacity and thermal diffusivity of the Mg-1, 3, and 5 Sn
alloys are shown in Fig. 2. In Fig. 2(a), the heat capacity of
the Mg-1, 3, and 5 Sn alloys at 298 K are 0.87, 0.84, and
0.79 J/g/K, respectively, and the heat capacity of the alloys
slowly decreases with increasing temperature. After 473 K,
the heat capacity of the alloys rapidly increases. Heat
capacity (CP) is defined as an equation of CP = Q/m/"T,
where Q, m, and "T represent heat-transferred energy, mass
of material, and temperature variation. According to the
equation, heat capacity depends on heat-transferred energy
which is lattice vibration. Alloying elements disturb the
constant vibration of the Mg lattice thereby the heat capacity
of the alloys is lower than that of pure Mg. With increase in
temperature, phonons with large wave-vectors are capable of
changing the constant vibration of the Mg lattice.6) In metals,
the contribution of phonons to thermophysical properties
increase with the increase of temperature thus heat capacity
1145
above a specific temperature is increased, while heat capacity
at low temperatures is decreased by phonon scattering.
Furthermore, since the specific heat capacity of Sn is very
low at 0.21 J/g/K, the heat capacity of the Mg-Sn alloys
is lower than that of pure Mg (1.02 J/g/K at 298 K).
Figure 2(b) shows the thermal diffusivity (D) and thermal
conductivity (k) of the Mg-1, 3, and 5Sn alloys at elevated
temperatures. The thermal diffusivity of Mg-1, 3, and 5 Sn
alloys at 298 K is 53.52, 48.31, and 42.62 mm2/s and the
diffusivity of the alloys gradually increases with increasing
temperature. Sn solute atoms and intermetallic compounds in
the Mg-Sn alloys are both scattering sources of electrons and
phonons. Thus thermal diffusivity decreases with the increase
of Sn content. However, electronic thermal resistivity is
inversely proportional to temperature thereby the thermal
diffusivity of the alloys increases with temperature.7) The
temperature dependence associated with the alloy densities
can be determined using the equation µ = µ0(l + "l/l0)¹3,
where "l/l0 is a relative elongation of material which can be
disregarded because the difference between the longitudinal
strain values of the alloys at 298 and 573 K is very small
("l/l0 = 26 µ/K).8) The densities of Mg-1, 3, and 5Sn alloys
at room temperature are measured as 1.75, 1.78, and 1.80
g/cm3, respectively, and these values are used to calculate
thermal conductivity at all temperatures. From the results
of the density, heat capacity, and thermal diffusivity of
the alloys, the thermal conductivity (k = µ © CP © D) of the
alloys are calculated with the temperature range of 298 to
573 K and shown in Fig. 2(b). The shapes of the thermal
conductivity curves of the alloys are similar to the heat
capacity of the alloys because the values are simply
calculated by the product of the component. The thermal
conductivity value of the Mg-1, 3, and 5 Sn alloys at 298 K
are 82.02, 72.29, and 61.27 W/m/K, respectively, and the
values at 573 K are 97.58, 89.15, and 70.61 W/m/K,
respectively.
Microstructure of the ZT51, ZT53, and ZT55 alloys is
observed using SEM and shown in Fig. 3. All the alloys
exhibits a typical dendritic microstructure. The dendrite
spacing is barely altered as the Sn contents increase. The
EDS results indicate that both the isolated and the
discontinuous phases correspond to an MgZn2 phase with
trace Sn content. From the magnified microstructure of the
alloys, the MgZn2 phases join with Mg2Sn, which is a
validated EDS analysis. In previous research,9) the Mg-Sn-Zn
phase diagram in the Mg2Sn-MgZn2-Zn-Sn regions showed
that the solubility of Zn in Mg2Sn is about 1 mass% along the
Mg2Sn-MgZn2 region. The solubility of Sn in the MgZn2
phase has also been reported.10) However, it is reported that
Sn does not demonstrate solubility in the Mg7Zn3 and
Mg4Zn7 phases. In Fig. 3(d), the MgZn2 phase shows a small
amount of overlap with the Mg2Sn-Zn phase. The portion of
the Mg2Sn-Zn phase in the MgZn2 phase increase as Sn
content increases. In the ZT55 alloy (Fig. 3(f )), the region
occupied by the Mg2Sn-Zn phase in the single MgZn2 phase
is more than half. Therefore, most of the Sn exists in the
MgZn2 phase and some of it forms the Mg2Sn phase at
arbitrary positions in the grain.
To evaluate the effect of Sn on the thermal conductivity of
the Mg-5Zn alloys, the temperature dependences relating to
1146
H. Kang, J. Y. Suh, S. W. Kang and D. H. Bae
(a)
(d)
(b)
(e)
(c)
(f)
and Mg2Sn-Zn phases in the Mg-Zn-Sn alloys are sources of
electrons and phonons scattering. However, the thermal
diffusivity values of the ZT53 and ZT55 alloys at elevated
temperatures are very similar with only a 0.38 mm2/s
difference between them since most of the Sn is precipitated
in the MgZn2 phase. The density of the ZT51, ZT53, and
ZT55 alloys at 298 K is 1.83, 1.87, and 1.88 g/cm3,
respectively. The heat capacity of the alloys is shown in
Fig. 4(a). Based on the density, heat capacity, and thermal
diffusivity results for the alloys, the thermal conductivity of
the ZT51, ZT53, and ZT55 alloys are calculated to be 111.10,
82.96, and 87.21 W/g/K at 25°C. The thermal conductivity
of the ZT55 alloy is slightly higher than that of the ZT53
alloy because the density and heat capacity of the ZT55 alloy
is higher than that of the ZT53 alloy although the thermal
diffusivity values for both alloys are similar.
4.
Fig. 3 SEM images of (a) ZT51, (b) ZT53, and (c) ZT55 alloys, and
magnified images of the (d) ZT51, (e) ZT53, and (f ) ZT55 alloys. EDS
spectrums of the regions marked with arrows are inserted in (d).
(a)
2.0
ZT51
ZT54
ZT55
Heat capacity, Cp/ J/g/K
1.8
1.6
1.4
1.2
1.0
0.8
0.6
300
350
400
450
500
550
600
160
D
180
k
ZT51
ZT53
ZT55
2
180
160
140
140
120
120
100
100
80
80
60
60
40
300
350
400
450
500
550
Thermal Conductivity, k/ W/m/K
(b)
Thermal Diffusivity, D/ mm /s
Temperature, T/ K
40
600
Temperature, T/ K
Fig. 4 Temperature dependences relating to (a) the heat capacity, (b) the
thermal diffusivity and thermal conductivity of the ZT51, ZT53, and
ZT55 alloys.
the thermal diffusivity and thermal conductivity of the ZT
alloys are shown in Fig. 4(a) and (b), respectively. Due to the
Mg-5Zn alloy with excellent thermal diffusivity (about
61 mm2/s),11) the thermal diffusivity of the Mg-Zn-Sn alloys
is not drastically dropped by the Mg2Sn phases and the
diffusivity values of the alloys at elevated temperatures is
increased by decreasing electronic thermal resistivity. The
fact that thermal diffusivity decreases when the Sn contents
increase can be easily understood by considering that the
more Sn impurity that is added to pure Mg, the greater
the opportunity for electrons to be scattered, and thermal
diffusivity to become lower. Furthermore, both the MgZn2
Conclusions
The effect of Sn on the thermal conductivity of pure Mg
and the Mg-Zn alloys at elevated temperatures was investigated. Due to the Mg2Sn phase, the heat capacity and
thermal diffusivity of the Mg-Sn binary alloys at elevated
temperatures decrease when Sn contents increase. In ZT alloy
systems, the volume fraction of the phases corresponding to
MgZn2, Mg2Sn, and Mg2Sn-Zn is largely not altered in any
of the alloys. Thus, the thermal diffusivity of the alloys is not
affected when Sn content increase. Furthermore, the thermal
diffusivity values for the ZT53 and ZT55 alloys are very
similar because the Sn element mostly exists in the MgZn2
phases which proportionally decrease in the Mg2Sn phase.
Therefore, the thermal conductivity of the ZT55 alloy is
slightly higher than that of the ZT53 alloy due to the higher
density of the ZT55 alloy.
Acknowledgements
This work was supported by the National Research
Foundation of Korea (2012-R1A1A-204-2329).
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