Aluminum-Lithium Alloys and Method of Making the Same

This invention relates to aluminum base alloy products, and more particularly, it relates to improved lithium containing aluminum base alloy products and a method of producing the same.

In the aircraft industry, it has been generally recognized that one of the most effective ways to reduce the weight of an aircraft is to reduce the density of aluminum alloys used in the aircraft construction. For purposes of reducing the alloy density, lithium additions have been made. However, the addition of lithium to aluminum alloys is not without problems. For example, the addition of lithium to aluminum alloys often results in a decrease in ductility and fracture toughness. Where the use is in aircraft parts, it is imperative that the lithium containing alloy have both improved fracture toughness and strength properties.

However, in the past, aluminum-lithium alloys have exhibited poor transverse ductility. That is, aluminum-lithium alloys have exhibited quite low elongation properties which has been a serious drawback in commercializing these alloys.

These properties appear to result from the anistropic nature of such alloys on working by rolling, for example. This condition is sometimes also referred to as a fibering arrangement, as shown in FIG. 9. The properties across the fibering arrangement are often inferior to properties measured in the direction of rolling, for example. Also, properties measured at 45.degree. with respect to the principal direction of working can also be inferior. By the use of 45.degree. properties herein is meant to include off-axis properties, i.e., properties between the longitudinal and long transverse directions, because the lowest properties are not always located in the 45.degree. direction. Thus, there is a great need to produce a lithium containing aluminum alloy having an isotropic type structure capable of maximizing the properties in all directions.

With respect to conventional alloys, both high strength and high fracture toughness appear to be quite difficult to obtain when viewed in light of conventional alloys such as AA (Aluminum Association) 2024-T3X and 7050-TX normally used in aircraft applications. For example, a paper by J. T. Staley entitled "Microstructure and Toughness of High-Strength Aluminum Alloys", Properties Related to Fracture Toughness, ASTM STP605, American Society for Testing and Materials, 1976, pp. 71-103, shows generally that for AA2024 sheet, toughness decreases as strength increases. Also, in the same paper, it will be observed that the same is true of AA7050 plate. More desirable alloys would permit increased strength with only minimal or no decrease in toughness or would permit processing steps wherein the toughness was controlled as the strength was increased in order to provide a more desirable combination of strength and toughness. Additionally, in more desirable alloys, the combination of strength and toughness would be attainable in an aluminum-lithium alloy having density reductions in the order of 5 to 15%. Such alloys would find widespread use in the aerospace industry where low weight and high strength and toughness translate to high fuel savings. Thus, it will be appreciated that obtaining qualities such as high strength at little or no sacrifice in toughness, or where toughness can be controlled as the strength is increased would result in a remarkably unique aluminum-lithium alloy product.

The present invention solves problems which limited the use of these alloys and provides an improved lithium containing aluminum base alloy product which can be processed to provide an isotropic texture or structure and to improve strength characteristics in all directions while retaining high toughness properties or which can be processed to provide a desired strength at a controlled level of toughness.

An object of this invention is to provide an aluminum lithium alloy and thermomechanical processing practice which greatly improves the short transverse properties of such alloy.

A second object of this invention is to provide an aluminum lithium alloy product and thermomechanical process for providing the same which results in an isotropic structure.

A further object of this invention is to provide a thermomechanical process which greatly improves the short transverse properties of aluminum-lithium alloys without detrimentally affecting properties in the other directions.

A principal object of this invention is to provide an improved lithium containing aluminum base alloy product.

Another obJect of this invention is to provide an improved aluminum-lithium alloy wrought product having improved strength and toughness characteristics.

Yet another object of this invention is to provide an aluminum-lithium alloy product capable of being worked after solution heat treating to improve strength properties without substantially impairing its fracture toughness.

And yet another object of this invention includes a method of providing a wrought aluminum-lithium alloy product and working the product after solution heat treating to increase strength properties without substantially impairing its fracture toughness.

And yet a further object of this invention is to provide a method of increasing the strength of a wrought aluminum-lithium alloy product after solution heat treating without substantially decreasing fracture toughness.

These and other objects will become apparent from the specification, drawings and claims appended hereto.

In accordance with these objects, disclosed is a method of making lithium containing aluminum base alloy products having improved properties particularly in the short transverse direction. The product comprises 0.5 to 4.0 wt. % Li, 0 to 5.0 wt. % Mg, up to 5.0 wt. % Cu, 0.03 to 0.15 wt. % Zr, 0 to 2.0 wt. % Mn, 0 to 7.0 wt. % Zn, 0.5 wt. % max. Fe, 0.5 wt. % max. Si, the balance aluminum and incidental impurities. The method of making the product comprising the steps of providing a body of a lithium containing aluminum base alloy and heating the body to a temperature for a series of low temperature hot working operations to put the body in condition for recrystallization. The low temperature hot working operations may be used to provide an intermediate product. Thereafter, the intermediate product is recrystallized and then hot worked to a final shaped product. After hot rolling, the product has a metallurgical structure generally lacking intense work texture characteristics normally attributable to the as-cast structure. That is, the structure is isotropic in nature and exhibits improved properties in the 45.degree. direction, for example. The final shaped product is solution heat treated, quenched and aged to provide a non-recrystallized product. Prior to the aging step, the product is capable of having imparted thereto a working effect equivalent to stretching an amount greater than 3% so that the product has combinations of improved strength and fracture toughness after aging. The degree of working as by stretching, for example, is greater than that normally used for relief of residual internal quenching stresses.

FIG. 1 shows that the relationship between toughness and yield strength for a worked alloy product in accordance with the present invention is increased by stretching.

FIG. 2 shows that the relatioship between toughness and yield strength is increased for a second worked alloy product stretched in accordance with the present invention.

FIG. 3 shows the relationship between toughness and yield strength of a third alloy product stretched in accordance with the present invention.

FIG. 4 shows that the relationship between toughness and yield strength is increased for another alloy product stretched in accordance with the present invention.

FIG. 5 shows that the relationship between toughness (notch-tensile strength divided by yield strength) and yield strength decreases with increase amounts of stretching for AA7050.

FIG. 6 shows that stretching AA2024 beyond 2% does not significantly increase the toughness-strength relationship for this alloy.

FIG. 7 illustrates different toughness yield strength relationships where shifts in the upward direction and to the right represent improved combinations of these properties.

FIG. 8 shows a metallurgical structure of an aluminum-lithium alloy processed in accordance with the invention.

FIG. 9 shows a metallurgical structure of an aluminum-lithium alloy processed in accordance with conventional practices.

FIG. 10 shows a graph of yield stress plotted against the orientation of the specimen.

FIG. 11 shows a micrograph of a typical recrystallized structure of an intermediate product at 100.times. of an aluminum alloy containing 2.0 Li, 3.0 Cu and 0.11 Zr processed in accordance with the invention.

FIG. 12 shows a micrograph taken in the longitudinal direction of a final product at 50.times. having isotropic type properties.

The alloy of the present invention can contain 0.5 to 4.0 wt. % Li, 0 to 5.0 wt. % Mg, up to 5.0 wt. % Cu, 0 to 1.0 wt. % Zr, 0 to 2.0 wt. % Mn, 0 to 7.0 wt. % Zn, 0.5 wt. % max. Fe, 0.5 wt. % max. Si, the balance aluminum and incidental impurities. The impurities are preferably limited to about 0.05 wt. % each, and the combination of impurities preferably should not exceed 0.15 wt. %. Within these limits, itis preferred that the sum total of all impurities does not exceed 0.35 wt. %.

A preferred alloy in accordance with the present invention can contain 1.0 to 4.0 wt. % Li, 0.1 to 5.0 wt. % Cu, 0 to 5.0 wt. % Mg, 0 to 1.0 wt. % Zr, 0 to 2 wt. % Mn, the balance aluminum and impurities as specified above. A typical alloy composition would contain 2.0 to 3.0 wt. % Li, 0.5 to 4.0 wt. % Cu, 0 to 3.0 wt. % Mg, 0 to 0.2 wt. % Zr, 0 to 1.0 wt. % Mn and max. 0.1 wt. % of each of Fe and Si.

In the present invention, lithium is very important not only because it permits a significant decrease in density but also because it improves tensile and yield strengths markedly as well as improving elastic modulus. Additionally, the presence of lithium improves fatigue resistance. Most significantly though, the presence of lithium in combination with other controlled amounts of alloying elements permits aluminum alloy products which can be worked to provide unique combinations of strength and fracture toughness while maintaining meaningful reductions in density. It will be appreciated that less than 0.5 wt. % Li does not provide for significant reductions in the density of the alloy and 4 wt. % Li is close to the solubility limit of lithium, depending to a significant extent on the other alloying elements. It is not presently expected that higher levels of lithium would improve the combination of toughness and strength of the alloy product.

With respect to copper, particularly in the ranges set forth hereinabove for use in accordance with the present invention, its presence enhances the properties of the alloy product by reducing the loss in fracture toughness at higher strength levels. That is, as compared to lithium, for example, in the present invention copper has the capability of providing higher combinations of toughness and strength. For example, if more additions of lithium were used to increase strength without copper, the decrease in toughness would be greater than if copper additions were used to increase strength. Thus, in the present invention when selecting an alloy, it is important in making the selection to balance both the toughness and strength desired, since both elements work together to provide toughness and strength uniquely in accordance with the present invention. It is important that the ranges referred to hereinabove, be adhered to, particularly with respect to the upper limits of copper, since excessive amounts can lead to the undesirable formation of intermetallics which can interfere with fracture toughness.

Magnesium is added or provided in this class of aluminum alloys mainly for purposes of increasing strength although it does decrease density slightly and is advantageous from that standpoint. It is important to adhere to the upper limits set forth for magnesium because excess magnesium can also lead to interference with fracture toughness, particularly through the formation of undesirable phases at grain boundaries.

The amount of manganese should also be closely controlled. Manganese is added to contribute to grain structure control, particularly in the final product. Manganese is also a dispersoid-forming element and is precipitated in small particle form by thermal treatments and has as one of its benefits a strengthening effect. Dispersoids such as Al.sub.2 OCu.sub.2 Mn.sub.3 and Al.sub.12 Mg.sub.2 Mn can be formed by manganese. Chromium can also be used for grain structure control but on a less preferred basis. Zirconium is the preferred material for grain structure control. The use of zinc results in increased levels of strength, particularly in combination with magnesium. However, excessive amounts of zinc can impair toughness through the formation of intermetallic phases.

Toughness or fracture toughness as used herein refers to the resistance of a body, e.g. sheet or plate, to the unstable growth of cracks or other flaws.

Improved combinations of strength and toughness is a shift in the normal inverse relationship between strength and toughness towards higher toughness values at given levels of strength or towards higher strength values at given levels of toughness. For example, in FIG. 7, going from point A to point D represents the loss in toughness usually associated with increasing the strength of an alloy. In contrast, going from point A to point B results in an increase in strength at the same toughness level. Thus, point B is an improved combination of strength and toughness. Also, in going from point A to point C results in an increase in strength while toughness is decreased, but the combination of strength and toughness is improved relative to point A. However, relative to point D, at point C, toughness is improved and strength remains about the same, and the combination of strength and toughness is considered to be improved. Also, taking point B relative to point D, toughness is improved and strength has decreased yet the combination of strength and toughness are again considered to be improved.

As well as providing the alloy product with controlled amounts of alloying elements as described hereinabove, it is preferred that the alloy be prepared according to specific method steps in order to provide the most desirable characteristics of both strength and fracture toughness. Thus, the alloy as described herein can be provided as an ingot or billet for fabrication into a suitable wrought product by casting techniques currently employed in the art for cast products, with continuous casting being preferred. It should be noted that the alloy may also be provided in billet form consolidated from fine particulate such as powdered aluminum alloy having the compositions in the ranges set forth hereinabove. The powder or particulate material can be produced by processes such as atomization, mechanical alloying and melt spinning. The ingot or billet may be preliminarily worked or shaped to provide suitable stock for subsequent working operations. Prior to the principal working operation, the alloy stock is preferably subjected to homogenization, and preferably at metal temperatures in the range of 900.degree. to 1050.degree. F. for a period of time of at least one hour to dissolve soluble elements such as Li and Cu, and to homogenize the internal structure of the metal. A preferred time period is about 20 hours or more in the homogenization temperature range. Normally, the heat up and homogenizing treatment does not have to extend for more than 40 hours; however, longer times are not normally detrimental. A time of 20 to 40 hours at the homogenization temperature has been found quite suitable. In addition to dissolving constituent to promote workability, this homogenization treatment is important in that it is believed to precipitate the Mn and Zr-bearing dispersoids which help to control final grain structure.

After the homogenizing treatment, the metal can be rolled or extruded or otherwise subjected to working operations to produce stock such as sheet, plate or extrusions or other stock suitable for shaping into the end product.

In the present invention, it has been discovered that short transverse properties can be improved by carefully controlled thermal and mechanical operations as well as alloying of the lithium-containing aluminum base alloy. Accordingly, for purposes of improving the short transverse properties, e.g. toughness and ductility in the short transverse direction, the zirconium content of lithium-containing aluminum base alloy should be maintained in the range of 0.03 to 0.15 wt. %. Preferably, zirconium is in the range of 0.05 to 0.12 wt. %, with a typical amount being in the range of 0.08 to 0.1 wt. %. Other elements, e.g. chromium, cerium, manganese, scandium, capable of forming fine dispersoids which retard grain boundary migration and having a similar effect in the process as zirconium, may be used. The amount of these other elements may be varied, however, to produce the same effect as zirconium, the amount of any of these elements should be sufficiently low to permit recrystallization of an intermediate product, yet the amount should be high enough to retard recrystallization during solution heat treating.

For purposes of illustrating the invention, an ingot of the alloy is heated prior to an initial hot working operation. This temperature must be controlled so that a substantial amount of grain boundary precipitate, i.e., particles present at the original dendritic boundaries, not be dissolved. That is, if a higher temperature is used, most of this grain boundary precipitate would be dissolved and later operations normally would not be effective. If the temperature is too low, then the ingot will not deform without cracking. Thus, preferably, the ingot or working stock should be heated to a temperature in the range of 600.degree. to 950.degree. F., and more preferably 700.degree. to 900.degree. F. with a typical temperature being in the range of 800.degree. to 870.degree. F. This step may be referred to as a low temperature preheat.

If it is desired, the ingot may be homogenized prior to this low temperature preheat without adversely affecting the end product. However, as presently understood, the preheat may be used without the prior homogenization step at no sacrifice in properties.

After the ingot has been heated to this condition, it is hot worked or hot rolled to provide an intermediate product. That is, once the ingot has reached the low temperature preheat, it is ready for the next operation. However, longer times at the preheat temperature are not detrimental. For example, the ingot may be held at the preheat temperature for up to 20 or 30 hours; but, for purposes of the present invention, times less than 1 hour, for example, can be sufficient. If the ingot were being rolled into plate as a final product, then this initial hot working can reduce the ingot to a thickness 1.5 to 15 times that of the plate. A preferred reduction is 1.5 to 5 times that of the plate with a typical reduction being two to three times the thickness of the final plate thickness. The preliminary hot working may be initiated at a temperature in the range of the low temperature preheat. However, this preliminary hot working can be carried out at a temperature in the range of 950.degree. to 400 .degree. F. While this working step has been referred to as hot working, it may be more conveniently referred to as low temperature hot working for purposes of the present invention. Further, it should be understood that the same or similar effects may be obtained with a series or variation of temperature preheat steps and low temperature hot working steps, singly or combined, and such is contemplated within the present invention.

After this initial low temperature hot working step, the intermediate product is then heated to a temperature sufficiently high to recrystallize its grain structure. For purposes of recrystallization, the temperature can be in the range of 900.degree. to 1040.degree. F. with a preferred recrystallization temperature being 980.degree. to 1020.degree. F. It is the recrystallization step, particularly in conjunction with the earlier steps, which permits the improvement in short transverse properties of plate, for example, fabricated in accordance with the present invention. If too much zirconium is present, then recrystallization will not occur. By the use of the word recrystallization is meant to include partial recrystallization as well as complete recrystallization.

It is believed that recrystallization, in conjunction with the low temperature preheat and the low temperature hot work, initiated at the grain boundary precipitates present at the original dendritic boundaries operate to occlude these particles, as well as segregated impurities at the dendritic boundary. Therefore, these impurities can no longer present weak sites or links for intergranular fracture. Thus, it can be seen why recrystallization must be initiated and why the control of zirconium which retards recrystallization must be controlled. That is, zirconium or its equivalent, along ith the low temperature hot working conditions, determine the nature of the recrystallized texture.

After recrystallization, the intermediate product is further hot worked or hot rolled to a final product shape. As noted earlier, to produce a sheet or plate-type product, the intermediate product is hot rolled to a thickness ranging from 0.1 to 0.25 inch for sheet and 0.25 to 10.0 inches for plate, for example. For this final hot working operation, the temperature should be in the range of 1000.degree. to 750.degree. F., and preferably initially the metal temperature should be in the range of 900.degree. to 975.degree. F. With respect to this last hot working step, it is important that the temperatures be carefully controlled. If too low a temperature is used, too much cold work can be transferred to the final product which can result in an adverse effect during the next thermal treatment, i.e., solution heat treating, as explained below.

In order to obtain improved short transverse properties, solution heat treating is performed as noted before, and care must be taken to ensure a substantially unrecrystallized grain structure. Thus, the alloy in accordance with the invention must contain a minimum level of zirconium to retard recrystallization of the final product during solution heat treating. In addition, it is for the same reason that care must be taken during the final hot working step to guard against using too low temperatures and its attendant problems. That is, unduly high amounts of work being added in the final hot working step can result in recrystallization of the final product during solution heat treating and thus should be avoided.

If it is required that the end product be less anisotropic or more isotropic in nature, i.e., properties more or less uniform in all directions, then the low temperature hot working operation can require further control. That is, if the end product is required to be substantially free or generally lacking an intense worked texture so as to improve properties in the 45.degree. direction, then the low temperature hot working operations can be carried out so as to attain such characteristic. For example, to improve 45.degree. properties, a step low temperature hot working operation can be employed where the working operation and the temperature is controlled for a series of steps. Thus, in one embodiment of this operation, after the low temperature preheat, the ingot is reduced by about 5 to 35% of thickness of the original ingot in the first step of the low temperature hot working operation with preferred reductions being in the order of 10 to 25% of the thickness. The temperature for this first step should be in the range of about 665.degree. to 925.degree. F. In the second step of the operation, the reduction is in the order of 20 to 50% of the thickness of the material from the first step with typical reductions being about 25 to 35%. The temperature in the second step should not be greater than 660.degree. F. and preferably is in the range of 500.degree. to 650.degree. F. In the third step, the reduction should be 20 to 40% of the thickness of the material from the second step, and the temperature should be in the range of 350.degree. to 500.degree. F. with a typical temperature being in the range of 400.degree. to 475.degree. F. These steps provide an intermediate product which is recrystallized, as noted earlier. A typical recrystallized structure of the intermediate product is shown in FIG. 11. For convenience of the present invention, the low temperature preheat, low temperature hot working coupled with temperature control and the recrystallization of the intermediate product are referred to herein as a recrystallization effect which, in accordance with the present invention, makes it possible to control the antistropy of the mechanical characteristics, and if desired, produce a final product isotropic in nature. While the invention has illustrated this embodiment of their invention by referring to a three-step process, it will be noted that the scope of their invention is not necessarily limited thereto. For example, there can be a number of low temperature hot working operations that may be employed to control antistropy depending on which property is desired, and this is now attainable as a result of the teachings herein, particularly utilizing the low temperature hot working operations and recrystallization of an intermediate product. The control can be even more effective if combined with small variations in composition of the aluminum-lithium alloys. For example, a two-step low temperature hot working operation may be employed. It is believed that in the three-step process, the last two steps of low temperature hot working are more important in producing the desired microstructure in the intermediate product. Or, the temperature direction may be reversed for each step, or combinations of low and high temperatures may be used during the low temperature hot working operations. These illustrations are not necessarily intended to limit the scope of the invention but are set forth as illustrative of the new process and aluminum-lithium products which may be attained as a result of the new processes disclosed herein.

To further provide for the desired strength and fracture toughness necessary to the final product and to the operations in forming that product, the product should be rapidly quenched to prevent or minimize uncontrolled precipitation of strengthening phases referred to herein later. Thus, it is preferred in the practice of the present invention that the quenching rate be at least 100.degree. F. per second from solution temperature to a temperature of about 200.degree. F. or lower. A preferred quenching rate is at least 200.degree. F. per second in the temperature range of 900.degree. F. or more to 200.degree. F. or less. After the metal has reached a temperature of about 200.degree. F., it may then be air cooled. When the alloy of the invention is slab cast or roll cast, for example, it may be possible to omit some or all of the steps referred to hereinabove, and such is contemplated within the purview of the invention.

After solution heat treatment and quenching as noted herein, the improved sheet, plate or extrusion and other wrought products can have a range of yield strength from about 25 to 50 ksi and a level of fracture toughness in the range of about 50 to 150 ksi in. However, with the use of artificial aging to improve strength, fracture toughness can drop considerably. To minimize the loss in fracture toughness associated in the past with improvement in strength, it has been discovered that the solution heat treated and quenched alloy product, particularly sheet, plate or extrusion, must be stretched, preferably at room temperature, an amount greater than 3% of its original length or otherwise worked or deformed to impart to the product a working effect equivalent to stretching greater than 3% of its original length. The working effect referred to is meant to include rolling and forging as well as other working operations. It has been discovered that the strength of sheet or plate, for example, of the subject alloy can be increased substantially by stretching prior to artificial aging, and such stretching causes little or no decrease in fracture toughness. It will be appreciated that in comparable high strength alloys, stretching can produce a significant drop in fracture toughness. Stretching AA7050 reduces both toughness and strength, as shown in FIG. 5, taken from the reference by J. T. Staley, mentioned previously. Similar toughness-strength data for AA2024 are shown in FIG. 6. For AA2024, stretching 2% increases the combination of toughness and strength over that obtained without stretching; however, further stretching does not provide any substantial increases in toughness. Therefore, when considering the toughness-strength relationship, it is of little benefit to stretch AA2024 more than 2%, and it is detrimental to stretch AA7050. In contrast, when stretching or its equivalent is combined with artificial aging, an alloy product in accordance with the present invention can be obtained having significantly increased combinations of fracture toughness and strength.

While the inventors do not necessarily wish to be bound by any theory of invention, it is believed that deformation or working, such as stretching, applied after solution heat treating and quenching, results in a more uniform distribution of lithium-containing metastable precipitates after artificial aging. These metastable precipitates are believed to occur as a result of the introduction of a high density of defects (dislocations, vacancies, vacancy clusters, etc.) which can act as preferential nucleation sites for these precipitating phases (such as T.sub.1 ', a precursor of the Al.sub.2 CuLi phase) throughout each grain. Additionally, it is believed that this practice inhibits nucleation of both metastable and equilibrium phases such as Al.sub.3 Li, AlLi, Al.sub.2 CuLi and Al.sub.5 CuLi.sub.3 at grain and sub-grain boundaries. Also, it is believed that the combination of enhanced uniform precipitation throughout each grain and decreased grain boundary precipitation results in the observed higher combination of strength and fracture toughness in aluminum-lithium alloys worked or deformed as by stretching, for example, prior to final aging.

In the case of sheet or plate, for example, it is preferred that stretching or equivalent working is greater than 3% and less than 14%. Further, it is preferred that stretching be in the range of about a 4 to 12% increase over the original length with typical increases being in the range of 5 to 8%.

After the alloy product of the present invention has been worked, it may be artificially aged to provide the combination of fracture toughness and strength which are so highly desired in aircraft members. This can be accomplished by subjecting the sheet or plate or shaped product to a temperature in the range of 150.degree. to 400.degree. F. for a sufficient period of time to further increase the yield strength. Some compositions of the alloy product are capable of being artificially aged to a yield strength as high as 95 ksi. However, the useful strengths are in the range of 50 to 85 ksi and corresponding fracture toughnesses are in the range of 25 to 75 ksi in. Preferably, artificial aging is accomplished by subjecting the alloy product to a temperature in the range of 275.degree. to 375.degree. F. for a period of at least 30 minutes. A suitable aging practice contemplate a treatment of about 8 to 24 hours at a temperature of about 325.degree. F. Further, it will be noted that the alloy product in accordance with the present invention may be subjected to any of the typical underaging treatments well known in the art, including natural aging. However, it is presently believed that natural aging provides the least benefit. Also, while reference has been made herein to single aging steps, multiple aging steps, such as two or three aging steps, are contemplated and stretching or its equivalent working may be used prior to or even after part of such multiple aging steps.

The following examples are further illustrative of the invention.

An aluminum alloy consisting of 1.73 wt. % Li, 2.63 wt. % Cu, 0.12 wt. % Zr, the balance essentially aluminum and impurities, was cast into an ingot suitable for rolling. The ingot was homogenized in a furnace at a temperature of 1000.degree. F. for 24 hours and then hot rolled into a plate product about one inch thick. The plate was then solution heat treated in a heat treating furnace at a temperature of 1025.degree. F. for one hour and then quenched by immersion in 70.degree. F. water, the temperature of the plate immediately before immersion being 1025.degree. F. Thereafter, a sample of the plate was stretched 2% greater than its original length, and a second sample was stretched 6% greater than its original length, both at about room temperature. For purposes of artificially aging, the stretched samples were treated at either 325.degree. F. or 375.degree. F. for times as shown in Table I. The yield strength values for the samples referred to are based on specimens taken in the longitudinal direction, the direction parallel to the direction of rolling. Toughness was determined by ASTM Standard Practice E561-81 for R-curve determination. The results of these tests are set forth in Table I. In addition, the results are shown in FIG. 1 where toughness is plotted against yield strength. It will be noted from FIG. 1 that 6% stretch displaces the strength-toughness relationship upwards and to the right relative to the 2% stretch. Thus, it will be seen that stretching beyond 2% substantially improved toughness and strength in this lithium containing alloy. In contrast, stretching decreases both strength and toughness in the long transverse direction for alloy 7050 (FIG. 5). Also, in FIG. 6, stretching beyond 2% provides added little benefit to the toughness-strength relationship in AA2024.

An aluminum alloy consisting of, by weight, 2.0% Li, 2.7% Cu, 0.65% Mg and 0.12% Zr, the balance essentially aluminum and impurities, was cast into an ingot suitable for rolling. The ingot was homogenized at 980.degree. F. for 36 hours, hot rolled to 1.0 inch plate as in Example I, and solution heat treated for one hour at 980.degree. F. Additionally, the specimens were also quenched, stretched, aged and tested for toughness and strength as in Example I. The results are provided in Table II, and the relationship between toughness and yield strength is set forth in FIG. 2. As in Example I, stretching this alloy 6% displaces the toughness-strength relationship to substantially higher levels. The dashed line through the single data point for 2% stretch is meant to suggest the probable relationship for this amount of stretch.

An aluminum alloy consisting of, by weight, 2.78% Li, 0.49% Cu, 0.98% Mg, 0.50 Mn and 0.12% Zr, the balance essentially aluminum, was cast into an ingot suitable for rolling. The ingot was homogenized as in Example I and hot rolled to plate of 0.25 inch thick. Thereafter, the plate was solution heat treated for one hour at 1000.degree. F. and quenched in 70.degree. water. Samples of the quenched plate were stretched 0%, 4% and 8% before aging for 24 hours at 325.degree. F. or 375.degree. F. Yield strength was determined as in Example I and toughness was determined by Kahn type tear tests. This test procedure is described in a paper entitled "Tear Resistance of Aluminum Alloy Sheet as Determined from Kahn-Type Tear Tests", Materials Research and Standards, Vol. 4, No. 4, 1984 April, p. 181. The results are set forth in Table III, and the relationship between toughness and yield strength is plotted in FIG. 5.

Here, it can be seen that stretching 8% provides increased strength and toughness over that already gained by stretching 4%. In contrast, data for AA2024 stretched from 2% to 5% (FIG. 6) fall in a very narrow band, unlike the larger effect of stretching on the toughness-strength relationship seen in lithium-containing alloys.

An aluminum alloy consisting of, by weight, 2.72% Li, 2.04% Mg, 0.53% Cu, 0.49 Mn and 0.13% Zr, the balance essentially aluminum and impurities was cast into an ingot suitable for rolling. Thereafter, it was homogenized as in Example I and then hot rolled into plate 0.25 inch thick. After hot rolling, the plate was solution heat treated for one hour at 1000.degree. F. and quenched in 70.degree. water. Samples were taken at 0%, 4% and 8% stretch and aged as in Example I. Tests were performed as in Example III, and the results are presented in Table IV. FIG. 4 shows the relationship of toughness and yield strength for this alloy as a function of the amount of stretching. The dashed line is meant to suggest the toughness-strength relationship for this amount of stretch. For this alloy, the increase in strength at equivalent toughness is significantly greater than the previous alloys and was unexpected in view of the behavior of conventional alloys such as AA7050 and AA2024.

An aluminum alloy consisting of, by weight, 2.25% Li, 2.98% Cu, 0.12% Zr, the balance being essentially aluminum and impurities, was cast into an ingot suitable for rolling. The ingot was homogenized in a furnace at a temperature of 950.degree. F. for 8 hours followed immediately by a temperature of 1000.degree. F. for 24 hours and air cooled. The ingot was then preheated in a furnace for 30 minutes at 975.degree. F. and hot rolled to 1.75 inch thick plate. The plate was solution heat treated for 2 hours at 1020.degree. F. followed by a continuous water spray quench with a water temperature of 72.degree. F. The plate was stretched at room temperature in the rolling direction with 4.9% permanent set. Stretching was followed by an artificial aging treatment of 18 hours at 325.degree. F. Tensile properties were determined in the short transverse direction in accordance with ASTM B-557. These values are shown in Table V. The ultimate tensile strength and the yield tensile strength were equal, and the resulting elongations are zero.

An aluminum alloy consisting of, by weight, 2.11% Li, 2.75% Cu, 0.09% Zr, the balance being essentially aluminum and impurites, was cast into an ingot suitable for rolling. The ingot was homogenized in a furnace at a temperature of 1000.degree. F. for 24 hours and air cooled. The ingot was then preheated in a furnace for 30 minutes at 975.degree. F. and hot rolled to 1.75 inch thick plate. The plate was solution heat treated for 1.5 hours at 1000.degree. F. and then quenched in a continuous water spray (72.degree. F.). The plate was stretched at room temperature in the rolling direction with 6.3% permanent set. Stretching was followed by an artificial aging treatment of 8 hours at 300.degree. F. Tensile properties were determined in the short transverse direction in accordance with ASTM B-557. These values are shown in Table VI. The ultimate tensile strength and the yield strength were equal, and the resulting elongations are zero.

An aluminum alloy consisting of, by weight, 2.0% Li, 2.55% Cu, 0.09% Zr, the balance being essentially aluminum and impurities, was cast into an ingot suitable for rolling. The ingot was homogenized in a furnace at a temperature of 950.degree. F. for 8 hours followed immediately by a temperature of 1000.degree. F. for 24 hours and air cooled. The ingot was then preheated in a furnace for 6 hours at 875.degree. F. and hot rolled to a 3.5 inch thick slab. The slab was returned to a furnace for reheating at 1000.degree. F. for 11 hours and then finish hot rolled to 1.75 inch thick plate. The plate was solution heat treated for 2 hours at 1020.degree. F. and continuously water spray quenched with water at 72.degree. F. The plate was stretched at room temperature in the longitudinal direction with 5.9% permanent set. Stretching was followed by an artificial aging treatment of 36 hours at 325 .degree. F. Short transverse tensile properties were determined in accordance with ASTM B-557 and are shown in Table VII. In addition to these tests, samples were cut after stretching and aged in the laboratory at 300.degree. and 325.degree. F. for various times. This data is shown in Table VIII. Regardless of the strength of the material fabricated with the standard or conventional process, the resulting elongations are zero. Material fabricated using the new process shows a clear increase in elongation with decreasing strength.

While the invention has been described in terms of preferred embodiments, the claims appended hereto are intended to encompass other embodiments which fall within the spirit of the invention.