Effect of addition of Fe on the occurrence of serration in Al-12mass%Zn alloys was investigated. Specimens aged at 293K for various periods after quenching from various temperatures (T(Q)), 398K to 823K, were tensile-tested at room temperature. Serration occurred more easily and more remarkably with decreasing T(Q) for as-quenched specimens: in the case that T(Q)=448K serration was observed both for the binary and Fe added alloys, while in the case that T(Q)=573K none of the three alloys showed serration. For the binary alloy serration was observed only when the aging period was short enough, but addition of Fe to the binary alloy prolonged the aging period where serration could be recognized. Aging rate measured by hardness was remarkably retarded with the increase of Fe addition. These results confirm the interpretation that the serration in Al-Zn alloy occurs in the early stage of aging where small GP zones or solute clusters are formed

Kanadani, Teruto

Sakakibara, Akira

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Serration in A l-Zn Alloys Containing a Small Amount of Fe

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Memoirs of the Faculty of Engineering, Okayama University, Vol.37, No.2, pp.l·4, March 2003Serration in A l-Zn A lloys Containing a Small Amount ofFeAkira SAKAKIBARA*Dept. of Mechanical EngineeringOkayama University3-1-1 Tsushimanaka, Okayama, JapanTeruto KANADANIFaculty of EngineeringOkayama University of ScienceI-I Ridaicho, Okayama, JapanEffect of addition of Fe on the occurrence of serration in AI-12mass%Zn alloys wasinvestigated. Specimens aged at 293K for various periods after quenching fromvarious temperatures (TQ), 398K to 823K, were tensile"tested at roomtemperature. Serration occurred more easily and more remarkably withdecreasing TQ for as-quenched specimens: in the case that TQ=448K serration wasobserved both for the binary and Fe added alloys, while in the case that TQ=573Knone of the three alloys showed serration. For the binary alloy serration wasobserved only when the aging period was short enough, but addition of Fe to thebinary alloy prolonged the aging period where serration could be recognized.Aging rate measured by hardness was remarkably retarded with the increase ofFe addition. These results confirm the interpretation that the serration in AI-Znalloy occurs in the early stage of aging where small GP zones or solute clustersare formed.1. IntroductionAppearance of serration in the stress-straincurve of AI-Zn alloys has been studied since thework by Chossat[ll Saito et aL[2] studiedthe mechanism of serration and explained theappearance of serration in terms of the Cottrelleffect[3] involving interaction between soluteatoms and dislocations. Besides that,explanations in terms of the interaction ofdislocations with super-jogs, which were madefrom lattice defects and dislocations[41, or interms of the interaction of dislocations withcoherent particles such as GP zones[5] werepresented. Recently the present authorsstudied systematically the relation betweenthe appearance of serration and the heattreatment, and found that in this alloyserration appeared at the initial stage of agingwhere GP zones with several nm in size orclusters of solute atoms were formed[6,71.Aging process of AI-Zn alloys is remarkablychanged with a small addition of some kind ofthird element. Addition of Ag, for example,*E'mail sakaki@mech.okayama·u.ac.jp1elevates dissolution temperature of GP zonesand addition of Fe slows down aging rate dueto refinement of grains[g,91. Fatigue strengthis likely to be fairly increased with addition ofFe[10].In this report effect of addition of Fe on theappearance of serration is investigated.2. Experimental ProceduresAlloys, AI-12mass%Zn, AI-12mass%Zn·O.1-mass%Fe and AI-12mass%Zn-O.5mass%Fe innominal composition, were obtained by meltingpure metals of Al and Zn and a mother alloy ofAI-5mass%Fe in a high alumina crucible in air.Ingots, 15mm in diameter and about 150mm inlength, were homogenized for 180ks at 723K,pealed by a lathe, and forged at 723K to platesof 5mm in thickness. The plates werecold-rolled, with appropriate intermediateannealings, to strips of about Imm inthickness. Specimens, shape and dimensionof which were reported previously[6,91, wereprepared from these strips.Specimens were solutionized at 823K for2 Akira SAKAKIBARA et al. MEM.FAC.ENG.OKA.UNI. VoI.37. No.2Table 1 Chemical composition (mass%) of alloys used.Alloys Zn Fe Cu Si AIAH2%Zn 11.6 0.001 0.002 0.002 bal.AI-12%Zn-0.l %Fe 11.8 0.107 0.001 0.001 bal.AI-12%Zn-0.5%Fe 11.5 0.509 0.003 0.002 bal.200 .......-------------,three specimens quenched from 398K. Solidmarks represent the state which showedserration in the tensile test. Rate ofhardening is markedly reduced by the additionof Fe to the binary alloy. Retarding of agehardening is considered to be due to therefinement of grains caused by the addition ofTO=473 KAs -quenchedAI-12 OfoZn-O.5%Fe------~\. / ..~~ -- Al-l2"/oln-Ql0J0Fe. AI-120f0Zn501501003.6ks, furnace·cooled to the quenchingtemperature (TcV, 823K to 398K, kept there for3.6ks and then quenched into iced water.Aging was carried out by immersing specimensfor various periods in an ethanol bath kept atthe aging temperature (TtJ, 293K. Accuracyof the TA measurement was within 0.5K.Specimens heat·treated were examined bytensile test with Instron testing machine, 4505,at room temperature, 293K, strain rate being2x 1O·4s· l • Aging process was followed byhardness with micro·Vickers hardness tester.Details of the measurement were described inthe previous papers[6,9]. Average grain sizesof the binary alloy, 0.1 %Fe added alloy and0.5%Fe added alloy are 120~m, 70~m and30~m, respectively.Strain. f.~3020E(%)10OL-__-----l ----l. ---'-_--'oFig.! stress(cr)-strain(£) curves of the binary andFe-added alloy specimens as-quenched from473K and tensile-tested at room temperatureat a strain rate of 2 X 1O-4s- 1.Fig.2 Schematic diagram of the typical serrationobserved in the Al-Zn alloys.3. Results and DiscussionFig.1 shows the stress(o)·strain(f:) curves ofthe specimens as·quenched from 413K. As isshown in Fig.2, serration observed in thesecurves fluctuates; therefore it is classified intoso·called A-type serration[ll]. A smoothcurve without serration was obtained for thebinary alloy. In the case of 0.5%Fe addedalloy, on the other hand, serration appeared inthe strain range between about 1% and 8.5%.In the case of 0.1 %Fe added alloy serration isalso observed but the range of its occurrence islimited between 2% and 4% strain.When the quenching temperature, TQ, was573K or above, serration did not appearirrespective of the addition of Fe. When TQ;:;;448K, serration could be observed in allspecimens. As the quenching temperaturedecreased, the range of strain where theserration was observed widened and theserration amplitude increased. Serrationobserved in the Fe added alloy specimen wasmore remarkable than that of the binary alloyfor the same quenching temperature.. Dependence of the appearance of serrationon the aging time (t~ was examined. Fig.3shows age hardening curves at 293K of theMarch 2003Fe, and/or to the increase of vacancy sinks,such as dislocations involved with theformation of intermetallic compounds likeAbFe. Serration was observed even at theSerration in Al-Zn Alloys Containing a Small Amount of Fe 3state aged for 480ks in the 0.5%Fe added alloyspecimen, while serration could be recognizedonly within 7.2ks of aging time (tA) in thebinary alloy.>I6040TQ=398 KTA =293 K"\Al-12%Zn-0.5%Fe2010-1 100 101 102 103Aging time,tAI ks104Fig.3 Variation of isothermal age-hardening curve of the binary and Fe-addedspecimen aged at 293K after quenching from 398K. Solid and opensymbols indicate whether the stress-strain curve showed serration ornot.It might be thought that gram refinementand/or intermetallic compound formationthemselves promote the appearance ofserration, but the grain size and the state ofintermetallic compound did not alter duringthe aging. It should be noted that themaximum hardness of the specimen whichshowed serration coincided each other. Thissuggests that serration essentially occurs inthe same stage of aging and that addition of Feincreased vacancy sinks and slowed down theaging rate; therefore serration appeared evenin longer aging time. In the case of theas-quenched specimens (Fig. 1), the result thatserration could be observed only in the Feadded alloys for the intermediate quenchingtemperature may be due to the retardation ofgrowth of GP zones or solute atom clustersduring the quench, caused by the addition ofFe. Lowering the quenching temperature hadthe same effect of retardation as Fe additionand made the serration occur even in thebinary alloy.4. ConclusionsEffect of addition of a small amount of Fe on theappearance of serration in tensile deformationwas examined for AI-12mass%Zn, AI-12mass%Zn-O.lmass%Fe and AI-12mass%Zn-0.5mass%Fealloy specimens aged at 293K after quenchingfrom various temperatures between 398 and823K.(1) Serration was not observed for the specimensquenched from high temperatures, TQ ~ 573K,irrespective of the addition of Fe. On the otherhand, when TQ ~ 448K serration occurred andbecame pronounced with decreasing TQ.(2) Addition of Fe to the binary alloy promoted theserration.(3) Lowering TQ and/or addition of Fe made theaging rate slower, which caused serration to occureasily because serration was allowed to occur inthe early stage of aging, where small GP zones orsolute clusters existed.References[1] H. Chossat, Rev. Metall., 47(1950), 343.[2] K. Saito, Y. Hosoi and T. Hikage, J. Japan lnst.Metals, 49(1985), 507.[3] A. H. Cottrell, Phil. Mag., 44(1953), 829.[4] S. Yoshioka, Y. Nakayama and T. Hosokawa, J.Japan lnst. Light Metals, 20(1970), 509.4 Akira SAKAKIBARA et al.[5] E. Pink and J. Krol, Acta Metall., 43(1995),2351.(61 T. Kanadani, K. Yoshioka and T. Tashiro, J.Japan lnst. Light Metals, 471997), 110.[7] A. Sakakibara, T. Kanadani, T. Tashiro and K.Yoshioka, Mem. Fac. Eng. Okayama Univ.,36(2002), 1.[8] Y. Baba, J. Japan lnst. Metals, 29(1965), 1075.[9] T. Kanadani, Y. Kakuno, K. Nakagawa, N.Hosokawa and H. Yamada, phys. stat. sol. (a),144(1994), K69.[10] K. Nakagawa, T. Kanadani, Y. Kakuno, N.Hosokawa and T. Tanimoto, phys. stat. sol.(a), 158(1996), K17.[11] S. lkeno, T. Watanabe and S. Tata, J. JapanInst. Metals, 47(1983), 231.MEM.FAC.ENG.OKA.UNI. Vol.37, No.2I

Serration in A l-Zn Alloys Containing a Small Amount of Fe

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