Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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RAPID SOLIDIFICATION ROUTE ALUMINIUM ALLOYS
CONTAINING LITHIUM -
This invention relates to aluminium based alloys
containing lithium, made by the rapid solidification rate ~RSR)
route.
It is well known that lithium can be included in an
aluminium alloy to reduce its density and increase its modulus
of elasticity. Much work has been performed in this area on
materials made by RSR routes as well as the more conventional
wrought ingot route. RSR routes to the production of high
modulus low density alloys based on aluminium-lithium offer at
least the three following potential attractionæ:-
(1) a more uniform and refined microstructure including ;
elimination of ingot-derived coarse metallic second phases
that can act as sites for crack initiation and corrosion: -
(11) incorporation of dispersoid phases that are more
effective in homogenising ~lip than are phases
incorporated via the ingot route, so improving ductility
and toughnes 8;
and;
(111) incorporation of higher lith1um contents than can be
accommodated by the wrought ingot route (by avoidance of
segregation li~itations) thereby offering the prospect of
more signiflcant reductions in density and increases in
I modulus and strength.
This invention concerns especlally the dispersoid phase
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aspect of the RSR route alumlnium-lithium art. Several
dispersoid-forming additions have been inveætlgated previously.
One prior art investigatlon looked at the effects of 0.2 to 0.
weight percent additions of manganese, chromlum, iron, cobalt,
titanium and zlrconlum on an aluminium - 3 weight percent lithium
alloy. Other additions which we know to have been investigated
previously (in a variety of documents) are as follows (all
proportions being by weight percent): 1 to 7 sillcon; 0.2
~' titanium; 0.4 chromium; 0.2 to 3 manganeæe; 0.S iron; 0.2-1
cobalt, 0.04 yttrium; and 0.2 to 1 æirconium. -
The problem i5 one of developing a RSR route aluminium- ~-
lithium alloy having a good balance of those properties desirable
especially for aerospace structural application, such desirable
properties including: strength, high modulus, ductility and
fracture toughness together with low density; and the present
invention tackles this problem by providing new additives for such
alloys, that resist coarsening in the aluminium-base matrix at
elevated temperatures of the level that is likely to be
experienced in solution treatment, in artificial ageing or in
service.
The invention comprlses an aluminium alloy formed by a
rapid solidification rate process which alloy consists of the
following constituents in proportions by weight:
lithium 1 to 5%,
one or more of the following refractory elements within the
individual proportlons stated below and in total proportion not
exceeding 5% when present in combination
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nioblum 0.2 to 5.3%,
molybdenum 0.2 to 5%,
hafnium 0.2 to 5%,
tantalum 0.2 to 5%, and
tungsten 0.2 to 5%,
with aluminium as balance save for lncidental lmpuritles and save
for up to 5% in total of one or more of the following conventional
strengthening ingredients within the individual proportions -
stated:
copper 0 to 5%,
magnesium 0 to 5%, and
zinc 0 to 5%.
Note that all compositions given hereinafter are
expressed in proportions by weight.
The refractory metal element twhich may be represented
by X~-hereinafter) is preferably one of the elements rather than
more than one, and X is preferably present in the alloy in
proportion w1thin the range 0.2 to 5.0 percent in order to achieve
satisfactory effect without side effect or too great an increase
in density. ~
Lithium i8 preferably present in the alloy in proportion ~- `;
within the range 1 to S percent. If lithium present in ~uch
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j greater proportion it would be llkely to render the alloy
overly brittle.
It has beea found that the inclusion of such a refractory
metal element in an aluminium-lithium base alloy confers
improved strength to same (as reflected in the microhardness of
RSR splats) and Improved thermal stability (as reflected in the
change of ~icrohardness with exposure to temperatures
representative of solution treatment temperature and artificial
ageing temperatures). It is believed that these improved
properties are consequent partly upon dispersoids formed within
the aluminium matrix by the refractory metal element. It is
expected that this benefit will not be significantly diminished
by the presence in the base alloy of those ingredients other
than lithium which have been added to aluminium-lithium alloys
for strengthening purposes etc. A dispersion of the
aluminium-refractory metal element compounds act to inhibit
grain growth and thus help to retain the fine grain size
inherent in RSR alloys. This fine grain size is important for
the development of high strength and ductility. Solution heat
20 treatment, quenching and ageing of the alloy results in the ;
precipitation of aluminium-lithium based compounds as it would
for dispersoid-free aluminium-lithium alloys. The presence of
other elements such as copper, zinc and magnesium in an
aluminium-lithium alloy which promote precipitation hardening
in a solution treated alloy, is unlikely to markedly affect the
grain growth inhibiting properties of the aluminium-refractory
metal element dispersoid.
I; Various RSR methods well established in the art are
j suitable for the practice of the invention. RSR methods
possess in common the imposition of a hi8h cooling rate on an
aIloy from the liquid. RSR methods such as melt spraying,
chill methods and weld methods are described in some depth in
~ Rapid Solidification of Metals ant Alloys by H Jones tpublished
I as Monograph No 8 by The Institution of Metallurgists) and in
many other texts. The varlous RS routes differ from one
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another in their abilities in regard to control of cooling
rate. The degree of dispersold refinement and the extension of
~ solid solubility are dependent on the rate of cooling from the
;, melt.
Much of the laboratory work in connection with the current
invention has been performed using a twin piston splat
quenching method as described at pages 11 and 12 of the
aforementioned text by H Jones. This tec&nique is not
susceptible to scale up to an industrial scale. For such
larger scale use alternative well established RSR methods such
as gas atomising or planar flow casting would be suitable.
The alloy may contain an ingredient or ingredients other
than aluminium, lithium, and X such as those common in
aluminium-lithium alloys, eg copper magnesium or zinc.
The alloy is exemplified by reference to the specific
compositions given in Tables 1~2 and 3 are for the alloys when
3 produced as RSR splats within an argon atmosphere by the twin
~ piston technique. The splats produced were typically around
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50mm thick and the cooling rate developed by the RSR process
was of the order of lOa to 106 degrees Celcius per second. The
recorded composition~ shown in the first column of Tables 1 and
2 represent the measured composition of the source ingot. This
i will correspond closely to the composition of the splst at all
, times in the tests documented save in respect of the lithium
i~ 25 content. The measured lithium content for most of the splats
3 is given in parenthesis adjacent the relevent microhardness
¦ entry.
~ Table 1 below gives measurements of microhardness
.I (expressed in kg/mm2) as a function of time of exposure at 540
i 30 degrees Celcius this being a temperature representative of a
solution treatment regime. The fipecimens were encapsulated
prior to exposure within a quartz enclosure fllled with argon.
At completion of the exposure period the specimens welre removed
I from the encapsulation and water-quenched to room temperature.
! 35 Tantalum containing alloys are not documented ln the quoted
,~ figures but are expected to have co~parable properties.
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TABLE 1
Composition As splatted lhrlOhrlOOhr LOOOhr
~of ingot)
Al-2.9Li-1.6Nb 64+3 60+666+6 61+6 28~6
(2-9) (2.8) (2.7) (2.3 (1.6)
Al-3.6Li-3.3Nb 78+2 14+488+17 77+16 19.1+1
~, (3.6) (3.4) (3.3) (2.7) (1-9)
Al-3.6Li-5.3Nb 104+1 127+1118+10 85+6 36+5
I (3-6) (3.1) (2.4) (1.7~ (0.7)
¦ Al-5Li-1.7Nb123+5143+6 124+4119+6 48+8
I Al-3Li-0.6Mo105+3113+5 106+1189+6 34+6
¦ Al-3.9Li-0.5Mo 102+9 117+6116+7 112+8 70+1
(3-9) (3.6) (3.0) (3.2) (1-4)
Al-4.9Li-0-5Mo104+8140+25 120+8120+5 68+5
(4.1) (4.0) (3.4) (3.3) (1-9)
Al-5Li-0.6Mo108+1124+7 114+591+9 42+13
Al-2.8Li-5Hf109+7146+7 118+688+6 37+7
(2.8)
Al-3.5Li-4.2Hf 129+8 148+8124+5 104+5 38+7
(3.5)
¦ Al-2.5Li-l.lW103+5122+7 76+952+3 25+7
! (2.5) (2.6) (2.5) (1.9) (1-2)
Al-2.9Li-1.8W76+2102+3 78+967+5 25+4
(2.9) (2.8) (1.2) (0.8) (1-5)
I Al-3Li-2.8W118+6129+5 162+1288+3 44+3
I (3.0) (3.0) (2.9) (2.3) (0.6)
I Al-3Li-1.5Zr*60+488+3 67+642+7 26+5
I (2-7) (2.6) (2.5) (2.0) (1.1) ~ -
I Al-2.6Li*57+1 62+4 53+643+4 22+4
(2.6) (~.4) (2.5) (1.6) (1.0) i
*Indicate~ a prior art alloy included ~or comparison purposes. -~
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Table 2 below documents variations ~n microhardness and lithium
content for 8 test similar to that in Table 1 save that it was
conducted at a temperature of 160 degrees Celcius which is a
temperature representative of artificial ageing conditions.
Some of the specimens (those indicated) were subjected to a
solution treatment of 1 hour at 540 degrees Celcius followed by
water-quenching to room temperature prior to ageing. The
remainder were sub~ected to the ageing treatment from the
'as-splatted' condition.
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TABLE 2
Composition As splatted lhr 10hr lOOhr lOOOhr
(of ingot)
Al-3.6LI-3.3Nb80+5105+10 117+8 125+5
Al 3.6Li-5.3Nb 103+1 146+4 153~4 172+8 127+11
(3-6) (2.8) (2.9) (2.8) (2.7)
Al-5.0Li-1.7Nb123+5154+2 169+8 179+4 130+9
Al-3Li-0.6Mo 105+3 150+3 158+4 165+4 103+8
Al-3.9Li-0.5Mo102+9112+10 133+6 134+7
(3.9) (3.5) (3.2) (2.3)
Al-3.9Li-0.5Mo102+9133+5 129+12 134+7 - -
(3.9) (3.2) (2.9) (2.6)
Al-4.1Li-0.5Mo104+8130+12 148+15 190+22
(4-1) (3.6) (3.1) (2.5)
Al-4.1Li-0.5Mo104+9125+11 140_15 156+11
(3-9) (3-4) (3.0) (2.3)
Al-5Li-0.6Mo109+10 159+8 162+5 171+6 114+2
Al-2.8Li-5.0Hf108+7148+4 159+6 164+5 129+10 --
Al-3.5Li-4.2Hf129+8153+10 178+7 185+7 133+10
Al-2.9Li-1.8W72+4 116+6 119+9 120+8
(2.9)
Al-3Li-2.8W 117+6 144+9 163+2 165+3 131+7 '
(2.8) (2.8) (2.8) (2.53 (2.8)
Al-3Li-1.5Zr 62+5 90+5 99+6 104+5
Al-2.6Li* 60_3 94+3 108f8 116+7
(2.6) (2.5) (2.5) (2.5) ;-
Al-2.6Li* 60+3 86+11 105+9 107+12
(2.6) (2.1) (2.3) (2.1) -~
Second entered figures for alloys Al-3.9Li-0.5No,
Al-4.1Li-0.5Mo and Al-2.6Li are for those specimens subjected
to a prior 1 hour solution treatment.
*Indicates a prior art alloy included for comparison purposes.
It is recognised that the experimental results presented
above document the properties of the claimed alloy only by
reference to microhardness measurement6 of splat speciments.
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It is expected that these figures will to a large extent be a
valid indication of the strength and stability propertles of -
~ the claimed alloy when produced on an indu6trial scale under
3 suitable RS~ condition.
Table 3 below documents one comparative test of an alloy
~; of the invention against a reference alloy. The aluminium - 4
lithium - 0.6 molybdenum alloy was produced as RSR powder by
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¦ inert gas atomising. The powder was canned and then extruded
¦ (without the usual intermediate degassing treatment) to round
bar at a 25:l reduction ratio. The extruded bar was solution
treated at 540 degrees Celcius for one hour, water quenched,
then aged for one hour at 160 degrees Celcius. The comparitive
data relates to a comparable prior art alloy and the figures
given are taken from a published work - a paper by P J Meschter
et al at page 85 et seq of Aluminium-Lithium III (the -
proceedings of the Third International Aluminium-
10 Lithium Conference) published by The Institute of Metals. The -
data given is for a alloy of aluminium - 4 lithium - 0.2 -~
zirconium extruded from RSR powder, solution treated at 588 ~ ~
degrees Celcius then aged for one hour at 160 degrees --
Celcius.
TABLE 3
Composition 0.2% Proof Stress Tensile Stress Elongation
. Al-4Li-0.6Mo 472 MPa 519 MPa lZ
Al-4Li-0.2Zr 390 MPa ~ 475 MPa 10%
The above quoted alloy of the invention is unlikely to be -
representative of the true~merits of the invention for the
igures document a process not optimised with regard to the RSR
powder production and without the important degassing stage.
25 ~owever the quoted alloy, even in this condition, shows a - -
. useful increase in strength over the reference alloy. It
should be recognised that although this comparison is fair with
regard to the equivalence of the materials documented i~ does
not document the best of prior art materials nor is it likely
~ 30 to document the best materials of the invention.
l~ As mentioned previously the alloy of the invention 1s not
¦~ limited to an aluminium - lithium - X system for it is possible
that additions other than lithium and X will be incorporated
within the aluminium base in order to yield a material having a
,~ 35 better blend of properties than can be secured through a ~
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ternary alloy. It is postulated that the alloy could include
p to S percent or thereabouts of one or more ingredients
selected from the group consisting of copperJ zinc r~og r~ ~ and
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magnesium.
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