CA1047901A - Rapid high temperature aging of al-zn-mg-cu alloys - Google Patents

Abstract

Abstract of the Disclosure A method of thermally treating an article composed of an alloy consisting essentially of aluminum, 4 to 8% zinc, 1.5 to 3.5% magnesium, 1 to 2.5% copper, and at least one element selected from the group consisting of 0.05 to 0.3% chromium, 0.1 to 0.5% manganese, and 0.05 to 0.3% zirconium, which method includes solution heat treating the article and subsequently sub-jecting the article to a time and temperature effective for increas-ing the resistance to corrosion of the alloy over its resistance in the T6 condition, the time and temperature being from 10 seconds to 10 minutes and from 350 to 520°F, or else 435 to 520°F, respec-tively. An alloy consisting essentially of aluminum, 4 to 8% zinc, 1.5 to 3.5% magnesium, 1 to 2.5% copper, and at least one element selected from the group consisting of 0.05 to 0.3% chromium, 0.1 to 0.5% manganese and 0.05 to 0.3% zirconium, having a corrosion resistance increased over that of its T6 condition, with a solution potential lying in the range minus 825 to minus 935 millivolts, a yield strength lying in the range 46 to 72 ksi, a dislocation den-sity above that exhibited by aluminum alloy 7075 in the T73 condi-tion, denuded grain boundary regions, and grain boundary precipitate.

Description

~0 47 9 ~ 1 .
The present invention relates to a method of ~hermally treating articles containing an alloy based on aluminum and to an aluminum alloy in a particular heat ~rea~ed condition.
The precipitation hardened condition o~ aluminum alloy 7075, referred to as the T6 condition of alloy 7075, ha~ no~
given su~ficient resistance ~o corrosion under certain service conditions. The T73 temper improves ths resistance of precipi-tation hardened 7075 alloy to stress eorrosion cracking. The process needed to obtain a T73 temper considerably increases the time required for heat treatîng the 7075 alloyO
An object of the present invention is to provide a new heat treating method to produce an alumlnum alloy in a unique heat treated condltion for providing favorable re~istance to corrosion.
A further object of the invention is to provide a new heat treating method for creating in an aluminum alloy favorable resistance to corrosion whlle allowing reduction of the heat ~reating time needed to reach the T73 conditLon.
Another object is to provide a new method for providing resistance to stress corrosion cracking in 7075 aluminum alloy.
Yet another ob~ect is to obtain an aluminum alloy, such as 7075, in a unique heat treated condition.
These as well as obher objects which will become apparent in the discu~sion which follows are achieved, according to the present invention, by 1) the metho~ of thermally treating an article composed of an alloy consisting essentlally of aluminum, 4 to 8% zinc, 1.5 to 3.5% magnesium, 1 to 2~5~/o copper, and at least one element selected ~rom the group consisting of 0.05 to 0.3% ~
chromium~ 0~1 to 0.5% manganese, and 0.05 to 0.3% zirconium, `
which method comprises solution heat treating the article and subsequently subjecting the article to a time and temperature : .. . . : . , . ." . . .

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effective for increaslng the resistance to corrosion of ~ e alloy over its resistance in the T6 condition, the time and temperature being from 10 seconds to 10 minutes and from 350 to 520F
respectively:

2) the alloy consisting essentially of aluminum, 4 to 8% zinc, 1.5 to 3.5% magnesium, 1 to 2.5% copper, and a~ least one element selected from the group consisting of 0.05 to 0,3~/0 chromium,sO.l to 0.5% manganese and 0.05 to 0.3% zirconium, having a corrosion resistance increased over ~hat of its T6 condition, with a solution potential lying in the range 830 to minus 935 millivolts, a yield strength lying in ~he range 46 to 72 ksi, a dislocation density above that exhibited by 7075 in the T73 condition, denuded grain boundary regions 9 and grain boundary precipitate. ;
Figures 1-7 are transmission electron micrographs of sections in a plate of aIuminum alloy 7075. The distance equivalent to 0.1 micron is indicated on the micrographs. The metal surfaces reproduced in the micrographs all were perpen- -~
dicular to the direction of rolling of the plate.
Figure 1 shows a prior art solution heat treated and stress relieved condition referred to as the W51 condition.
Figure 2 shows the prior art precipitation hardened condition referred to as the T6 condition.
Figure 3 shows the prior art stress corrosion cracking r~sistan~ condition re~erred to as the T73 condition.
Figures 4 and 5 illustrate one embodiment of the present invention. -~
Figures 6 and 7 illustrate a second embodimen~ of the present invention, Figure 8 is a graph of data taken from Examples 31 to 42 illustrating the present invent~on.
Figure 9 is a graph ~ho~ng additional characteris~ics of the invention.
The alloys in the present inventlon have a composition con~aining 4 to 8% zinc, 1.5 to 3.S% magnesium, l to 2.S% copper, and at least one element selected from the group made up by chromium at 0O05 to 0.3%, manganese at O.l to 0.5%, and zirconium at 0.05 to 0.3%. The balance of the compo~ition is essentially aluminum.
Alloys designated 7075 by the aluminum indu&try are preferred for the present invention and have a composition containing 5.1 to 6.lV/o zinc, 211 to 2.9% magnesium, 1.2 to 2.0~/O
copper, 0~18 to 0.35% chromium, 0.30/O maximum manganese, 0.40% -~
maximum silicon, 0.50% maximum iron9 0.20~o maximum titanium, others each 0.05% maximum and others to~al 0.15% maximum, ~ ;
balance aluminum.
The alloys used in the present inventlon may also contain one or more of the group of grain refining elements including titanium at O.Ol to 0.2% and boron at 0.0005 to 0.002%. ~ ~ -These elements serve to produce a fine grain si2e in the cast form o~ the alloy. This is generally advantageous to mechanical properties.
In addition, it may be helpful to add O.OOl to 0.005% ;
beryllium for the purpose of minimizing oxidation at times when the alloy i8 molten.
Iron and silicon are generally present as impurities.
Up to 0~5% iron can be tolerated, and the silicon content should not exceed 0.4%, Ln order to avoid ths formation of any sub-stantial amount of the intermetallic compound Mg2Sio A preferred heat treatment accordlng to the present invention for obtaining improved stress-corrosion resistance is to immerse alloy, as above defined, in the precipitation~
hardened, T6 condition into m~lten metal at 400 to 500F for l to 7 millutes.

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In i~s broader aspects, a T6 condition may be obtained by precipitation hardening soLu~ion heat treated alloy at 175 ~o 325F Typical conditions may be:
a. For alloys conta~ning les~ than 7.5~/O
zinc, heating a solution heat treated article to 200 to 275F and holdlng or a period o~ 5 to 30 hours;
b. For alloys containing more than 7.5%
~inc3 heating a solution heat treated article to 175 to 275F and holding or a period of 3 to 30 hours.
Preferably, the T6 condition i5 obtained by heating a specimen for 24 hours at 250F in a circulatory-air furnace.
Th~ article of J. T. Staley e~ al. entitled "Heat Treating Characteristics of High Strength Al-Zn-Mg-Gu Alloys With and Without Silver Additions" appearing at pages 191 to 1~9 in the January, 1972 issue of Metallurgical Tran~a ons, published by ASM/AIME, shows that solution heat treat quench rate, the lapse of time between the solution heat ~rea~ quench and the beginning o heating for precipitation hardening, and the heating rate ~or precipitation hardening may affect the ~ ,.
maximum yield strength obtainable in 7075 aluminum~alloys. It is intended that, within the concepts of the present invention, the teachings of Staley et al. be used in the present lnvention for optimizing results. Thus, it may be advantageous for increasing strength to immerse specimens, which have had their .~
Bolution heat treatment quench, for example, 1-1/2 years ago, into molten Wood's metal according to the invention.
Referring now to Figures 1 to 7, ~ransmission electron ;~
micrographs of various microstruetures important for consideration of the present invention are presented. All o~ Figures 1 to 7 were taken from a single l/4-inch thick 7075 aluminum alloy plate. Figures 1 to 3 are micro~tructures of prior art `
conditions of 7075 aluminum. In Figure 1, an example of the W51 ;~
solution heat treated condition is given. A W51 solution heat ~-- 4 ~ ~

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trea~ed mlcrostructure is ob~ained in 7075 aluminum alloy plate by heating to 900F and then quenchlng in water at room temperature. The plate ma~erial is then stretched to ~rom 1-1/2 to 3% permanent se~ for stress relief. This gives the micro-structure shown in Figure 1, including E-phase particles of Al-Mg-Cr precipitate, ma~rix regions R of single phase aluminum solid-solution material, grain boundaries B and dislocatlons D.
The mottling effect appearing in the matrix region of ~igure l is an ar iact of the action of the thinning solution used in preparing thinned material for transition electron microscopy.
The specimen for Figure 1 was taken rom the same 7075 alloy plate used in Examples 1 to 29 below.
Figure 2 shows the 7075 alloy material of Figure 1 after it has been brought to the T6, in particular ~he T651, temper by heating W51 material in a circulatory-air furnace for 24 hours at 250F. E-phase remains ~ubstantially unchanged.
Dislocations D and a grain boundary B are shown. Now in the matrix there has appeared many small black dots; these are referred to as G.P. zones and are clusterings of magnesium and zinc atoms generally in the ratio two zinc atoms for each magnesium atom.
Figure 3 shows a specimen taken from the same plate o Figures 1 and 2 in the T73 condition, which is produced from W51 material by heating in circula~ory-air ~urnaces for, first, 24 hours at 250F and, second, 8 hours at 350F. Grain boundary precipitate 10 has appeared, and the G.P. zones have grown to greater size. The G.P. zones have begun to exhibit crystallinity by giving rise to X-ray diffraction patterns and are referred to by those in the art as M' and M phase. Solution potential studies indicate that the M' and M phases contain some copper ~ -atoms. It is believed that the G.P. zones progress toward crystallinity by becoming first M' phase, which is still ~047~
partlally coherent with the matrlx crystal structure. The M' phase then changes to M phase, which has a crystal structure different from the matrlx. It is believed also that the pro-gression through the M' phase to the M phase makes the original G.P. zones increasi~gly anodic wi~h respect to the matrix and that the resulting anodic particuLate ma~ter in the matrix protects against stress-corrosion cracking.
The microstructure of Figure 4 was obtained according to the invention by aging a l/~" x 3/8" x 4" blank of the W51 material of Figure l first to the T6 condition using 24 hours at 250F in a circulatory-air furnace and then immersing the blank for lO seconds in Wood's me~al molten ~t 490F. Upon removal from the molten Wood's metal the blank was allowed to air cool. Appearing in Figure 4 are G.P. zones 3 E-phase, grain boundary precipitate ll and denuded ~free of G.P. zones) grain boundary material 12. Because of the particular orientation of the grains in Figure 4, dislocations do not show. They are, , : ~
however, present, as is clear ~rom the presence of dislocations D shown in Figure 5 illustrating another gra1n in the same blank used ~or Figure 4. The grain of Figure 5 is more favorably oriented than that of Figure 4 for showing dislocations. ~
Figure 6 illustrates a blank of the same size as used ,! ' for Figures 4 and 5, heat treated in the same manner except that, upon removal from the Wood's metal, the blank was quenched in cold water. Present again are grain boundary precipitate 13, denuded grain boundary material 14, E-phase, and G.P. zones.
Dislocations D appear in the lower, favorably oriented grain in ~ -.. .
Figures6. Figure 7 shows another grain in the same blank as used for Figure 6 for further illustrating the dislocation density. ;
Further illustrative of the present invention are the ~;` ;
following examples. Examples l to 29 use as starting material the same plate used for obtaining Figures l to 7.
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~(~479t)1 Examples 1 to 3 Data for Examples 1 to 3 appear ln Table I. Examples 1 to 3 represent different prior art processes and conditions or a 7075 aluminum alloy composition, an alloy composltion whioh may be used in the presen~ invention. The alloy composition was as given in Table II for alloy A. Data W~9 collected from blanks measuring l/4-inch thick by 4-inches. These blanks were taken from a l/4-inch thick plate of alloy A in the W51 condition.
The longest, 4-inch dimension o ~he blanks was parallel to the longitudinal direction of the plate, i.e., the direction of rolling. The T6 temper was obtained by heating W51 blanks in a circulating-air furnace for 24 hours at 250F. The T73 temper was carried out also in circulating-air furnaces, first at 250F
for 24 hours and then for 8 hours at 350F. Measured were solution potential, yield strength, and degree of ex~olia~ion9 as given in Table I.

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Table II.
Composition o:f Alloys, in Weight-%.
. . _ _ Alloy _ . . .
Element A B
____ ~ . ,_ Cu 1~45 1~81 Fe 0.19 0.31 S~ 0.09 0.08 Mn 0.02 0.02 Mg 2.40 2.38 -~
Zn 5.92 6.02 ~ `
Ni 0.00 ____ Cr 0.18 0.19 Ti 0.02 0.03 Be 0.001 0 00 Examples 4 to 21 ;
Blanks as in Examples 1 to 3 were aged to a T6 temper using 24 hours at 250F. They were then vapor degreased and subjected~to additional treatmen~ in molten Wood's metal as ~;
indicated in Table I. Measured were solution potential, yield stren8th9 and degree of exfoliation.
Examples 22 to 29 Specimens as in Example 1 of Alloy A were subjected to various treatments in molten Wood's metal without firs~ being brou~ht to the T6 temper. Measured were solution potential, yield strength, cnd degree o ex~oliation. ; `
* * * ;~
A plot o~ yield strength versus solution potential or the dcta of Examples 1 to 29 reveals thct the data for Examples -

4 to 29 according to the present invention lie in an area occupied neither by the T6 data of Example 2 nor by the T73 data . ;; '' ~ 47 g ~ ~
o~ Example 3. Examples 7 ~o 29 had exfoliation resistances better than that for the T651 data of Example 2. Examples 8, 9, 10 to 15, 17 to 19, 21 to 23, 25, 26, and 27 to 29 had exfolia- :
tion resistances better than the data ~or the T73 condition of Example 3. Each o Examples 7 to 22, 25~ 2B, 27, and 28 had both higher yield strength and more anodic (more negative milli-volt value) solution potential than the corresponding values for '.!
the T73 condition o~ Example 3. The solu~ion poten~ial obtained by any particular heat treatment followed by a cold water quench was considerably more anodic than that obtained by air cooling.
In general9 resistance to exfoliation and pitting increases as solution potential becomes more anodic, i.e.~ progresses toward greater negative value. ~.
* * . : .
Examplas 30 to 35 - Cold Water Quenched ;
For each example, four tensile blanks of dimensions , :.~ . .. .
3/8 inch by 3/8 inch by 2-1/2 inches were cu~ rom a sin~le lot ~ ~ :
of 2-l/2 fnch thick 7075-T651 (metallurgical history as described for Figure 2) alloy plate such that their lengths were in the short-transverse direction7 i,e., in the direction perpendicular to the surace of the plate. The mechanical properties of this .
material were as presented in Table III. ~
Table III. . ~.
Mechanical Properties of Plate . .
Used for Examples 30 to 41.
... .
Tensile Yield Strength Strength ~/O
ksi ksi Elongation .... . . .
Long 80.2 71.7 8.0 Transverse Short : 74.8 66.6 2.0 Transverse The chemical composition of the alloy is as presented for alloy B in Table II. The tensile blanks for each example were ~.
..
immersed in 445F molten Wood's metal of composition 50% bismuth, - 12 - ~ -~ , .

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25% lead, 12.5% tin and 12.5% cadmium. The immersion times ~or Examples 30 to 35 were 30, 60, 90, 120, 240, and 420 second6, respec~lvely. Following immersion in the molten Wood's metal, the specimens were quenched in cold water Erom the tap. The dlfference between cold water temperature in ~he summer and in the winter does not affect the results to any significan~ extent.
Two tensile blanks were machined to a 0.125 inch diame~er tensile bar for exposure to 3-1/2% sodium chloride solution by alternate ~;
immersion at stress levels o~ 42 and 35 ksi, respectively, according to Military Specification MIL-A-22771B. The specimens were held until failure under a given stress level with successive immersions for 10 minutes in the salt solu~ion ~ollowed by 50 minutes in air~ Examples 31-35 lasted more than 30 days under such treatment, thus meeting the standard of the Military Specification. The remaining ~o blanks of each example were tested for yield strength and solution potential, respectively. ~;
The yield strength and solution potential data for Examples 30 to 35 are presented in Figure 8 as curves originating respectively, from the yield strength and solution pstential of the plate in the T651 condition. It is to be noted that the yield strength data passes through a minimum, termed herein a "first minlmum", at Example 30. Measurements of the conductivities of Examples ~0 to 35 showed that a stress corrosi.on resistance (as measured by the alternate immersion test~ obtainable only at a conductivity ~;
of 38% IACS with the T73 temper is obtained at 35 to 37% IACS in the present invention. Conductivity data appears in Table IV.
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~6347~ : 7 ' Table IV. ~ -Conductivity Data for Examples 30 to 41.
. . ~ . _ ":, Cold Water Quench . _ _ , .: .
Immersion . -Time In Electrical ..
Seconds ExampleConductivity ~:
No. ~/o IACS
~ :---~ -. ': ' 33 3 ~:-31 34 7 -~
32 35.2 120 33 35.8 -~.
~40 34 36.7 420 35 38.2 ``'i -~ :
_ ~ _. . .
Air CooIed ElectricaI :~
ExampleConductivity No. % IACS : ::~
: . - ~ .... ..
: 30 36 34.2 ~ ;
37 35.2 ~:

38 36.4 ~ ~ :
. 120 39 36~7 :: ` :
: 240 4Q 37.7 : 4~0 41 38.8 .
..... .. :~ ~ . .. ,-:
''~` ' `-amples 36 to_41_- Air Cooled Tests~were~run as for Examples 30 to 35, the only differ~nce being that the specimens were allowed to air cool ~: following~immersioD, Diferences in room temperature from:day ....
: ~ to day~or season to aeason do ~ot produce significant variations in results. The data for yield strength and solution potential are presented in Figure 8. Here, all of Examples 37 to 41 .
passed the alternate immersion, aqueous sal~-solution test of ;:

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the Military Speciication mentloned in Examples 30 to 35. Here again, lt was noted that a stres~ corrosion resistance (as measured by the al~ernate immersion test) obtainable only at a conductivity of 38% IACS with the T73 temper is obtained at 35 to 37% IACS in the present invention. The data for conductivity is presented in Table IV. All of Example~ 37 to 41 lying beyond the fir~t minimum at Example 36 in Figure 8 in the yield strength curve passed the 30-day standard of the Mil~ary Speclfication.
Additional Examples 4~ to 55 Tests were run as for Examples 36 to 41 using additional variations in time and temperature of immersion in mol~en Wood's metal. The points for these additional tests (as presented in Table V) plus the ~ests of Examples 36 to 41 are plotted in Flgure 9. Above every point in the Figure, the mean time to failure in the alternate immersion, aqueous salt-solution test ~ -of the Military Specification mentioned in ~xamples 30 to 35 is ;
glven in days. Below every point, the yield s~reng~h is given, expressed in percent of the T651 yield strength. Times and temperatures of the Wood's metal immersion according to one aspect of the present invention showing combined high yield strength and resistance to stress corrosion cracking fall within the perimeter of irregular pentagon ABCDE in Figure 9. Preferably~
the times and temperatures lie within the perimeter of the quadrilateral FGHI.

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Table V.
Tlmes and Temperatures in :: ~
Wood's Metal for Example3 42 to 55 ;- -and the Coordinates of Point~ A to I
. . _ ~ " ' .
Example No., Timeg Temperature, or Point min. F
. .. , . . __ 42 0.5 500 4~3 0.75 500 44 1.0 500 1. 5 500 46 2. (3 500 47 0. 5 475 b,8 1. 0 475 49 0.5 400 S0 ~. 0 400 51 1.5 400

5 3 4 . 0 ~ 4 0 0 54 6 . 0 400 5 5 ~ 7 . 0 ~ 375 A 3. 0: 390 : B 0. 2 500 C 1. 0 500 D : 10.0 438 E lO. 0 390 F 4.0 400 ~ G 0.67 ~ 476 : H ~ 1.05 476 I 8. 0 400 ~ . . i ::

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The ~ollowing definitions hold herein:
a. The term "ksi" is equivalent to kilopounds per square inch.
b. Wherever percentages are given, re~erence is to % by weight, unless indicated otherwise, c. The initials "G.P,-I stand for Guinier-Preston.
Various modi~lcations may be made in the inventlon without departing from the sp-lrit thereof, or the scope of the claims, and, therefore7 the exact orm shown is to be taken as illustrative only and not în a limiting sense~ and it is desired that only such limitationsshall be placed thereon as are imposed by the prior art, or are specifically set forth in the appended claims. `~
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Claims (18)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a method of heat treating an article composed of an alloy consisting essentially of aluminum, 4 to 8% zinc, 1.5 to 3.5% magnesium, 1 to 2.5% copper, and at least one element selected from the group consisting of 0.05 to 0.3% chromium, 0.1 to 0.5% manganese, and 0.05 to 0.3% zirconium, said method com-prising the steps of solution heat treating said article and sub-sequently subjecting said article to a time and temperature effective for increasing the resistance to corrosion of said alloy over its resistance in the T6 condition, the time and temperature being from 10 seconds to 10 minutes and from 435 to 520°F, respec-tively.

2. The method as claimed in Claim 1, further comprising, after the step of solution heat treating and before the step of subjecting, the step of precipitation hardening said article at 175 to 325°F.

3. The method as claimed in Claim 2, the step of sub-jecting being at 435 to 500°F for 1 to 7 minutes.

4. The method as claimed in Claim 1, said alloy con-sisting essentially of 5.9% zinc, 2.4% magnesium, 1.45% copper, 0.18% chromium, balance aluminum, wherein, following the step of solution heat treating and before the step of subjecting, the method further comprises precipitation hardening the articles for 24 hours at 250°F, wherein the step of subjecting comprises immersing said article in molten metal at 470°F for 18 seconds, and wherein the method further comprises air cooling the articles following the immersing.

5. The method as claimed in Claim 1, wherein the step of subjecting comprises immersing said article in molten metal having said temperature.

6. A method of heat treating an article composed of an alloy consisting essentially of aluminum, 4 to 8% zinc, 1.5 to 3.5% magnesium, 1 to 2.5% copper, and at least one element selected from the group consisting of 0.05 to 0.3% chromium, 0.1 to 0.5% manganese, and 0.05 to 0.3% zirconium, said method com-prising the steps of solution heat treating said article, precipi-tation hardening said article at 175 to 325°F, and subsequently subjecting said article to a time and temperature lying within the perimeter BCDL in Figure 9.

7. A method as claimed in Claim 6, wherein the step of subjecting comprises immersing said article in molten metal having said temperature.

8. A method as claimed in Claim 6, wherein the step of subjecting is for a time and temperature lying within the peri-meter GEJK in Figure 9.

9. An alloy consisting essentially of aluminum, 4 to 8% zinc, 1.5 to 3.5% magnesium, 1 to 2.5% copper, and at least one element selected from the group consisting of 0.05 to 0.3% chromium, 0.1 to 0.5% manganese and 0.05 to 0.3% zirconium, having a corro-sion resistance increased over that of its T6 condition, with a solution potential lying in the range minus 825 to minus 935 millivolts, a yield strength lying in the range 46 to 72 ksi, a dislocation density above that exhibited by 7075 aluminum alloy in the T73 condition, denuded grain boundary regions, and grain boundary precipitate.

10. An alloy consisting essentially of aluminum, 4 to 8% zinc, 1.5 to 3.5% magnesium, 1 to 2.5% copper, and at least one element selected from the group consisting of 0.05 to 0.3% chromium, 0.1 to 0.5% manganese, and 0.05 to 0.3% zirconium, having a stress corrosion resistance at a conductivity of 35% IACS at least equiva-lent to the stress corrosion resistance of 7075 aluminum alloy in the T73 condition at a conductivity of 38% IACS.

11. The method as claimed in Claim 1, the temperature in the step of subjecting being at least 445°F.

12. A method as claimed in Claim 6, the temperature being at least 445°F.

13. The method as claimed in Claim 1, the temperature being 445°F.

14. The method as claimed in Claim 1, the temperature being 470°F.

15. The method as claimed in Claim 1, the temperature being 475°F.

16. The method as claimed in Claim 1, the temperature being 490°F.

17. The method as claimed in Claim 1, the temperature being 500°F.

18. The method as claimed in Claim 1, the temperature being 520°F.