ALLIAGES DE MAGNESIUM

MAGNESIUM ALLOYS

Abrégé anglais

ABSTRACT


A magnesium-based alloy for casting contains at least
88% magnesium, 1.6-3.5% silver, 0.1-2.3% of rare earth metals
comprising at least 60% neodymium, 0-2.3% thorium and 0.1-2.5%
yttrium. When no more than 0.5% of yttrium is present the mini-
mum amount of thorium is given by the equation [Th] = Image.

Other elements may be present to improve the alloy properties.
Alloys of these compositions have advantagous mechanical properties
such as resistance to creep at elevated temperatures.

Revendications

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OF PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A magnesium-based alloy containing by weight (other
than iron and other impurities):

Image




and the balance of the composition being magnesium, the amount
of rate earth metals and Th together not exceeding 3.0% and when
no more than 0.5% of Y is present the minimum amount of Th is
defined by the equation [Th] = Image where [Th] and [Y] are

the amounts % of Th and Y respectively, the maximum contents of
Zr and Mn together being limited by their mutual solubility.

-10-


2. An alloy according to claim 1, in which the rare earth
metals comprise at least 75% by weight of neodymium.


3. An alloy according to claim 1, in which the rare earth
metals comprise not more than 15% by weight of lanthanum plus
cerium.

4. An alloy according to claim 3, in which the rare earth
metals comprise not more than 3% by weight of lanthanum plus
cerium.


5. An alloy according to claim 4, in which the rare earth
metals comprise substantially no lanthanum or cerium.


6. An alloy according to claims 1, 2 or 3, in which the
minimum amount of thorium is defined by the equation

[Th] = Image when less than 1% of yttrium is present.



7. An alloy according to claims 1, 2 or 3, which contains
at least 0.4% by weight of zirconium.


8. An alloy according to claims 1, 2 or 3, which contains
from 2 to 3% by weight of silver.



9. An alloy according to claims 1, 2 or 3 when heat treated.


10. A method for making a heat treated metal article from a
magnesium based alloy containing by weight (other than iron and
other impurities):

-11-





Image




and the balance of the composition being magnesium, the amount
of rare earth metals and Th together not exceeding 3.0% and when
no more than 0.5% of Y is present the minimum amount of Th is
defined by the equation [Th] = Image when [Th] and [Y]

are the amounts % of Th and Y respectively, the maximum contents
of Zr and Mn together being limited by their mutual solubility, the
method comprising forming the alloy, subjecting the article to
solution heat treatment 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.

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11. A method according to claim 10, in which the article is
solution heat treated at a temperature of about 525°C for about
8 hours.


12. A method according to claim 10, in which the alloy contains
over 0.1% by weight of copper and the article is solution heat
treated at a temperature not exceeding 485°C followed by solution
heat treatment at a higher temperature.


13. A method according to claims 10, 11 or 12, in which the
article is aged at a temperature of about 200°C for a period of
about 16 hours.

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Description

7~57

This invention relates to magnesium-based alloys.
Magnesium alloys find numerous applications where light
weight is essential, especially in aerospace technology.
Magnesium alloys are known having good mechanical properties,
particularly high yield strength, which are well maintained at
elevated temperatures. Such alloys contain silver - usually
2-3% by weight - and neodymium, which may be added in the form of
a mixture of rare earth metal.
Known alloys which contain silver, neodymium and thorium,
and optionally, yttrium; the yttrium is believed to improve the
stability of the alloys' tensile properties at high temperatures
(of the order of 250C) and also the resistance to creep. These
alloys, containing yttrium and thorium, contain at least 3% of
yttrium by weight. Yttrium is an expensive material.
It has now been found that alloys suitable for casting
having advantageous mechanical properties such as resistance to
creep at elevated temperatures can be obtained by the addition
of smaller quantities of yttrium to magnesium alloys containing
silver and neodymium. ~en the yttrium content is less than
0.5% by weight thorium should be present also.
~ccording to one aspect of the invention, there is
provided a magnesium-based alloy containing by weight
(other than iron and other impurities):




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- Mg at least 88%
Ag 1.6 - 3.5 %
Rare earth metals 0.1 - 2.3 %
of which at least
60% is neodymium
Th 0 - 2.3 %
Y 0.1 - 2.5 %
Zn 0 _ 0 05 %
Cd 0 - 1.0 %
Li 0 - 6.0 %
Ca 0 - 0.8 %
Ga 0 - 2.0 %
In 0 - 2.0 %
Tl 0 5 0
Pb 0 - 1.0 %
Bi 0 - 1.0 %
Cu 0 - 0.15%
Zr 0 - 1.0 %
Mn 0 - 2'.0% :`~
the amount of rare earth metals and Th together not
exceeding 3.0% and when no more than 0.5% of Y is present
the minimum amount of Th is defined by the equation [Th] - ~ .
4 ~ ] where [Th] and [Y] are the amounts % of Th and Y
respectively. : -
The minimum quantity of thorium is such that it may .
be nil at yttrium contents of 0.5% or above and increases :
linearly to a value of 0.1% at the minimum yttrium content
of 0.1% in accordance with the above equation.
It should be noted that yttrium is not classed as a ;.~;. . .

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rare earth metal.
According to one embodiment, when less than 1% of
yttrium is present the minimum amount of thorium is
defined by the equation:
[Th] = 4 5
In this embodiment the minimum quantity of thorium is
nil at yttrium contents of 1% or above and increases
linearly to a value of 0.2% at the minimum yttrium content
of 0.1%.
The rare earth metals preferably comprise at least
75% by weight of neodymium. They preferably contain not
more than 15% of cerium and lanthanum taken together, most
preferably not more than 3%, as these elements may have a
deleterious effect on the mechanical properties of the
alloy. Cerium and lanthanum may with advantage be
substantially absent.
zirconium may be present in an amount of up to 1.0%,
preferably at least 0. 46, for grain refining purposes. Up
to 2.0% of manganese may also be present, but the maximum
amount of zirconium and manganese together is limited by -
their mutual solubility.
Other elements soluble in magnesium may be present ~ -
provided that they do not, by forming compounds, interfere
with hardening treatment or depress the melting point
sufficiently to prevent dissolution of the rare earth
metals on heat treatment. These elements include:
Zinc 0 - 0.5 -6
Cadmium 0 - 1.0 %
Lithium 0 - 6.0 ~ -



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4~57

1 Calcium 0 - 0.8 %
Gallium 0 - 2.0 ~
Indium 0 - 2.0 %
Thallium 0 - 5.0 %
Lead 0 - 1.0
Bismuth 0 - 1.0 ~
Copper 0 - 0.15%
To obtain optimum mechanical properties the silver
content is preferably 2-3%.
Heat treatment is normally required to obtain optimum
mechanical properties in the cast alloy. The heat
treatment generally comprises solution heat treatment at
an elevated temperature followed by quenching and ageing
to achieve precipitation hardening. Solution treatment
may be carried out at a temperature ~rom 485C to the
solidus of the alloy and ageing at from 100C to 275C.
Typical conditions are solution treatment at about 525C
for about 8 hours and ageing at about 200C for 16 hours.
If the alloy contains above 0.1% Cu the high-
temperature treatment should be preceded by treatment at
a temperature not exceeding 485C, for example 465C, to
avoid incipient melting.
Alloys according to the invention will be described in
the following Examples.
Example
Alloys having the compositions shown in the Table were ~
prepared: alloys 1, 2 and 3 are comparative examples. ~ -
The silver was added as pure silver or a silver/ -
magnesium alloy. The rare earth metals were added as a

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~41~'7

1 "mischmetal" or a magnesium/rare earth hardener alloy; in
either case at least 60% by weight of the rare earth metal
is neodymium and not more than 3% is lanthanum plus
cerium. The thorium was added as a magnesium/thorium
alloy or as pure thorium. Zirconium was added as
magnesium/zirconium hardener or introduced via a reducible
zirconium halide. Yttrium was added as pure yttrium or
as a magnesium yttrium hardener alloy.
The case specimens were heat treated at 525%C for 8
hours following by quenching and ageing for 16 hours at
200C.
The yield, and ultimate tensile strengths and ~ -~
elongation were measured at 250C according to British
Standard 3688. The creep at 250C was measured by the
method of British Standard 3500 part 3. The room
temperature mechanical properties were measured in
accordance with British Standard 18. The results are
shown in the Table.




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It can be ~een that whereaQ a~ldition of yttrium gave
virtually no adverse effect on the ten.~ile properties of
the alloy it gave a notable improvement in rssi~tance to
creep.
It can be seen from Alloy 3,;that the creep properties
of the alloy cont&ining lesq thanØ5~ yttrium and no
thorium were worqe than for ~imilar alloy~ containin~
thorium and yttrium.
The following ganeralisations may be made regarding
alloys having compositions according to the invention:
~a) The addition of rela*iYely ~mall amounts of
yttrium to magne~ium alloy~ containing silver, neodymium
and thorium i~ beneficial in raising creep resistance at
elevated temperatures,
(b) Good mechanical properties at elevated
temperature.~ may be obtained with alloys containing yttrium
plus thorium or at least 0.5 yttrium. ~~
The yttrium may be added to the alloy~ of the
invention a~ pure yttrium~ but it may al~o be added at
lower C09t in the form of a mixture of yttrium and rare
earth metals containing at lea~t 60%~ preferably at least
65%, of yttrium.




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