Note: Descriptions are shown in the official language in which they were submitted.
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Al-Cu-Li ALLOY PRODUCT SUITABLE FOR AEROSPACE APPLICATION
FIELD OF THE INVENTION
The invention relates to an aluminium alloy, in particular an Al-Cu-Li type
alloy
product, more in particular an Al-Cu-Li-Mg-Ag-Mn alloy product, for structural
members, the aluminium alloy product combining a high strength with high
toughness. Products made from this aluminium alloy product are very suitable
for
aerospace applications, but not limited to that. The alloy can be processed to
various
product forms, e.g. sheet, thin plate, thick plate, extruded or forged
products.
BACKGROUND TO THE INVENTION
As will be appreciated herein below, except as otherwise indicated, alloy
designations and temper designations refer to the Aluminum Association
designations in Aluminum Standards and Data and the Registration Records, as
published by the Aluminum Association in 2007.
For any description of alloy compositions or preferred alloy compositions, all
references to percentages are by weight percent unless otherwise indicated.
As used herein, the term "about" when used to describe a compositional range
or amount of an alloying addition means that the actual amount of the alloying
addition may vary from the nominal intended amount due to factors such as
standard
processing variations as understood by those skilled in the art.
The term "substantially free" means having no significant amount of that
component purposely added to the alloy composition, it being understood that
trace
amounts of incidental elements and/or impurities may find their way into a
desired
end product.
It is generally well known in the aerospace industry that one of the most
effective ways to reduce the weight of an aircraft is to reduce the density of
aluminium alloys used in aircraft construction. This desire led to the
addition of
lithium, the lowest density metal element, to aluminium alloys. Aluminum
Association
alloys, such as AA2090 and AA2091 contain about 2.0 wt.% lithium, which
translates
into about a 7% weight savings over alloys containing no lithium. Aluminum
alloys
AA2094 and AA095 contain about 1.2 wt.% lithium. Another aluminium alloy,
AA8090
contains about 2.5 wt.% lithium, which translates into an almost 10% weight
savings
over alloys without lithium.
However, casting of such conventional alloys containing relatively high
amounts
of lithium is difficult. Furthermore, the combined strength and fracture
toughness of
such alloys is not optimal. A trade-off exists with conventional aluminium-
lithium
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alloys in which fracture toughness decreases with increasing strength.
Another important characteristic of aerospace aluminium alloys is fatigue
crack
growth resistance. For example, in damage tolerant applications in aircraft,
increased
fatigue crack growth resistance is desirable. Better fatigue crack growth
resistance
means that cracks will grow slower, thus making airplanes much safer because
small
cracks can be detected before they achieve critical size for catastrophic
propagation.
Furthermore, slower crack growth can have an economic benefit due to the fact
that
longer inspection intervals can be utilized.
Some other prior art documents are:
US-2004/0071586 discloses a broad ranges for an aluminium alloy comprising,
in wt.%, 3 to 5% of Cu, 0.5 to 2% of Mg, and 0.01 to 0.9% of Li. It is
disclosed that
the Li content should remain at a low level in combination with having
controlled
amounts of Cu and Mg to provide the desired levels of fracture toughness and
strength. Preferably the Cu and Mg are present in the alloy in a total amount
below a
solubility limit of the alloy.
WO-2004/106570 discloses another AI-Cu-Li-Mg-Ag-Mn-Zr alloy for use as a
structural member. The alloy comprises, in wt.%, 2.5 to 5.5% Cu, 0.1 to 2.5%
Li, 0.2
to 1% Mg, 0.2 to 0.8% Ag, 0.2 to 0.8% Mn, and up to 0.3% Zr, balance
aluminium.
US-2007/0181229 discloses an alumunium alloy comprising, in wt.%, 2.1 to
2.8% Cu, 1.1 to 1.7% Li, 0.1 to 0.8% Ag, 0.2 to 0.6% Mg, 0.2 to 0.6% Mn, a
content
of Fe and Si less or equal to 0.1% each, balance impurities and aluminium, and
wherein the alloy is substantially zirconium free. The low Zr content is
reported to
increase the toughness.
A need therefore exists for an aluminium alloy that is useful in aircraft
application which has high fracture toughness, high strength and excellent
fatigue
crack growth resistance.
DESCRIPTION OF THE INVENTION
It is an object of the present invention to provide AlCuLi-type alloy product,
ideally for structural members, having a balance of high strength and high
toughness.
It is yet another object of the present invention to provide a method of
manufacturing such an aluminium alloy product.
These and other objects and further advantages are met or exceeded by the
present invention in which there is provided an aluminium alloy product for
structural
members having a chemical composition comprising, in wt.%: Cu 3.4 to 5.0, Li
0.9 to
1.7, Mg about 0.2 to 0.8, Ag about 0.1 to 0.8, Mn about 0.1 to 0.9, Zn maximum
1.5,
one or more elements selected from the group consisting of: (Zr about 0.05 to
0.3, Cr
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about 0.05 to 0.3, Ti about 0.03 to 0.3, Sc about 0.05 to 0.4, Hf about 0.05
to 0.4), Fe
<0.15, Si <0.5, normal and unavoidable impurities and balance aluminium.
The alloy product can contain normal and/or inevitable elements and
impurities,
typically each <0.05% and the total <0.2%, and the balance is made by
aluminium.
Optionally the alloy product may contain 0 to 1%, and preferably 0 to 0.1%, of
a grain refiner elements selected from the group consisting of B, TiB2, Ce,
Nb, Er,
and V.
Copper is one of the main alloying elements in the alloy products and is added
to increase the strength of the alloy product. Care must be taken, however, to
not
add too much copper since the corrosion resistance can be reduced. Also,
copper
additions beyond maximum solubility will lead to low fracture toughness and
low
damage tolerance. A preferred upper-limit for the Cu-content is for that
reason about
4.4%, and more preferably about 4.2%. A preferred lower-limit is about 3.6%,
and
more preferably about 3.75%, and most preferably about 3.9%.
Magnesium is another main alloying element in the alloy product and is added
to increase strength and reduce density. Care should be taken, however, to not
add
too much magnesium in combination with copper since additions beyond maximum
solubility will lead to low fracture toughness and low damage tolerance. A
more
preferred lower-limit for the Mg addition is 0.3%, and a more preferred upper-
limit is
0.65%. It has been found that at a level of above about 0.8% the further
addition of
Mg may result in a decrease in toughness of the alloy product.
Lithium is another important alloying element in the product of this invention
and to added together with the copper to obtain an improved combination of
fracture
toughness and strength. This means the present alloy either posses higher
fracture
toughness and equivalent or higher strength, or possess higher strength and
equivalent or higher fracture toughness, in at least one temper in comparison
with
similar alloys having no lithium or greater amounts of lithium. A preferred
lower-limit
for the Li addition is 1.0%. A preferred upper-limit for the Li addition is
about 1.4%,
and more preferably 1.25%. A too high Li content has adverse effect on the
damage =
tolerance properties of the alloy product in particular with the relatively
high Cu levels
in the alloy product of this invention.
The silver addition is to further increase strength and should not exceed
about
0.8%, and a preferred lower limit is about 0.1%. A preferred range for the Ag
addition
is about 0.2 to 0.6%, and more preferably of about 0.25 to 0.50%.
The manganese addition is to control the grain structure by providing a more
uniform distribution of the main precipitating phases and thereby further
increases
strength in particular. The Mn addition should not exceed about 0.9% and
should be
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at least about 0.1%. A preferred lower-limit for the manganese addition is at
least
about 0.2%, and more preferably at least about 0.3%, and more preferably at
least
0.35%. A preferred upper-limit for the Mn addition is about 0.7%.
In addition to aluminium, copper, magnesium, lithium, silver, manganese, and
preferably also zinc, the alloy of the present invention contains at least one
element
selected from the group of Zr, Cr, Ti, Sc, Hf.
If added zirconium should be present in a range of 0.05 to 0.3%, and
preferably
in a range of 0.07 to 0.2%. A too low Zr addition has an adverse on the unit
propagation energy of the alloy product.
The Cr addition can be made to increase in particular the unit propagation
energy (UPE) of the alloy product. The UPE is typically measured in the Kahn-
tear
test and is the energy needed for crack growth. It is commonly believed that
the
higher the UPE, the more difficult to grow the crack, which is a desired
feature of the
material. The Cr addition should be in a range of 0.05 to 0.3%, and preferably
in a
range of 0.05 to 0.16%. The purposive addition of Cr to lithium containing
aluminium
alloy products has been reported previously as having adverse effect on
engineering
properties.
The effect of the Cr addition on the UPE is significantly enhanced with a
combined addition of Cr and Ti. The Ti should be in a range of 0.05 to 0.3%
also, and
preferably in a range of 0.05 to 0.16%. The combined addition of Cr and Ti has
also a
positive effect of the intergranular corrosion resistance of the alloy
product.
The scandium addition can be made to significantly increase in particular the
unit propagation energy (UPE) of the alloy product. The Sc addition should be
in a
range of 0.05 to 0.4%, and preferably in a range of 0.05 to 0.25%.
The scandium can be replaced in part or in whole by the addition of hafnium.
The Hf addition should be made in similar compositional ranges as the
scandium.
In a preferred embodiment of the alloy product of this invention there are
combined additions of at least Cr, Ti, and Sc.
And in a more preferred embodiment of the alloy product of this invention
there
is combined addition of Zr, Cr, Ti, and Sc.
The Si content in the alloy product should be less than 0.5% and can be
present as a purposive alloying element. In another embodiment silicon is
present as
an impurity element and should be present at the lower-end of this range, e.g.
less
than about 0.10%, and more preferably less than 0.07%, to maintain fracture
toughness properties at desired levels.
The Fe content in the alloy product should be less than 0.15%. When the alloy
product is used for aerospace application the lower-end of this range is
preferred,
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e.g. less than about 0.1%, and more preferably less than about 0.07% to
maintain in
particular the toughness at a sufficiently high level. Where the alloy product
is used
for commercial applications, such as tooling plate, a higher Fe content can be
tolerated.
5 In a further embodiment of the alloy product the zinc is present as an
impurity
element which can be tolerated to a level of at most 0.1%, and preferably at
most
about 0.05%, e.g. at about 0.02% or less. Thus the alloy product may be
substantially free from Zn.
In another preferred embodiment of the alloy product the zinc is purposively
added to improve strength and it has a small effect on the damage tolerance
properties of the alloy product. In this embodiment the zinc is typically
present in a
range of about 0.1 to 1.5%, and more preferably in a range of about 0.2 to
1.0%. As
a particular example, zinc in an amount of about 0.5% is being added.
In the embodiment of the alloy product having the purposive addition of zinc
also one or more alloying elements selected from the group consisting of (Zr,
Cr, Ti,
Sc, Hf) is added. In a more preferred embodiment only one of the elements of
this
group is being added, while still having a desirable balance in strength and
toughness. For example the alloy product may contain Ti in a range of 0.03 to
0.3%,
whereas it is substantially free from each of Zr, Cr, Sc, and Hf. In another
example
the alloy product may contain Zr in a range of 0.05 to 0.3%, preferably in a
range of
0.05 to 0.25%, whereas it is further substantially free from each of Cr, Ti,
Sc, and Hf.
In yet another example the alloy product may contain Cr in a range of 0.05 to
0.3%,
whereas it is further substantially free from each of Zr, Ti, Sc, and Hf.
In an embodiment of the alloy product the product is in the form of a rolled,
extruded or forged product, and more preferably the product is in the form of
a sheet,
plate, forging or extrusion as part of an aircraft structural part. In a more
preferred
embodiment the alloy product is provided in the form of an extruded product.
When used as part of an aircraft structural part the part can be for example a
fuselage sheet, upper wing plate, lower wing plate, thick plate for machined
parts,
forging or thin sheet for stringers.
Resistance to intergranular corrosion of the products of the present invention
is
generally high, for example, typically only pitting is detected when the metal
is
submitted to corrosion testing. However, the sheet and light gauge plate may
also be
clad, with preferred cladding thickness of from about 1% to about 8% of the
thickness
of the sheet or plate. The cladding is typically a low composition aluminium
alloy.
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In a further aspect of the invention it relates to a method of manufacturing a
wrought aluminium alloy product of an Al-Cu-Li alloy, the method comprising
the
steps of:
a. casting stock of an ingot of an AlCuLi-alloy according to this
invention,
b. preheating and/or homogenizing the cast stock;
c. hot working the stock by one or more methods selected from the group
consisting of rolling, extrusion, and forging;
d. optionally cold working the hot worked stock;
e. solution heat treating ("SHT") of the hot worked and/or optionally cold
worked stock, the SHT is carried out at a temperature and time sufficient to
place into
solid solution the soluble constituents in the aluminium alloy;
f. cooling the SHT stock, preferably by one of spray quenching or immersion
quenching in water or other quenching media;
g. optionally stretching or compressing the cooled SHT stock or otherwise
cold working the cooled SHT stock to relieve stresses, for example levelling
or
drawing or cold rolling of the cooled SHT stock; and
h. ageing, preferably artificial ageing, of the cooled and optionally
stretched
or compressed or otherwise cold worked SHT stock to achieve a desired temper.
The aluminium alloy can be provided as an ingot or slab or billet for
fabrication
into a suitable wrought product by casting techniques regular in the art for
cast
products, e.g. DC-casting, EMC-casting, EMS-casting. Slabs resulting from
continuous casting, e.g. belt casters or roll casters, also may be used, which
in
particular may be advantageous when producing thinner gauge end products.
Grain
refiners such as those containing titanium and boron, or titanium and carbon,
may
also be used as is known in the art. After casting the alloy stock, the ingot
is
commonly scalped to remove segregation zones near the cast surface of the
ingot.
Homogenisation treatment is typically carried out in one or multiple steps,
each
step having a temperature in the range of about 475 C to 535 C. The pre-heat
temperature involves heating the hot working stock to the hot-working entry
temperature, which is typically in a temperature range of about 440 C to 490
C.
Following the preheat and/or homogenisation practice the stock can be hot
worked by one or more methods selected from the group consisting of rolling,
extrusion, and forging, preferably using regular industry practice. The method
of hot
rolling is preferred for the present invention.
The hot working, and hot rolling in particular, may be performed to a final
gauge, e.g. 3 mm or less or alternatively thick gauge products. Alternatively,
the hot
working step can be performed to provide stock at intermediate gauge, typical
sheet
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or thin plate. Thereafter, this stock at intermediate gauge can be cold
worked, e.g. by
means of rolling, to a final gauge. Depending on the alloy composition and the
amount of cold work an intermediate anneal may be used before or during the
cold
working operation.
Solution heat-treatment ("SHT") is typically carried out within the same
temperature range as used for homogenisation, although the soaking times that
are
chosen can be somewhat shorter. A typical SHT is carried out at a temperature
of
480 C to 525 C for 15 min to about 5 hours. Lower SHT temperatures generally
favour high fracture toughness. Following the SHT the stock is rapidly cooled
or
quenched, preferably by one of spray quenching or immersion quenching in water
or
other quenching media.
The SHT and quenched stock may be further cold worked, for example, by
stretching in the range of about 0.5 to115 A) of its original length to
relieve residual
stresses therein and to improve the flatness of the product. Preferably the
stretching
is in the range of about 0.5 to 6%, more preferably of about 0.5 to 5%.
After cooling the stock is aged, typically at ambient temperatures, and/or
alternatively the stock can be artificially aged.
The alloy product according to this invention is preferably provided in a
slightly
under-aged T8 condition to provide the best balance in strength and damage
tolerance properties.
A desired structural shape is then machined from these heat treated plate
sections, more often generally after artificial ageing, for example, an
integral wing
spar. SHT, quench, optional stress relief operations and artificial ageing are
also
followed in the manufacture of thick sections made by extrusion and/or forged
processing steps.
In one embodiment of the present invention comprising a welding step, the
ageing step can be divided into two steps: a pre-ageing step prior to a
welding
operation, and a final heat treatment to form a welded structural member.
The AlCuLi-alloy product according to this invention can be used amongst
others as in the thickness range of at most 0.5 inch (12.5 mm) the properties
will be
excellent for fuselage sheet. In the thin plate thickness range of 0.7 to 3
inch (17.7 to
76 mm) the properties will be excellent for wing plate, e.g. lower wing plate.
The thin
plate thickness range can be used also for stringers or to form an integral
wing panel
and stringer for use in an aircraft wing structure. When processed to thicker
gauges
of more than 2.5 inch (63 mm) to about 11 inch (280 mm) excellent properties
have
been obtained for integral part machined from plates, or to form an integral
spar for
use in an aircraft wing structure, or in the form of a rib for use in an
aircraft wing
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structure. The thicker gauge products can be used also as tooling plate, e.g.
moulds
for manufacturing formed plastic products, for example via die-casting or
injection
moulding. The alloy products according to the invention can also be provided
in the
form of a stepped extrusion or extruded spar or extruded stiffeners for use in
an
aircraft structure, or in the form of a forged spar for use in an aircraft
wing structure.
When applied in the form of a sheet product the yield strength or proof
strength
of the product should be at least 460 MPa, and preferably at least 480 MPa.
When
applied in the form of an extruded product, e.g. as a stringer, or in the form
of a plate
product the yield strength or proof strength of the product should be at least
480
MPa, and more preferably at least 500 MPa. These strength levels can be
obtained
by a selecting the alloy composition within the claimed ranges, and preferably
within
the preferred narrower ranges, in combination with the artificial ageing
practice.
In the following, the invention will be explained by the following non-
limitative
example.
Example.
On an laboratory scale eight aluminium alloys were cast to prove the principle
of the current invention and processed into 2 mm sheet. The alloy compositions
are
in listed Table 1, and wherein alloy no. 2 is a comparative alloy due to a
lower Li-
content. For all ingots the balance were inevitable impurities and aluminium.
Rolling
blocks of approximately 80 by 80 by 100 mm (height x width x length) were sawn
from round lab cast ingots of about 12kg. The ingots were homogenised at 520 5
C
for about 24 hours and consequently slowly air cooled to mimic an industrial
homogenisation process. The rolling ingots were pre-heated for about 4 hours
at
450 5 C and hot rolled to a gauge of 8 mm and subsequently cold rolled to a
final
gauge of 2 mm. The hot-rolled product were solution heat treated (SHT) for 30
min at
520 5 C and quenched in water. The quenched products were cold stretched for
about 1.5%. On the SHT and quenched sheet two ageing practices were carried
out:
(1) an under-aged condition by ageing for 20 hours at 170 C, and only for
alloys 1, 7,
and 8: (2) a peak-aged condition by ageing for 48 hours at 170 C.
Following the ageing the tensile properties have been determined according to
EN10.002, and whereby "Rp" represents the yield strength in MPa, "Rm"
represents
the tensile strength in MPa and "Ag" the uniform elongation in % in the L- and
LT-
direction. For all alloys also the tear-strength have been determined
according to
ASTM B871-96, and the test direction of the results are for the T-L and L-T
direction.
The so-called notch-toughness can be obtained by dividing the tear-strength,
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obtained by the Kahn-tear test, by the tensile yield strength ("TS/Rp"). This
typical
result from the Kahn-tear test is known in the art to be a good indicator for
true
fracture toughness. The mechanical properties as tested are shown in Table 2
and
3. If the tensile strength is given in the L-direction then the corresponding
direction for
the notch-toughness is the L-T direction, and if the tensile strength is given
in the LT-
direction then the corresponding direction for the notch-toughness is the T-L
direction.
Table 1. Chemical composition of the aluminium alloys tested. All alloying
additions are by wt.%, the balance is made by unavoidable impurities and
aluminium.
For all alloys Fe 0.03%, Si 0.03%.
Alloy Alloying element
No. Li Cu Mg Ag Mn Zr Cr Ti Sc Zn
1 1.1 3.9 0.5 0.4 0.5 0.11 0.11 0.10
0.15 -
2 0.6 3.9 0.5 0.4 0.5 0.11 0.11 0.10
0.15 -
3 1.3 3.9 0.5 0.4 0.5 0.11 0.11 0.10
0.15 -
4 1.1 3.6 0.5 0.4 0.5 0.11 0.11 0.10
0.15 -
5 1.1 4.4 0.5 0.4 0.5 0.11 0.11 0.10
0.15 -
6 1.6 3.6 0.5 0.4 0.5 0.11 0.11 0.10
0.15 -
7 1.1 3.9 0.5 0.4 0.5 - 0.10 - 0.5
8 1.1 3.9 0.5 0.4 0.5 0.11 -
1.0
Table 2. Mechanical properties of the rolled alloy product after
aging for 16
hours at 170 C.
Alloy L-direction LT-direction
No. Rp Rm Ag TS TS/ Rp Rm Ag TS TS/
Rp Rp
1 502 536 6.1 654 1.30 442 509 6.8 580 1.31
2 346 443 - 9.3 668 1.93 362 449 8.4 611
1.69
3 527 565 5.6 598 1.13 471 542 5.6 454 0.96
4 479 518 7.0 678 1.42 414 482 8.5 621 1.50
5 508 549 6.5 578 1.14 477 541 7.7 505 1.06
6 456 516 6.8 565 1.24 -
7 574 611 5.5 571 0.99 542 600 5.9 479 0.88
8 570 606 5.4 483 0.85 514 550 3.4 451 0.88
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Table 3. Mechanical properties of the rolled alloy products after aging for 24
hours at 170 C.
Alloy L-direction LT-direction
No. Rp Rm Ag TS TS/ - Rp Rm Ag
TS TS/
Rp Rp
1 510 543 5.9 647 - 1.27 461 535 7.2
546 1.18
7 582 617 4.9 547 603 4.3
8 564 604 4.9 536 592 5.0
5 From the results of Table 2 it can ben seen from a comparison between
Alloy
no. 1 (according to the invention) and Alloy no. 2 (comparative) that lowering
the Li-
content has a significant adverse effect on the yield strength and the tensile
strength.
For that reason the lower-limit for the Li-content in the alloy product
according to this
invention is at least 0.9%, and more preferably at least 1.0%.
10 From a comparison between Alloy no. 1 and Alloy no. 3 it can be seen
from
Table 2 that raising the Li-content increases the strength levels, but has an
adverse
effect on the toughness of the alloy product. In order to obtain a good
balance in
strength and toughness in the alloy product according to this invention, the
Li-content
should not exceed 1.7%, and preferable not more than 1.4%, and more preferably
should not exceed 1.25%.
From a comparison between Alloy no. 1 and Alloy no. 4 it can be seen from
Table 2 that lowering the Cu-content has an adverse effect on the strength
levels.
For that reason the Cu-content in the alloy product according to this
invention should
not be less than 3.4%, and preferably not be less than 3.6% in order to
maintain
sufficient strength levels. Whereas from a comparison between Alloy No. 1 and
Alloy
no. 5 it can be seen that increasing the Cu-content results only in a small
increase of
the strength levels, but has a significant adverse effect on the toughness of
the alloy
product. In order to obtain a good balance in strength and toughness in the
alloy
product according to this invention, the Cu-content should preferably not
exceed
4.4%, and more preferable should not exceed 4.2%.
From a comparison between Alloy no. 1 and Alloy no. 6 it can be seen that
significantly increasing the Li content while reducing the Cu content results
in a
decrease in strength while significantly reducing the toughness in the alloy
product
according to this invention.
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From a comparison between Alloy no. 1 and Alloy no. 7 it can be seen that
adding only about 0.5% of zinc significantly increases the strength of the
alloy
product. This strength increase is obtained in this example even in the
absense
of the purposive combined addition of Zr, Cr, and Sc.
From a comparison between Alloy no. 7 and Alloy no. 8 it can be seen that
increasing the Zn-content does not necessarily lead to a further increase of
strength or toughness and may have an adverse effect on other engineering
properties. For that reason a preferred upper-limit of the Zn content is about
1.0%. The alloy products having a purposive addition of zinc represent a
preferred embodiment of the alloy product according to this invention.
From the results in Table 2 for Alloy no. 7 and Alloy no. 8 it can be seen
that high strength levels are obtained while purposively adding only one
element
selected from the group of (Zr, Cr, Ti, Sc, and Hf).
From the results of Table 2 and Table 3 it can be seen that depending on
the artificial ageing practice a further Increase in strength can be obtained.
Having now fully described the invention, it will be apparent to one of
ordinary skill in the art that many changes and modifications can be made
without departing from the scope of the invention as herein described.
