CA2079327A1 - Double aged rapidly solidified aluminum-lithium alloys - Google Patents
Double aged rapidly solidified aluminum-lithium alloysInfo
- Publication number
- CA2079327A1 CA2079327A1 CA 2079327 CA2079327A CA2079327A1 CA 2079327 A1 CA2079327 A1 CA 2079327A1 CA 2079327 CA2079327 CA 2079327 CA 2079327 A CA2079327 A CA 2079327A CA 2079327 A1 CA2079327 A1 CA 2079327A1
- Authority
- CA
- Canada
- Prior art keywords
- component
- aluminum
- lithium
- mpa
- ranges
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
- 229910001148 Al-Li alloy Inorganic materials 0.000 title claims abstract description 17
- JFBZPFYRPYOZCQ-UHFFFAOYSA-N [Li].[Al] Chemical compound [Li].[Al] JFBZPFYRPYOZCQ-UHFFFAOYSA-N 0.000 title claims abstract description 16
- 239000001989 lithium alloy Substances 0.000 title claims abstract description 14
- 230000032683 aging Effects 0.000 claims abstract description 25
- 229910052749 magnesium Inorganic materials 0.000 claims abstract description 10
- 239000011777 magnesium Substances 0.000 claims abstract description 10
- FYYHWMGAXLPEAU-UHFFFAOYSA-N Magnesium Chemical compound [Mg] FYYHWMGAXLPEAU-UHFFFAOYSA-N 0.000 claims abstract description 9
- 229910052802 copper Inorganic materials 0.000 claims abstract description 9
- 239000010949 copper Substances 0.000 claims abstract description 9
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 claims abstract description 8
- 229910052726 zirconium Inorganic materials 0.000 claims abstract description 8
- QCWXUUIWCKQGHC-UHFFFAOYSA-N Zirconium Chemical compound [Zr] QCWXUUIWCKQGHC-UHFFFAOYSA-N 0.000 claims abstract description 7
- 239000012530 fluid Substances 0.000 claims abstract description 5
- 229910045601 alloy Inorganic materials 0.000 claims description 25
- 239000000956 alloy Substances 0.000 claims description 25
- XAGFODPZIPBFFR-UHFFFAOYSA-N aluminium Chemical compound [Al] XAGFODPZIPBFFR-UHFFFAOYSA-N 0.000 claims description 12
- 229910052782 aluminium Inorganic materials 0.000 claims description 12
- 238000000034 method Methods 0.000 claims description 10
- 229910052744 lithium Inorganic materials 0.000 claims description 8
- WHXSMMKQMYFTQS-UHFFFAOYSA-N Lithium Chemical compound [Li] WHXSMMKQMYFTQS-UHFFFAOYSA-N 0.000 claims description 7
- 239000000203 mixture Substances 0.000 claims description 7
- 238000010791 quenching Methods 0.000 claims description 3
- 230000000171 quenching effect Effects 0.000 claims description 2
- 239000002244 precipitate Substances 0.000 description 6
- 229910000838 Al alloy Inorganic materials 0.000 description 4
- 230000015572 biosynthetic process Effects 0.000 description 4
- 238000007669 thermal treatment Methods 0.000 description 4
- 238000007792 addition Methods 0.000 description 3
- 238000005242 forging Methods 0.000 description 3
- 239000005457 ice water Substances 0.000 description 3
- 238000001556 precipitation Methods 0.000 description 3
- 238000012545 processing Methods 0.000 description 3
- 238000005728 strengthening Methods 0.000 description 3
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 3
- 229910001093 Zr alloy Inorganic materials 0.000 description 2
- 238000010521 absorption reaction Methods 0.000 description 2
- 229910052790 beryllium Inorganic materials 0.000 description 2
- ATBAMAFKBVZNFJ-UHFFFAOYSA-N beryllium atom Chemical compound [Be] ATBAMAFKBVZNFJ-UHFFFAOYSA-N 0.000 description 2
- 239000013078 crystal Substances 0.000 description 2
- 239000000463 material Substances 0.000 description 2
- 238000010899 nucleation Methods 0.000 description 2
- 230000006911 nucleation Effects 0.000 description 2
- 229910016343 Al2Cu Inorganic materials 0.000 description 1
- 229910017073 AlLi Inorganic materials 0.000 description 1
- ZOXJGFHDIHLPTG-UHFFFAOYSA-N Boron Chemical compound [B] ZOXJGFHDIHLPTG-UHFFFAOYSA-N 0.000 description 1
- 229910000733 Li alloy Inorganic materials 0.000 description 1
- FCVHBUFELUXTLR-UHFFFAOYSA-N [Li].[AlH3] Chemical compound [Li].[AlH3] FCVHBUFELUXTLR-UHFFFAOYSA-N 0.000 description 1
- PCYQNJRTSDSDLR-UHFFFAOYSA-N [Li][Cu][Mg] Chemical compound [Li][Cu][Mg] PCYQNJRTSDSDLR-UHFFFAOYSA-N 0.000 description 1
- AZDRQVAHHNSJOQ-UHFFFAOYSA-N alumane Chemical class [AlH3] AZDRQVAHHNSJOQ-UHFFFAOYSA-N 0.000 description 1
- -1 aluminum-lithium-copper-magnesium-zirconium Chemical compound 0.000 description 1
- 230000009286 beneficial effect Effects 0.000 description 1
- 229910052796 boron Inorganic materials 0.000 description 1
- 238000005266 casting Methods 0.000 description 1
- 230000001427 coherent effect Effects 0.000 description 1
- 230000000052 comparative effect Effects 0.000 description 1
- 238000009749 continuous casting Methods 0.000 description 1
- 238000007796 conventional method Methods 0.000 description 1
- RKTYLMNFRDHKIL-UHFFFAOYSA-N copper;5,10,15,20-tetraphenylporphyrin-22,24-diide Chemical compound [Cu+2].C1=CC(C(=C2C=CC([N-]2)=C(C=2C=CC=CC=2)C=2C=CC(N=2)=C(C=2C=CC=CC=2)C2=CC=C3[N-]2)C=2C=CC=CC=2)=NC1=C3C1=CC=CC=C1 RKTYLMNFRDHKIL-UHFFFAOYSA-N 0.000 description 1
- 230000002939 deleterious effect Effects 0.000 description 1
- 230000001627 detrimental effect Effects 0.000 description 1
- 238000000113 differential scanning calorimetry Methods 0.000 description 1
- 238000004090 dissolution Methods 0.000 description 1
- 238000009826 distribution Methods 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- 108010052322 limitin Proteins 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- AHLBNYSZXLDEJQ-FWEHEUNISA-N orlistat Chemical compound CCCCCCCCCCC[C@H](OC(=O)[C@H](CC(C)C)NC=O)C[C@@H]1OC(=O)[C@H]1CCCCCC AHLBNYSZXLDEJQ-FWEHEUNISA-N 0.000 description 1
- 239000002245 particle Substances 0.000 description 1
- 238000004663 powder metallurgy Methods 0.000 description 1
- 230000001376 precipitating effect Effects 0.000 description 1
- 238000001953 recrystallisation Methods 0.000 description 1
- 238000010008 shearing Methods 0.000 description 1
- 238000009864 tensile test Methods 0.000 description 1
Classifications
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
- C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
- C22F1/04—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon
- C22F1/047—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon of alloys with magnesium as the next major constituent
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C21/00—Alloys based on aluminium
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
- C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
- C22F1/04—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
- C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
- C22F1/04—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon
- C22F1/057—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon of alloys with copper as the next major constituent
Landscapes
- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Materials Engineering (AREA)
- Mechanical Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Physics & Mathematics (AREA)
- Thermal Sciences (AREA)
- Crystallography & Structural Chemistry (AREA)
- Forging (AREA)
- Lens Barrels (AREA)
- Continuous Casting (AREA)
Abstract
A component consolidated from a rapidly solidified aluminum-lithium alloy containing copper, magnesium and zirconium is subjected to a preliminary aging treatment at a temperature of about 400 ·C
to 500 ·C for a time period of about 0.5 to 10 hours; quenched in a fluid bath; and subjected to a final aging treatment at a temperature of about 100 ·C to 250 ·C for a time period ranging up to about 40 hours. The component exhibits increased strength and elongation, and is especially suited for use in lightweight structural parts for land vehicles and aerospace applications.
to 500 ·C for a time period of about 0.5 to 10 hours; quenched in a fluid bath; and subjected to a final aging treatment at a temperature of about 100 ·C to 250 ·C for a time period ranging up to about 40 hours. The component exhibits increased strength and elongation, and is especially suited for use in lightweight structural parts for land vehicles and aerospace applications.
Description
2 i~ 2;,' WO91/17281 PCT/~'S91/005~9 . , Double aged rapidly solidified aluminium-lithium alloys.
,: ,:
1. ~ield of Inv~n~iQ~ :
The invention relates to rapidly solidified aluminum-lithium coppermagnesium-zirconium powder metallurgy components having a combination of high ductility and high tensile strength; and more ~ lO particularly to a process wherein the components are subjected to thermal treatment which improves yield and ultimate strengths thereof with minimal loss in t~n,Silf? rll~ctility.
:~ 2. ~ie~ ~e~ ~ or ~rt The need for structural aerospace alloys o~
improved specific strength and specific modulus has . -long ~een present. It is known that the elements lithium, beryllium, boron, and magnesium can be added to an aluminum alloy to decrease its density.
;, 20 Conventional methods for producing aluminum alloys, such as direct chill (DC), continuous and semi-continuous casting, yield aluminum alloys having -up to 5 wt~ magnesium or beryllium; but such alloys are inade~uate for use in applications requiring a - .
combination of high strength and low density.
Lithium contents o~ about 2.5 wt~ have been sati~actorily in~orporated into the :.
lithium-copper-magnesium family of aluminum alloys, including those alloys designated 8090, 8091, 2090 and 2091. The~ alloys have copper and magnesium additions in the l to 3 wt% and 0.25 to l.5 wt%
range, respectively. In addition, zirconium is also added at levels up to 0.16 wt~.
The above alloys deriqe strength and toughness through the formation of sev~ral precipitate phases, , which are described in detail in the Conference Proceedings of Aluminum-Lithium V, edited by T.H.
.. . .
-:' , .
:
' . ' ' ' . " ' :. ' WO91/17281 ~ 3 ~J r~ PCT/VS91/00549 ' ~
Sanders and E.A. Starke, pub. MCE, (19~9). An important strengthening precipitate in aluminum-lithium alloys is the metastable ~ phase which has a well defined solYus line. Thus, - 5 aluminum-lithium alloys are heat treatable, their strength increasing as ~' homogeneously nucleates from the supersaturated aluminum matri~.
The ~' phase consists of the ordered Ll2 crystal structure and the composition Al3Li. The phase has a very smail lattice misfit with the surrounding aluminum matri~ and thus a coherent interface with the matri~. Dislocations easily shear t'.c ~.a~lrit3te~ durir.~ dgform~tion, re~ul tina; n th~
buildup of planar slip bands. This, in turn, reduces the toughness of aluminum lithium alloys. In binary aluminum-lithium alloys where this is the only - strengthening phase employed, the slip planarity results in reduced toughness.
The addition of copper and magnesium to aluminum-lithium alloys has two beneficial effects.
First, the elements reduce the solubility of lithium in aluminum, increasing the amount of strengthening precipitates available. More importantly, however, the copper and magne~ium allow the formation of additional precipitate phases, most importantly the orthorhombic S' phase (A12MgLi) and the he3agonal T
phase (Al2Cu~i). Unlike 8', these phases are ; re~istant to shearing by dislocations and are e~fective in mini~izing slip planarity. The ~; 30 resulting homogeneity of the deformation results in improved toughness, increasing the applicability of these alloys over binary aluminum-lithium.
; Unfortunately, these phases form sluggishly, precipitating primarily on heterogeneous nucleation sites such as dislocations. In order to generate these nucleations ~ites, the alloys must be cold worked prior to aging.
.
2~32;7 WO91/17281 PCT/~S91/0~49 -Zirconium, at levels under appro~imately 0.15 wt%, is typically added to the alloys to form the metastable A13Zr phase for grain size control an~ to retard recrystallization. Metastable A13Zr consists of an L12 crystal structure which is essentially isostructural with ~' (A13Li). Additions of zirconium to aluminum beyond 0.15 wt% using conventional casting practice result in the formation o~ relatively large dispersoids of equilibrium A13Zr -~ 10 having the tetragonal DO23 structure which are detrimental to toughness.
~ Much work has been done to develop the - ~forementioned alloys, which are currently near commercialization. However, the processing lS constraint imposed by the need for cold deformation has limited the application o~ these alloys to thin, low dimensional shapes such as sheet and plate. ~ -Comple~, shaped components such as forgings are not amenable to such proressing. Hence, conventional aluminum-lithium alloy forgings lack the combination -~ of strength, ductility, and low density required for aerospace structural applications.
The invention provide~ a method for increasing the tensile stre~gth of a component composed of a rapidly solidiied aluminum-lithium-copper-magnesium-zirconium alloy by subjecting the component to a multi-step aging trea~ment. Generally stated, ~he component is a consolidate~ article, ~ormed from an alloy that is rapidly solidified and consists essentially of the formula AlbalLiaCUbM9~Zrd wherein "a" ranges from about 2.1 to 3.4 wt%, "b~ ranses from 0.5 to 2.0 wt~, c" ranges from 0.2 to 2.0 wt~, and "d~ ranges from ,-about 0.9 to 1.8 wt%, the balance being aluminum.
The aging treatment to which the component is subjected comprises the step~ of subjecting the ,' .
.
.. . , ~
.
~ W091/17281 ~7~7~ ~ PCT/~S91/00~49 .. 4 component to a preliminary aging treatment at a temperature of about 400C-500C for a time period ranging from about 0.5 to lO hours; quenching th~
component in a fluid bath; and subjecting the component to a final aqing treatment at a temperature of about 100C-250C for a time period ranging up to about 40 hours.
In addition, the invention provides a component consolidated from a rapidly solidified aluminum-lithium alloy of the type delineated, which component has been subjected to the multi-step aging treatment specified hereinabove.
It has been found that when specific components consolidated from rapidly solidified lS alloys of the composition ~elineated are subjected to the multi-step aging treatment speciied , they exhibit inereased strength and elongation, as ~ compared with components that are t~ermally processed in a conventional manner. The improved combination of propertles afforded by components of the invention : renders them especially suited for lightweight structural parts used in automobile, aircra~t or spacecraft applications.
The invention will ba more fully understood and ~urther ad~antages will become apparent when reere~ce is made to the following detailed description of the preferred embodiment of the invention and the accompanying drawings in which:
FIG. l is a graph depicting the heat evolution/absorption vs. temperature as measured by differential scanning calorimetry for an Al-2.6Li-l.OCu-0.5Mg-l.OZr alloy aged at 590C for 2 hours and ice water quenched;
FIG. 2 is a graph of the yield strength vs.
aging temperature o~ a transverse specimen cut from an extruded bar aged for 2 hrs. followed by an ice water quench and subsequent aging for 16 hrs. at WO91/17281 PCT/I 79~/30~4~
.
- _ 5 135C, the open rectangle providing data for a transverse specimen cut from an Al-2.34Li-1.07Zr e~truded bar; the specimen being aged at 500C for l hr. was water quenched and subsequently aged at 190C
for 2 hours;
FIG. 3 is a graph of the ultimate tensile strength vs. aging temperature for specimens aged in the manner of the specimens of Fig. 2;
; FIG. 4 is a graph of the tensile elongation ; lO vs. aging temperature for specimens aged in the manner of the specimens of Fig. 2; and FIG. 5 is a graph depicting the ultimate -tr~ th ~c ~lonn~tion Çor the alloys of Fig. 2 illustrating the improvement in properties estant along the diagonal away from the origin.
De~ri~L ~f t~e Pr& ~9L~=es~
The in~ention provides a thermal treatment that increases the tensile strength of a low density rapidly solidified aluminum-base alloy, consisting essentially of the formula AlbalLiaCUb~9cZrd wherein .~ ~a" ranges from 2.1 to 3.4 wt~, ~b~ ranges from about 0.5 to 2.0 wt~, ~c~ ranges from 0.2 to 2.0 wt~, "d~
; ranges from about 0.4 to l.8 wt~ and the balance is ~' aluminum.
In accordance with the invention, the compacted alloy or comp,onent is sub~ected to a preliminary th~rmal treatment at temperatures ranging ~' from a~out 400C to 500C for a period o~
appro~imately 0.5 to lO hours. While not being bound by theory, it is believed that this treatment dissolYes elements such as Cu, Mg, and Li which may be microsegregated in precipitated phases such as ~', ~, T, and S. In addition, the thermal treatment produces an optimized distribution of cubic Ll2 particles ranging f rom about 5 to 50 nanometers in size. The alloy article is then quenched in a fluid bath, preferabl~ held between 0 and 60'C. ~s used ..
~'.
., , ,,: , ;, ~, ,~ . . ..
.. . . . ~ -. , - - ~: .
: . . : ; . . ' .
. . .
, , : .
"
: ~ ;
WO91/17281 ~ 3~ ~ PCT/~'S91/~0549 hereinafter in the specification and claims, the term ~preliminary aging" is intended to define the thermal treatment described in the first sentence of this paragraph. The compacted article is then aged at a temperature ranging from about 100C to 250OC. for a time period ranging up to about 40 hours to provide selected strength/toughness tempers. No cold deformation step is required during this thermal processing, with the result that comple~ shaped components such as forgings produced from the aged component have escellent mechanical properties.
Preliminary aging below approsimately 400C
results in a deleterious drop in tensile properties due to the formation of undesirable phases such as the ~ (AlLi) phase. PrPliminary aqing above appro~imately 500C results in an acceptable combination of tensile properties but does not result in the attainment of the optimum tensile strength !~since the volume fraction of precipitates is -;20 reduced. Grain coarsening may also oceur at temperature beyond 550C, further reducing strength.
Consolidated articles aged in accordance with the invention e~hibit tensile yield strength ranging from about 400 MPa (58 ksi) to 545 MPa (79 ksi), ultimate tensile stren~th ranging from about 510 MPa (74ksi~ to MPa ~B3 ksi) and ~elongation to fracture ranging f rom about 4 to 9 % when measured at room temperature (20C).
The following esamples are presented to proYide a more complete understandin~ of the invention. The specific techniques, conditions, materials, proportions and reported da~a set forth to illustrate the principles and practice of the invention are e~emplary and should not be construed as limitin~ the scope of the invention.
.
.
~ v ~
. - 7 _ Thermal processing in accordance with the invention was carried out rn e~truded bar made ~rom rapidly solidified alloys having compositions (in wtS) listed in Table I.
. T~E I
__ l.Al-2.lLi-l.OCu-0.5Mg-0.2Zr 2.Al-2.6Li l.OCu-O.SMg-0.6Zr .
lO3.Al-2.6Li-l.OCu-0.5Mg-l.OZr . , _ _ , .
,, ~a~5~E 4 '"
Al-2.6Li-l.OCu-0.5Mg-l.OZr, made via rapid solidiication and formed into an e~trusion, was ... .
given a preliminary age at 590C for 2 hours and ice ` water quenched. The heat evolution/absorption as a function of temperature was then measured using the technique of di~ferential scanning ~alorimetry (DSC), shown in Figure l. The peak~ in Figure l represent the dissolution of precipitate phases during heating .1 while the troughs represent precipitation. A :, precipitation reactio~ is represented hy the trough centered at 453C. It is this precipitation rea~tion 25 which is re~ponsible for the e~hanced strength --:. resulting ~ro~ the preliminary aging treatment.
The tensile properties of eonsolidated . article~ form~d by e~trusion of the alloys listed in ; 30 Table I and thermally processed in accord3nee with -.
the method of the invention are listed in Table II.
The ~struded bars were given a preliminary age for 2 hours at temperatures between 400C and 600C and quenched into an ice water bath; subsequently, they were aged at 135C for 16 hours. Trans~erse specimens were th~n cut and machined into round tensile specimons having a gauge diameter of 3~B
. ~.
`. -' ' ' ' ' ' .- .
., ,',, - , , ~
..
.
. . .
,: ,:
1. ~ield of Inv~n~iQ~ :
The invention relates to rapidly solidified aluminum-lithium coppermagnesium-zirconium powder metallurgy components having a combination of high ductility and high tensile strength; and more ~ lO particularly to a process wherein the components are subjected to thermal treatment which improves yield and ultimate strengths thereof with minimal loss in t~n,Silf? rll~ctility.
:~ 2. ~ie~ ~e~ ~ or ~rt The need for structural aerospace alloys o~
improved specific strength and specific modulus has . -long ~een present. It is known that the elements lithium, beryllium, boron, and magnesium can be added to an aluminum alloy to decrease its density.
;, 20 Conventional methods for producing aluminum alloys, such as direct chill (DC), continuous and semi-continuous casting, yield aluminum alloys having -up to 5 wt~ magnesium or beryllium; but such alloys are inade~uate for use in applications requiring a - .
combination of high strength and low density.
Lithium contents o~ about 2.5 wt~ have been sati~actorily in~orporated into the :.
lithium-copper-magnesium family of aluminum alloys, including those alloys designated 8090, 8091, 2090 and 2091. The~ alloys have copper and magnesium additions in the l to 3 wt% and 0.25 to l.5 wt%
range, respectively. In addition, zirconium is also added at levels up to 0.16 wt~.
The above alloys deriqe strength and toughness through the formation of sev~ral precipitate phases, , which are described in detail in the Conference Proceedings of Aluminum-Lithium V, edited by T.H.
.. . .
-:' , .
:
' . ' ' ' . " ' :. ' WO91/17281 ~ 3 ~J r~ PCT/VS91/00549 ' ~
Sanders and E.A. Starke, pub. MCE, (19~9). An important strengthening precipitate in aluminum-lithium alloys is the metastable ~ phase which has a well defined solYus line. Thus, - 5 aluminum-lithium alloys are heat treatable, their strength increasing as ~' homogeneously nucleates from the supersaturated aluminum matri~.
The ~' phase consists of the ordered Ll2 crystal structure and the composition Al3Li. The phase has a very smail lattice misfit with the surrounding aluminum matri~ and thus a coherent interface with the matri~. Dislocations easily shear t'.c ~.a~lrit3te~ durir.~ dgform~tion, re~ul tina; n th~
buildup of planar slip bands. This, in turn, reduces the toughness of aluminum lithium alloys. In binary aluminum-lithium alloys where this is the only - strengthening phase employed, the slip planarity results in reduced toughness.
The addition of copper and magnesium to aluminum-lithium alloys has two beneficial effects.
First, the elements reduce the solubility of lithium in aluminum, increasing the amount of strengthening precipitates available. More importantly, however, the copper and magne~ium allow the formation of additional precipitate phases, most importantly the orthorhombic S' phase (A12MgLi) and the he3agonal T
phase (Al2Cu~i). Unlike 8', these phases are ; re~istant to shearing by dislocations and are e~fective in mini~izing slip planarity. The ~; 30 resulting homogeneity of the deformation results in improved toughness, increasing the applicability of these alloys over binary aluminum-lithium.
; Unfortunately, these phases form sluggishly, precipitating primarily on heterogeneous nucleation sites such as dislocations. In order to generate these nucleations ~ites, the alloys must be cold worked prior to aging.
.
2~32;7 WO91/17281 PCT/~S91/0~49 -Zirconium, at levels under appro~imately 0.15 wt%, is typically added to the alloys to form the metastable A13Zr phase for grain size control an~ to retard recrystallization. Metastable A13Zr consists of an L12 crystal structure which is essentially isostructural with ~' (A13Li). Additions of zirconium to aluminum beyond 0.15 wt% using conventional casting practice result in the formation o~ relatively large dispersoids of equilibrium A13Zr -~ 10 having the tetragonal DO23 structure which are detrimental to toughness.
~ Much work has been done to develop the - ~forementioned alloys, which are currently near commercialization. However, the processing lS constraint imposed by the need for cold deformation has limited the application o~ these alloys to thin, low dimensional shapes such as sheet and plate. ~ -Comple~, shaped components such as forgings are not amenable to such proressing. Hence, conventional aluminum-lithium alloy forgings lack the combination -~ of strength, ductility, and low density required for aerospace structural applications.
The invention provide~ a method for increasing the tensile stre~gth of a component composed of a rapidly solidiied aluminum-lithium-copper-magnesium-zirconium alloy by subjecting the component to a multi-step aging trea~ment. Generally stated, ~he component is a consolidate~ article, ~ormed from an alloy that is rapidly solidified and consists essentially of the formula AlbalLiaCUbM9~Zrd wherein "a" ranges from about 2.1 to 3.4 wt%, "b~ ranses from 0.5 to 2.0 wt~, c" ranges from 0.2 to 2.0 wt~, and "d~ ranges from ,-about 0.9 to 1.8 wt%, the balance being aluminum.
The aging treatment to which the component is subjected comprises the step~ of subjecting the ,' .
.
.. . , ~
.
~ W091/17281 ~7~7~ ~ PCT/~S91/00~49 .. 4 component to a preliminary aging treatment at a temperature of about 400C-500C for a time period ranging from about 0.5 to lO hours; quenching th~
component in a fluid bath; and subjecting the component to a final aqing treatment at a temperature of about 100C-250C for a time period ranging up to about 40 hours.
In addition, the invention provides a component consolidated from a rapidly solidified aluminum-lithium alloy of the type delineated, which component has been subjected to the multi-step aging treatment specified hereinabove.
It has been found that when specific components consolidated from rapidly solidified lS alloys of the composition ~elineated are subjected to the multi-step aging treatment speciied , they exhibit inereased strength and elongation, as ~ compared with components that are t~ermally processed in a conventional manner. The improved combination of propertles afforded by components of the invention : renders them especially suited for lightweight structural parts used in automobile, aircra~t or spacecraft applications.
The invention will ba more fully understood and ~urther ad~antages will become apparent when reere~ce is made to the following detailed description of the preferred embodiment of the invention and the accompanying drawings in which:
FIG. l is a graph depicting the heat evolution/absorption vs. temperature as measured by differential scanning calorimetry for an Al-2.6Li-l.OCu-0.5Mg-l.OZr alloy aged at 590C for 2 hours and ice water quenched;
FIG. 2 is a graph of the yield strength vs.
aging temperature o~ a transverse specimen cut from an extruded bar aged for 2 hrs. followed by an ice water quench and subsequent aging for 16 hrs. at WO91/17281 PCT/I 79~/30~4~
.
- _ 5 135C, the open rectangle providing data for a transverse specimen cut from an Al-2.34Li-1.07Zr e~truded bar; the specimen being aged at 500C for l hr. was water quenched and subsequently aged at 190C
for 2 hours;
FIG. 3 is a graph of the ultimate tensile strength vs. aging temperature for specimens aged in the manner of the specimens of Fig. 2;
; FIG. 4 is a graph of the tensile elongation ; lO vs. aging temperature for specimens aged in the manner of the specimens of Fig. 2; and FIG. 5 is a graph depicting the ultimate -tr~ th ~c ~lonn~tion Çor the alloys of Fig. 2 illustrating the improvement in properties estant along the diagonal away from the origin.
De~ri~L ~f t~e Pr& ~9L~=es~
The in~ention provides a thermal treatment that increases the tensile strength of a low density rapidly solidified aluminum-base alloy, consisting essentially of the formula AlbalLiaCUb~9cZrd wherein .~ ~a" ranges from 2.1 to 3.4 wt~, ~b~ ranges from about 0.5 to 2.0 wt~, ~c~ ranges from 0.2 to 2.0 wt~, "d~
; ranges from about 0.4 to l.8 wt~ and the balance is ~' aluminum.
In accordance with the invention, the compacted alloy or comp,onent is sub~ected to a preliminary th~rmal treatment at temperatures ranging ~' from a~out 400C to 500C for a period o~
appro~imately 0.5 to lO hours. While not being bound by theory, it is believed that this treatment dissolYes elements such as Cu, Mg, and Li which may be microsegregated in precipitated phases such as ~', ~, T, and S. In addition, the thermal treatment produces an optimized distribution of cubic Ll2 particles ranging f rom about 5 to 50 nanometers in size. The alloy article is then quenched in a fluid bath, preferabl~ held between 0 and 60'C. ~s used ..
~'.
., , ,,: , ;, ~, ,~ . . ..
.. . . . ~ -. , - - ~: .
: . . : ; . . ' .
. . .
, , : .
"
: ~ ;
WO91/17281 ~ 3~ ~ PCT/~'S91/~0549 hereinafter in the specification and claims, the term ~preliminary aging" is intended to define the thermal treatment described in the first sentence of this paragraph. The compacted article is then aged at a temperature ranging from about 100C to 250OC. for a time period ranging up to about 40 hours to provide selected strength/toughness tempers. No cold deformation step is required during this thermal processing, with the result that comple~ shaped components such as forgings produced from the aged component have escellent mechanical properties.
Preliminary aging below approsimately 400C
results in a deleterious drop in tensile properties due to the formation of undesirable phases such as the ~ (AlLi) phase. PrPliminary aqing above appro~imately 500C results in an acceptable combination of tensile properties but does not result in the attainment of the optimum tensile strength !~since the volume fraction of precipitates is -;20 reduced. Grain coarsening may also oceur at temperature beyond 550C, further reducing strength.
Consolidated articles aged in accordance with the invention e~hibit tensile yield strength ranging from about 400 MPa (58 ksi) to 545 MPa (79 ksi), ultimate tensile stren~th ranging from about 510 MPa (74ksi~ to MPa ~B3 ksi) and ~elongation to fracture ranging f rom about 4 to 9 % when measured at room temperature (20C).
The following esamples are presented to proYide a more complete understandin~ of the invention. The specific techniques, conditions, materials, proportions and reported da~a set forth to illustrate the principles and practice of the invention are e~emplary and should not be construed as limitin~ the scope of the invention.
.
.
~ v ~
. - 7 _ Thermal processing in accordance with the invention was carried out rn e~truded bar made ~rom rapidly solidified alloys having compositions (in wtS) listed in Table I.
. T~E I
__ l.Al-2.lLi-l.OCu-0.5Mg-0.2Zr 2.Al-2.6Li l.OCu-O.SMg-0.6Zr .
lO3.Al-2.6Li-l.OCu-0.5Mg-l.OZr . , _ _ , .
,, ~a~5~E 4 '"
Al-2.6Li-l.OCu-0.5Mg-l.OZr, made via rapid solidiication and formed into an e~trusion, was ... .
given a preliminary age at 590C for 2 hours and ice ` water quenched. The heat evolution/absorption as a function of temperature was then measured using the technique of di~ferential scanning ~alorimetry (DSC), shown in Figure l. The peak~ in Figure l represent the dissolution of precipitate phases during heating .1 while the troughs represent precipitation. A :, precipitation reactio~ is represented hy the trough centered at 453C. It is this precipitation rea~tion 25 which is re~ponsible for the e~hanced strength --:. resulting ~ro~ the preliminary aging treatment.
The tensile properties of eonsolidated . article~ form~d by e~trusion of the alloys listed in ; 30 Table I and thermally processed in accord3nee with -.
the method of the invention are listed in Table II.
The ~struded bars were given a preliminary age for 2 hours at temperatures between 400C and 600C and quenched into an ice water bath; subsequently, they were aged at 135C for 16 hours. Trans~erse specimens were th~n cut and machined into round tensile specimons having a gauge diameter of 3~B
. ~.
`. -' ' ' ' ' ' .- .
., ,',, - , , ~
..
.
. . .
3-i' W091/17281 PCT/~S91/0054 inches and a gauge length of 3/4 inches. Tensile testing was performed at room temperature at a strain rate of 5.5~10 ~4Sec-l.
TAB~E II
Composltlon (wtX) 0.2% YS UTSElong. to Aa_n~ Tem~. _ (MPa2 (MPa~frac~. (%) _ A1-2 . 6L1 -t . OCu-0. 5Mg-0 . 2Zr 400-C ~00 435 4 . 9 1 04~0-C 41 0 495 7 . 2 490''C 395 S00 6 . 3 540C 350 470 8.0 sqnor 3~0 460 6.9 Al -2 . 6Ll -1 . OCu-0 . SMg-0 . 6Zr 15400~ 500 510 5.2 440-C 490 535 6 . 9 490C 470 540 7 . 7 540~C 440 520 5 . 8 590C 395 51 5 8. 3 2 oA 1- 2 6 ! 1-1 . OCu-0 . 5Mg- 1 . OZr 400-~ 540 555 3 . 7 440-C 535 570 7 . 1 490-~ S l 0 560 6 . 3 540~C 465 545 8 . 4 _ 5~Q C 4Q~ ~Q~ . 7.3 Figure~ Z, 3, and 4 are graphs of the data li~ted in Table II. The graphs illustrate that the peak ultimate tensile strength (UTS) is a function of both zir~onium content and temperature of the first aging treatment. For e~ample, a peak UTS of 570 MPa is obtained for 440C preliminary aged Al-2.6Li-l.Cu-0.5Mg-l.OZr while a peak UTS of 540 MPa is obtained for a 490C preliminary aged 35 Al-2~6Li-l~oMs-o~6zr~
Also included for re~erenoe in the Figures 2, 3, and 4 is the transver~e tensile data for an .
, , . , ' , ,, . , ' ' , W09¦/l728] PCT/~S91 70 0549 Al-2.34Li-1.07Zr e~trude bar processed in the ~anner disclosed by U.S.P. 4,747,~84 to Gayle et al. It is clear that the tensile properties of materials processed in accordance with the present invention are superior to those of conventionally processed alloy chemistries. Gayle et al. have also included data on rod produced from this alloy which displays, qualitatively, a similar strength behavior with preliminary aging temperature. The rod, due to enhanced deformation compared with bar, displays a strength somewhat higher than the bar stock, as shown in'the data of Gayle et al. It will be appreciated h~ th~.e~ .skill~d in the art that similar comparative ~ gains in strength induced by enhanced deformation ; 15 will be achieved in rod made from Al-Li-Cu-Mg-Zr alloys. Data is also included for the ingot , aluminum~lithium alloy 8090, is taken from Damerval et. al., "4th International Aluminum-Lithium Conference", J. de Physique; pg. C3 661, (1987), ~or transverse sections o~ e~truded bar. The bar was solutionized at 540C-l hr; cold water quenched, given a 2~ cold ~tretch, and peak aged at 190C for 12 hours. It will be seen that the properties of RS ~-Al-Li-Cu-Mg-Zr bar stock given a preliminary age in accordance with the invention are superior to those of the alloy 8090, without the cold deformation -~
commonly employed on ingot aluminum-lithium alloys.
E~a~oe~ 6 This ~$ample illustrates th~t the enhanced strength resulting ~rom control o~ the preliminary age is greater than and thus distinct from merely extending the aging time of the second low temperature aging treatment. The tensile yield strengths for an Al-2.6LiOl.OCu-0.5Mg-0.6Zr es~rusion measured in the manner set forth in Esample 5 are listed in Table III. Reducing the preliminary aging temperature ~rom 540~C to 400C results in a 14~
. .
. .. . , .
' ' .
c~7~3 j~ ~
WO91~17281 PCT/~S91/00~49 increase in tensile strength compared with only 4~
increase in strength when a 540C preliminary aged specimen is aged for double the time 135C.
Table III
S
i` Therm~1 Treatment _ . YS(MPa) 400C-2hr ice WQ;135C-16hr 500 5~0C-2hr ice WQ;135C-16hr 440 10 540C~2hr ice WQ;135C 32hr 460 Having thus described the invention in rather full detail, it will be understood that these details need not be strictly adhered to but that various changes and modifications may suggest themselves to one skilled in the art, all falling within the scope of the invention as defined by the su~joined claims.
.~
.
. 25 ' ' -; ,. , : .. ~, . .. ..' ' ': ' . '' . : . . .
, . , , : . . . . . .. . .
. . ..
.:,.. ,,. . . . , ... :' . : ' ' : , . ' .'' "' ', ' ,. :'' ' , .. ' . .: ,.:
: : ' . .: ,, :
TAB~E II
Composltlon (wtX) 0.2% YS UTSElong. to Aa_n~ Tem~. _ (MPa2 (MPa~frac~. (%) _ A1-2 . 6L1 -t . OCu-0. 5Mg-0 . 2Zr 400-C ~00 435 4 . 9 1 04~0-C 41 0 495 7 . 2 490''C 395 S00 6 . 3 540C 350 470 8.0 sqnor 3~0 460 6.9 Al -2 . 6Ll -1 . OCu-0 . SMg-0 . 6Zr 15400~ 500 510 5.2 440-C 490 535 6 . 9 490C 470 540 7 . 7 540~C 440 520 5 . 8 590C 395 51 5 8. 3 2 oA 1- 2 6 ! 1-1 . OCu-0 . 5Mg- 1 . OZr 400-~ 540 555 3 . 7 440-C 535 570 7 . 1 490-~ S l 0 560 6 . 3 540~C 465 545 8 . 4 _ 5~Q C 4Q~ ~Q~ . 7.3 Figure~ Z, 3, and 4 are graphs of the data li~ted in Table II. The graphs illustrate that the peak ultimate tensile strength (UTS) is a function of both zir~onium content and temperature of the first aging treatment. For e~ample, a peak UTS of 570 MPa is obtained for 440C preliminary aged Al-2.6Li-l.Cu-0.5Mg-l.OZr while a peak UTS of 540 MPa is obtained for a 490C preliminary aged 35 Al-2~6Li-l~oMs-o~6zr~
Also included for re~erenoe in the Figures 2, 3, and 4 is the transver~e tensile data for an .
, , . , ' , ,, . , ' ' , W09¦/l728] PCT/~S91 70 0549 Al-2.34Li-1.07Zr e~trude bar processed in the ~anner disclosed by U.S.P. 4,747,~84 to Gayle et al. It is clear that the tensile properties of materials processed in accordance with the present invention are superior to those of conventionally processed alloy chemistries. Gayle et al. have also included data on rod produced from this alloy which displays, qualitatively, a similar strength behavior with preliminary aging temperature. The rod, due to enhanced deformation compared with bar, displays a strength somewhat higher than the bar stock, as shown in'the data of Gayle et al. It will be appreciated h~ th~.e~ .skill~d in the art that similar comparative ~ gains in strength induced by enhanced deformation ; 15 will be achieved in rod made from Al-Li-Cu-Mg-Zr alloys. Data is also included for the ingot , aluminum~lithium alloy 8090, is taken from Damerval et. al., "4th International Aluminum-Lithium Conference", J. de Physique; pg. C3 661, (1987), ~or transverse sections o~ e~truded bar. The bar was solutionized at 540C-l hr; cold water quenched, given a 2~ cold ~tretch, and peak aged at 190C for 12 hours. It will be seen that the properties of RS ~-Al-Li-Cu-Mg-Zr bar stock given a preliminary age in accordance with the invention are superior to those of the alloy 8090, without the cold deformation -~
commonly employed on ingot aluminum-lithium alloys.
E~a~oe~ 6 This ~$ample illustrates th~t the enhanced strength resulting ~rom control o~ the preliminary age is greater than and thus distinct from merely extending the aging time of the second low temperature aging treatment. The tensile yield strengths for an Al-2.6LiOl.OCu-0.5Mg-0.6Zr es~rusion measured in the manner set forth in Esample 5 are listed in Table III. Reducing the preliminary aging temperature ~rom 540~C to 400C results in a 14~
. .
. .. . , .
' ' .
c~7~3 j~ ~
WO91~17281 PCT/~S91/00~49 increase in tensile strength compared with only 4~
increase in strength when a 540C preliminary aged specimen is aged for double the time 135C.
Table III
S
i` Therm~1 Treatment _ . YS(MPa) 400C-2hr ice WQ;135C-16hr 500 5~0C-2hr ice WQ;135C-16hr 440 10 540C~2hr ice WQ;135C 32hr 460 Having thus described the invention in rather full detail, it will be understood that these details need not be strictly adhered to but that various changes and modifications may suggest themselves to one skilled in the art, all falling within the scope of the invention as defined by the su~joined claims.
.~
.
. 25 ' ' -; ,. , : .. ~, . .. ..' ' ': ' . '' . : . . .
, . , , : . . . . . .. . .
. . ..
.:,.. ,,. . . . , ... :' . : ' ' : , . ' .'' "' ', ' ,. :'' ' , .. ' . .: ,.:
: : ' . .: ,, :
Claims (10)
1. A process for increasing the strength of a rapidly solidified aluminum-lithium alloy component, comprising the steps of:
a. subjecting the component to a preliminary aging treatment at a temperature of about 400°C to 500°C for a time period from about 0.5 to 10 hours;
b. quenching the component in a fluid bath;
and, c. subjecting the component to final treatment at a temperature of about 100°C to 250°C
for a time period ranging up to about 40 hours, said component being a consolidated article formed from an aluminum-lithium alloy that is rapidly solidified and consists essentially of the formula AlbalLiaCubMgcZrd wherein "a" ranges from about 2.1 to 3.4 wt%, "b"
ranges from about 0.5 to 2.0 wt%, "c" ranges from about 0.2 to 2.0 wt% and "d" ranges from about 0.4 to 1.8 wt%, the balance being aluminum.
a. subjecting the component to a preliminary aging treatment at a temperature of about 400°C to 500°C for a time period from about 0.5 to 10 hours;
b. quenching the component in a fluid bath;
and, c. subjecting the component to final treatment at a temperature of about 100°C to 250°C
for a time period ranging up to about 40 hours, said component being a consolidated article formed from an aluminum-lithium alloy that is rapidly solidified and consists essentially of the formula AlbalLiaCubMgcZrd wherein "a" ranges from about 2.1 to 3.4 wt%, "b"
ranges from about 0.5 to 2.0 wt%, "c" ranges from about 0.2 to 2.0 wt% and "d" ranges from about 0.4 to 1.8 wt%, the balance being aluminum.
2. A process as recited by claim 1, wherein said component has the composition 2.6 wt% lithium, 1.0 wt% copper, 0.5 wt% magnesium and 0.6 wt%
zirconium, the balance being aluminum.
zirconium, the balance being aluminum.
3. A process as recited by claim 2, wherein said component, after final aging, has a 0.2% tensile yield strength of 440 MPa, ultimate tensile strength of 530 MPa, and elongation to fracture of 7%.
4. A process as recited by claim 1, wherein said component has the composition 2.6 wt% lithium, 1.0 wt% copper, 0.5 wt% magnesium and 1.0 wt%
zirconium, the balance being aluminum.
zirconium, the balance being aluminum.
5. A process as recited by claim 4, wherein said component, after final aging, has 0.2% tensile yield strength of about 535 MPa, ultimate tensile strength of 570 MPa, and elongation to fracture of 7%.
6. A component consolidated from an alloy that is rapidly solidified and consists essentially of the formula AlbalCubMgcZrd wherein "a" ranges from about 2.1 to 3.4 wt%, "b" ranges from about 0.5 to 2.0 wt%, "c" ranges from about 0.2 to 2.0 wt%, and "d" ranges from about 0.4 to 1.8 wt%, the balance being aluminum, said component having been subjected to a preliminary aging treatment at a temperature of about 400°C to 500°C for a time period of about 0.5 to 10 hours, quenched in a fluid bath and subjected to a final aging treatment at a temperature of about 100°C to 250°C for a time period ranging up to about 40 hours.
7. A component as recited by claim 6, wherein said alloy has the composition 2.6 wt% lithium, 1.0 wt% copper, 0.5 wt% magnesium and 0.6 wt% zirconium, the balance being aluminum.
8. A component as recited by claim 7, having a 0.2% tensile yield strength of 440 MPa, ultimate tensile strength of 530 MPa, and elongation to fracture of 7%.
9. A component as recited by claim 6, wherein said alloy has the composition 2.6 wt% lithium, 1.0 wt% copper, 0.5 wt% magnesium and 1.0 wt% zirconium, the balance being aluminum.
10. A component as recited by claim 9, having 0.2% tensile yield strength of 535 MPa, ultimate tensile strength of 570 MPa and elongation to fracture of 7%.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US51777490A | 1990-05-02 | 1990-05-02 | |
| US517,774 | 1990-05-02 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2079327A1 true CA2079327A1 (en) | 1991-11-03 |
Family
ID=24061176
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2079327 Abandoned CA2079327A1 (en) | 1990-05-02 | 1991-01-25 | Double aged rapidly solidified aluminum-lithium alloys |
Country Status (5)
| Country | Link |
|---|---|
| EP (1) | EP0528811A1 (en) |
| JP (1) | JPH05506271A (en) |
| AU (1) | AU7582291A (en) |
| CA (1) | CA2079327A1 (en) |
| WO (1) | WO1991017281A1 (en) |
Families Citing this family (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5178695A (en) * | 1990-05-02 | 1993-01-12 | Allied-Signal Inc. | Strength enhancement of rapidly solidified aluminum-lithium through double aging |
| US5277717A (en) * | 1992-02-20 | 1994-01-11 | Alliedsignal Inc. | Rapidly solidified aluminum lithium alloys having zirconium for aircraft landing wheel applications |
| GB9424970D0 (en) * | 1994-12-10 | 1995-02-08 | British Aerospace | Thermal stabilisation of Al-Li alloy |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4806174A (en) * | 1984-03-29 | 1989-02-21 | Aluminum Company Of America | Aluminum-lithium alloys and method of making the same |
| US4747884A (en) * | 1985-04-03 | 1988-05-31 | Massachusetts Institute Of Technology | High strength aluminum-base alloy containing lithium and zirconium and methods of preparation |
| US4721537A (en) * | 1985-10-15 | 1988-01-26 | Rockwell International Corporation | Method of producing a fine grain aluminum alloy using three axes deformation |
-
1991
- 1991-01-25 CA CA 2079327 patent/CA2079327A1/en not_active Abandoned
- 1991-01-25 WO PCT/US1991/000549 patent/WO1991017281A1/en not_active Ceased
- 1991-01-25 EP EP19910906823 patent/EP0528811A1/en not_active Ceased
- 1991-01-25 AU AU75822/91A patent/AU7582291A/en not_active Abandoned
- 1991-01-25 JP JP91507111A patent/JPH05506271A/en active Pending
Also Published As
| Publication number | Publication date |
|---|---|
| WO1991017281A1 (en) | 1991-11-14 |
| JPH05506271A (en) | 1993-09-16 |
| EP0528811A1 (en) | 1993-03-03 |
| AU7582291A (en) | 1991-11-27 |
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