Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~280342
DESCRIPTION
IGH STRENGTH, DUCTILE, ',OW DENSITY AL~MINUM
ALLOYS AND PROCESS FOR MAKING SAME
5 1. Field of the Invention
The invention relates to a process for making high
strength, high ductility, low density aluminum-based
alloys, and, in particular, to the alloys that are
characteriæed by a homogeneous distribution of composite
10 precipitates in the aluminum matrix thereof. The
microstructure is developed by heat treatment method
consisting of initial solutionizing treatment followed
by multiple aging treatments.
2. Background of the Invention
There is a growing need for structural alloys with
improved specific strength to achieve substantial weight
savings in aerospace applications. Aluminum-lithium
alloys offer the potential of meeting the weight savings
due to the pronounced effects of lithium on the
20 mechanical and physical properties of aluminum alloys.
The addition of one weight percent lithium t ~3.5 atom
percent) decreases the density by ~3~ and increases the
elastic modulus by ~6~ , hence giving a substantial
increase in the specific modulus ~/ p ). Moreover,
heat treatment of alloys results in the precipitation of
a coherent, metastable phase, ~' (A13Li) which offers
considerable strengthening. Nevertheless, development
and widespread application of the Al-Li alloy system
have been impeded mainly due to its inherent
brittleness,
It has been shown that the poor toughness of alloys
in the Al-Li system is due to brittle fracture along the
grain or subgrain boundaries. The two dominant micro-
structural features responsible for their brittleness
appear to be the precipitation of intermetallic phases
along the grain and/or subgrain boundaries and the
marked planar slip in the alloys, which create stress
concentrations at the grain boundaries. The inter-
`.~
~2~)342
granular precipitates tend to embrittle the boundary,and simultaneously extract Li from the boundary region
to form precipitate free zones which act as sites of
strain localization. The planar slip is largely due to
the shearable nature of ~' precipitates which result in
decreased resistance to dislocation 51ip on plane~
containing the sheared ~' precipitates.
Several metallurgical approaches have been under-
taken to circumvent these problems. It has been found
lQ that the PFZ (precipitate free zone) and precipitate-
induced intergranular fracture can be reduced by
controlling processing to avoid the intergranular
precipitation of stable Al-Li, Al-Cu-Li, Al-Mg-Li
phases. The problem of planar slip can be partly
15 alleviated by promoting slip dispersion through the
addition of dispersoid forming elements and the
controlled co-precipitation of Al-Cu-Li, Al Cu-Mg and/or
Al-Li-Mg intermetallics. The dispersoid forming
elements include Mn, Fe, Co, etc. The co-precipitation
20 f Cu and/or Mg containing intermetallics appears to be
relatively effective in dispersing the dislocation
movement. However, the sluggish formation of these
intermetallics requires the thermomechanical treatments
involving stretching operations and multiple aging
treatments ~P.J. Gregson and M.M. Flower, Acta
Metallurgica, vol. 33, pp. 527-~37, 1985), or a high Cu
content which adversely affects the density of alloys ~B
van der Brandt, P.J. von den Brink, H.F. de Jong, L.
Katgerman, and H. Kleinjan, in "Aluminum-Lithium Alloy
II", Metallurgical Society of AIME, pp. 433-446,
1984). Moreover, the properties of alloys thus
processed were less than satisfactory.
Recently, a new approach has been suggested to
modify the deformation behavior of Al-Li alloy system
through the development of Zr modified ~' precipitate.
This approach is based on the observation that the
metastable A13Zr phase in the Al-Zr alloy system is
highly resistant to dislocation shear and is of the same
3~
--3--
crystal structure (L12) as ~'. In this regard, attempts
have been made to produce a ternary ordered composite
A13(Li, zr) phase in the aluminum matrix with an alloy
of Al-2.34 Li-1.07Zr (F.W. Gayle and J .B . vander Sande,
5 Scripta Metallurgica, vol. 18, pp. 473-478, 1984).
However, the process for developing a homogeneou~ dis-
tributlon of such phase has required the strict control
of processing parameters during the thermomechanical
processing, as well as prolonged solutionizing and/or
10 aging treatments. From the practical point of view,
this process is quite undesirable and may also result in
undesirable microstructural features such as recrystal-
lization and wide precipitate free zones. Moreover, the
process cannot be effectively applied to low Zr ~e.g.,
15 0.2 wt% Zr) containing alloys which produce a small
volume fraction of heterogeneously distributed coarse
composite precipitates (P.L. Makin and B. Ralph, Journal
of Materials Science, vol. 19, pp. 3835-3843, 1984; P.J.
Gregson and H.M. Flower, Journal of Materials Science
20 Letters, vol. 3, pp. 829-834, 1984; P.L. Makin, D.J
Lloyd, and W.M. Stobbs, Philosophical Magazine A,
vol. 51, pp. L41-L47, 1985).
Despite considerable efforts to develop low density
aluminum alloys, conventional techniques, such as those
25 discussed above, have been unable to provide low density
aluminium alloys having the sought for combination of
high strength, high ductility and low density. As a
result, conventional aluminum-lithium alloy systems have
not been entirely satisfactory for applications such as
aircraft structural components, wherein high strength,
high ductility and low density are required.
SUMMARY OF THE INVENTION
The present invention provides a process for making
aluminium-lithium alloys containing a high density of
substantially uniformly distributed shear resistant
dispersoids which markedly improve the strength and
ductility thereof. The low density aluminum-base
alloys, of the invention consist essentially of the
~28~33~L2
-4-
formula AlbalZraLibXc, wherein X is at least one element
selected from the group consisting of Cu, Mg, Si, Sc,
Ti, U, Hf, Cr, V, Mn, Fe, Co and Ni, "a" ranges from
about 0.15-2 wt~, ~b" ranges from about 2.5-S wt~, ~c"
5 ranges from about 0-s wt% and the balance is aluminum.
The microstructure of these alloys is characterized by
the precipitation of composite A13(Li, Zr) phase in the
aluminum matrix thereof. This microstructure is
developed in accordance with the process of the present
10 invention by subjecting an alloy having the formula
delineated above to solutionizing treatment followed by
multiple aging treatments. An improved process for
making high strength, high ductility, low density
aluminum-based alloy is thereby provided wherein the
15 aluminum-based alloy produced has an improved
combination of strength and ductility (at the same
density).
The high strength, high ductility, low density
aluminum-based alloy produced in accordance with the
20 present invention has a controlled composite A13(Li, Zr)
precipitate which, advantageously, offers a wide range
of strength and ductility combinations.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be more fully understood and
further advantages will become apparent when reference
is made to the following detailed description and the
accompanying drawings, in which:
Fig. 1 is a dark field transmission electron micro-
~raph of an alloy having the composition Al-3.1Li-2Cu-
lMg-0.5Zr~ the alloy having been subjected to double
aging treatments (170C for 4 hrs. followe~ by 190C for
16 hrs.) to develop a composite precipitate in the
aluminum matrix thereof;
Fig. 2 is a weak beam dark field micrograph of an
alloy having the composition Al-3.7Li-O.SZr,
illustrating the resistance of the composite precipitate
to dislocation shear during deformation;
Fig. 3(a) shows the planar slip observed in an
~L28~3~2
--5--
alloy having the composition Al-3.7Li-0.5Zr, the alloy
having been subjected to a conventional aging treatment
(180C for 16 hours);
Fig. 3(b) shows the beneficial effect of subjecting
5 the alloy of Fig.3(a) to treatment in accordance with
the claimed process tl60C for 4 hr5. followed by 180C
for 16-hrs.), thereby promoting the homogeneous
deformation thereof;
Fig. 4 shows the sheared ~' precipitates observed
10 in an alloy having the composition Al-3.1Li-2Cu-lMg-
0.5zr, the alloy havin~ been subjected to a conventional
aging treatment (190C for 16 hours); and
Fig. 5 shows the development of composite precipi-
tates in an alloy having the composition Al-3.2Li-3Cu-
15 1.5Mg-0.2Zr~ the alloy having been subjected to
treatment in accordance with the claimed process tl70C
for 4 hrs. followed by 190C for 16 hrs).
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In general, the present invention relates to the
2~ process of making high strength, high ductility, and low
density Al-Li-Zr-X alloys. The process involves the use
of multiple aging steps during heat treatment of the
alloy. The alloy is characterized by a unique micro-
structure consisting essentially of "compositen
25 A13(Li~Zr) precipitate in an aluminum matrix (Fig. 1)
due to the heat treatment as hereinafter described. The
alloy may also contain other Li, Cu and/or Mg containing
precipitates provided such precipitates do not
significantly deteriorate the mechanical and physical
properties of the alloy.
The factors governing the properties of the Al~
zr-X alloys are primarily its I.i content and micro-
structure and secondarily the residual alloying
elements. The microstructure is determined largely by
the composition and the final thermomechanical
treatments such as extusion, forging and/or heat
~reatment parametersO Normally, an alloy in the as-
processed condition (~ast, extruded or forged) has large
. . .
~Z3~3~:
intermetallic particles. Further processing is required
to develop certain microstructural features for certain
characteristic properties.
The alloy is given an initial solutionizing treat-
5 ment, that is, heating a~ a temperature (Tl) for a
period of time sufficient to substantially dis501ve most
of the intermetallic particles present during the
forging or extrusion process, followed by cooling to
ambient temperature at a sufficiently high rate to
10 retain alloying elements in said solution~ Generally,
the time at temperature Tl, will be dependent on the
composition of the alloy and the method of fabrication
~e.g., ingot cast, powder metallargy processed) and will
typically range from about 0.1 to 10 hours. The alloy
15 is then reheated to an aging temperature, T2, for a
period of time sufficient to activate the nucleation of
composite A13(Li, Zr) precipitates, and cooled to
ambient temperature, followed by a second aging
treatment at temperature, T3, for a period of time
20 sufficient for the growth of the composite A13(Li, Zr)
precipitate and a dissolution of ~' precipitate whose
nucleation is not aided by Zr. The alloy at this p~int
is characterized by a unique microstructure which con-
sists essentially of composite A13(Li, zr) precipi-
tate. This composite A13 (Li, Zr) precipitate is re-
sistant to dislocation shear and quite effective in
dispersing dislocation motion (Fig. 2). The result is
that the alloy containing an optimum amount of composite
A13(Li, Zr) precipitate deforms by a homogeneous mode of
deformation resulting in improved mechanical
properties. Fig. 3(b) clearly shows the h~mogeneous
mode of deformation in an alloy subjected to the process
claimed in this invention, while Fig. 3(a) shows the
severe planar slip observed in a conventionally
processed alloy due to the shearing of ~' precipitates
by dislocations (see Fig. 4). The combination of
ductility with high strength is best achieved in
accordance with the invention when the density of the
--7--
shear resistant dispersoids ranges from about 10 to 60
percent by volume, and preferably from about 20-40
percent by volume.
The exact temperature, Tl, to which the alloy is
5 heated in the solutionizing step is not critical as long
as there is a dissolution of intermetallic particles at
this temperature. The exact temperature, T2, in the
first aging ~tep where the nucleation of composite
A13 ~Li, Zr) precipitate is promoted, depends upon the
10 alloying elements present and upon the final aging
step The optimum temperature range for T2, is ~rDm
about 100C to 180C. The exact temperature, T3, whose
range is from 120C to 200C, depends on the alloying
elements present and mechanical properties desired.
15 Generally, the times at temperatures T2 and T3 are
different depending upon the composition of the alloy
and the thermomechanical processing history, and will
typically range from about 0.1 to 100 hours.
EXAMPLE 1
The ability of composite A13(Li, Zr) precipitates
to modify the deformation behavior of Al-Li-Zr alloys is
illustrated as follows:
Fig. 2 is a weak beam dark field transmission`
electron micrograph showing microstructure of a deformed
alloy (Al-3.7Li-0.5Zr) which has been solutionized at
540C for 4 hrs. and subseguently aged at 160C for 4
hrs. followed by final aging at 180C for 16 hrs. Such
heat treatment promotes the precipitation of composite
A13(Li, Zr) which is highly resistant to dislocation
shear and is quite effective in dispersing the
dislocation movement.
Fig~ 3(a) shows a bright field electron micrograph
showing microstructure of a deformed alloy (Al-3.7Li-
O.SZr) which has not been given the claimed process~
The alloy had been aged, for 16 hrs. at 180QC after
solutionizing at 540 for 4 hrs. This alloy showed the
pronounced planar slip which is the common deformation
characteristic of brittle alloy.
34~2
--8--
In contrast, Fig. 3(b) illustrates the beneficial
effect of the claimed process on the deformation
benavior of an alloy having the composition Al-3.7Li-
0.5Zr. After solutionizing at 540C for 4 hrs., the
5 alloy had been subjected to the double aging treatment
of 160C for 4 hrs. and 180C for 16 hrs. The
deform~tion mode of this alloy is quite homogeneous
indicating high ductility.
Example 2
An alloy having a composition of Al-3.1Li-2Cu-lMg-
0.5Zr was developed for medium strength applications as
shown in Table I. The alloy was solutionized at 540C
for 2.5 hrs./ quenched into water at about 20C and
given conventional single aging and the claimed double
15 aging treatments.
~28~3~2
TABLE I
0.2~ Yield Ultimate Tensile Elongation to
Strength (MPa) Strength (MPa) Failure (~)
Aged at 190C
for 16 hrs. 524 592 3.6
Aged at 170C --
for 4 hrs. and
10190C for 16 hrs. 530 606 6.1
Conventional aginy treatment (19ODC for 16 hrs.)
showed poor ductility t3.6%) due to the shearing
of ~' precipitate (Fig. 4), while composite precipitate
developed by double aging (Fig. 1) improve both strength
15 and ductility (6.1% elongation).
Example 3
A high strength Al-Li alloy was made to satisfy the
requirements for high strength applications for aero-
space structure. An alloy having a composition of Al-
3.2Li-2Cu-2Mg-0.5Zr was solutionized at 542C for 4
hrs. As shown in Table II, conventional aging treatment
(190C for i6 hrs.) showed lower strength (yield
strength of 521 MPa) and ductility ~3.6%). However,
double aging of the alloy (160C for 4 hrs. followed by
180C for 16 hrs.) gave significantly higher strength
Iyield strength of 554 MPa) and ductility t5.5%), which
meets property requirements for high strength alloys
needed for aerospace structural applications.
TABLE II
0.2~ Yield Ultimate Tensile Elongation
Strength (Mæa) Strength (MRa) to Failure (~)
Aged at 190C
for 16 hrs. 521 595 3.6
Aged at 160C for
4 hrs. and 180C
for 16 hrs. 554 631 5.5
- ~ -. .
'
.
33~
--10--
Example 4
This example illustrates the beneficial effect of
the claimed process on the mechanical properties of a
simple ternary alloy Al-3.~Li-0.5Zr. The alloy was
5 solutionized at 540C for 4 hrs., and subseg~ently aged
as shown in Table III. The resulting tensile properties
show that the claimed process results in improved
~trength and ductility compared to the conventional
process.
TABLE III
Aging Treatment 0.2% Yield Ultimate Tensile ~longation to
Strength (MPa) Strength (Mæa) Fract~e (~)
. _
15 140C, 16 hr. 424 442 4.2
120C, 4 hr. +
140C, 16 hr. 434 460 6.0
160C, 16 hr. 419 431 3.
20 140C, 4 hr. ~
160C, 16 hr. 425 448 4.8
140C, 16 hr. +
160C, 16 hr. 426 451 4.6
Example 5
A wide range of mechanical properties can be
achieved by using multiple aging conditions. For
example, a triple aging treatment (120C, 4 hrs. ~
140C, 16 hrs. ~ 160C, 4 hrs.) produced yield strength
of 446 MPa and ultimate tensile strength of 464 MPa with
4.6~ elongation. As a result, a variety of heat
treatments of the alloys according to the claims can be
employed to produce alloys having a variety of
mechanical properties.
Example 6
This example illustrates the potential of the
claimed process for the development of composite pre-
cipitate in low Zr containing Al-Li alloysO Fig. 5
shows the dark field electron micrograph of a typical
:...
8633~2
alloy Al-3.2Li-3Cu-1.5Mg-0.2Zr which had been
solutionized at 540C for 4 hrs., reheated to 170C for
4 hrs. followed by final aging at 190C for 16 hrs. The
large volume fraction of composite A13 (Li, Zr) precipi-
5 tate observed in such an alloy indicates that theclaimed process is also quite effective in Al-Li alloys
having low ~r content of 0.2~. Having thus described
the invention in rather full detail, ~t will ~e
understood that such detail need not be strictly adhered
10 to but that further changes and modifications may
suggest themselves to one skilled in the art, all
falling within the scope of the present invention as
defined by the subjoined claims.
