Materials Transactions, Vol. 61, No. 11 (2020) pp. 2101 to 2106
© 2020 The Japan Institute of Metals and Materials

他誌発表論文

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公益社団法人日本金属学会, Materials Transactions, Vol. 61 (11) p. 2101-2106 (2020)

Hot Compression Behavior of Mg­14Li­6Al­1Ca Alloy
Ryo Tsukane1, Hiroyasu Tamai1, Masahiro Nogawa2, Yoshihisa Harada3, Kunio Matsuzaki3 andTakashi Nagoshi3
1

Tottori Institute of Industrial Technology, Yonago 689-3522, Japan
Kikusui Forging Ltd., Yonago 683-0851, Japan
3
National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8564, Japan2

The hot deformation behavior of a Mg­14Li­6Al­1Ca alloy was studied using the hot compression true stress­strain curves corresponding
to the temperature range of 473­673 K at strain rates of 1 © 10¹1­1 © 10¹3 s¹1. The true stress­strain curves indicated dynamic softening under
the test conditions. The peak stress during deformation could be correlated with the temperature and strain rate using a hyperbolic-sine equation.
The activation energy of the Mg­14Li­6Al­1Ca alloy was determined to be 193 kJ mol¹1. The Zener-Hollomon parameter (Z) for the Mg­14Li­
6Al­1Ca alloy was determined. The tendency for dynamic recrystallization increases at low strain rates and high temperatures, corresponding
to low Z values. The hot deformation behavior of the Mg­14Li­6Al­1Ca alloy was modelled by a suitable constitutive equation. Furthermore,
the size of the equiaxed grains in the hot-deformed and quenched specimens was estimated from the Z value.
[doi:10.2320/matertrans.MT-M2020157](Received May 19, 2020; Accepted August 3, 2020; Published October 25, 2020)
Keywords: magnesium­lithium alloy, hot deformation, Zener-Hollomon parameter, activation energy, dynamic recrystallization

1.

Introduction

Magnesium alloys are known for their low density andhigh strength to weight ratio, resulting in a high demand
from industries such as transportation and electric appliances.
Magnesium­lithium (Mg­Li) alloys have a lower density
than other magnesium alloys. The addition of lithium(>³11 mass% (30 at%)) to magnesium causes the crystal
structure to transform from hcp ¡ phase to single bcc ¢ phase.
The bcc ¢ phase is soft and ductile, leading to increased
deformability. The mechanical properties and the plastic
deformation of Mg­Li alloys at room temperature havelong been investigated.1­13) However, high temperatures arenecessary for any deformation involving complex and large
shapes, even if the original Mg­Li alloys are soft and ductile.
To date, there is very little information about the hotcompression deformation behavior of Mg­Li alloys. If
dynamic recrystallization occurs during high-temperature
deformation, the ﬂow stress will decrease and thus, plastic
deformation becomes easier. Further, the achievement of
an equiaxed crystal grain via recrystallization can improvevarious mechanical properties of the alloy such as its strengthand ductility.
To study the hot compression behavior, it is important tounderstand the ﬂow stress and microstructure evolution. The
Zener-Hollomon parameter is often used to understand theﬂow stress of high-temperature compression deformation,14,15) since it establishes a correlation between
temperature, strain rate, and ﬂow stress. Several studies havereported that it is possible to optimize the microstructural
evolution.16­19) The processing conditions by which thegrains become equiaxed after deformation were estimatedusing the Zener-Hollomon parameter.
This study is, therefore, devoted to the investigation of thehot deformation behavior of a Mg­Li­Al­Ca four-elementbased alloy (Al and Ca were added to improve the alloy’s
mechanical properties). Analysis was performed by tuning

the Zener-Hollomon parameter to control the grain structure
obtained by deformation and recrystallization and to improvethe mechanical properties of the alloy. The relationshipbetween the Zener-Hollomon parameter and the microstructural evolution in the hot-deformed specimens has also
been determined.
2.

Experimental Procedure

2.1 Process and materials
2.1.1 Materials
The as-received Mg­14Li­6Al­1Ca alloy was an extruded
round rod with a diameter of 15 mm, obtained from Santoku
Co. Ltd., Japan. The chemical composition of the alloy isgiven in Table 1. Cylindrical specimens of 10 mm diameterand 15 mm height were machined from the as-received rod.
Phase information were obtained by a X ray diﬀraction(XRD), using a Ultima IV (Rigaku, Japan).
2.1.2 Process
Hot compression tests of the specimens were performed on
an MTS-810 system (MTS, US) by maintaining the furnace
within a temperature range of 473­673 K at a strain rate inthe range of 1 © 10¹1­1 © 10¹3 s¹1. After soaking for 5 minin the above temperature range, hot compression wasperformed up to a true strain of 0.7. Then, the specimens
were immediately quenched in water to preserve the hot
deformation microstructure. The hot-deformed specimens
were cut in a direction parallel to the compression direction
and were examined by optical microscopy after etching in
2% nitric acid in ethanol. The average grain size of thesamples was measured by the linear intercept method using
the following equation:20)
D ¼ 1:74L=nL ;ð1Þwhere L is the total length of the lines used and nL is thenumber of grains crossed through by the lines.

2102

R. Tsukane et al.
Table 1 Chemical compositions of Mg­Li­Al­Ca alloy (in mass%).

Fig. 1

3.

Initial microstructure of the Mg­Li­Al­Ca alloy.

Results and Discussion

Microstructure and composition of the as-received
alloy
The initial microstructure of the as-received alloy is shownin Fig. 1. The microstructure exhibits a mixture of coarse andﬁne grains, with the average grain size of the coarse grains
being approximately 50­80 µm. Figure 2 displays the X-ray
diﬀraction pattern of the as-received Mg­14Li­6Al­1Caalloy, where the basic microstructural constituent of the alloy
is seen to be the ¢ phase (Li).

Fig. 2

X-ray diﬀraction pattern of Mg­14Li­6Al­1Ca alloy.

3.1

3.2 True stress­strain curves
The true stress­strain curves, obtained by the hotcompression testing, are presented in Fig. 3. Since the ﬂow
stress decreases with the increase in temperature and decreasein strain rate, it is evident that the ﬂow stress is aﬀected by
the strain rate and temperature. At a given strain rate, byincreasing the strain, the ﬂow stress ﬁrst increases up to amaximum and then decreases to a steady state. It is suggested
that dynamic softening21) occurred in a temperature rangeof 473­673 K. Further, it may also be considered thatsome stress oscillations occur under particular processingconditions.
3.3

Microstructures of the deformed Mg­14Li­6Al­1Ca
alloy
The typical microstructures of the Mg­14Li­6Al­1Ca
alloy deformed under the true strain value of 0.7 at a strain
rate of 1 © 10¹3 s¹1 in the temperature range of 473­673 K
are shown in Fig. 4. At the lowest temperature of 473 K
and a strain rate of 1 © 10¹3 s¹1, the original grains wereelongated perpendicular to the compression direction andthere was no apparent recrystallization (Fig. 4(a)).
Figures 4(b)­(d) illustrate the microstructures of the specimens hot deformed at 573­673 K and 1 © 10¹3 s¹1 strain
rate. In this case, compared with the grain size of the primary

microstructure in Fig. 1, the average grain size changed, andthe grains were equiaxed. At the temperature of 573 K(Fig. 4(b)), the grain sizes were almost the same as thoseobserved in 623 K (Fig. 4(c)). Coarsening occurred at thetemperature of 673 K. Grain reﬁnement occurred during thehot deformation process owing to dynamic recrystallization,
thereby implying that grain reﬁnement may also occur inMg­14Li­6Al­1Ca alloys depending on the processingconditions.
Constitutive equations for the ﬂow stress of the Mg­
14Li­6Al­1Ca alloy
Constitutive equations were used to calculate the activationenergy and hot deformation constants of the Mg­14Li­6Al­1Ca alloy. The base equation is shown in eq. (2). In thisequation, the Zener-Hollomon parameter (Z) is the temperature-compensated strain rate.

3.4Z ¼ ¾_ expðQ=RT Þ ¼ Fð·Þð2ÞHere, ¾_ is the strain rate, Q is the activation energy ofdeformation, R is gas constant (8.314 J·mol¹1·K¹1), and T is
the temperature.
As seen in eq. (2), the Z parameter is also a function ofstress. The relationship between Z and stress is dependent
on the stress regime. The power law description of stress(eq. (3)) is preferred for relatively low stress values.
Conversely, the exponential law (eq. (4)) is only suitablefor high stresses. However, the hyperbolic sine law (eq. (5))
can be used for a wide range of temperatures and strainrates.22)
Fð·Þ ¼ A0 · p n1
ð3Þ
Fð·Þ ¼ A00 e¢· pð4ÞnFð·Þ ¼ A½sinhð¡· p Þð5ÞHot Compression Behavior of Mg­14Li­6Al­1Ca Alloy

Fig. 3 Compression true stress­strain curves for the Mg­14Li­6Al­1Ca
alloy under various deformation conditions.® (a) ¾_ ¼ 0:001 s¹1, (b) ¾_ ¼0:01 s¹1, and (c) ¾_ ¼ 0:1 s¹1.

Here A, AA, AAA, ¡ (where ¡ µ ¢/n1), ¢, n, and n1 are apparent
material constants, and ·p is the peak stress. For eqs. (3)­(5),
the steady stress is generally used, however, the peak stress
may also by used.15,16,21) By substitution of F(·) fromeqs. (3)­(5) into eq. (2) and by taking the natural logarithm
from each side of the resulting equations, the following
expressions were derived for the peak stress:ln ¾_ þ Q=RT ¼ lnðA0 Þ þ n1 lnð· p Þ;00ln ¾_ þ Q=RT ¼ lnðA Þ þ ¢· p ;ð6Þð7Þ

2103

Fig. 4 Microstructures of alloy subjected to diﬀerent deformation conditions.® (a) 473 K, 1 © 10¹3 s¹1, (b) 573 K, 1 © 10¹3 s¹1, (c) 623 K, 1 ©
10¹3 s¹1, (d) 673 K, 1 © 10¹3 s¹1, where C.A. means the compressionaxis.

ln ¾_ þ Q=RT ¼ lnðAÞ þ n ln½sinhð¡· p Þ:ð8Þ

At a constant deformation temperature, and assuming theactivation energy as a constant parameter, partial diﬀerentiation of eqs. (6)­(8) yielded the following equations,
respectively:@ ln ¾_
n1 ¼ð9Þ@ ln · p T2104

R. Tsukane et al.


¢¼


@ ln ¾_@· p T@ ln ¾_n¼@ ln½sinhð¡· p Þ Tð10Þ
ð11Þ

The slope of the plot of ln ¾_ versus ln ·p, and the slope of
the plot of ln ¾_ versus ·p were used to obtain the values of nA
and ¢, respectively. These plots are shown in Fig. 5(a) and(b). The linear regression of these data resulted in the averagevalues of 5.9 and 0.10 for n1 and ¢, respectively, yielding avalue of ¡ = ¢/n1 = 0.017. According to eq. (11), the slopeof the plot of ln ¾_ versus ln sinh(¡·p) was used to obtaining
the value of n (Fig. 5(c)). The average value of n wasdetermined to be 4.96.
At a constant strain rate, the partial diﬀerentiation ofeq. (8) yields the following equation:@ ln½sinhð¡· p ÞQ ¼ Rn¼ Rnp
ð12Þ@ð1=T Þ¾_
Thus, p is the value of the slope of the plot of ln sinh[(¡·p)]versus the reciprocal of the absolute temperature. This plot isshown in Fig. 5(d). The linear regression of this data resulted
in an average value of 4.68 for p and a value of 193 kJ mol¹1for the activation energy from eq. (12).
Taking the natural logarithm of eq. (5), the following
expression was derived:ln Z ¼ ln A þ n ln½sinhð¡· p Þð13Þ

The resulting plot is shown in Fig. 6. The linear regression ofthis data resulted in values of 5.11 and 31.8 for n and ln A,respectively. Hence, the relation between Z and ·p wasobtained as follows:
Z ¼ ¾_ expð193000=RT Þ ¼ e31:8 ½sinhð0:017· p Þ5:11 ð14Þ
From this equation, the ﬂow stress was estimated from thetemperatures and strain rates. Figure 7 shows the correlation
between calculated and measured stress values, where theﬂow stress was accurately estimated.
3.5

Relationship between the Zener-Hollomon parameter and the microstructures of the deformed Mg­
14Li­6Al­1Ca alloy
The softening processes of recovery and recrystallization
occur during deformation at high temperatures in many
alloys, and the resulting microstructure of the deformed
alloys is dependent on the processing conditions. The truestress­strain curves of the Mg­14Li­6Al­1Ca alloy (Fig. 3)
show a steady-state ﬂow stress, indicating that softening
occurred as a result of dynamic restoration which cancelledwork hardening. Dynamic recovery may occur at higher Z
values because at these values the alloy is composed ofthe deformed initial grains (Fig. 4(a)). On the other hand,
dynamic recrystallization may occur at lower Z values
because the alloy is composed of equiaxed grains surroundedby clear grain boundaries instead of initial grains (Fig. 4(b)­(d)). The possibility of expressing dynamic recrystallization
using Z was examined. Figure 8 depicts the relationshipbetween Z and the strain rate, temperature, and deformation
structure. As outlined by the solid lines in Fig. 8, theFig. 5 Plots depicting (a) ln ¾_ vs. ln ·p, (b) ln ¾_ vs. ·p, (c) ln ¾_ vs.
ln[sinh(¡·p)], and (d) ln[sinh(¡·p)] vs. 1/T.

Hot Compression Behavior of Mg­14Li­6Al­1Ca AlloyFig. 6

Relationship between ln Z and ln[sinh(¡·p)].

2105

Fig. 9 Relationship between the Zener-Hollomon parameter and theequiaxed grain sizes in hot-deformed and quenched specimens.

The relationship between Z and the equiaxed grain sizes in
hot-deformed and quenched specimens is shown in Fig. 9.
From these results, the following relationship between grain
size and Z was estimated:
D ¼ AZnFig. 7

Comparison between calculated and measured stress.

where D is the grain size, A is a constant, and n is the powerlaw exponent.
This equation has been used to predict the dynamic
recrystallization grain size in other Mg alloys. In the presentwork, the values of A and n were determined to be 270 and0.10, respectively. Values of A and n were reported in therange of 498­760 and 0.09­0.16, respectively, for alternative
Mg alloys.23­25) This indicates that grain reﬁnement for aMg­14Li­6Al­1Ca alloy can be performed at the samedeformation conditions necessary for popular Mg alloys such
as AZ31 or AZ91.
4.

:Equiaxed grains:Deformed initial grains

Fig. 8 Relationship between deformation structure and the ZenerHollomon parameter for the following conditions.

boundary Zc between the deformation condition regions
where the grains are elongated perpendicular to thecompression direction and the deformation condition regions
where the grains become equiaxed, exists in the range of
4.02 © 1016 s¹1 < Zc. Under the deformation condition ofZ > Zc, the elongated initial grains were retained, whereas
under the deformation condition of Z < Zc, dynamic
recrystallization occurred, changing the shape of grain toequiaxed. Thus, the microstructure of the deformed Mg­
14Li­6Al­1Ca alloy could be determined using Z.

ð15Þ

Conclusions

This work has focused on the inﬂuence of the deformation
conditions on the hot deformation behavior of a Mg­Li­Al­
Ca four-element-based alloy. The key conclusions of thestudy are as follows:(1) The true stress­strain curves of the Mg­14Li­6Al­1Caalloy, over the wide range of temperatures and strain
rates investigated in this study, exhibited a single peakstress, followed by a gradual fall towards a steady-state
stress.
(2) The activation energy of the Mg­14Li­6Al­1Ca alloy
was determined to be 193 kJ mol¹1.
(3) The peak stress of the Mg­14Li­6Al­1Ca alloy in atemperature range of 573­673 K and a strain rate rangeof 1 © 10¹1 s¹1­10¹3 s¹1 could be accurately estimatedfrom the temperatures and strain rates. The following
constitutive equation was used to express the hot
working characteristics of the investigated alloy:
Z ¼ ¾_ expð193000=RTÞ ¼ e31:8 ½sinhð0:017· p Þ5:11
ð16Þ
(4) Dynamic recrystallization occurred under the conditionof Z < 4.02 © 1016 s¹1.

R. Tsukane et al.

2106(5) The equiaxed crystal grain size could be estimated fromthe following equation:
D ¼ 270Z0:10 :ð17Þ

Acknowledgments
This research was partially supported by project “Chiikisangyo kasseika jinzai ikusei shien jigyo” of the National
Institute of Advanced Industrial Science and Technology(AIST).
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