Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02765484 2012-01-25
CA Application
Agent Ref. 71404/00010
1 SINGLE-PHASE SOLID SOLUTION CAST
2 OR WROUGHT MAGNESIUM ALLOYS
3
4 FIELD OF THE INVENTION
The present invention relates to cast and wrought single-phase solid solution
magnesium alloys
6 with high mechanical properties, formability and corrosion resistance.
7
8 BACKGROUND OF THE INVENTION
9 Magnesium alloys have not yet been widely accepted by car manufacturers.
Most of the
technical barriers preventing magnesium alloys from widespread applications
arise from the low
11 ductility and toughness at low temperatures, poor corrosion and creep
resistance at high
12 temperatures. Their present commercial products are normally fabricated by
high pressure die
13 casting. The use of wrought magnesium alloys is limited because of its poor
formability and
14 corrosion resistance.
16 It will be necessary to improve the low-temperature formability of wrought
magnesium alloys in
17 order to obtain a higher acceptance of these alloys in industry. Low
ductility and low toughness
18 are due to the intrinsically brittle nature of the hexagonal close-packed
crystal structure. A
19 further issue which hinders the acceptance of wrought magnesium alloys is
their poor corrosion
resistance.
21
22 Most commercial wrought magnesium alloys belong to magnesium-aluminium (Mg-
Al) and
23 magnesium-zinc (Mg-Zn) series. The later developed magnesium-rare earth (Mg-
RE) series
24 such as WE43 (Mg-4.1Y-2.2Nd-1 HRE-0.5Zr) and WE54 (Mg-5.2Y-1.7Nd-1.7HRE-
0.4Zr) alloys
were not accepted by the industry due to their high price arising from the
high content of rare
26 earth elements.
27
28 The alloys of magnesium-aluminium series are the most commonly used in
wrought applications
29 for their relative ease of extrusion and adequate mechanical properties,
but they suffer from
both a pronounced asymmetry in the yield behaviour and a relatively narrow
processing
31 window. Due to the lower eutectic temperature 437 C for magnesium-aluminium
alloys, the hot
32 processing temperatures are normally selected below 350 C and the
processing speeds are not
33 so high. If selecting high temperatures more than 350 C with high
processing speeds, the
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CA Application
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1 eutectic phases dissolve again, leading to the occurrence of hot cracking
and bad surface
2 quality of the products. In addition, until now, the methods for refining
the as-cast
3 microstructures of magnesium-aluminium alloys are not satisfying and not
widely accepted by
4 the industry.
6 Since magnesium-zinc series contain no aluminium, their as-cast
microstructure can be
7 effectively refined by the addition of zirconium. However, these magnesium-
zinc alloys still have
8 very limited applications because they are susceptible to microporosity
during casting. The
9 addition of zinc in magnesium increases the susceptibility to hot tearing.
Moreover, due to the
high content of zinc, it was considered that these alloys are difficult to be
welded.
.11
12 Therefore, at present only AZ31 (Mg-2.9AI-0.8Zn) alloy is used in industry
to an significant
13 extent. However, AZ31 (Mg-2.9AI-0.8Zn) alloy exhibits some problems with
recrystallisation
14 during the hot working and has insufficient mechanical and corrosion
properties.
16 It is therefore the object of the present invention to develop new
magnesium alloys with high
17 corrosion resistance and formability using innovative alloy design concept.
18
19 SUMMARY OF THE INVENTION
Accordingly, the present invention provides a magnesium alloy comprising 0.5
wt.% to less than
21 5.0 wt.% of at least two elements selected from the group consisting of La,
Ce, Pr, Nd, Pm, Sm,
22 Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y, wherein the content of each of
said elements La, Ce,
23 Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y, if present, is
from 0.05 to 2.0 % by
24 weight, based on the total weight of the alloy.
26 Preferably, the amount of the at least two elements selected from the group
consisting of La,
27 Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and Y, is from 1.0
wt.% to less than 5.0
28 wt.%.
29
BRIEF DESCRIPTIONS OF THE DRAWINGS
31 The above and other characteristics and advantages of the invention will be
more readily
32 apparent through the following examples, and with reference to the appended
drawings, where:
33
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CA 02765484 2012-01-25
CA Application
Agent Ref. 71404/00010
1 Figure 1 compares the optical microstructure of the investigated, as cast
alloys ((a) Mg, (b) Mg-
2 0.4Y, (c) Mg-0.4Gd-0.4Y, (d) Mg-0.4Gd-0.4Y-0.4Dy, (e) Mg-0.4Gd-0.4Y-0.4Dy-
0.2Zr, (f) Mg-
3 0.4Gd-0.4Y-0.4Dy-0.2Zn, (g) Mg-0.4Gd-0.4Y-0.4Dy-0.2Ca, (h) Mg-0.4Gd-0.4Y-
0.4Dy-0.2Zn-
4 0.2Zr and (I) AZ31);
6 Figure 2 shows the grain size, hardness and corrosion properties of the
investigated alloys;
7
8 Figure 3 shows the tensile properties of selected as-cast alloys; and
9
Figure 4 shows the microstructural situation and the micro-segregation of the
alloying elements.
11
12 DETAILED DESCRIPTION OF THE INVENTION
13 The strengthening effects of rare earths in the previous magnesium alloys
have been explained
14 by two mechanisms, precipitate strengthening and solid solution
strengthening. Precipitate
strengthening, especially the age hardening, has been emphasised to improve
the mechanical
16 properties. Without being bound to any theory, it is believed that in
alloys of the present
17 invention, precipitate strengthening is avoided and that solid solution
strengthening is the main
18 mechanism which improves the mechanical properties in the magnesium alloys
according to the
19 present invention.
21 It is further believed that the solid solution strengthening depends on the
contents of alloying
22 elements in the matrix of magnesium and the difference in atomic radius
between the alloying
23 elements and magnesium such that a high content of alloying elements and
large difference in
24 atomic radius increase the effect of solid solution strengthening.
26 In addition, it has been found that there exists a synergistic effect
caused by the interaction of
27 the different rare earth elements. With the same total content of rare
earth elements in the
28 magnesium alloy, the improvement in mechanical properties is higher when
two different rare
29 earth elements are present in comparison to the improvement achieved with
the presence of
only on e rare earth element.
31
32 Furthermore, the addition of rare earth elements can purify the melt during
casting. The addition
33 of rare earth elements can remove impurity elements such as hydrogen,
oxygen, chlorine, etc.
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CA 02765484 2012-01-25
CA Application
Agent Ref. 71404/00010
1 Moreover, they interact with iron, cobalt, nickel or copper elements during
melting, and these
2 elements are removed by the formation of intermetallic compounds which
settle at the bottom of
3 the ingot. The decrease of impurities in the matrix also contributes to the
high corrosion
4 resistance.
6 Preferably, the magnesium alloy of the present invention further comprises
an element selected
7 from the group consisting of Zr, Ca, Zn, and mixtures thereof. The stress
corrosion of
8 magnesium alloys could be alleviated by the addition of zirconium (Zr) and
rare earth elements.
9 Zirconium (Zr) can be used as an element to decrease the stress corrosion
cracking.
11 Preferably, the magnesium alloys according to the present invention contain
no aluminium;
12 therefore, their as-cast microstructure can effectively be refined by the
addition of zirconium or
13 calcium.
14
In principle, two groups of rare earth elements can be classified in periodic
table: light rare earth
16 elements and heavy rare earth elements. In each group, rare earth elements
have the similar
17 chemical and physical properties. Due to the similar properties of yttrium
and scandium to heavy
18 rare earth elements, for the purposes of the present invention Y and Sc are
treated as they were
19 heavy rare earth elements. The light rare earth elements include samarium,
lanthanum, cerium,
neodymium, and praseodymium, and the heavy rare earth elements include
gadolinium, yttrium
21 and dysprosium. Besides the rare earth elements, zirconium and/or calcium
are preferably
22 added as a grain refiner.
23
24 The magnesium alloys of the present invention comprise 0.5 wt.% to less
than 5.0 wt.% of at
least two rare earth elements with a content of 0.05 to 2.0 % by weight of
each of the rare earth
26 elements. The total content of rare earths is maintained below 5 wt.%,
mainly for economical
27 reasons. The content of grain refiner calcium and/or zirconium is
preferably in the range of 0.05-
28 0.6% by weight.
29
The manufacturing processes of the magnesium alloys according to the present
invention are
31 not restricted. The alloys can be prepared by die casting, permanent
casting, chill casting, semi-
32 solid processes, continuous casting or continuous twin roll casting.
33
22195545.1 4
CA 02765484 2012-01-25
CA Application
Agent Ref. 71404/00010
1 The magnesium alloys according to the present invention exhibit excellent
room temperature
2 ductility with a value of about 25%.
3
4 Tensile tests show that as-cast alloy MgO.4GdO.4YO.4DyO.2Zr and
MgO.4GdO.4YO.4DyO.2ZnO.2Zr exhibit excellent ductility. The elongation is more
than 20%,
6 which is much higher than that of AZ31 alloy. These two alloys have shown a
good
7 deformability.
8
9 EXAMPLES
Three rare earth elements gadolinium, yttrium, dysprosium with high solubility
in magnesium
11 were selected to develop the single-phase solid solution magnesium alloys.
Table 1 lists the
12 compositions of the investigated alloys. A conventional alloy, Mg-3AI-1 Zn
(AZ31), was included
13 for comparison.
14
All alloys were prepared by zone solidification. Their optical microstructures
are shown in Figure
16 1. The average grain size decreases with the increment in the content of
rare earths. Compared
17 to the gadolinium and dysprosium, the yttrium element is the most effective
element to decrease
18 the grain size. The average grain sizes of E and H alloys containing
zirconium are 55 pm and 67
19 pm. The average grain size of Mg-3AI-lZn (AZ31 is 480 pm.
21 Table 1. Nominal compositions of the investigated alloys
22 Alloys (Composition (weight percent, wt.%)
23 Mg Gd Y Dy Zn Al Zr Ca
24 A-Pure Mg 100 - - - - - -
B-MgO.4Y Bal* - 0.4 - - - -
C-MgO.4GdO.4 Y Bal 0.4 0.4 - - - -
D-MgO.4GdO.4 Y0.4Dy Bal 0.4 0.4 0.4 - - -
26 E-MgO.4GdO.4YO.4DyO.2Zr Bal 0.4 0.4 0.4 - - 0.2
27 F-MgO.4GdO.4 YO.4DyO.2Zn Bal 0.4 0.4 0.4 0.2
G-MgO.4GdO.4 YO.4DyO.2Ca Bal 0.4 0.4 0.4 0.2
28 H-MgO.4GdO.4 Bal 0.4 0.4 0.4 0.2 - 0.2
YO.4DyO.2ZnO.2Zr
29 I-AZ31 Bal - - - 1.0 3.0 -
* Balance.
31
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