of the known alcu or alcumg alloys those with high hot strength exhibit either low strength at room temperature , for example aa 2219 - t6 , or low toughness e . g . aa 2618 - t6 . the alloy of the invention on the other hand exhibits both high strength at room temperature and high hot strength , and also good toughness . the alloy of the invention can be used above all in constructional parts which are highly stressed as for example occurs in aircraft constructions . the evaluation of numerous trials with constructional parts made of aluminum alloys with copper as the main alloying element has led to the knowledge that , by introducing a material dependent construction factor s k in keeping with the equation a is a weighting factor between 2 and 2 . 5 which relates to the construction , and a material can be characterized with respect to its suitability for highly stressed constructional parts . this empirical construction factor s k shows , in the case of the alloy of the invention , an extreme dependence on the copper content , and is likewise affected by the cadmium content . the maximum value of s k as a function of the copper content , representing the optimum combination of strength and toughness , lies at a copper content of about 4 %. for economic reasons a copper content of less than 4 % is of no interest as the time required for aging is too long . on the other hand , at a copper content of more than approx . 4 . 7 % s k drops markedly . the practical , useful copper range lies therefore between 4 and 5 %. raising the cadmium content likewise increases the strength of the alloy without decreasing the toughness ; the upper cadmium limit of 0 . 2 % is determined by the tendency towards hot tearing at high cadmium contents and by the marked diminution in corrosion resistance . to achieve higher strength values , it has been found useful to limit both the iron and silicon contents to 0 . 5 % max ., preferably to 0 . 17 %. strength and toughness -- the latter expressed as the crack propagation energy -- show a pronounced dependence on the temperature and duration of artificial aging . there is therefore the possibility to change , within certain limits , the combination of strength and toughness ( expressed as the construction factor s k in terms of the above equation ) by appropriate choice of temperature and duration of artificial ageing . thermomechanical treatments play a role here too . at a first approximation manganese , zirconium and vanadium have no effect on the strength . manganese , zirconium and vanadium however increase the hot strength and creep resistance of the alloy of the invention . this is due to the thermally stable aluminides formed by the elements mn , zr and v . the particle diameter of these aluminides is between 0 . 1 and 1 μm . they increase the toughness very markedly in that they improve slip inside the grains and inhibit grain growth . the alloy of the invention , like all alloys of the alcu type , exhibits a certain susceptibility to pitting corrosion . the resistance to stress corrosion cracking depends greatly on the heat treatment given i . e . from the age hardening treatment . it was therefore found that the resistance to stress corrosion is also very satisfactory in the air cooled condition i . e . after slow cooling from the solution treatment temperature and artificial aging for 2 to 30 hours , preferably 15 to 26 hours at 170 ° to 195 ° c . it is known that the extrudability of alcu alloys , in particular alcumg alloys , is much poorer than that of the easily formable alloys such as , for example , alznmg alloys . it was therefore for the expert fully unexpected , when the alloy of the composition in keeping with the invention was found to exhibit a deformation behavior in terms of extrudability i . e . both in formability and resistance to deformation which is comparable with that of alznmg alloys . this opens up a broad field of application for the alloy of the invention in areas utilizing highly stressed constructional parts . a further advantage over the known alcu alloys lies in the possibility of extrusion welding , which -- in combination with the good formability -- permits the manufacture of complicated hollow sections via the extrusion process . the advantages of the alloy of the invention will now be explained in greater detail with the help of four examples . four series of alloys a , b , c and d with different copper contents in the range 2 . 0 to 5 . 5 % were prepared keeping the concentrations of cd , mn and zr constant in each of the series . the four series are listed in table i . table i______________________________________cu cd mn zr______________________________________a 2 . 0 - 5 . 5 % 0 . 05 % 0 . 50 % 0 . 20 % b 2 . 0 - 5 . 5 % 0 . 10 % 0 . 50 % 0 . 20 % c 2 . 0 - 5 . 5 % 0 . 15 % 0 . 50 % 0 . 20 % d 2 . 0 - 5 . 5 % 0 . 15 % 0 . 10 % 0 . 10 % ______________________________________ the alloys were solution treated at 530 ° c . for 6 hours , quenched into water at room temperature and then artificially aged at 190 ° c . to maximum hardness . on the copper content c cu for the four series of alloys , all of which were artificially aged at 190 ° c . to maximum hardness . it can be seen from fig1 that at a constant copper content , increasing the concentration of cadmium , manganese and zirconium increases the s k factor . it is also clear that the maximum permissible copper content which provides a favorable combination of strength and toughness , lies at about 5 %. extrusion billets 216 mm in diameter and 410 mm in length were cast with an alloy composition in accordance with the invention , and in an alloy of the type aa 2017 . the compositions of both alloys are given in table ii . the billets were then extruded to a section of cross section 200 mm × 4 mm . table ii______________________________________ cu cd mg mn zr______________________________________alloy acc . to invention 4 . 5 % 0 . 15 % 0 . 50 % 0 . 20 % aa 2017 4 . 1 % 0 . 5 % 0 . 5 % ______________________________________ while the alloy in keeping with the invention could be extruded without problem with an extrusion force of 225 bar -- the exit speed of the section was 5 m / min -- the alloy aa 2017 could not be extruded , in spite of raising the applied force to 270 bar . the hot strength and creep resistance of an alloy of the invention with the composition given in table ii were measured with the material in the heat treated t6 condition . for this the conventional testing of the 0 . 2 % proof stress r p0 . 2 1000h after 1000 hours at the testing temperature , and the creep fracture strength r m 1000h after 1000 hours of loading at the testing temperature were measured . values for the alloys aa 7075 - t6 and aa 2618 - t6 were taken from the technical literature for comparison purposes . table iii______________________________________ r . sub . p0 . 2 . sup . 1000h ( n / mm . sup . 2 ) 150 ° c . 200 ° c . 250 ° c . ______________________________________aa 2618 - t6 320 220 180aa 7075 - t6 270 150 80alloy acc . toinvention 340 200 160______________________________________ table iv______________________________________ r . sub . m . sup . 1000h ( n / mm . sup . 2 ) 150 ° c . 200 ° c . 250 ° c . ______________________________________aa 2618 - t6 250 140 80aa 7075 - t6 170 60 40alloy acc . toinvention 250 150 100______________________________________ three versions of the heat treatment condition t6 ( as shown in table v ) were carried out on an alloy of the invention with the composition given in table ii . table v______________________________________ artificial age hardening______________________________________a 175 ° c ./ 8 hb 190 ° c ./ 8 hc 160 ° c ./ 48 h______________________________________ a known stress corrosion test with elastically bent samples was then carried out with the material which had been heat treated this way . the load applied during the test was in each case 0 . 75 × r p0 . 2 . fig2 shows the lifetime ( in days ) reached by the samples as a function of the applied load 0 . 75 × r p0 . 2 . each point represents the average of 10 samples ; an arrow indicates that no fracture occurred after the maximum test period of 90 days . for comparison , the range of scatter for the alloys aa 7075 - t6 and aa 2014 - t6 , obtained from technical literature , is also shown ( applied load r ). it can be seen clearly in fig2 that the alloy of the invention exhibits greater resistance to stress corrosion cracking than the alloys aa 7075 and aa 2014 .