U.S. patent application number 13/476308 was filed with the patent office on 2012-11-22 for aluminum alloys.
Invention is credited to Abhijeet Misra, James A. Wright.
Application Number | 20120291926 13/476308 |
Document ID | / |
Family ID | 47174045 |
Filed Date | 2012-11-22 |
United States Patent
Application |
20120291926 |
Kind Code |
A1 |
Misra; Abhijeet ; et
al. |
November 22, 2012 |
ALUMINUM ALLOYS
Abstract
The disclosure relates to an alloy comprising, by weight, about
5.8% to about 6.8% zinc, about 2.5% to about 3.0% magnesium, about
1.5% to about 2.3% copper, 0% to about 0.2% scandium, 0% to about
0.2% zirconium, and optionally less than about 0.50% silver, the
balance essentially aluminum and incidental elements and
impurities. In embodiments, the alloy has a stress-corrosion
cracking threshold stress of at least about 240 MPa using an ASTM
G47 short-transverse test specimen and a yield strength of at least
about 510 MPa using an ASTM E8 longitudinal test specimen.
Inventors: |
Misra; Abhijeet; (Chicago,
IL) ; Wright; James A.; (Wilmette, IL) |
Family ID: |
47174045 |
Appl. No.: |
13/476308 |
Filed: |
May 21, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61488713 |
May 21, 2011 |
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Current U.S.
Class: |
148/552 ;
148/439; 148/549; 420/532 |
Current CPC
Class: |
C22C 21/10 20130101;
C22F 1/053 20130101 |
Class at
Publication: |
148/552 ;
420/532; 148/549; 148/439 |
International
Class: |
C22C 21/10 20060101
C22C021/10; C22F 1/053 20060101 C22F001/053 |
Goverment Interests
FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT
[0002] Activities relating to the development of the subject matter
of this invention were funded at least in part by U.S. Government,
Office of Naval Research Contract Nos. N00014-09-M-0400 and
N00014-11-C-0080, and thus the U.S. may have certain rights in the
invention.
Claims
1. An alloy comprising, by weight, about 5.8% to about 6.8% zinc,
about 2.5% to about 3.0% magnesium, about 1.5% to about 2.3%
copper, 0% to about 0.2% scandium, 0% to about 0.2% zirconium, and
optionally less than about 0.50% silver, the balance essentially
aluminum and incidental elements and impurities, wherein the alloy
has a stress-corrosion cracking threshold stress of at least about
240 MPa using an ASTM G47 short-transverse test specimen and a
yield strength of at least about 510 MPa using an ASTM E8
longitudinal test specimen.
2. The alloy of claim 1, wherein the alloy comprises dispersed
L1.sub.2 phase particles including at least one of scandium and
zirconium, constituting about 0.1% by volume of the alloy.
3. The alloy of claim 1, wherein the alloy comprises an
.eta.-MgZn.sub.2 phase that constitutes about 3% to about 8% by
volume of the alloy.
4. The alloy of claim 1, wherein the alloy comprises 6.3 Zn, 2.7
Mg, 1.6 Cu, 0.10 Sc, 0.05 Zr, and balance Al, in wt %, with the
composition including a variation of ten percent of the nominal
values.
5. The alloy of claim 1, wherein the alloy comprises 5.8 Zn, 3.0
Mg, 2.2 Cu, 0.05 Sc, 0.05 Zr, and balance Al, in wt %, with the
composition including a variation of ten percent of the nominal
values.
6. The alloy of claim 1, wherein the alloy comprises 6.3 Zn, 2.7
Mg, 1.6 Cu, 0.12 Zr, and balance Al, in wt %, with the composition
including a variation of ten percent of the nominal values.
7. A method for producing an alloy comprising: preparing a melt
that includes, by weight, about 5.8% to about 6.8% zinc, about 2.5%
to about 3.0% magnesium, about 1.5% to about 2.3% copper, 0% to
about 0.2% scandium, 0% to about 0.2% zirconium, and optionally
less than about 0.50% silver, the balance essentially aluminum and
incidental elements and impurities; cooling the melt to room
temperature; and homogenizing the alloy by heating it from room
temperature to 400.degree. C. at 1.degree. C. per minute, holding
it at 400.degree. C. for 12 hours, heating it from 400.degree. C.
at 1.degree. C. per minute, and holding it at 460.degree.
C.-480.degree. C. for 24-48 hours.
8. The method of claim 7, further comprising: hot-working the alloy
to a change in cross section.
9. The method of claim 7, further comprising: solution
heat-treating the alloy at 460.degree. C.-480.degree. C. for 1-4
hours.
10. The method of claim 7, further comprising: aging the alloy at a
first temperature of 100.degree. C.-120.degree. C. for 6-12 hours,
then heating the alloy to a second temperature of 160.degree.
C.-180.degree. C. and holding the alloy at the second temperature
for 8-30 hours, and quenching the alloy with water.
11. An alloy produced according to the method of claim 7.
12. A manufactured article comprising an alloy according to claim
11.
13. A method for producing an aluminum alloy comprising: providing
an alloy comprising an aluminum matrix; adding to the aluminum
matrix amounts of zinc, magnesium, and copper according to an SCC
index of the equation: (SCC index)=2.times.wpZn+wpMg-wpCu where
wpZn, wpMg, and wpCu are the weight percentages of Zn, Mg, and Cu,
respectively, in the matrix of the alloy, and wherein the SCC index
of the alloy is less than or equal to 1.6.
14. The method of claim 13, wherein the alloy comprises about 5.8%
to about 6.8% zinc, about 2.5% to about 3.0% magnesium, about 1.5%
to about 2.3% copper, 0% to about 0.2% scandium, 0% to about 0.2%
zirconium, and optionally less than about 0.50% silver, the balance
essentially aluminum and incidental elements and impurities.
15. The method of claim 13, further comprising: homogenizing the
alloy by heating it from room temperature to 400.degree. C. at
1.degree. C. per minute, holding it at 400.degree. C. for 12 hours,
heating it from 400.degree. C. at 1.degree. C. per minute, and
holding it at 460.degree. C.-480.degree. C. for 24-48 hours.
16. The method of claim 13, further comprising: hot-working the
alloy to a change in cross section.
17. The method of claim 13, further comprising: solution
heat-treating the alloy at 460.degree. C.-480.degree. C. for 1-4
hours.
18. The method of claim 13, further comprising: aging the alloy at
a first temperature of 100.degree. C.-120.degree. C. for 6-12
hours, then heating the alloy to a second temperature of
160.degree. C.-180.degree. C. and holding the alloy at the second
temperature for 8-30 hours, and quenching the alloy with water.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of priority to U.S.
Provisional Patent Application No. 61/488,713, filed May 21, 2011,
the content of which is incorporated herein by reference in its
entirety.
BACKGROUND
[0003] Aluminum alloys, such as the 7XXX Al--Zn-based alloys, are
commonly used in structural applications demanding high specific
strength. For example, the commercial aluminum alloy 7050 is widely
used for aerospace applications. When aged to near the peak of
strength, commercial aluminum alloys are susceptible to
stress-corrosion cracking (SCC). Thus, there has developed a need
for aluminum alloys which show a high strength and yet are
resistant to SCC.
SUMMARY
[0004] In an aspect the disclosure relates to an alloy comprising,
by weight, about 5.8% to about 6.8% zinc, about 2.5% to about 3.0%
magnesium, about 1.5% to about 2.3% copper, 0% to about 0.2%
scandium, 0% to about 0.2% zirconium, and optionally less than
about 0.50% silver, the balance essentially aluminum and incidental
elements and impurities. In embodiments, the alloy has a
stress-corrosion cracking threshold stress of at least about 240
MPa using an ASTM G47 short-transverse test specimen and a yield
strength of at least about 510 MPa using an ASTM E8 longitudinal
test specimen.
[0005] In another aspect the disclosure relates to a method for
producing an alloy, the method comprising preparing a melt that
includes, by weight, about 5.8% to about 6.8% zinc, about 2.5% to
about 3.0% magnesium, about 1.5% to about 2.3% copper, 0% to about
0.2% scandium, 0% to about 0.2% zirconium, and optionally less than
about 0.50% silver, the balance essentially aluminum and incidental
elements and impurities. In embodiments of the method, the melt can
be cooled to room temperature. In further embodiments of the
method, the alloy is homogenized by heating it from room
temperature to 400.degree. C. at 1.degree. C. per minute, holding
it at 400.degree. C. for 12 hours, heating it from 400.degree. C.
at 1.degree. C. per minute, and holding it at 460.degree.
C.-480.degree. C. for 24-48 hours.
[0006] In another aspect the disclosure relates to a method for
producing an alloy that comprises an aluminum matrix. The method
comprises adding to the aluminum matrix amounts of zinc, magnesium,
and copper according to an SCC index of the equation: (SCC
index)=2.times.wpZn+wpMg-wpCu where wpZn, wpMg, and wpCu are the
weight percentages of Zn, Mg, and Cu, respectively, in the matrix
of the alloy. In embodiments, the SCC index of the alloy is less
than or equal to 1.6.
[0007] Other aspects and embodiments will become apparent in light
of the following description and accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a graph plotting short-transverse SCC threshold
stress and typical longitudinal yield strength of some embodiments
of alloys in comparison to conventional aluminum alloys.
[0009] FIG. 2 is a graph plotting maximum applied stress as a
function of life (cycles to failure) of one of the embodiments of
FIG. 1 in comparison to a conventional aluminum alloy.
DETAILED DESCRIPTION
[0010] Aspects relate to an alloy as described herein. Surprisingly
the inventors have produced aluminum alloys that exhibit improved
physical properties relative to existing aluminum alloys, and have
developed methods for making the same. It should be understood that
the claims are not limited in application to the details of
construction and the arrangements of the components set forth in
the following description. Other aspects and embodiments will be
apparent in light of the following detailed description.
[0011] As used herein, terms such as L1.sub.2 phase, fracture
toughness (K.sub.Ic), and stress-corrosion-cracking resistance
(K.sub.ISCC) include definitions that are generally known in the
art such as those found in ASM MATERIALS ENGINEERING DICTIONARY (J.
R. Davis ed., ASM International 1992).
[0012] "Homogenizing" as used herein refers to a process in which
high-temperature soaking is used at a suitable temperature for a
suitable dwell time to reduce chemical or metallurgical
segregation, which occurs as a natural result of solidification in
some alloys. In some embodiments, the high-temperature soaking is
conducted for a dwell time of about 8 hours to about 48 hours.
[0013] "Extrusion" or "extruding" as used herein refers to a
conversion of a metal ingot or billet into lengths of uniform cross
section by forcing the metal to flow plastically through a die
orifice.
[0014] "Aging temperature" as used herein refers to an elevated
temperature at which an alloy is kept for heat treatment. Such heat
treatment may suitably induce a precipitation reaction. In some
embodiments, the heat treatment may be conducted at two distinct
temperatures for two distinct times.
[0015] "Yield strength" as used herein refers to the stress level
at which plastic deformation begins.
[0016] Any recited range described herein is to be understood to
encompass and include all values within that range, without the
necessity for an explicit recitation.
[0017] Aspects of the disclosure relate to aluminum alloys which
show acceptably high strength and yet are resistant to SCC. Without
being necessarily limited by any mechanism or mode of operation, it
may be that segregation of zinc to grain boundaries in aluminum
alloys can make the alloy susceptible to SCC. According to one
aspect, the disclosed alloys can minimize the elemental segregation
of zinc to the grain boundaries, and thereby reduce the
susceptibility of the alloy to SCC. It is contemplated that
segregation of zinc to the grain boundaries in Al--Zn-based alloys
can be prevented by using the zinc to instead form the MgZn.sub.2
phase. The MgZn.sub.2 phase forms both within the grain and at the
grain boundary, as either discrete or linked particles.
[0018] In the course of this work, an "SCC index" was developed,
and it was determined that compositions that minimize the SCC index
are generally effective in minimizing the segregation of zinc to
the grain boundaries. This index is as follows:
(SCC index)=2.times.wpZn+wpMg-wpCu [1]
where wpZn, wpMg, and wpCu are the weight percentages of Zn, Mg,
and Cu, respectively, in solution in the matrix of the alloy. The
SCC index is calculated at the aging temperature, and is based on
the equilibrium composition of the aluminum matrix at the aging
temperature, after accounting for the phase fraction of
precipitates present at the aging temperature. The matrix
composition can be computed with any suitable thermodynamic
database and calculation packages such as Thermo-Calc.RTM. software
version N offered by Thermo-Calc Software (McMurray, Pa.).
[0019] According to one aspect, an alloy can be produced by adding
zinc, copper, and magnesium to an aluminum matrix, in amounts
calculated using the SCC index, such that the SCC index is
maintained at or below about 1.6 (e.g., about 1.6, about 1.5, about
1.4, about 1.3, about 1.2, about 1.1, about 1.0, about 0.9, about
0.8, about 0.7, about 0.6, about 0.5, about 0.4, about 0.3, about
0.2, about 0.1, or less). The alloy may contain other components
and/or additives, including other components and/or additives as
specified herein, and may be further processed using a variety of
processing techniques known in the art, and also including the
processing techniques described herein, such as press-forging,
homogenizing, aging, and the like.
[0020] In one embodiment, the alloy can be first homogenized after
solidification from the melt by heating it from room temperature to
400.degree. C. at 1.degree. C. per minute, holding it at
400.degree. C. for 12 hours, heating it from 400.degree. C. at
1.degree. C. per minute, and holding it at 460.degree.
C.-480.degree. C. for 24-48 hours. The homogenized alloy can then,
in another embodiment, be hot-worked, e.g., extruded to a change in
cross section, then solution heat-treated at 460.degree.
C.-480.degree. C. for 1-4 hours, then aged at a first temperature
of 100.degree. C.-120.degree. C. for 6-12 hours, then heated to a
second temperature of 160.degree. C.-180.degree. C. and held at the
second temperature for 8-30 hours, and quenched with water. These
heat treatments can assist in forming the .eta.-MgZn.sub.2 phase as
discrete particles rather than linked particles, as explained
further herein. In other embodiments, different homogenization,
forging, aging, and/or other forming or heat treatment techniques
may be used. In further embodiments, the alloy may be optionally
subjected to a stress-relief treatment between the solution
heat-treatment and the aging heat-treatment. The stress-relief
treatment can include stretching the alloy, compressing the alloy,
or combinations thereof.
[0021] According to a further aspect, the disclosed alloys
incorporate dispersoid forming elements in amounts sufficient to
inhibit recrystallization. Such dispersoid formers may include
scandium and zirconium. To this end, the dispersoid formers may
form dispersed L1.sub.2 phase particles in the alloy, wherein the
L1.sub.2 phase constitutes about 0.1% by volume of the alloy.
[0022] According to a still further aspect, the alloys are hardened
by the .eta.-MgZn.sub.2 phase. The .eta.-MgZn.sub.2 phase may
constitute about 3% to about 8% by volume of the alloy. The
.eta.-MgZn.sub.2 phase may form within grains and/or at grain
boundaries, and may form as discrete particles and/or linked
particles. Linked particles are often more likely to form at grain
boundaries, adversely affecting the SCC resistance. Accordingly, in
one embodiment, the alloy contains .eta.-MgZn.sub.2 that is formed
primarily as discrete particles. Various heat treatments that are
known in the art or otherwise disclosed herein can be used to guide
the formation of .eta.-MgZn.sub.2 as discrete particles, rather
than linked particles.
[0023] According to one embodiment, the composition of an alloy
includes, by weight, about 5.8% to about 6.8% zinc, about 2.5% to
about 3.0% magnesium, about 1.5% to about 2.3% copper, 0% to about
0.2% scandium, 0% to about 0.2% zirconium, and optionally less than
about 0.50% silver, the balance essentially aluminum and incidental
elements and impurities. In one embodiment, the alloy may include
the elements in the nominal composition, as well as additional
elements; in another embodiment, the alloy may consist essentially
of the elements in the nominal composition; and in a further
embodiment, the alloy may consist only of the elements in the
nominal composition. Incidental elements and impurities in the
disclosed alloys may include, but are not limited to, silicon,
iron, chromium, nickel, vanadium, titanium, or mixtures thereof,
and may be present in the alloys disclosed herein in amounts
totaling no more than 1%, no more than 0.9%, no more than 0.8%, no
more than 0.7%, no more than 0.6%, no more than 0.5%, no more than
0.4%, no more than 0.3%, no more than 0.2%, no more than 0.1%, no
more than 0.05%, no more than 0.01%, or no more than 0.001%.
Additionally, in one embodiment, the alloy has a predominately
face-centered cubic crystal structure, with additional phases and
precipitates, such as those disclosed herein.
[0024] In one embodiment, the alloy has a stress-corrosion cracking
threshold stress of at least about 240 MPa using an ASTM G47
short-transverse test specimen and a yield strength of at least
about 510 MPa using an ASTM E8 longitudinal test specimen. ASTM G47
covers the test method of sampling, type of specimen, specimen
preparation, test environment, and method of exposure for
determining the susceptibility to SCC of aluminum alloys. ASTM E8
covers the testing apparatus, test specimens, and testing procedure
for tensile testing.
[0025] Some samples exemplary of embodiments of the alloy disclosed
herein were prepared and tested for physical properties.
Additionally, a counter-example (alloy B) was also prepared and
tested for comparison. These examples are described in greater
detail below as illustrative non-limiting embodiments.
Example 1
Alloy A
[0026] A melt for alloy A was prepared by heating a charge of
starting materials, the charge having the nominal composition of
6.3 Zn, 2.7 Mg, 1.6 Cu, 0.10 Sc, 0.05 Zr, and balance Al, in wt %.
The alloy includes a variance in the constituents in the range of
plus or minus ten percent of the nominal (mean) value. The melt
weighed about 450 grams. After being cooled to room temperature,
the alloy was homogenized by heating it from room temperature to
460.degree. C. at 1.degree. C. per minute and holding it at
460.degree. C. for 8 hours. The homogenized alloy was press-forged
down to 50% reduction in height, to about 5 cm in short-transverse
thickness. Specimens were excised in the short-transverse direction
to measure the fracture toughness, K.sub.Ic, and the SCC
resistance, K.sub.ISCC.
[0027] The excised specimens were aged at 107.degree. C. for 6
hours, then heated to 177.degree. C. and held at 177.degree. C. for
8 hours, and quenched with water. This aging heat-treatment is also
called the "T7x" heat treatment hereinafter. During the SCC test,
the specimens were coupled to the stainless steel 17-4PH in a 3.5%
NaCl solution. Because the specimens were notched by machining
instead of conventional pre-cracking, the measured K.sub.Ic and
K.sub.ISCC values were appropriately discounted. In a head-to-head
comparison, alloy A was found to have hardness better than that of
7050, and an SCC resistance about 2.4 times greater than that of
7050, as shown in the following Table 1. Table 1 also indicates the
SCC Index of the alloy, calculated using the equation above. The
alloy 7050 was subjected to a heat treatment identical to alloy A
and was tested using the same procedures. The tensile strength was
also measured, and the results are listed in Table 2.
Example 2
Alloy B
[0028] A melt for alloy B was prepared by heating a charge of
starting materials, the charge having the nominal composition of
6.5 Zn, 1.5 Mg, 1.6 Cu, 0.50 Ag, 0.10 Sc, 0.05 Zr, and balance Al,
in wt %. The melt weighed about 450 grams. Alloy B is a
counterexample. Although alloy B includes Zn and Cu in amounts
similar to alloy A, the lower Mg content raises the SCC index,
undesirably lowering K.sub.ISCC. A comparison of the properties of
alloy B and 7050 is shown in Table 1. Table 1 also indicates the
SCC Index of the alloy, calculated using equation [1] above. The
alloy 7050 was subjected to a heat treatment identical to alloy B,
which was also identical to the heat treatment and processing
described above with respect to alloy A (EXAMPLE 1). The tensile
strength was also measured, and the results are listed in Table
2.
Example 3
Alloy C
[0029] A melt for alloy C was prepared by heating a charge of
starting materials, the charge having the nominal composition of
5.8 Zn, 3.0 Mg, 2.2 Cu, 0.05 Sc, 0.05 Zr, and balance Al, in wt %.
The alloy C includes a variance in the constituents in the range of
plus or minus ten percent of the nominal (mean) value. The melt
weighed about 450 grams. After being cooled to room temperature,
the alloy was homogenized by heating it from room temperature to
460.degree. C. at 1.degree. C. per minute and holding it at
460.degree. C. for 8 hours. The homogenized alloy was press-forged
down to 50% reduction in height, to about 4 cm in short-transverse
thickness. Specimens were excised in the short-transverse direction
to measure the K.sub.Ic and K.sub.ISCC. The excised specimens were
aged at 107.degree. C. for 6 hours, then heated to 177.degree. C.
and held at 177.degree. C. for 8 hours, and quenched with water.
During the SCC test, the aluminum specimens were coupled to the
stainless steel PH17-4 in a 3.5% NaCl solution. In a head-to-head
comparison, alloy C was found to have hardness better than that of
7050, and also an SCC resistance better than that of 7050, as shown
in Table 1. Table 1 also indicates the SCC Index of the alloy,
calculated using equation [1] above. The alloy 7050 was subjected
to a heat treatment identical to alloy C. The tensile strength was
also measured, and the results are listed in Table 2.
TABLE-US-00001 TABLE 1 Cal- Calculated Vickers culated phase
Converted Converted hard- SCC fraction of K.sub.ISCC K.sub.Ic
K.sub.ISCC/ ness Index .eta.-MgZn.sub.2 (MPa m) (MPa m) K.sub.Ic
number Alloy A 1.5 0.07 32.3 32.7 0.99 165 Alloy B 3.2 0.05 13.3
31.5 0.42 152 Alloy C 1.5 0.08 15.9 20.7 0.77 177 7050 1.9 0.08
13.3 34.1 0.39 157
TABLE-US-00002 TABLE 2 0.2% Ultimate Yield Tensile Reduction Stress
Stress Elongation of Area (MPa) (MPa) (%) (%) Alloy A 370 .+-. 30
370 .+-. 30 3 5 Alloy B 350 .+-. 70 380 .+-. 90 8 .+-. 2 13 .+-. 9
Alloy C 470 500 5 4 7050 410 .+-. 40 430 .+-. 30 4 .+-. 1 5 .+-.
3
[0030] As seen from Tables 1 and 2, the alloys according to the
disclosed aspects and embodiments (e.g., alloys A and C) produce
physical properties that are comparable or superior to those of
alloy 7050, and in particular, the alloys A and C have a lower SCC
Index compared to alloy 7050, which indicates a superior resistance
to SCC. For alloy A, the hardness is superior to that of alloy
7050, and the SCC resistance is also superior to alloy 7050.
Additionally, the fracture toughness (K.sub.Ic), yield stress,
ultimate tensile stress, and ductility are all comparable to those
of alloy 7050. For alloy C, the hardness, yield stress, ultimate
tensile stress, and SCC resistance are superior to those of alloy
7050, and the ductility is comparable. The fracture toughness
(K.sub.Ic) of alloy C was found to be slightly lower than that of
alloy 7050. It is noted that the K.sub.ISCC of alloys A and C are
very close to the theoretical limit (i.e. the K.sub.Ic value).
Example 4
Alloy A-1
[0031] A melt was prepared by heating a charge of starting
materials, the charge having the nominal composition of 6.3 Zn, 2.7
Mg, 1.6 Cu, 0.12 Zr, and balance Al, in wt %, which is the same as
alloy A. The as-cast alloy A-1 was generally shaped like a
cylinder, measuring about 18 cm in diameter and 56 cm in height,
and weighing about 50 kg. After being cooled to room temperature,
the as-cast alloy A-1 was homogenized by heating it in a furnace
from room temperature to 400.degree. C. at 1.degree. C. per minute,
holding it at 400.degree. C. for 12 hours, heating it from
400.degree. C. at 1.degree. C. per minute, and holding it at
460.degree. C.-480.degree. C. for 24-48 hours. The homogenized
alloy A-1 was extruded to a cylindrical billet, reducing the
diameter to about 8 cm in diameter. This represents an extrusion
ratio of about 51/2:1. Specimens were excised and subjected first
to a solution heat-treatment ("SHT"), and then to an aging
heat-treatment. The solution heat-treatment was conducted by
subjecting the specimens to a temperature of 460.degree. C. or
465.degree. C. for 2 hours. The aging heat-treatment was conducted
by subjecting the specimens to 107.degree. C. for 6 hours, then
heating to 177.degree. C., holding at 177.degree. C. for 8 hours,
and quenching with water. Tensile strength was measured at room
temperature according to ASTM E8 with a longitudinal test specimen.
The results are listed in Table 3 in comparison to a conventional
aluminum alloy, namely, QT-7050-T74. The yield strength ("YS") of
alloy A-1 is about 10% higher than that of QT-7050-T74 in the
longitudinal and transverse directions, with comparable elongations
and reduction-of-area percentages ("% RA").
TABLE-US-00003 TABLE 3 QT-7050-T74 A-1-T7x Properties 475.degree.
C. SHT 465.degree. C. SHT 460.degree. C. SHT Longitudi- UTS (ksi)
79.4 85.5 84.4 nal 0.2% YS (ksi) 72.4 81.1 78.7 % elongation 13.5
14 13 % RA 42.5 39 39.5 Transverse UTS (ksi) 70.6 76.8 77.2 0.2% YS
(ksi) 63.4 70.5 68.8 % elongation 4 5 6 % RA 9.5 6.2 7.8
[0032] The SCC resistance was measured according to a rising step
load (RSL) method developed by Lou Raymond & Associates in
Newport Beach, Calif., generally as follows. Machined notched
samples in the fully heat-treated condition were used for the
testing. Initial fracture toughness (K.sub.Ic) testing was
performed in air at a rapid loading rate to first determine the
maximum breaking load. The test specimen geometry was changed to
increase the amount of constraint. An effective stress intensity
K.sub..rho. was calculated, since the specimen had a machined notch
instead of a fatigue pre-crack as required by ASTM E399. Previous
testing of 7075-T6 aluminum alloy in a similar way found that the
value for K.sub..rho. was approximately 1.5 times the value for
K.sub.Ic. Having measured the maximum breaking load, the RSL method
was employed to measure the K.sub.ISCC of the samples. During the
SCC test, the aluminum specimens were anodically charged by
coupling them to PH17-4 adapters in a 3.5% salt-water environment.
Alloy A-1 showed a K.sub.Ic value of 38.8 ksi-in.sup.1/2 and a
K.sub.ISCC value greater than 38 ksi-in.sup.1/2.
Example 5
Alloy D
[0033] A melt for alloy D was prepared by heating a charge of
starting materials, the charge having the nominal composition of
6.3 Zn, 2.7 Mg, 1.6 Cu, 0.12 Zr, and balance Al, in wt %. The alloy
D preferably includes a variance in the constituents in the range
of plus or minus ten percent of the nominal (mean) value, and is
substantially free of scandium. The as-cast alloy D was generally
shaped like a cylinder, measuring about 18 cm in diameter and 56 cm
in height, and weighing about 50 kg. After being cooled to room
temperature, the as-cast alloy D was homogenized by heating it from
room temperature to 400.degree. C. at 1.degree. C. per minute,
holding it at 400.degree. C. for 12 hours, heating it from
400.degree. C. at 1.degree. C. per minute, and holding it at
460.degree. C.-480.degree. C. for 24-48 hours. The homogenized
alloy D was extruded to a cylindrical billet, reducing the diameter
to about 8 cm in diameter. This represents an extrusion ratio of
about 51/2:1. Specimens were excised and subjected first to a
solution heat-treatment, and then to an aging heat-treatment. The
solution heat-treatment was conducted by subjecting the specimens
to a temperature of 460.degree. C., 465.degree. C., or 470.degree.
C. for 2 hours. The aging heat-treatment was conducted according to
the T7x heat treatment. Tensile strength was measured at room
temperature according to ASTM E8 with a longitudinal test specimen.
The results are listed in Table 4 in comparison to the conventional
QT-7050-T74. Alloy D has about 20% higher YS than 7050-T74 in the
longitudinal direction and about 13% to about 15% higher YS than
7050-T74 in the transverse and 45.degree. direction, with
comparable elongations and % RA. The strength values of alloy D
represent a significant improvement over 7050-T74.
TABLE-US-00004 TABLE 4 QT-7050-T74 D-T7x Properties 475.degree. C.
SHT 470.degree. C. SHT 465.degree. C. SHT 460.degree. C. SHT
Longitudinal UTS (ksi) 79.4 90.9 89.1 90.7 0.2% YS (ksi) 72.4 86
84.7 85.3 % elongation 13.5 12.5 12.5 13.5 % RA 42.5 35 37.5 37
Transverse UTS (ksi) 70.6 79.6 74.5 74.3 0.2% YS (ksi) 63.4 72.5 70
70.7 % elongation 4 5 4 3.8 % RA 9.5 5.2 3.1 4.0 45.degree.
orientation UTS (ksi) 69.1 76 73.9 76.3 0.2% YS (ksi) 62.1 70.5
68.9 68.6 % elongation 5 3.5 4.5 5.9 % RA 5.6 5.5 5.2 7.3
[0034] The SCC threshold stress of alloy D was measured by a 30-day
accelerated stress corrosion testing according to ASTM G47.
Short-transverse samples of alloy D were solution heat-treated at
460.degree. C. for 2 hours, and heat-treated according to the T7x
heat treatment. FIG. 1 shows the SCC threshold stress and typical
longitudinal yield strength of alloy D in comparison to
conventional aluminum alloys. The samples of alloy D passed a
stress level of about 380 MPa, which is above the highest SCC
temper designation currently in use, namely, T73. Thus, the
combination of strength and SCC resistance of alloy D is
substantially improved over that of conventional aluminum
alloys.
[0035] The SCC resistance was measured according to the RSL method.
Machined notched samples in the fully heat-treated condition were
used for the testing. K.sub.Ic testing was performed in air at a
rapid loading rate to first determine the maximum breaking load.
The test specimen geometry was changed to increase the amount of
constraint. An effective stress intensity K.sub..rho. was
calculated. Having measured the maximum breaking load, the RSL
method was employed to measure the K.sub.ISCC of the samples.
During the SCC test, the aluminum specimens were anodically charged
by coupling them to PH17-4 adapters in a 3.5% salt-water
environment. Alloy D specimens that were solution heat-treated at
460.degree. C. for 2 hours and heat-treated according to the T7x
heat treatment showed a K.sub.Ic value of 47.8 ksi-in.sup.1/2 and a
K.sub.ISCC value of 20.0 ksi-in.sup.1/2. On the other hand, alloy D
specimens solution heat-treated at 470.degree. C. for 2 hours and
heat-treated according to the T7x heat treatment showed a K.sub.Ic
value of 55.4 ksi-in.sup.1/2 and a K.sub.ISCC value of 15.0
ksi-in.sup.1/2.
[0036] Smooth bar fatigue testing was carried out according to ASTM
E466 at four different maximum stress levels: 250 MPa, 280 MPa, 340
MPa, and 400 MPa. An R-ratio, i.e., the ratio of the minimum peak
stress to the maximum peak stress, of 0.1 and frequency of 20 Hz
was used for the test. Transverse alloy D specimens were solution
heat-treated at 470.degree. C. and heat-treated according to the
T7x heat treatment, and compared to 7050-T74 samples. FIG. 2 shows
the maximum applied stress as a function of life (cycles to
failure) of alloy D in comparison to 7050-T74. Alloy D shows
fatigue behavior comparable to 7050-T74. Notably, at a low stress
range, e.g., maximum stress below about 35 ksi (about 250 MPa), the
difference in life between alloy D and 7050-T74 is expected to be
minimal.
[0037] It is understood that the disclosure may embody other
specific forms without departing from the spirit or central
characteristics thereof. The disclosure of aspects and embodiments,
therefore, are to be considered in all respects as illustrative and
not restrictive, and the claims are not to be limited to the
details given herein. Accordingly, while specific embodiments have
been illustrated and described, numerous modifications come to mind
without significantly departing from the spirit of the invention
and the scope of protection is only limited by the scope of the
accompanying claims. Unless noted otherwise, all percentages listed
herein are weight percentages.
* * * * *