Effect of addition of a small amount of copper to Al- 12mass % Zn binary alloy on the process of low temperature aging is studied by hardness test and transmission electron microscopy. Age hardening rate after quenching from various temperatures is slowed down by addition of 0.5mass % copper to the binary alloy. The hardness obtained after long aging, however, is increased a little by the addition of copper. The extent of soft surface layer formed by aging in the copper-added alloy is nearly
equal to that obtained in the binary alloy

Kanadani, Teruto

Nakagawa, Keiyu

Sakakibara, Akira

Okayama University Scientific Achievement Repository

Memoirs of the Faculty of Engineering,Okayama UniversitY,Vol.29, No.2, pp. 1- 4, March 1995Low Temperature Age-Hardening of AI-12mass%Zn-O.5mass%Cu AlloyAkira SAKAKIBARA*, Keiyu NAKAGAWA**, Teruto KANADANI**(Received January 27 , 1995)Effect of addition of a small amount of copper to AI-12mass%Zn binary alloy on theprocess of low temperature aging is studied by hardness test and transmissionelectron microscopy. Age hardening rate after quenching from various temperaturesis slowed down by addition ofO.5mass% copper to the binary alloy. The hardnessobtained after long aging, however, is increased a little by the addition of copper.The extent of soft surface layer formed by aging in the copper-added alloy is nearlyequal to that obtained in the binary alloy.1. INTRODUCTIONAI-Zn alloy, which is a base binary alloy of ultra-super Duralumin, is a typical age-hardenable alloy andhas been studied by many workers, relating to the microscopic structure, on its mechanical and physicalproperties obtained with various heat treatments. When the alloy is held at around room temperatureafter quenching from, say, 673K (so-called low temperature aging), many fine GP zones enriched with solutezinc atoms form and contribute to the age hardening. Until recently GP zones had been thought to beformed uniformly in the specimen except for the proximity to grain boundaries. According to the detailedexamination by Ohta et al. ofthe age hardening process at various regions in the specimen, regions near thespecimen surfac~ and near the grain boundary in the vicinity of surface remained, even after a long aging,soft compared with the other hardened regions.(l) They later explained the result by use of X-ray small-angle scattering and other methods as follows: Specimen surface and grain boundaries behave as efficientsinks for quenched excess vacancies and the vacancy concentration in the vicinity of them decreases morerapidly than in the other regions, which retards or virtually stops GP zone formation in the region.(2) Thisregion with a little lower hardness near the surface obtained after aging will be called "soft surface layer"hereafter. .The present authors performed fatigue test of the Al-Zn alloys with various compositions andheat treatments, paying attention to the effect of this soft surface layer on fatigue, and found that theexistence of soft surface layer improved fatigue strength under repeated tensile mode. (3-5)On the other hand, it is well known in many cases that the formation and growth of GP zones areremarkably influenced by addition of a third element.(6) Addition of silver, for instance, elevates the solvustemperature of.GP zones and promotes the formation and growth of the zones, while addition of copper does* Department of Mechanical Engineering** Okayama University of Science12 Akira SAKAKIBARA, Keiyu NAKAGAWA and Teruto KANADANInot change the solvus temperature of GP zones.(7,8) In this report, effect of addition of a small amount ofcopper to Al-12mass%Zn alloy on the age hardening process and on the formation of the soft surface layer isexamined.2. EXPERIMENTAL PROCEDURES2.1. SpecimensAlloys,nominal compositions of which were Al-12mass%Zn and Al-12mass%Zn-0.5mass%Cu (calledhereafter binary alloy and Cu-added alloy, respectively), were made by melting 99.99%AI, 99.999%Zn and99.999%Cu in the alumina crucible in atmosphere. Chemical analysis ofthese alloys were shown in Table 1.Ingots obtained were homogenized by annealing at 723K for 180ks, peeled mechanically, and then hot forgedand cold rolled with intermediate annealing procedures to a strip of 1.1mm in thickness and a strip ofO.7mmin thickness, from which were prepared the specimens for hardness test and the ones for electron microscopy,respectively. Shapes and dimensions of the specimens were the same as reported previously.(1,3) Crystalgrains of the specimen for hardness test were coarsened to about 4mm in diameter by the strain annealingmethod.Table 1 Chemical composition of alloys used (mass%)Alloys Zn Cu Si Fe AlAl-12%Zn 11.3 0.003 0.004 0.003 balanceAl-12%Zn-0.5%Cu 11.2 0.491 0.002 0.001 balance2.2. Heat TreatmentsThe sequence of quenching was as follows:The specimen was inserted into the slit of analuminum block in an electric furnace and washeld at 773K for 3.6ks for solution treatment.Then it was cooled in the furnace to the quenchingtemperature (TQ), held there for 3.6ks, and finallyquenched into iced water.Aging was carried out at 273K or 293K inan ethanol bath. The sequence of the heat treat-ment is schematically shown in Fig. 1.2.3. Measurementssolutiontreatmenta inTimeFig.! Schematic diagram of the sequence of heat treatment..Hardness was tested at room temperature using Vickers microhardness tester and ultramicrohardnesstester (Akashi Co. Ltd.). Age hardening process was followed at the load of 1.96N, and fully aged state wasexamined at various penetration loads, 0.01 to 9.8N, at the position more than 200fIDl apart from the grainboundary before and after each removal of the surface layer about 50 fIDl in thickness. Surface layer wasremoved by electropolishing in a perchloric acid-ethanol (1:4) solution.Microstructure of the aged specimen was observed with a transmission electron microscope, JEM-2000EX, operated at 200kV.Low Temperature Age-Hardening of Al-Zn-Cu Alloy3. RESULTS AND DISCUSSIONFigure 2 shows isothermal age hardening curves of the binary and Cu-added alloys at 293K afterquenching from 673K. Average values and standard deviations of more than eight measurements arepresented in the figure for the aging time (tA) equal to or more than 4.8ks, and only average values of four orless measurements are presented for the tA less than 4.8ks. Hardness increases with increase of tA andreaches a stationary value in either alloy. The stationary value of Cu-added alloy isa little higher thanthat of the binary alloy, but the time to reach the value is longer. Baba reported a similar result to thepresent one for an alloy containing 6%Zn, a comparatively low concentrated alloy.(9) Many spherical GPzones about 6um in diameter were observed with TEM in either alloy, independent of the copper addition,aged at 293K for 300ks after quenching from 673K.The result of the same experiment as Fig.2 but of a higher TQ, 773K, is shown in Fig.3. Similarbehavior to Fig.2 was obtained for this higher TQ. Time to reach the stationary value, however, is shortertha,n that of Fig.2 for either alloy, which is considered to be due to faster diffusion of solute atoms withhigher concentration of vacancies quenched from higher temperature.Difference in the age hardening process between the two alloys shown in Figs.2 and 3 is considered to beexplained by the copper atom-vacancy binding energy which is a little larger than the zinc atom-vacancy3120>::t:10080lQ=673K t TA=293K-0AI-12°/oZn-0 5%Cu~AI-12°/oZnFig.2 Isothermal age hardening curves of the Al-12mass%Zn binary alloy and Cu-added alloy aged at293K after quenching from 673K.Fig.3 Isothermal age·hardening curves of the Al-12mass%Zn binary alloy and Cu-added alloy aged at293K after quenching from 773K.80 -l00r-~120 f->::t:AI-12°/oZnTQ=-573KTA=273K-120 ks-0 ~lJm.fr5-0-10~150 >::t:12010080AI-12% Zn -0.5%CuTQ =573 KTA =-m K-120 kso OlJm-6- 500100-va-150I10-1 100 101Load t NFig.4 Dependence of hardness on the indentation loadfor the binary alloy specimen just aged at 273K for120ks after quenching from 573K and the ones agedand electropolished to remove the surface layers.Fig.5 Dependence of hardness on the indentation load forthe Cu-added alloy specimen just aged at 273K for120ks after quenching from 573K and the ones agedand electropolished to remove the surface layers.4 Akira SAKAKIBARA, Keiyu NAKAGAWA and Teruto KANADANIbinding energy, and by the formation of a kind of copper atom clusters in additiolrto the UP zone formationproper to the binary alloy.Figures 4 and 5 show the variation along the depth from the surface of the hardness of the fully agedspecimens of binary alloy and Cu-added alloy, respectively. Open circles represent the result of thehardness test carried out at various penetration loads, 0.01 to 9.8N, of the specimen just aged at 273K for120ks after quenching from 573K. The fact that hardness decreases with decrease ofthe load indicates thathardness near the surface is lower than that of the interior. Hardness of the Cu-added alloy is slightlylower than that of the binary alloy when measured at smaller loads, but they coincide with each other whenmeasured at 4.9N or more of the load. Surface layers of 50Jllll in thickness were removed successively andhardness was measured after each removal (.6,0, \7). Removal of the layer more than 100/Wl in thicknesswas required in either alloy to obtain the hardness independent of the load. It may be considered thereforethat the thickness of the soft surface layer in this case has 100Jllll or more for either alloy. Similar resultwas obtained for the specimen quenched from different temperatures. Addition of a small amount of copper(0.5%) did not alter the thickness of the soft surface layer. The average grain size of the Cu-added alloyspecimen was almost the same as that of the binary alloy specimen. Taking account of the previouslyreported fact that addition of a small amount of silver to AI-Zn alloys, which elevates the solvus temperatureof GP zones, suppress the formation of the soft surface layer, (10) the result that change was hardly observedin the thickness of the soft surface layer by the addition of copper is considered to be due to little change insolvus temperature by the addition.ACKNOWLEDGMENTThe authors would like to express their hearty thanks to Mr. Y Takabayashi and Mr. H. Taki for theircooperation in the experiment.REFERENCES(1) M.Ohta, H.Yamada, T.Kanadani, ASakakibara, and M.Yamada: J. Japan Inst. Light Metals, 33(1983),212.(2) M.Ohta, T.Kanadani, M.Yamada, and ASakakibara: J. Japan Inst. Metals, 50(1986), 887.(3) T.Kanadani: Phys. Stat. Sol(a), 115(1989), K147.(4) T.Kanadani: Synopses of 106th Meeting of Japan Inst. Metals, Tokyo(1990), p252.(5) T.Kanadani, T.Matsushima, N.Hosokawa, T.Kaneeda and ASakakibara: Phys. Stat. Sol.(a), 124(1991),K97.(6) T.Fujikawa, K.Hirano, and YBaba: Bull. Japan Inst. Metals, 7(1968), 494.(7) M.Ohta, H.Hashimoto, T.Tanimoto, and H.Maeda: J. Japan Inst. Metals, 32(1968), 1103.(8) M.Murakami, M.Kikuchi, O.Kawano and YMurakami: J. Japan Inst. Metals, 36(1972), 1164.(9) YBaba: J. Japan Inst. Metals, 30(1966), 679.(10) T.Kanadani, ASakakibara, N.Hosokawa and T.Kaneeda: Phys. Stat. So1.(a), 129(1992), Kl.

Low Temperature Age-Hardening of Al-12mass % Zn-0.5mass % Cu Alloy

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