CA1042686A - Aluminum alloy system - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Wrought articles of Al-Cu-Mg alloy containing up to about 5%
copper and up to about 2% magnesium as the principal alloying ele-ments by weight, within limits effective to achieve substantially single phase structure, and exhibiting improved fracture toughness in -T8XX condition; also, related practices and improved alloy com-positions for making such articles, including plate.

Description

The present invention relates to improvements in wrought articles made of an aluminum base alloy con-taining copper and magnesium as the principal alloying elements by weight, in amounts up to about 5% Cu and up to about 2~ Mg, including at least 0.3% magnesium and at least about 3% total of copper and magnesium; and it further relates to practices for making such articles to achieve improved properties, and to alloy compositions of the Al-Cu-Mg type.
Alloy 2024 as currently produced commercially con-tains a large percentage of second phase particles, such as (i) e phase - CuA12, (ii) S phase - A12CuMg, and (iii) FeMnA17. Cu2FeA17 is another probable phase. These par-ticles apparently add little to the strength of the alloy, but tend to impair both ductility and toughness. In ad-; dition to 3.8% to 4.9% copper and i.2 to 1.8% magnesium, alloy 2024 contains 0.3 to 0.9% manganese, with iron up to 0.50~, silicon up to 0.50%, chromium up to 0.10%, zinc up to 0.25%, others .05% max. each and 0.15% total, balance aluminum. Thus, there are three major sources of second phase particles: the minor addition element Mn, undis-solved soluble phases containing Cu and Mg, and trace elements such as Si and Fe. Alloy 2124 is similar, except for lower maximum amounts of silicon and iron.
As far as the reasons for originally including man-ganese in 2024 alloy are concerned, it seems to have been generally recognized that Mn ties up Fe and so avoids the pre-sence of long FeA13 needles which tend to cause embrittlement.
Manganese may also reduce the amount of Cu tied up by Fe, 1~)4Z686 and so increase the availability of Cu for precipitation. The presence of manganese also tends to inhibit recrystallization, although in other systems it is less effective in this re~pect than 'Cr, or Zr. Further, manganese contributes to strength of the alloy in two ways, in that the portion in solid solution has a direct influence and its effect on recrystallization may also result in increased strength due to the presence of substructure.
In connection with previous work on the Al-Zn-Mg-Cu system, especially 7075 alloy, it has been noted that various properties 10 are improved by using prolonged solution heat treatments and limi-ting the occurrence of second phases by using higher purity (low Si, Fe, and Mn) aluminum base metal. In trying to apply the same general approach to producing a homogeneous 2024-type alloy, how-ever, it became apparent that use of high purity aluminum and pro-longed homogenization alone would not suffice.
As regards Al-Cu-Mg alloys in particular, it has been deter-mined in accordance with the present invention that limitations must be imposed specifically on the major alloying elements copper and magnesium, in order to achieve the desired results.' mus, a 20 fir~t aspect of the invention is providing an aluminum base alloy containing copper and magnesium essentially within thëir solubility limit, as indicated by a solidus temperature (incipient melting point) of at least about 945 F. or higher in homogenized condition.
This criterion alone distinguishes the alloys of the present inven-tion from commercial grades of 2024 alloy, having a solidus temper-ature of about 935F. (Metals Handbook, Vol. 1, 8th Edition, page 938).

1~4Z686 A second aspect of the invention involves con-tro~ling the impurity and minor alloying elements to an ext~nt sufficient to make possible the attainment of sub-stantially single phase structure. As used herein, a wrought article has "substantially single phase structure"
- whr~n its VPSP (volume percent second phase) does not ex-ceed 1~ on the basis of second phase particles which are vistble and resolvable in unetched condition under optical ma~nification up to lOOOX.
As a result of preliminary work it also became ap-parent that excessive suppression of the minor alloying element manganese could lead to a loss in strength, impaired ductility, and possibly inferior stress corrosion resistance due to inability to maintain an elongated grain structure.
Accordin~y, a third aspect of thq present invention is that of providing Al-Cu-Mg alloys which contain up to about 0.4~ manganese in an amount effective approximately to saturate the matrix without producing undesirable amounts of insoluble particles. The use of chromium and/or zir- ,-2Q conium in at least partial replacement of or in additionto manganese is also contemplated.
The alloy may further include incidental amounts of silicon, iron, zinc and titanium, ordinarily in minor fractional amounts each, as hereinafter discussed in greater detail, but is substantially free of other impurities (i.e.
typically .05 max. each, 0.15 max. total).
When all of the foregoing considerations are taken into account the resulting wrought articles of Al-Cu-Mg alloy in -T8XX temper not only have suitable homogeneity and mor~ nearly isotropic properties, but also exhibit improved fracture toughness at least 50~ greater than and up to ahout double that of conventional 2024-T851 alloy.

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In summary, therefore, a wrought article of Al-Cu-Mg alloy in accordance with the present'invention is character-ized compositionally by containing up to about 5% copper and up to about 2% magnesium as the major alloying elements, and up to about 0.4% manganese, within limits effective to - achieve a solidus of 945F. or higher in homogenized condi-tion: and is further characterized and distinguished by sub-stantially single phase structure. Such articles advantageous-ly are made of alloy compositions affording a yield strength of at least 45,000 psi in -T8XX temper, preferably at least 52,000 psi. Plate products of this type may exhibit a short transverse'yield strength of about 45 to 60 Ksi, and up to 10% elongation.
Improved alloy compositions of the present inven-tion may include silicon and iron in amounts up to about0.2% each (typically about .05-0.3% total), as well as minor alloying additions and incidental elements including, for example, up to about 0.2~ titanium, up to about 0.2~ chromium, up to about 0.4% manganese, up to 0.25% or even somewhat more zinc, and up to about 0.25% zirconium; but all of these elements and other impurities ordinarily will not exceed 1%
in the aggregate. Such alloys contain at least about 3%
total of copper and magnesium, including at ~east about 0.6~
magnesium, in order to achieve adequate strength, preferably about 4-5.5% total of Cu and Mg when maximum strength is desired, with copper typically in the range of about 3-4.5%
by weight. Optimum alloys are those containing at least 1.2% magnesium and less than 3.8~ copper, preferably about

2.9-3.7% Cu and about 1.3-1.7% Mg, thus further distinguish-ing over conventional 2024 alloy~
The accompanying Figs. 1-6 are graphical representa-tions of certain data discussed in the examples; and Fig. 7 shows a comparison of alloy microstructures hereinafter described.
The following exemplary practices of the inven-tion are provided for purposes of illustration.
Example 1 Ingots of various Al-Cu-Mg alloys prepared for processing in accordance with method aspects of the pre-sent invention were semicontinuously cast using a "CC"
mold. The majority were 50 lb. ingots with a 3 X 8" cross section. A few 400 lb. ingots (4 X 14") were also used.
Differential Thermal Analysis (DTA) samples were taken from replicate chemical analysis buttons. Chemical analyses were performed spectrochemically and the results are presented in Table I.
The ingots were homogenized in dry air (about -40F Dew Point) for 48 hours at about 10F below the incipient melting point, as determined by DTA techniques.
After homogenization 1/8" was scalped from the two large surfaces of each ingot. The ingots were preheated to 800F
for rolling. Reductions of 1/4" per pass were taken, with reheating to 800F after four passes or when the tempera-ture dropped to 650F.
The final heat treatments involved solution heat treatment for 5 hours at about 10 below the melting point (DTA basis), and cold water quenching. After a one day incubation at room temperature, portions of the plate were stretched 2% or 8%. The 2% stretch plates then were aged for 12 hours at 375F and the 8% plates for 8 hours at 375F. A controlled heating rate of 25F/hr. was used to reach the aging temperature. Tensile (standard .505" for L and LT and compact specimens for ST) and compact ten-sion KIC ~pecimens were obtained.

Mechanical properties are listed in Table II, fo~
the compositions indicated by asterisk in Table I.

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TABLE II
Mechanical Properties of Heat Treated Alloys %

S# Dir. Stretch UTS Y.S. O/G El. Cu M~ Mn Fe zr ST 2 55.6 44 11.9 ST 8 62.4 55.6 9.3 3.83 .65.29 .17 ~.01 24261L 2 55.8 45.5 12.1 L 8 62.8 58.3 11.4 ST 2 51.3 39.4 16.6 24262ST 8 54.0 45.9 12.7 3.83 .29 .30 .09 <.01 L 2 51.4 40.8 15.7 L 8 51.0 40.8 14.8 ST 2 55.7 43.7 15.4 24353ST 8 63.9 55.5 12.9 3.87 .65.19 .03 .16 L 2 55.1 45.2 15.5 L 8 63.4 58.4 14.0 L 2 64.9 55.3 10.0 23094L 8 72.6 68.5 9.3 3.87 1.04 .17 .01 <.01 L 2 62.6 49.1 14.8 LT 2 60.8 44.8 12.9 24742ST 2 62.6 46.7 11.0 3.94 .96.15 .02 .14 L 8 65.2 53.0 12.7 LT 8 66.2 52.3 9.1 ST 8 64.6 50.4 7.7 At 0.65~ Mg content it may be noted with reference to Table II that the Zr-containing a~loy (S~24353) is as strong as the Zr-free version (S#24261), but exhibits even greater elongation, perhaps due partly to its lower iron content. Also of interest is the relatively high shor~
transverse (ST) elongation obtained in both instances.
At the 1% Mg level the Zr-free alloy had higher strength. On the basis of hardness, however, the effect of Zr does not seem significant on either the rate of aging or the peak-hardness.
Fracture toughness tests of 3/4-inch plate in -T8XX tempér ~2% stretch) indicated that these alloys were so tough as to require plate thicknesses of between 1.5 and 1.75 inches for dependable results. Other tests (8% stretch) provided KQ values considered to be within 10% of the true KIC, and indicating essentially a doubling of toughness over 2024 alloy. Fracture toughness results were determined in accordance with ASTM practice E399-70T, Part 31, May 1970, pages 911-927.
Although the data given in Table I on incipient -' ,melting points include results for certain compositions in cast condition, and for others after homogenizing treat-ment, the latter approach has been found more reliable and more definitive in distinguishing the compositions effective for purposes of the present invention. Such compositions are susceptible to solution treatment at temperatures higher than conventional 2024 alloy.
The following additional data summarize the results of various tests on laboratory produced materials, and also on five plant-produced heats.
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/o Example 2 I. Chemistrv and Processinq a. LaboratorY Heats Several lots of laboratory produced materials were prepared from 4 x 14 x 72 inch ingots of the following compositions:
S# Cu Fe Si Mn Mq Zn Ni Cr Ti zr 26497 3.42 .07.06 0.30 1.25 ~.02 <.02 <.02 .02 ~ .02 26498 3.60 .06.05 0.29 1.23 .02 <.02 C.02 .02 0.11 26499 3.46 .07.04 <.02 1.18 .03 <.02 <.02 .01 0.10 28137 2.68 .08.04 0.28 0.99 .01 <.01 <.01 .02 ~.01 28138 2.76 .08.04 0.28 1.37 .01 <.01 <.01 .02 ~.01 These ingots were stress relieved for 24 hours at 550 F.
and then cut into 24 inch long sections. After homogenizing in dry air (about -40 F dew point) for 48 hours at 925 F., the ingot sections were scalped to 3.25 inch thickness and hot rolled at 800 F. Reductions of 1/4 inch per pass were taken, with reheating to 800 F. after four passes or when the temperature dropped to 650 F. Except as otherwise noted these materials were finished at 1.25 inch thickness.
b. Plant Heats Five plant produced heats were cast as 16 x 60" ingots.
The first three involved alloy variations without zr (-A) and with zr (-B). The chemical compositions of these ingots were:
Lot #Cu Fe Si Mn Mq Ti Zr I-A 3.76 0.12 .06 0.33 1.33 .02 --I-B 3.66 0.12 .06 0.29 1.28 .02 .06 II-A 3.56 0.12 .05 0.29 1.39 .02 --II-B 3.66 0.12 .06 0.29 1.28 .02 .06 III-A3.34 0.09 .04 0.37 1.11 .02 --III-B3.25 0,09 .05 0.68 1.04 .01 .06 ~V-A 3.12 0.09 .03 0.38 1.31 .01 --V-A 3.37 0.08 .03 0.33 1.43 .01 --The casting and scalping operations in-volved normal 2024 practice. Lots I-A and I-B were homo-genized for 48 hours at 935F. (about 10 above the normal 2024 ingot homogenizing temperature) All subsequent lots were homogenized for 16 hours at 940 to 960F. The ability of these alloys to be treated at such high temperatures greatly contributed to their low second phase content and high fracture toughness.
Special care was exercised to cool the ingots directly from homogenization temperature to the hot rolling temperature (approximately 890F.) to minimize the repreci-pitation of second phase particles. Rolling to either 3 or 6 inch plate thickness involved normal 2024 practices.
Final solution heat treatment was performed at 910 to 925F.
for 3 hours. The plates were stretched immediately after quenching. For Lot I, both 2 and 6% stretch were used and the final aging time at 375F. was adjusted according to the level of stretching (See Table VI). For the remaining four heats (Table VII) only 3" plate, 2% stretch, and a final age of 12 to 14 hours at 375F. were employed.
II. Mechanical Properties and Fracture Toughness a. 1.25" Laboratory Plate Tables III and IV present additional data for laboratory produced heats of the subject alloy, for compositions indicated by a double or triple asterisk in Table I. Typical results for laboratory material are sum-marized in Figure 1, compared to 2024 alloy.
b. Thin gage ~ Kc values and Kahn Tear Testing While the majority of testing concerned plate - 30 products, some testing of thin gage material was conducted.

.

~)42686 Eight sheets of the first two compositions (S~26497 and -498) were rolled to .080 inch for plane stress fracture toughness tests, by hot rolling as noted above to a thick-ness of 1/4 inch, etching and cold rolling to .080 inch.
With some exceptions (as noted) the subse-quent thermal treatment of these materials involved a 24 hour solution heat treatment ~t~ ~. and cold water quench-ing. After a one day room temperature incubation the samples were stretched. Those pieces stretched 2% were aged 12 hours at 375F., and those stretched 8% were aged 8 hours at 375F.
A 25F./hr. heating rate was used in all cases.
The results of .080 gage fracture toughness tests indicate that:
- (a) with 2~ stretch the alloy is as tough as 2024-T3, but has about 20 Ksi higher yieId strength;
(b) with 8% stretch its strength and toughness are comparable to 7475-T61;
(c) with 8% stretch there is strength comparable to 2024-T86, as well as about double its toughness.
* * *
Xahn tear tests in both the L-T and T-L
directions also showed good toughness (see Table V) and only slight directionality of properties.
- 25 c. Plant Produced Plate.
Results for the first plant heat with two plat~ thicknesses and two levels of stretch are presented in Table VI. Although a substantial improvement occurred ~ith respect to 2024, the full potential of the alloy system was not realized until the higher ingot homogenization . . .

practice was employed, as for the four subsequent heats, cf. Table VII.
III. Fatigue Fatigue results, for material taken from Lot S II-A, are presented in Figures 2-5. These results indicate that the alloy is comparable to 2024 or 2124 in this property.
For the short transverse direction, in laboratory produced material, the alloy has even higher fatigue resistance than 2024 or 2124 ~cf. Figure 6), particularly for the low cycle (high stress) rangè.
IV. Elevated Temperature Stability The effect on residual strength of the alloy (Lot I-A and -B) after exposure for 100 hours at 400F. is presented in Table VIII. This property is important parti-cularly for supersonic aircraft where air friction can cause temperatures to about 275F. It appears that the present alloy has a comparable advantage regarding thermal stability as 2024 with respect to the 7000 series high strength alloys.
V. Stress Corrosion.
2024-T851 specifications require passing 30 days of alternate immersion in a 3% NaCl solution at 50% of the yield strength, in the ST direction. For alloys of the present invention in its preferred operating range (4.0-5.5 total of copper and magnesium, including at least 1.2% Mg and less than 3.8~ Cu) most specimens pass a 90 day exposure (cf. Table VII). Many samples also pass testing at 75% of the ST yield strength. The Zr-free alloy apparently is superior to the Zr-containing alloy in stress corrosion resistance.

~042686 TABLE I II
Fracture Toughness of 1.25" Laboratory Produced Plate Aging KQ
% Time Direc- uTS YS % (KsiJ~in.) S # VPSP Stretch 375 F tion (Ksi) (Ksi) El. L-T T-L

L 66.1 59.710.3 54.0 **25938 .41 2 12 LT 65.8 58.6 9.0 52.0 L 68.1 62~510.5 49.8 **25942 .46 2 12 LT 67.9 61.6 8.5 48.6 L 70.5 63.6 8.7 42.1 **25943 .23 2 12 LT 70.6 62.9 6.8 45.3 L 76.5 73~9 7.5 35.9 **25983 .44 8 8 LT 76.2 71.1 5.0 26.8 ** For composition see Table I

~04Z686 TABLE IV

Mechanical Properties and Fracture Toughhess of Laboratory Produced Alloys with Different zr and/or Mn Additions Direc- L-T or (T-L) S # % zr % Mn tion UTS YS % El. IC VPSP
L 70.0 65.9 7.3 48.1 25499 .10 .02 LT 66.6 61.1 7.2 (40.2) .11 ST 67.5 61.5 5.7 N.D.

L 69.9 64.0 8.8 44.3 ***25939 .02 .28 LT 69.4 62.3 7.9 N.D. .22 ST 68.9 61.6 6.0 N.D.

L 70.2 65.9 7.4 37.4 26498 .11 .29 LT 69.9 65.1 7.1 (35.9) .53 ST 70.5 65.1 6.3 N.D.

*27913 .11 .29 LT 68.7 62.5 7.3 (45.8) .14 * Material originally from ingot #26498 but homogenized an extra 24 hours at 925F. to decrease the VPSP.
*** For composition see Table I.

1~2686 TABLE V
Kahn Tear Test Data MD-148 Compared to Other High Strength Alloys (0.090") Yield Direc- % Strength UTS %
AlloY tion Stretch (Ksi) (Ksi) El. UIE UPE UTE T/Y
L-T 2 66.7 70.1 8.0 605.7 429 1034.6 1.30 T-L 2 63.1 68.3 8.0 496.7 531.8 1028.5 1.32 S#26497 L-T 8 71.4 73.8 7.0 362.3 215.3 577.6 1.03 (w/o Zr) T-L 8 68.3 72.4 6.0 398.7 218.8 617.5 1.17 L-T 2 64.8 69.9 8.0 467.5 446.6 914 1.30 S#26498 T-L 2 64.2 69.5 8.0 555.8 559.4 1115.2 1.35 (with zr) L-T 8 71.5 75.1 7.7377.0 276.4 653.4 1.11 T-L 8 68.0 72.9 7.0 346.8 211.0 557.8 1.12 2024-T3 LJT O 52.4 69.6 19.5 710 1.46 T-L 0 46.4 67.4 19.7 600 1.59 L-T 0 74.7 79.4 13.0 569.9 369.8 939.6 1.24 7475-T61 T-L 0 73.3 80.2 13.0 505.0 227.1 732.2 1.21 L-T 0 60.8 70.2 13.0 739.3 796.2 1548 1.56 7475'T761 T-L 0 59.7 70.7 12.5672.1 590.2 1262 1.54 - (1) Data of Kaufman and Holt - Fracture Characteristics of Aluminum Alloys (Paper No. 18).

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p ~ o o o o o o o O o 1~42686 TABLE VII
Strength, Toughness and Stress Corrosion Resistance of MD-148 Plant Heats (Lots II-V) AlloY ~ion Yield Tensile /O El. ( ) 50/O 75%
L 63.2 69.2 6.5 37.8 3 NF

II-A LT 62.6 69.2 5.5 34.0 2NF
ST 59.4 66.0 5.7 28.6 90D
L 59.6 67.2 9.5 42.8 3 NF
II-B LT 60.5 66.7 7.5 29.2 90D 6, 7, 10 ST 59.0 65.0 4.0 24.2 L 58.4 65.5 10 (47,3) LT 57.9 65.2 8.3 36.0 ST 57.1 63.5 6.0 27.1 3, 3, 4 2, 2, 2 L 55.9 61.6 11 38.2 III-B LT 55.2 61.6 8.8 30.7 ST 55.4 60.9 5.7 23.2 ~27, 78, ~ 4, 7, 30 L 60.4 65.5 10 41.3 IV-A LT 60.6 66.5 8 33.8 ST 59.9 66.7 8 28.0 3~F 3NF
L 61.6 67.6 8.0 40.5 V-A LT 61.2 67.6 7.3 33 ST 58.6 65.5 6.0 27.9 3NF 3NF

TABLE VIII
Effect of 100 Hr. Exposure at 400F.
on Mechanical Properties; Testing at Room Temperature .
UTS YS %
Alloy (Ksi) (Ksi)El.
I.-A (Before Expo~
sure) 68.963.1 6.5 I-A (After Expo- 61.7 51.7 7.0 sure) I-B (Before Expo- ' : sure) 69.2 62.99.0 I-B (After Ex-posure) 59.2 48.0_10.3 2124 After Ex-posure` 62.2 50.77.7 ; 2024-T86* - 52.

* ASTM Publication 291 Finally, in Fig. 7, a compari~son is shown bet-ween the microstructure of conventional 2024 alloy and a substantially single phase structure obtained in ac-cordance with the present invention.
-~ 5 In conclusion, it has been found that alloy MD-148 (X2048) consisting essentially of aluminum, about 2.9-3.7%
copper, about 1.3-1.7% magnesium and about 0.1-0.4% man-ganese, adapted for homogenizing treatment at 940-960F., is useful in making wrought articles such as hot rolled I0 plate which, in -T8XX temper, exhibit better strength than -2219 alloy and better fracture toughness than 2024 and 2124 alloys, particularly the combination of a short trans-verse yield strength of at least 55 Ksi and an L-T plane strain fracture toughness value of at least 35 Ksi~~n., compared to typical values of 22.5 for 2024 alloy and 28 ' for 2124 alloy.
While present preferred embodiments of the inven-tion have been described it will be recognized that the in-vention may be otherwise variously embodied and practiced within the scope of the following claims.

Claims (18)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In the art of processing an aluminum base alloy con-taining copper and magnesium as the principal alloying elements by weight, including casting the alloy to form an ingot, the method which comprises:
providing an ingot composed of an alloy consisting essentially of aluminum, copper and magnesium, in amounts up to about 5% by weight copper and up to about 2% by weight magnesium not exceeding their limit of solubility, including at least 0.3% by weight magnesium and at least 3% by weight total of copper and magnesium, with no more than one percent by weight total of minor alloying elements and incidental impurities from the group consisting of silicon, iron, manganese, chromium, zinc, titanium and zirconium, including about .05-0.3% by weight total of silicon and iron, others up to about 0.4% by weight in the case of manganese, up to about 0.25% by weight in the case of zinc and zirconium, and up to about 0.2% by weight in the case of chromium and titanium, said alloy being further characterized by a solidus temperature of at least 945°F. or higher in homogenized condition;
heating said ingot to homogenize the alloy:
and hot rolling the ingot to obtain a wrought article exhibiting substantially single phase structure, having a solid solution matrix of copper and magnesium in aluminum and a volume percent of second phase particles not exceed-ing one percent based on particles that are visible and resolvable in unetched condition under optical magnification up to 1000X.

2. The method of claim 1, wherein said processing includes solution heat treating the wrought article.

3. The method of claim 1 wherein said processing includes solution heat treating and artificially aging the wrought article.

4. The method of claim 3 wherein said processing includes cold working the solution treated article at least 1 1/2% prior to said aging treatment.

5. The method of claim 1 including homogenizing the ingot at about 940-960°F.

6. The method of claim 5 including cooling the homogenized ingot directly to hot rolling temperature.

7. An aluminum base alloy in the form of plate prepared by casting and rolling the alloy, including hot rolling the casting, said plate exhibiting substantially single phase structure, having a solid solution matrix of copper and magnesium in aluminum and a volume percent of second phase particles not ex-ceeding one percent based on particles that are visible and re-solvable in unetched condition under optical magnification up to 1000X, said alloy consisting essentially of aluminum, copper and up to 2% by weight magnesium, including about 4-5.5% by weight total of copper and magnesium, at least 1.2% by weight magnesium and less than 3.8% by weight copper, and containing by weight zero to 0.4% manganese, zero to 0.25% zirconium, zero to 0.2% chromium, zero to 0.2% titanium, zero to 0.2% silicon, zero to 0.2% iron, zero to 0.25% zinc, and having a solidus temperature of at least 945°F. or higher in homogenized condition.

8. The article of claim 7, said alloy containing about .05 - 0.3% by weight total of silicon and iron.

9. The article of claim 7, said alloy containing about 2.9 - 3.7% by weight copper, about 1.3 - 1.7% by weight magnesium and about 0.1 - 0.4% by weight manganese.

10. A solution heat treated, cold worked and artificially aged aluminum base alloy in -T8XX temper, exhibiting substantially single phase structure having a solid solution matrix of copper and magnesium in aluminum and a volume percent of second phase particles not exceeding one percent based on particles that are visible and resolvable in unetched condition under optical magni-fication up to 1000X; said alloy consisting essentially of aluminum, about 0.6 - 2% magnesium by weight, about 4 - 5.5% by weight total of copper and magnesium, and up to about 0.4% by weight manganese in an amount effective substantially to saturate its matrix and having a solidus temperature of at least 945°F. or higher in homogenized condition.

11. The alloy of claim 10 containing at least about 1.2% by weight magnesium and less than 3.8% by weight copper.

12. The alloy of claim 10 containing about 2.9 - 3.7% by weight copper and about 1.3 - 1.7% by weight magnesium, in the form of plate having an L-T plane strain fracture toughness of at least 35 Ksi in. and a short transverse yield strength of at least 55 Ksi.

13. The alloy of claim 10 containing about .05 - 0.3% by weight total of silicon and iron.

14. The method of claim 1 including hot rolling the ingot to a plate thickness of about 3 inches.

15. The method of claim 1 including hot rolling the ingot to a plate thickness of about 6 inches.

16. The method of claim 4 including hot rolling the ingot to plate thickness, wherein said cold working includes stretching the plate about 2%.

17. The method of claim 4 including hot rolling the ingot to plate thickness, wherein said cold working includes stretching the plate about 6%.

18. The method of claim 4 including hot rolling the ingot to plate thickness, wherein said cold working includes stretching the plate about 8%.