WDS'05 Proceedings of Contributed Papers, Part III, 643–648, 2005.
ISBN 80-86732-59-2 © MATFYZPRESS
Mg-RE Alloys and Their Applications
N. Žaludová
Charles University, Faculty of Mathematics and Physics, Department of Low Temperature Physics,
Prague, Czech Republic.
Abstract. This contribution deals with magnesium and magnesium-rare earth alloys
and their applications. Investigation of structure and identification of transient and
equilibrium phases in Mg-RE alloys is presented here.
Magnesium and its application
Magnesium is the lightest of the structural metals. Today, magnesium is used in a diverse range of
markets and applications, each one exploiting the unique physical and mechanical properties of the
element and its alloys. World production of magnesium totals around 400,000 tonnes per annum and
the figure is increasing annually as the lightweight properties of magnesium alloys are used
increasingly in the automotive industry as a means of reducing weight, increasing fuel efficiency and
reducing greenhouse gas emissions.
The commercial possibilities of the electrolytic method of production were first exploited in 1909
by a German company, Chemische Fabrik Griesheim Elektron. By the 1920’s, the electrolytic process
had been worked out on an industrial scale and the metal became available in commercial quantities to
justify its use as a structural material.
To service this growing demand, there is potentially no limit to magnesium output since
magnesium is the eighth most common element in the world and the sixth most abundant metal,
comprising about 2.5% of the earth's surface. Seawater contains 0.14% magnesium and the element is
abundant in minerals Carnallite (MgCl.KCl.6H2O), Dolomite (MgCO3.CaCO3), and Magnesite
(MgCO3). Demand for magnesium is also being met by an expanding magnesium recycling industry.
Alloys used for structural applications can be recycled back into products displaying the same
chemical, physical and mechanical characteristics as primary material. This attribute is being actively
encouraged within the industry, given its positive impact upon the environment. Recycling requires
only 5% of the energy necessary to produce the primary product.
In Europe, the increase in using magnesium as a structural lightweight material is being led by the
Volkswagen Group of companies, with the material also being used by other leading manufacturers
including DaimlerChrysler (Mercedes Benz), BMW, Ford and Jaguar. Presently, around 14kgs of
magnesium are used in the VW Passat, Audi A4 & A6. All those vehicles use magnesium transmission
casings cast in AZ91DT(9wt.%Al, 1wt.%Zn), offering a 20%-25% weight saving over aluminium.
Other applications include instrument panels, intake manifolds, cylinder head covers, inner boot lid
sections and steering components which utilise the more ductile AM50A & AM60B (5&6wt.% Al,
0,5wt.%Mn) alloys. In North America, the use of magnesium for car applications is more advanced.
The GM full sized Savana & Express vans use up to 26kg of magnesium alloy.
Figure 1. Elektron WE43 alloy Helicopter Gearbox
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ŽALUDOVÁ: Mg-RE ALLOYS AND THEIR APPLICATIONS
Magnesium-rare earth alloys
Alloying magnesium with rare-earth elements (RE), inclusive Y and Sc, is used to develop light
construction alloys for the applications at elevated and high temperatures. Generally, RE elements in
Mg have relatively high solubility decreasing significantly with decreasing temperature. Therefore age
hardening is possible in these alloys to improve mechanical properties.
We can divide rare-earth elements in:
• light (Ce, Pr, Pm)
• heavy (Er, Dy, Gd),
according to of a number of valence electrons:
• Ce group (La-Eu with two valence electrons)
• Y group (Y, Gd-Lu with three valence electrons)
or accordingly what forges phase diagram:
• Eutectic (Dy, Er, Eu, Gd, Y)
• Peritectic (Sc)
Solubility of rare-earth Ce group is essentially lower than elements of Y group. G-P zones are
formed in Mg alloy with RE of Ce group in early stage of the decomposition of supersaturated solid
solution. These zones are not observed Mg alloys with RE of Y group [1].
Two basic sequences of the decomposition of supersaturated solid solution are known in binary
Mg-RE alloys, namely
• Mg-Gd (Y) type [2, 3]: α´(cph) → β´´(D019) → β´(cbco) → β(fcc, bcc)
• Mg-Ce type (Nd, Ce) [2, 3]: α´(cph) → β´´(D019) → β´(fcc) → β(bct)
The β´´transient phase has hexagonal D019 structure (a = 2 . aMg, c = cMg) and is coherent to the
close packed hexagonal α´matrix in both decomposition sequences.
The β´ (Cbco - C base-centred orthorhombic, a = 2 . aMg, b ~ 8 . d(1100)Mg, c = cMg) is transient
phase semicoherent to the α'-Mg matrix in MgGd type sequence.
The equilibrium β phase in this sequence is either face centred cubic (Mg5Gd, a ~ 8.d(1100)Mg) or
body centred cubic (Mg24 Y5, a ~ 4.d(1100)Mg).
The equilibrium β phase following D019 phase in the Mg-Ce type sequence has fcc structure (D03,
a = 0.74 nm, d(220) ~ d(0002)Mg).
Most of transient and equilibrium phases of the decomposition sequences of Mg-RE alloys
precipitate as plates with the habit plane parallel to the first or second order prismatic planes of the
α'-Mg matrix. G-P zones reported to form in the pre-precipitation stage of Mg-Ce decomposition
sequence [2, 3] were also plates or rods lying in 1100 prismatic planes of the α'-Mg matrix. Also
other decomposition sequences are reported in Table 1.
These decomposition sequences can be modified in Mg alloys with combination of RE from
different groups (Y and Ce) and in complex alloys with the addition of another elements [4, 5, 6].
Modification of precipitation sequences and phase morphology may depend not only on the alloy
composition but also on casting technology and thermomechanical treatment. Transmission electron
microscopy (TEM) and electron diffraction (ED), energy dispersive analysis of X-ray (EDAX), are the
essential methods for determination of phase composition, structure and morphology, especially in the
case of low and very low volume fractions and nano-size of phases involved.
{ }
Development of microstructure in WE alloys
One of a combination of RE in Mg, which has a commercial using is the WE alloy, containing Y
and Nd. For a grain refinement there small amount of Zr is added. Two types of these alloys are
prepared by the MEL Ltd. company (Magnesium electron) - WE43 (Mg-4wt.%Y-3,3 wt.%R.E.-0,5
wt.%Zr ) and WE54 (Mg-5,5 wt.%Y-2 wt.%Nd-2 wt.%R.E.-0,4 wt.%Zr ).
We can observe significant hardening by isothermal annealing, which depends on temperature of
annealing. Both alloys previously homogenized at 515°C for 24h exhibit a large ageing response
(exceeding 30%) if annealed at 200°C – Figs. 2 and 3. A faster response is observed at higher
temperatures but overageing occurs during rolongated annealing.
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ŽALUDOVÁ: Mg-RE ALLOYS AND THEIR APPLICATIONS
Fast and effective mapping of the temperature ranges of phase transformations can be done by the
measurement of electrical resistivity response to the isochronal annealing. The resistivity of WE54
alloy measured at 77 K decreases in two stages – Fig. 4. The first resistivity decrease starts at 90°C
reaching minimum at 200°C. The D019 phase precipitates in this stage. The second decrease starts at
250°C after a slight increase. The precipitation of equilibrium phase (fcc, isomorphous with Mg5Gd) is
responsible for this decrease.
Very similar response of electrical resistivity to analogous annealing was observed in the WE43
alloy - Fig. 5. The same phases were revealed by TEM observations. A very good correlation between
resistivity and hardness response was presented in [9].
The phase composition of WE type alloys and their morphology after various thermal treatments
are summarized in Tables 2 - 4.
Table 1. Decomposition sequences according to Polmear [7]:
Mg-RE(Nd)
SSSS
→G-P zones
(Mg-Nd)
plates
_
→β´´
Mg3Nd?
cph
→β´
Mg3Nd
fcc
→β
Mg12Nd
bct
D019 superlattice
plates
(0001)β´´ II(0001)Mg
_
_
{1010}β´´ II{1010}Mg
plates
a=1,03nm, c=0,593nm
(incoherent)
II(1010)Mg
(coherent)
Mg-Y-Nd
SSSS
→?
→β´´
D019 superlattice
hexagonal
plates
a=0,736nm
(011) β´ II(0001)Mg
_ _
_
(111) β´ II(2110)Mg
(semicoherent)
→β´
Mg12NdY?
bc orthorombic
plates
(011)β´´ II(0001)Mg
_
_
[1010] β´´II[1010] Mg
Mg-Th
SSSS
Mg-Ag-RE(Nd)
→β´´
Mg3Th?
cph
D019
superlattice
discs
_
II{1010}Mg
(coherent)
rod-like
G-P zones
┴(0001)Mg
(coherent)
→→
º
º
(011)β´ II(0001)Mg
_
[010] β´II[2110]Mg
_
[010] β´ [0110] Mg
→→
→β
Mg11NdY2?
fcc
(011)β II(0001)Mg
_
_
(111) (1210)Mg (incoherent)
→β
Mg23Th6
fcc
a=1,43 nm (incoherent)
ºMg2Th¸
→→
(i) β1´hexagonal
(ii) β2´fcc
(both semi-coherent)
→γ
hexagonal rods
a=0,963nm, c=1,024nm
II to [0001]Mg (coherent)
SSSS¸
º
ellipsoidal
→→
G-P zones
II to (0001)Mg
(coherent)
→β
→Mg12Nd2Ag
hexagonal
complex
equiaxed
hexagonal
a=0,556nm,
c=1,024nm
(011)β II(0001)Mg
_
_
(1120) β II(1010)Mg
(semi-coherent)
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lathes (incoherent)
ŽALUDOVÁ: Mg-RE ALLOYS AND THEIR APPLICATIONS
Figure 2. Influence of ageing at various
temperatures on hardness HV 20 for
homogenized Mg-Y-Nd alloy [8]
Figure 3. Influence of ageing at various
temperatures on hardness HV 20 for
homogenized Mg-Y-Nd-Zr alloy [8]
76
0
HV3
-5
72
∆ρ/ρ0[%]
-10
68
-15
64
∆ρ/ρ0[%]
-20
HV3
60
-25
WE43 homogenized at 525°C/4h
-30
0
Figure 4. Relative electrical resistivity ρ/ρ0
annealing curves of homogenized Mg-Y-Nd and
Mg-Y-Nd-Zr alloys. Annealing rate 20K/10 min;
ρ0 is the initial value of electrical resistivity [8].
100
200
300
400
T A [°C]
Figure 5. Resistivity and hardness
annealing curves of homogenized WE43.
Annealing rate 30K/30 min; ρ0 is the initial
value of electrical resistivity [9].
Table 2. Overview of the occurrence conditions and orientation of β´´ phase (D019).
Alloy
[wt.%]
56
500
composition Treatment; temperature
[°C]
Not plate-like shape
Mg-4Y-3,3R.E.Isothermal 150
0,5Zr(WE43)
Mg-4Y-3,3R.E.Isothermal 150;
0,5Zr(WE43)
Monolayer plates
Mg-4Y-3,3R.E.Isothermal 250
0,5Zr(WE43)
Mg-5,5Y-2Nd-2R.E.250, 1-4h
0,4Zr(WE54)
(D019 structure not
proved)
Habit plane of plates
References
_
{1120}Mg
_
{1100}Mg
[10]
[10]
[10]
_
{1120}Mg
646
[4, 11, 12]
ŽALUDOVÁ: Mg-RE ALLOYS AND THEIR APPLICATIONS
Table 3. Overview of the occurrence conditions and orientation of β´ phase (cbco).
Alloy
[wt.%]
composition Treatment; temperature
[°C]
Not plate-like shape
Mg-4Y-3,3R.E.Isothermal 150, 1896h
0,5Zr(WE43)
Mg-4Y-3,3R.E.Isothermal 250, 2-8h
0,5Zr(WE43)
Mg-4Y-3,3R.E.Isothermal 150, 1324h;
0,5Zr(WE43)
250, 2-16h; globular
WE54
Isothermal 250, 100h
cbco coexists with
D019
WE54
Isothermal 200, 72h
WE43
Isothermal 200, 300h
Habit plane of plates
References
_
{1120}Mg
_
{1120}Mg
[10]
[10]
[10]
[4, 11, 12]
plates
[12]
[13]
Table 4. Overview of the occurrence conditions and orientation of β1 phase (fcc).
Alloy
[wt.%]
WE54
WE54
WE54
WE43
composition Treatment; temperature Habit plane of plates
[°C]
Not plate-like shape
Isothermal 250, 4h
_
{1100}Mg
Cold rolled 6 % and 12
_
% - annealing 250, 48h {1100}Mg
Isothermal 250, 48h
_
{1100}Mg
Isothermal 250, 16h
_
Nucleate on globular {1100}Mg
cbco
References
[12]
[12]
[4, 11]
[10]
Aim of investigation
It was shown that precipitation sequences are rather complicated in Mg-RE alloys and they
include transient as well as equilibrium phases. The morphology, arrangement and volume fraction of
these phases are decisive for mechanical properties and following application of these alloys.
The aim of the detailed investigation is to study the kinetics of phase transformations during
isothermal annealing in complex Mg-RE alloys including WE43 and more complex alloys with RE
and Zn. Thermal stability of developed phases will be studied, too. The correlation of microstructure
and mechanical properties will be also performed. Electrical resistometry and hardness measurements
have been chosen as main experimental methods. They will be completed with direct microstructure
observations by TEM.
Acknowledgement. This work is partly supported by the Czech Scientific Foundation (GACR) in
the frame of the project # 106/03/0903. This financial support is gratefully acknowledged.
References
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[6] Kiehn, K., Smola, B., Vostrý, P., Stulíková, I., Kainer, K. U.: Phys. Stat. Sol. (a), 164, 1997, p. 709.
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ŽALUDOVÁ: Mg-RE ALLOYS AND THEIR APPLICATIONS
[7] Polmear I. J.: Physical Metallurgy of Magnesium Alloys, in: Magnesium alloys and Their Applications,
(Eds.: Mordike B. L., Hehmann F.), DGM Informationsgesellschaft, Oberursel 1992, p. 201.
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[12] Hilditch, T., Nie, J., F., Muddle, B., C.: In: Magnesium Alloys and their Application.
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