328 CHAPTER 10 Materials
propertyofthesealloysistheirretentionofstrengthathightemperatures,whichmakesthemparticularly
suitable for aero engine manufacture. A development of these alloys by Rolls-Royce and High Duty
AlloysLtdreplacedsomeofthenickelwithironandreducedthecoppercontent;theseRRalloys,as
theywerecalled,wereusedforforgingsandextrusionsinaeroenginesandairframes.
Thethirdgroupofalloysdependsontheinclusionofzincandmagnesiumfortheirhighstrengthand
hasatypicalcompositionof2.5%copper,5%zinc,3%magnesium,andupto1%nickel,withmechanical
properties of 0.1 percent proof stress 510N/mm^2 , tensile strength 585N/mm^2 , and an elongation of
8percent.Inamoderndevelopmentofthisalloy,nickelhasbeeneliminatedandprovisionmadefor
theadditionofchromiumandfurtheramountsofmanganese.
Alloys from each of the preceding groups have been used extensively for airframes, skins, and
otherstressedcomponents,thechoiceofalloybeinginfluencedbyfactorssuchasstrength(proofand
ultimatestress),ductility,easeofmanufacture(e.g.,inextrusionandforging),resistancetocorrosion
and amenability to protective treatment, fatigue strength, freedom from liability to sudden cracking
due to internal stresses, and resistance to fast crack propagation under load. Clearly, different types
of aircraft have differing requirements. A military aircraft, for instance, having a relatively short life
measuredinhundredsofhours,doesnotcallforthesamedegreeoffatigueandcorrosionresistanceas
acivilaircraftwitharequiredlifeof30000hoursormore.
Unfortunately,asoneparticularpropertyofaluminumalloysisimproved,otherdesirableproper-
tiesaresacrificed.Forexample,theextremelyhighstaticstrengthofthealuminum–zinc–magnesium
alloys was accompanied for many years by a sudden liability to crack in an unloaded condition due
to the retention of internal stresses in bars, forgings, and sheet after heat treatment. Although varia-
tionsincompositionhaveeliminatedthisproblemtoaconsiderableextent,otherdeficienciesshowed
themselves.Earlypostwarpassengeraircraftexperiencedlargenumbersofstress-corrosionfailuresof
forgingsandextrusions.Theproblembecamesoseriousthatin1953itwasdecidedtoreplaceasmany
aluminum–zinc–manganesecomponentsaspossiblewiththealuminum–4%copperAlloyL65andto
prohibittheuseofforgingsinzinc-bearingalloyinallfuturedesigns.However,improvementsinthe
stress-corrosionresistanceofthealuminum–zinc–magnesiumalloyshaveresultedinrecentyearsfrom
British,American,andGermanresearches.BothBritishandAmericanopinionsagreeonthebenefits
of including about 1% copper but disagree on the inclusion of chromium and manganese, while in
Germany, the addition of silver has been found extremely beneficial. Improved control of casting
techniques has brought further improvements in resistance to stress corrosion. The development
of aluminum–zinc–magnesium–copper alloys has largely met the requirement for aluminum alloys
possessinghighstrength,goodfatiguecrackgrowthresistance,andadequatetoughness.Furtherdevel-
opment concentrates on the production of materials possessing higher specific properties, bringing
benefitsinrelationtoweightsavingratherthanincreasingstrengthandstiffness.
The first group of alloys possesses a lower static strength than the preceding zinc-bearing alloys
butarepreferredforportionsofthestructurewherefatigueconsiderationsareofprimaryimportance,
such as the undersurfaces of wings, where tensile fatigue loads predominate. Experience has shown
that the naturally aged version of these alloys has important advantages over the fully heat-treated
formsinfatigueenduranceandresistancetocrackpropagation.Furthermore,theinclusionofahigher
percentageofmagnesiumwasfound,inAmerica,toproduce,inthenaturallyagedcondition,mechanical
propertiesbetweenthoseofthenormalnaturallyagedandartificiallyagedalloy.Thisalloydesignated
2024(aluminum–copperalloysformthe2000series)hasthenominalcomposition:4.5%copper,1.5%