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 , it is 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 20 cu 2 mn 3 and al 2 mg 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 . aluminum - lithium clad products may be processed in accordance with the invention . such clad products utilize a core of a lithium - containing aluminum base alloy and a cladding of higher purity alloy which protects the core . the cladding on the core may be selected from aluminum association alloys 1100 , 1200 , 1230 , 1135 , 1235 , 1435 , 1145 , 1345 , 1250 , 1350 , 1170 , 1175 , 1180 , 1185 , 1285 , 1188 , 1199 or 7072 . the core material can be aa 2090 or 2091 . 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 . 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 ° to 1050 ° 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 otherwise subjected to working operations to produce stock such as sheet or plate or other stock suitable for shaping into the end product . to produce a sheet or plate - type product , a body of the alloy is preferably hot rolled to a thickness ranging from 0 . 1 to 0 . 25 inch for sheet and 0 . 25 to 6 . 0 inches for plate . for hot rolling purposes , the temperature should be in the range of 1000 ° f . down to 750 ° f . preferably , the metal temperature initially is in the range of 900 ° to 975 ° f . when the intended use of a plate product is for wing spars where thicker sections are used , normally operations other than hot rolling are unnecessary . where the intended use is wing or body panels requiring a thinner gauge , further reductions as by cold rolling can be provided . such reductions can be to a sheet thickness ranging , for example , from 0 . 010 to 0 . 249 inch and usually from 0 . 030 to 0 . 10 inch . if a clad material is being produced , the alloy material is first affixed to the ingot prior to the rolling steps . after rolling a body of the alloy to the desired thickness , the sheet or plate or other worked article is subjected to a solution heat treatment to dissolve soluble elements . the solution heat treatment is preferably accomplished at a temperature in the range of 900 ° to 1050 ° f . and produces either a recrystallized or an unrecrystallized grain structure . solution heat treatment can be performed in batches or continuously , and the time for treatment can vary from hours for batch operations down to as little as a few minutes for continuous operations . basically , solution effects can occur fairly rapidly , for instance in as little as 30 to 60 seconds , once the metal has reached a solution temperature of about 950 ° to 1050 ° f . however , heating the metal to that temperature can involve substantial amounts of time depending on the type of operation involved . in batch treating a sheet product in a production plant , the sheet is treated in a furnace load and an amount of time can be required to bring the entire load to solution temperature , and accordingly , solution heat treating can consume one or more hours , for instance one or two hours or more in batch solution treating . in continuous treating , the sheet is passed continuously as a single web through an elongated furnace which greatly increases the heat - up rate . the continuous approach is favored in practicing the invention , especially for sheet products , since a relatively rapid heat up and short dwell time at solution temperature is obtained . accordingly , the inventors contemplate solution heat treating in as little as about 1 . 0 minute . as a further aid to achieving a short heat - up time , a furnace temperature or a furnace zone temperature significantly above the desired metal temperature provides a greater temperature head useful in reducing heat - up times . 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 ° f . per second from solution temperature to a temperature of about 200 ° f . or lower . a preferred quenching rate is at least 200 ° f . per second in the temperature range of 900 ° f . or more to 200 ° f . or less . after the metal has reached a temperature of about 200 ° 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 . when the use of the sheet product is aircraft wing or body panels , the sheet product is first subjected to a thermal treatment prior to stretching and aging ( sometimes referred to as preaging ). it is this thermal treatment which is so important in the present invention in preventing any substantial development of luder &# 39 ; s lines during stretching or forming . this preaging treatment must be carried out at a temperature sufficiently low such that it does not degrade the properties of the sheet product after the final aging treatment . thus , preferably , the preaging treatment is carried out at a temperature of less than 270 ° f . and greater than 150 ° f ., e . g ., 180 ° f . it is believed that for magnesium - containing aluminum - lithium alloys , the temperature can be even lower . for example , for aa2091 , the temperature may be as low as 125 ° f . with longer times , e . g ., over 50 hours and as high as 100 hours or more , being required . a suitable preaging temperature is in the range of 210 ° to 250 ° f . and typically at about 230 ° f . for example , time at the preaging temperature can be as low as 6 hours with typical times being greater than 18 hours . in the case of sheet , for example , it is preferred that stretching to provide a flat product is less than 3 % and typically in the range of about 1 % to about 2 %. in some instances , it has been found that controlled cold working may be employed after solution heat treating and prior to the thermal treatment . for example , sheet or plate may be cold rolled to provide up to 5 % reduction and preferably 3 % or less , e . g ., 1 . 0 %. after the alloy product of the present invention has been stretched , it may be artificially aged to provide the required combination of fracture toughness and strength . this can be accomplished by subjecting the sheet or plate to a temperature in the range of 150 ° to 400 ° 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 45 to 85 ksi and corresponding fracture toughnesses are in the range of 100 to 25 ksi √ in . preferably , artificial aging is accomplished by subjecting the alloy product to a temperature in the range of 275 ° to 375 ° 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 ° 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 . also , while reference has been made herein to single aging steps , multiple aging steps , such as two or three aging steps , are contemplated . an aluminum alloy consisting of 2 . 2 wt . % li , 2 . 6 wt . % cu , 15 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 ° f . for 24 hours and was subsequently hot rolled and cold rolled to 0 . 063 inch thick sheet . the sheet was then cut to a length and solution heat treated in a heat treating furnace for a 20 minute soak at 1020 ° f . and then quenched in 75 ° f . water . following quench , all pieces were roller leveled to remove quench distortion . four different finishing practices were then tried on the material . two pieces were stretched 1 . 5 % directly after leveling . both pieces exhibited luder &# 39 ; s lines . four pieces were pre - aged for 24 hours at 230 ° f ., air cooled and finished two ways . two of these pieces were stretched 1 . 5 % and showed no signs of luder &# 39 ; s lines . the other two pieces were given a cold reduction of 1 % and then stretched 0 . 5 %. these also exhibited no luder &# 39 ; s lines . the last finishing practice utilized a 0 . 75 % cold reduction prior to the pre - age . these two pieces were then given the same pre - age practice and stretched 0 . 75 %. no luder &# 39 ; s lines were observed . an aluminum alloy consisting of 2 . 3 wt . % li , 2 . 7 wt . % cu , 0 . 10 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 ° f . for 24 hours and was subsequently hot rolled to 0 . 162 inch thick . samples were then cut from the sheet and cold rolled to 0 . 063 inch thick sheet by 6 inch wide . solution heat treatment was done in a heat treating furnace for 60 minutes soak at 1020 ° f . and then quenched in 75 ° f . water . following quench , three pieces were stretched immediately at three different levels , 0 . 75 %, 1 . 0 % and 1 . 5 %. all pieces exhibited luder &# 39 ; s lines following the stretching operation . two other pieces from the same heat treatment load were pre - aged at 230 ° f ., one for 24 hours and one for 100 hours and air cooled . both pieces were stretched 1 . 0 %. the piece which was pre - aged for 24 hours showed only light or very slight luder &# 39 ; s lines on one end . the piece which was pre - aged for 100 hours showed no luder &# 39 ; s lines . it will be appreciated that this thermal treatment can be applied to aluminum - lithium alloys , e . g ., aluminum association ( aa ) alloys such as 2090 , 2091 , 8090 , x8192 , x8092 and 8091 . 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 .