The effect of heat-treatment conditions on occurrence of serration in Al-Zn alloys was investigated. Specimens were aged for various times up to 2.6Ms at 293K or 273K after quenching from various temperatures (T(Q)), 473K to 853K, and tensile-tested at room temperature. Serration occurred more easily according as T(Q) became lower and the aging time became shorter: in the case that T(Q)=473K serration was observed even after aging for 2.6Ms, while in the case that T(Q)=773K serration did not occur irrespective of aging conditions. Serration was also recognized when the specimens were furnace-cooled from 773K to room temperature. These results together with those obtained by the electrical resistometry suggest that the serration in the low'concentrated Al-Zn alloy is caused by the formation of small GP zones whose Guinier radius is less than 1nm or some sort of solute clusters

Kanadani, Teruto

Sakakibara, Akira

Tashiro, Tsutomu

Yoshioka, Kazumi

English

Okayama University Digital Information Repository

Effect of Heat Treatment on the Serration of Low-Concentrated Al-Zn Alloys

Okayama University Scientific Achievement Repository

Memoirs of the Faculty of Engineering. Okayama University. Vo1.36, No.2, pp.1-6, March 2002Effect ofHeat Treatment on the Serration ofLow-Concentrated A l-Zn AlloysAkira SAKAKIBARA*Dept. of Mechanical EngineeringOkayama University3'1'1 Tsushimanaka, Okayama, JapanTsutomu TASHIROFaculty of InformaticsOkayama University of Science1'1 Ridaicho, Okayama, JapanTeruto KANADANIFaculty of EngineeringOkayama University of Science1'1 Ridaicho, Okayama, JapanKazumi YOSHIOKAGraduate SchoolOkayama University of Science1·1 Ridaicho, Okayama, JapanThe effect of heat· treatment conditions on occurrence of serration in AI-Zn alloyswas investigated. Specimens were aged for various times up to 2.6Ms at 293K or273K after quenching from various temperatures (TQ), 473K to 853K, andtensile·tested at room temperature. Serration occurred more easily according asTQ became lower and the aging time became shorter: in the case that TQ=473Kserration was observed even after aging for 2.6Ms, while in the case that TQ=773Kserration did not occur irrespective of aging conditions. Serration was alsorecognized when the specimens were furnace' cooled from 773K to roomtemperature. These results together with those obtained by the electricalresistometry suggest that the serration in the low'concentrated AI'Zn alloy iscaused by the formation of small GP zones whose Guinier radius is less than Inmor some sort of solute clusters.1. INTRODUCTIONAging phenomena of AI'Zn alloys have been much investigated and many studies have beenmade on the change of mechanical properties on aging. Appearance of saw-tooth yielding(serration) in the stress'strain curve of tensile deformation has been reported in severalreports. According to Yoshioka et al. serration was found only for low-concentrated alloys,4%Zn or less, when aged after quenching from high temperature such as 773K. Theyinterpreted that the appearance of serration was caused by super-jogs formed by theinteraction of Frank sessile dislocation loops, which were made from quenched excessvacancies during aging, with mobile dislocationsW , This interpretation is different from thatof the Portevin-Le Chatelier effect, where the saw·tooth yielding is caused by the interactionbetween dislocations and solute atoms(2). Later Saito et al. determined the parameters ofdiscontinuous deformation, m and b, for furnace'cooled AI'10mass%Zn, considering thatdiscontinuous deformation of this alloy was due to the interaction between dislocations andsolute atoms(3). Recently Pink et al. suggested from the result of XSAS from AI-lOmass%Znalloy that the serration of this alloy was originated from the fomation of coherentprecipitatesW .On the other hand, it has been proposed that the serration ofAI-Ag alloy, in which sphericalGP zones are formed during aging as in Al'Zn alloy, is caused by cluster of solute atoms suchas GP zone(5). It was reported that the serration appears when GP zones of a certain range ofsize were formed(6,7) .*E'mail sakaki@mech.okayama·u.ac.jp1Akira SAKAKIBARA et al. MEM.FAC.ENG.OKA.UNI. Vo1.36. No.2Studies mentioned above have hardly dealt with the relation between the appearance of serrationand the heat"treatment condition, especially the aging condition systematically. Detailedexamination of the relation is considered to be required to reveal the mechanism of serration.In this paper stress"strain curve of aged AI-6mass%Zn and AI"4mass%Zn alloys, where theappearance of serration has been ambiguous previously, are investigated when aged at aroundroom temperature for various times after quenching from various temperatures and therelation between the appearance of serration and the heat treatment is discussed.2. EXPERIMENTAL PROCEDURESAlloys, AI"6mass%Zn and AI"4mass%Zn in nominal composition, were obtained by melting99.996%AI and 99.999%Zn in a high alumina crucible in air. Ingots, 15mm in diameter andabout 150mm in length, were homogenized for 180ks at 723K, pealed by a lathe, and forged at723K to plates of 5mm in thickness. The plates were cold"rolled, with appropriateintermediate annealings, to strips of O. 7mm and 1.1mm in thickness. Specimens for variousmeasurements were prepared from these strips. Shape and dimension of the specimen foreach measurement were the same as reported previously(6-S).Specimens were solutionized at 853K for 3.6ks, furnace"cooled to the quenchingtemperature (T~, 823K to 473K, kept for 3.6ks and then quenched to iced water. Aging wascarried out by immersing specimens for various times in an ethanol bath kept at the agingtemperature (TtJ, 273K or 293K. Accuracy of the TA measurement was within 0.5K. Allspecimens were equal in average grain size, about 150f.tm, due to the homogenization at 823K.Specimens heat·treated were examined by tensile test with Instron testing machine, 4505,at room temperature, strain rate being 2x10·4S· 1. Aging process was examined by measuringelectric resistance, of which the method was described in previous reports (6-8) .3. RESULTS AND DISCUSSIONFig.1 shows stress-strain curves of the AI"6%Zn specimen quenched from 773K and the onequenched from 473K. For the specimen TQ=473K, serration is recognized in a wide range ofstrain from about 1% to 18.5%, where the stress attains the maximum value, while thespecimen TQ=773K shows a smooth change with no serration.Fig.2 shows the collected results of the similar experiment which examined the relation ofappearance of serration and 0.2% proof-stress with TQ for the AI-6%Zn specimen. Full circlesrepresent the case where serration was recognized. Serration was recognized in the case TQ~673K. According to Yoshioka et al. saw"tooth yielding in AI-Zn alloys was caused by theinteraction of dislocations with secondary defects formed by quenching from high TQ such as773K. This interpretation is contradictory to the result shown in Fig.2; serration wasobserved in the case oflow TQ such as 523K and 473K where secondary defects should not havebeen formed. It was reported that secondary defects were hardly observed when thespecimen was quenched from 673K or lowerw. In Fig.2 the proof stress increases withincreasing TQ and is smaller when the serration is observed than when it is not observed.Fig.3 shows the results for AI-4%Zn specimen. The same fact is realized as for the AI-6%Znspecimen; serration appears for lower TQ and proof stress increases with increasing TQ.Fig.4 shows dependence of the appearance of serration and 0.2% proof-stress on the agingtime, tA, at 293K after quenching from 473K and 573K for the AI'6%Zn specimen. Solidsymbols represent the case where serration was recognized. In the case TQ=573K theserration was observed for short tA but it could not be found when aged for 10.8ks or longer.In the case TQ=473K the serration was observed even after long aging such as tA=2.6Ms.For both TQ's, the proof stress of the case where the serration was recognized was lower thanMarch 2002 Effect of Heat Treatment on the Serration of Low-Concentrated AI-Zn Alloys 3AI-6°loZn60 AI -6°loZnIf SO~.........J 403020500 600 700 800TQ / K20\~=473Kas-quenched10 15e (olo)5402080~ 60"'-'0Fig.l Stress'strain curves of the Al'6%Zn specimenswater-quenched from 773K or 473K andsubsequently tensile·tested.Fig.2 Effect of TQ on the occurrence of serration and0.2% proof stress for the as'quenchedAl-6%Zn specimens. Solid and open circlesrepresent the specimen which showed theserration and the one which did not show theserration, respectively.50 AI-So'.ZnAl-4·'.Zn 50•. 0 573 K}If 40 40 .... 6 473 K TQ,Z If T,,=293 K......... z.... 30"'" 30'0 J.......20 ........ .2010 10500 600 700 800 a 10 0 101 102 103To. /K t A / ksFig.3 Effect of TQ on the occurrence of serration and0.2% proof stress for the as' quenchedAl'4%Zn specimens_ Solid and open circlesrepresent the specimen which showed theserration and the one which did not show theserration, respectively.Fig.4 Effect of the aging time on the occurrence ofserration and the proof' stress for thespecimens aged at 293K after quenching from573K or 473K. Solid and open circlesrepresent the specimen which showed theserration and the one which did not show theserration, respectively.that of the case where it was not observed. For 773K or higher TQ (figures omitted in thispaper), the serration was not observed for any tA as was the case for the as'quenchedspecimen.Fig.5 shows variation of the strain from which the serration started, Ee, with tA at 293K afterquenching from 473K, where serration was observed for the longest tAo The serration wasretarded by longer aging. The maximum stress amplitude and the number of serration arepresented as a function tA in Figs.6 and 7, respectively. Both values remain constant for theshort tA but decrease gradually with tA longer than 102ks. These results indicate that theserration became weaker according as the aging proceeded.Fig.8 shows isothermal aging curves of resistivity when the Al'6%Zn specimen was aged at4 Akira SAKAKIBARA et al. MEM.FAC.ENG.OKA.UNI. Vol. 36. No.2oAI-6'I,ZnTQ= 473 KT.=293K10 102 103 104tA / ks1.2~ 0.8~'"J 0.4<]104 0 1A1-6%Zn4Fig.5 Relation between the strain where theserration starts and the aging time at 293K for theAI-6%Zn specimens after quenching from 473K.Fig.6 Variation of the maximum stress amplitude ofthe serration with the aging time at 293K for theAl-6%Zn specimens after quenching from 473K.160I 120~• 80'0 AI-6"1.Znj 40 fQ =473K~ f. =293Kz0Fig.7 Variation of number of serration with theaging time at 293K for the Al'6%Zn specimens afterquenching from 473K.10 102 103 104t. / ks293K after quenching from various TQ. Solid symbols represent the specimens which showedthe serration. Curves for TQ;;;;; 673K show the maximum resistivity that is typical for GP zoneformation. Resistivity for TQ=773K may have taken the maximum value during quenching orwithin 20s of aging since the high concentration of quenched vacancy accelerated the agingrate. Isothermal aging curves at 273K showed a similar behavior. Fig.9 shows anisothermal aging curve of the Al'4%Zn specimen aged at 273K after quenching form 673K. It100 101tA / ksFig.8 Variation of isothermal aging curve ofAI'6%Zn specimens in electrical resistivity aged at293K after quenching from various temperatures.Solid and open circles represent the specimen whichshowed the serration and the one which did not show it,respectively.103AI-4'/,ZnTQ= 673 KTA =273 K6.38E 6.32ICC"'- 6.26Cl.6.200 100 101tA / ksFig.9 Variation of isothermal aging curve of Al-6%Znspecimens in electrical resistivity aged at 293Kafter quenching from various temperatures. Solidand open circles represent the specimen which showedthe serration and the one which did not show it,respectively.o 773 K}TA=293 K. Y.17 673 K ""•• 0 573 K Q.... tJ. 473 KAI-6'/oZn6.5 -7.5E~ 6.0 -...........0..March 2002 Effect of Heat Treatment on the Serration of Low-Concentrated Al-Zn Alloys60AI-S·I.Zn40If~"'-..'020furnace -cooled60r-----------------,AI-4·I.Zn40b 20302010OL- ......L ....L... ---'o2010 15e ('1.)5Fig.10 Stress'strain curve of the Al'6%Zn specimenfurnace'cooled from 773K to room temperature.Fig. 11 Stress'strain curve of the Al'4%Zn specimenfurnace'cooled from 773K to room temperature.is clearly seen from Figs. 8 and 9 that serration can be observed for the specimen aged forshort time up to around the maximum resistivity. According to the studies of XSAS on theaging ofAI'Zn alloy Guinier radius ofGP zones formed during aging is about lnm at the stagecorresponding to the maximum resistivity(8.9). The present results might suggest that soluteatoms in the matrix decreased while GP zones grew with tA and therefore poL effect, whichwas caused by the interaction of dislocations with solute atoms, became weakened. However,the specimen furnace'cooled from 773K, where solute atoms in the matrix were considered tobe less than the specimen quenched from 773K, showed serration as in Figs.IO and 11,whereas the quenched specimen didn't show serration. This result is difficult to explain bythe interaction between dislocation and solute atom above described.Results of present experiment suggest a relation of the appearance of serration in AI-Znalloy to spherical GP zones whose Guinier radius is less than lnm or formation of someclustering of solute atoms(lO,lll, but it will be necessary for clear explanation of the mechanismof serration to study further the change of microstructure during deformation and the changeof solute concentration during aging. Moreover, effects oftest temperature and strain rate onthe appearance of serration should be examined because Ikeno et 01. (I2) indicated profoundeffects of these factors in AI·Mg alloy.4.CONCLUSIONSRelation of the appearance of serration in tensile deformation to the heat treatments wasexamined for AI'4%Zn and Al'6%Zn alloy specimens aged at around room temperature afterquenching from various temperatures between 473 and 853K.(1) Serration was not observed for the specimens quenched from high temperatures, TQ~773K.(2) In the case TQ~673K serration occurred. As 1Q became lower, the serration could be observedfor the specimen aged for longer time but it was restricted in the relatively initial aging stage up toaround the maximum of resistivity.(3) Serration was recognized for the specimen furnace cooled.(4) It is considered that serration occurs for this alloy when the specimen is heat·treated so thatsmall spherical GP zones, whose Guinier radius is as large as lnm, or clusters of solute atoms maybe formed.REFERENCES6 Akira SAKAKIBARA et al. MEM,fAC,ENG,OKA,UNI. Vol.36. No.2(1) S. Yoshioka, Y. Nakayama and T. Hosokawa, Light Metals, 20(1970), 509.(2) A. H. Cottrell, Phil. Mag., 44(1953), 829.(3) K. Saito, Y. Hosoi and T. Hikage, J. Japan Inst. Metals, 49(1985),507.(4) E. Pink and J. Krol, Acta Metall., 43(1995), 2351.(5) R. Onodera, H. Era, T. Ishibashi and M. Shimizu, Acta Metall., 31(1983), 1589.(6) T. Kanadani andA. Umada, Phys. Stat. Sol. (a), 148(1995), K23.(7) T. Kanadani and A. Umada, Phys. Stat. Sol. (a), 151(1995), K29.(8) M. Ohta, T. Kanadani and A. Sakakibara, J. Japan Inst. Metals, 47(1983), 375.(9) K. Osamura, Y. Hiraoka and Y. Murakami, Phil. Mag., 28(1973), 809.(10) H. Terauchi, N. Sakamoto, K. Osamura and Y. Murakami, Trans. JIM, 16(1975), 379.(11) M. Ohta, T. Kanadani and H. Maeda, J. Japan Inst. Metals, 40(1976), 1199.(12) S. Ikeno, T. Watanabe and S. Tata, J. Japan Inst. Metals, 47(1983), 231.

Effect of Heat Treatment on the Serration of Low-Concentrated Al-Zn Alloys

Abstract

Similar works

Full text

Available Versions

Okayama University Digital Information Repository

Okayama University Scientific Achievement Repository