ALLIAGE AU MAGNESIUM

MAGNESIUM ALLOYS

Abrégé anglais

A B S T R A C T
Magnesium alloys having favourable mechanical
properties at high temperatures contain silver, neodymium
and thorium, the permissible content of neodymium varying
inversely according to the content of thorium. The alloys
are subjected to solution heat treatment followed by ageing
to give optimum properties.

Revendications

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A cast magnesium-based alloy having a 0.2 proof
stress of at least 148 N/mm2 at 250°C consisting essen-
tially of the following constituents by weight (other
than iron and other impruities):
<IMG>
the maximum and permissible quantity of zirconium and man-
ganese being limited by their mutual solubility and the
total quantity of rare earth metals which contain at least
60% neodymium and thorium being from 1.5 to 2.4%.

2. An alloy according to claim 1, containing at least
0.3% by weight of thorium.
3. An alloy according to claim 1, containing at least
0.4% by weight of zirconium.
4. An alloy according to claims 1 or 2, containing
at least 0.4% by weight of zirconium and manganese
taken together.
5. An alloy according to claim 1, which contains at
least 1.5% silver by weight.
6. An alloy according to claim 5, which contains
from 2 to 2.5% by weight of silver, from 0.9 to 1.4% by
weight of rare earth metals, from 0.6 to 1.1% by weight
of thorium and at least 0.4% by weight of zirconium.
7. A method of making a heat-treated metal article
which comprises forming an alloy according to claim 1,
into shape subjecting the article to solution heat treat-
ment at a temperature from 485°C to the solidus of the
alloy, quenching the article and ageing the article at a
temperature from 100°C to 275°C for a period of at least
1/2 an hour.
8. A method according to claim 7, in which the article
is solution heat treated at a temperature of about 525°C
for 8 hours.
9. A method according to claim 7, in which the alloy
contains at least 0.1% by weight of copper and the article

is solution heat treated at a temperature not exceeding
485°C followed by treatment at a higher temperature.
10. A method according to claim 7, 8 or 9, in which
the alloy is aged at a temperature of about 200°C for
a period of about 16 hours.
11

Description

1053484
This invention relates to magl~esium alloy~.
Magne~ium alloy~ have a very low weight in
comparison with alloys of other metals and accordingly
find applications, particularly in the aero~pace industry,
where a low weight is important. Such alloys having
advantageous mechanical propertie~, in particular a high
proof ~tress, are described in British Patent
Specification No. o75,929.
~ lloys within the scope of the latter specification
have been used in aerospace components which are subject
to relatively high stress, such as aircraft compressor
housing~, helicopter main gearboxes and undercarriage
components. To obtain adequate mechanical properties
it is neces~ary to subject these alloys to a two-stage
heat treatment entailin$ solution treatment at a high
temperature, followed by quenching and ageing at a lower
` temperature to improve the mechanical properties by
precipitation hardening.
~ lechanical properties thus obtained are well
20 - maintained during exposure to elevated temperatures up to
200C. However, on exposure to temperature~ above 200C~
mechanical properties deteriorate significantly, limiting
the applications of such alloys in aircraft and other
machinery, especially in engine~ and gearboxes operating
in this temperature range~
There have now been found magnesium alloys having
satisfactory ten~ile propertie~ at room temperature which
retain their advant~geous propertieY, at least to some
desree~ at temperatures of the order of 250C.
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~r

1053484
According to one aspect of the pre~ent invention,
there i9 provided a magnesium-based alloy containing the
following con~tituents by ~eight (other than iron and
other impurities):
Silver 1.25 - 3.0%
~are earth metals, of which at
least 60% i~ neodymium 0.5 - 2.25
Thorium 0.2 - 1.9%
Zinc 5%
Cadmium 0 - 1~
Lithium 0 - 6%
Calcium 0 - o.8%
Gallium 0 - 2%
Indium 0 - 2%
Thallium 0 - 5%
Lead 0 - .1%
Bismuth 0 - 1%
Copper 0 -0.15%
Zirconiu~ 0 - 1%
Manganese 0 - 2%
Remainder Magnesium
the maximum and permissible quantity of zirconium and
manganese being limited by their mutual solubility and the
total quantity of rare earth metal and thorium being from
1.5 to 2.4%.
In a preferred embodiment of the invention the
proportion of rare earth metals is 0.5 - 2.1% and the
proportion of thorium i~ from 0.3 to 1.9%, the total
amount of thorium and rare earth metal3 being 1.5 - 2.4%.

iO5348~
The alloys may ~)e Inade l~ing pure neo(lyll~ m as the
rare earth metal but aY pure neodymium is very expensive
it i9 preferred to add it in the form of a rare earth ¦
mixture containinS at least` 60% neodymillm. The mixture
of rare earth metnls preferably contains not more than
25% of lanthamlm and cerium taken together. It should
be noted that yttrium is not classed a~ a rare earth metal. `i
In order to develop fully the tensile properties of
the alloys of the invention it is necessary to ~ubject them
to heat treatment~ firstly at a high temperature to achieve
dissolution of the alloying constituents and then at a
lower temperature to achieve "ageing" in which
precipitatlon hardening takes place. The solution
treatment should be carried out at a temperature from
~i85C to the solidus temperature of the alloy for a
sufficient time to effect ~olution which may be at least
2 hours. The alloy may then be quenched to room
témperature and then aged at a temperature from 100C to
275C for a period of at least ~ an hour; longer times are
required at lower temperatures within the stated range.
In general a solution treatment of 8 hours at 525C
is normally satiqfactory. IIo~Yevez the presence of copper
in amounts of over 0.19' affects the solidu~ ~o that initial
- treatment at a temperature not exceeding 485 C, for example
8 hours at 465 C 9 iS required before the higher
temperature treatment.
It has been found that alloys having the above-
mentioned quantities of rare earth metals and thorium have
advantageous properties both at room and at elevated

1053484
~1
(e.g. 250 C) temperatures. If the total alllount of rare
earth metal and thorium exceeds 2.4% low elongation at
fracture at room temperature i~ observed and if it falls
below 1.5% ~oor ca~tability is obtained. Poor room
temperature 0.2% proof strength is found at rare earth ~r
metal contents below 0.5%. The high temperature
mechanical properties deteriorate if the thorium content
falls ~elow 0.2%.
A preferred alloy according to the invention contains
2-2.5% silver, 0.9-1.4% rare earth metals, 0.6-l.lq6 thorium
and at least 0.4% zirconium, the balance being magnesium.
The desired amount of thorium may conveniently be
added in the form of a magnesium-thorium hardener alloy.
The silver content has an effect on the properties
of the alloy. The tensile properties deteriorate as the
silver decreases although the elongation at fracture
increases. The alloy should contain at lea~t 1. 25% of
silver and the preferred range is from 1.5 to 3.0%.
- The pre~ence of up to 1% zirconium in the alloy is
generally desirable to obtain satisfactory grain-refining.
.
In order to obtain satisfactory castings it is dcYirable
to incorporate at least 0.4% of zirconium. It may be
desirable to add manganese, but the content of manganese
i8 limited by its mutual solubility with zirconium. Part
of the desirable minimum of 0. 4% of the zirconium may be
replaced by manganese.
Preferred embodiments of alloys according to the
invention will be described in the follo-ring Examples.

105;~48~
1 BRIEF DISCUSSION OF THE DRAWINGS
Further details of the present invention will be
discussed with respect to the drawings, wherein:
Figures la through lf show the effect of Nd.Th
on tensile properties of QEH alloys with Figure la showing
the proof stress at room temperature; Figure lb showing the
ultimate tensile strength at room temperature; Figure lc
showing elongation at room temperature; Figure ld showing
the proof stress at 250C; Figure le showing ultimate ten-
sile strength at 250C; and Figure lf showing elongation
at 250C. Figures 2 and 3 show a comparison between QE22
and QEH alloys.
'- - 5a -

~053484
EXAMPLES
_
AlloyY having the compositionY ~iven below were
made by a conventional method. Silver was added either ~
ag pure silver or fro~ an ingot cont~ining 2.5% A~ 8% ~`
rare earthY, Zr o.36% and the rest magneYium. Rare
earths were added as a magnesium/neodymium hardener alloy.
Thorillm was added as a magnesium/thorium hardener alloy.
The alloys obtained were subjected to heat treatment
initially at a high temperature to effect solution J
followed by quenchins and ageing at a lower temperature.
Initial solution treatment was carried out either for 8
hours at 525C or, for alloys containing significant amounts
of copper, for 8 hours at ~65C followed by 8 hours at t
525C. The specimens were then quenched in hot w~ter and
aged for 16 hours at 200C.
The mechanical propertieY of the specimen~t thust
obtained (0. 2~/o proof stress, ultimate tensile stress and
elongation) were measured at room temperature according to
British Standard 18 and at 250C according to British
Standard 3688. 15 minute soak times at 250C were used.
- In order to investigate the over-aseinS resistance
of tlle alloys the same mechanical teYts were carried out
but with soak timeY varying from 15 to 120 minutes.
The fatigue resistance of the Yamples was measured
using standard Wohler U-notched and un-notched fatigue -~
te~ts. Creep behaviour was determined by plotting the
stresY/time relationship for 0.2% creep ~train at 200 C and
250C using a method according to British Standard 3600.

105348~ ,
l~esults of the tensile property tests are given in
Figure 1 which relates to alloys containing 2.5% silver and
o.6% zirconium. The rare earth metal content is plotted
as ordinate and thorium content as absci~a.
The alloys l~ithin the scope of the pre~ellt
invention are within the trape~ium shown on these plots.
It will be seen from the values quoted within the trapezium
that the alloys therein have favourable mechanical properties,
and those outside are generally inferior. Thuq alloys with
increa~ed total rare earth and thorium content (area A) have
poorer room temperature elongation (plot c) and those with f
a rare earth content below 0.5% have lower proof stress and
ultirnate stress (plots (a), (b), (d) and (e) ). AlloyY
having less than 0.2% thorium show inferior high
temperature propertie~ and those having a rare earth plus
thorium content below lo 5% have been found to have inferior
castability (more porosity).
The effect of the thorium content on over-ageing
resistance i~ shown in Table 1 belo-~. It will be seen
that the high temperature properties for a given degree
of ageing are improved by the presence of thorium~ and that
these properties are substantially retained on over-ageinS.
The results of Wohler fatigue tests, respectively
for un-notched and notched specimens~ are shown in ~`igures
2 and 3. The alloys of these figures are as follows:

1053484
,Appro~imate nnalysis %
Ag~are Earths Th Zr
2.5 2.2 - o.6 Qound dots ,3
2.5 o.6 1.3 o.6 Square dot~
2.5 1.0 1 o.6 Trian~ular dots
It can be seen that the thorium-containing alloys
show maximum stress value~ which are as good as or better
than those of the alloy not containing thorium, especially
for un-notched specimens. 3j
The creep properties of specimen~ were measured
at 200C and 250 C. The results ~ere as follows:
Composition %
6 value (100 hours) for2
0.2% creep strain N/mm
Agl~are earths Th Zr 200C 250C
2.5 2.2 0 o.6 75 2
2-5 o.8 1 o.6 96 39
It can be seen that the creep properties of the
thorium-containing alloy at ele~ated temperatures are
, considerably more favourable than those of the known alloy.
The addition of mansanese has no deloterious effect
on the tensile and creep properties of the alloy.
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