Reversion of the age-hardened Al-15mass % Zn alloy, in which ellipsoidal GP zones were formed, was investigated by hardness test. Ellipsoidal zones were reverted more quickly near the surface and grain boundary than in the interior, as spherical zones did. The results confirm their role as sources for vacancies in reversion

Kanadani, Teruto

Sakakibara, Akira

Okayama University Scientific Achievement Repository

Memoirs of the Faculty of Engineering,Okayama University,VoI.26, No.1, pp.l -7. November 1991Effect ofSurface and Grain Boundary on the ReversionofAge-Hardened AI-15mass%Zn AlloyAkira SAKAKIBARA* and Teruto KANADANI**(Received October 11 , 1991)SYNOPSISReversion of the age-hardened AI-15mass%Zn alloy, inwhich ellipsoidal GP zones were formed, was investi-gated by hardness test. Ell ipsoidal zones were re-verted more quickly near the surface and grain bound-ary than in the interior, as spherical zones did. Theresults confirm their role as sources for vacancies inreversion.l. I NTRODUCT I ONAge hardening of AI-Zn alloy after quenching develops inhomogene-)usly due to the effect of surface as a vacancy sink and grain bound-lry as a easy path. (1) It was found that, when age hardened AI-lOmass%Zn alloy was annealed at high temperatures. the reversion;tarted near the surface and delayed in the interior. (2) The factvas explained also by the effect of the surface, but in this case as1 source of vacancies.When AI-lOmass%Zn alloy was aged at 293K after quenching from723K or higher. as in the previous experiment,(2) GP zones formedvere small and spherical, which was easily dissolved again on rever-;ion. It is known that large ellipsoidal GP zones are formed in AI-~n alloys more concen tra ted in Zn and aged af ter quench i ng from~ Department of Mechanical Engineering~*Okayama University of Science12 Akira SAKAKIBARA and Teruto KANADANItemperatures lower than 650K. (3) These zones are considered to bestabilized by the shape change.This paper deals with the effect of the surface on the reversionof the large ellipsoidal GP zones, investigated by hardness test andconfirms the role of surface as a vacancy source on reversion.2. EXPERIMENTAL PROCEDURESPure aluminum, 99.996%, and pure zinc, 99.999%, in the nominalcomposition of l5mass%Zn, were melted in a high-alumina crucible inthe air. Ingots, 15mm in diameter and about 150mm in length, werehomogenized for 180ks at 723K. After peeling, they were hot-forgedrepeatedly at around 723K to plates of 5mm in thickness. The plateswere cold-rolled several times with appropriate intermediate anneal-ing at 723K to plates of 1mm thickness. Grains were coarsened by thestrain-annealIng method to about 5mm in diameter.Quenching was carried out in the same way as reported(4) andaging was done in ethyl alcohol at 293K±0.6K. Reversion was studiedboth by the isochronal and isothermal annealing in ethyl alcohol orsilicon oil. In the isochronal annealing experiment, the specimenwas annealed at every ten degrees for 0.6ks and hardness was measuredafter each annealing.Hardness was measured by Vickers microhardness tester (Akashi.MVK-E) and ultra-microhardness tester (Akashi, MZT-1). In order toexamine the variation wijh depth, hardne~s test was done at variousloads from 0.001 to 4.9N. To the same purpose surface layer wasremoved by electropolishing and then hardness was measured. Electro-polishing was carried out in the perchloric acid-ethyl alcohol <1:4)solution at 20V of voltage and 500A/m2 of current density.3. RESULTS AND DISCUSSIONFig.1 shows variation with the indentation load, of the hardnessof the alloy aged for 120ks at 293K after quenching from 723K. Thisheat treatment produces spherical GP zones in the alloy.(3) As-agedspecimen showed constant hardness from 4.9N down to 0.98N, but atlower loads hardness decreased with decreasing load. This suggeststhe existence of soft surface layer. After the surface layer 30~mEffect of Surface and Grain Boundary on the Reversion 3AI-15 mass"loZnTQ=723 KTA =293 K , tA=120 ks293 333 373 413 453Annealing Temperature (K)604012{),....--------------,AI-15mass"loZnTQ=723KTA=293K~=120ks+ -75,umPOlished:t 80101100Load (N)10-1100110>:r:Fig.l Variation with the thickness ofsurface layer removed, of the dependenceof hardness on the indentation load, forthe specimen aged for 120ks at 293Kafter quenching from 723K. Removedthickness: 0 O/lm, l:::. 30/lm, D75/lm.Fig.2 Variation of the isochronalannealing curve with the positionof indentation for the specimen agedas in Fig.l and electropolished toremove 75/lm thickness of surfacelayer. Indentation load was O.49N.o far from GB, l:::. near GB, 0 on GB.in thickness was removed, constant hardness was obtained down toO.49N, but the soft surface layer remained. When thickness of 75/lmwas removed hardness never decreased down to the smal lest load usedin this experiment, O.OOIN. These results indicate that the thick-ness of the soft surface layer after aging was less than 75/lm.The specimen aged for 120ks at 293K and electropolished by 75/lm,in which soft surface layer no longer existed, was annealed isochron-ally. Fig.2 shows variation of the isochronal curves in hardness atO.49N of load with the position of indentation in the grain. Thehardness near the grain boundary (abbreviated hereafter by GB), about70/lm distant from GB, fell more rapidly than that far apart, morethan 200/lm distant, from GB. and that at the GB still more rapidly.At the stages marked by arrows in this figure hardness at the posi-tion far from GB was measured at various loads. The results areshown in Fig.3. Although the hardness was constant irrespective ofthe load before the isochronal anneal i ng, those measured af ter 333Kannealing were lowered from the constant value at the load less thanO.98N. This indicates that the reversion began at the surface but4 Akira SAKAKlBARA and Teruto KANADANl60 ~10~ 100 101Load (N)Al-15massOfoZn1Q=623 K. TA=293 K. tA=l20ks1001101--In----iD---o-a:::-o--g.~>:r::1Q=723KTA=293K'tA=120ks-75J-1mpOlished70809010011 0 AI-15 massOfoZn>:r::Fig.3 Dependence of hardness at positionfar from GB on the indentation load atseveral stages of isochronal annealing(marked by arrows in Fig.2).o 293K, b. 333K, 0 373K, 'V393K, <> 413K.Fig.4 As Fig.l, but for the specimenaged after quenching from 623K.o O~m, b. IOO~m, 0 150~m.did not occur in the interior. As the annealing temperature waselevated, hardness of the large load was also lowered, indicating thereversion in the interior, but the surface layer was softened fasterat all stages up to 413K.Fig.4 shows variation with the load, of the hardness of the alloyaged for 120ks at 293K after quenching from 623K. Lowering ofquenching temperature leads to the formation of large ellipsoidal GPzones. (3) For the as-aged specimen. the hardness at O.98N of loadwas lower than those at larger loads. suggesting the thicker softlayer at the surface. Removal of IOO~m thickness from the surfacestill left soft layer, and after removal of 150~m thickness softsurface layer was completelY eliminated. Isochronal annealing curvesof the specimen of which the surface layer 150~m thick was removedare shown in Fig.5, classified by the position of indentation. Thereversion near GB was faster and that at GB the fastest. Hardnessmeasured at various loads was shown in Fig.6, tested at the positionof indentation far from GB for the specimen isochronally annealed upto various stages of isochronal annealing. As in the above case ofspherical GP zones, reversion of the large ellipsoidal GP zones beganEffect of Surface and Grain Boundary on the Reversion 54010010-190AI-15mass%Zn~=G23 K•TA=293K.t,.=120 ks.l!'D!J~lishaj11 OI---0-----<o--o---f,J.......-j/:l}-&.l1-O>100::c293 333Amealing60120.---------:-:-:-::-'-----.AI-15 mass%Zn~=623KTA=293K1A=120ks-150 lJmpoliShedl80Fig.5 As Fig.2, but for the specimen agedafter quenching from 623K.o far from GB, tl.nearGB, 0 on GB.Fig.6 As Fig.3, but for the isochro-nal annealing of Fig.5.o 293K, tl. 333K, 0 373K, \l 393K.120110AI-15 masso/oZnTQ=623 K •TA=293 K•tA=l20 ks..... l50!Jm. polished80o 10i 103 104Amealing Time. tA' (5).110°Load (N).1101Fig.7 Isothermal annealing curves at 373Kfor the specimen aged after quenching from623K and electropolished.o far from GB, tl. near GB, 0 on GB.Fig.8 Dependence of hardness on theload at several stages of isothermalannealing of Fig.7. 0 Os, tl.20s,o 180s, \l 3600s, <> 18000s.at the surface, and then proceeded to the interior. It is alsosuggested that the reversion of the ellipsoidal GP zones was slowerthan that of the spherical ones, perhaps due to the smaller surface-to-volume ratio. It may be considered from the thicker surface layerwhere reversion occurred fast than that in Fig.3 that the difference6 Akira SAKAKIBARA and Teruto KANADANI100 101Load (N)~120s~lXls30 10-140,I110 JL f\. '"' '"' Os~ ,-,v"'-J100 AI-15masso/oZnlQ=G23K90 TA=293K, tA=2400ks......150 IJm. polished80 TA'=453K. tA'>:x: F 20s70 tA'GO 40s50103AI-15 masso/oZnTQ=G23 KTA=293KtA=2400ks......150IJm•polishedTA'=453K5040300 102Annealing Time. tA'(s)\100 '.•90 \~......"11080>:x:70GOFig.9 As Fig.7, but for the annealingtemperature 453K.o far from GB, b. near GB, 0 on GB.Fig.l0 As Fig.S, but for the annealingtemperature 453K.o Os, b.20s, 0 408 \J 120s, <> 300s.in size of the ellipsoidal GP zones brought about larger differencein hardness than that of the spherical GP zones.Isothermal annealing was carried out at 373K for the specimenaged for 120Ks at 293K after quenching from 623K and electropolishedby 150",m. Variation of hardness with time shown in Fig.7 was de-pending on the position relative to GB corresponding to the isochron-al experiment. At several stages of the isothermal annealing depend-ence of hardness number on the load of indentation was measured. Theresults are shown in Fig.8. The hardness in the interior decreasedstead i I y but the hardness near the surf ace was lowered f as ter.Thickness of the layer of the fast reversion was corresponding to theload of 1.95N, which agreed well with the result of the isochronalannealing shown in Fig.5.In the above experiment of isochronal annealing and isothermalone at 373K, partial reversion was expected to occur because of thelow annealing temperature relative to the solvus of GP zones. Com-plete reversion at higher temperature was examined although it may bedifficult to detect the dependence on the location owing to the fastdevelopment of reversion through the specimen. Fig.9 shows isothermalannealing curves at 453K for the specimen aged for 2400ks at 293K andelectropolished by 150",m. Faster softening near GB was detected.Load dependence at several stages of the annealing is shown inFig.IO. Rapid reversion was observed, but it was detected that theEffect of Surface and Grain Boundary on the Reversion 7reversion was a little faster in the surface layer which, however,was thinner than that observed in 373K annealing. The reversion wasconsidered to be completed at 120s at the surface and at 300s in theinterior.From the results described above it is concluded that the rever-sion of the ellipsoidal GP zones was faster at the surface and at thegrain boundary. It is considered that the surface and the grainboundary plays a role of effective source for vacancies, in additionto the interior source such as dislocations, as in the case of thereversion of spherical GP zones.ACKNOWLEDGMENTThe present authors thanks Miki Co. Ltd. for the offer of conven-ience in using the equipments.REFERENCES(1) M. Ohta, T. Kanadani, M. Yamada and A. Sakakibara, J. Japan Inst.Metals, 50(1986), 887.(2) M. Ohta, A. Sakakibara, T. Kanadani and M. Yamada, J. Japan Inst.Metals, il<l987), 810.(3) S. Yoshioka, Y. Nakayama and N. Hosokawa, J. Japan Inst. Metals,31<l967), 860.(4) M. Ohta, J. Japan Inst. Metals, ll<l963), 197.

Effect of Surface and Grain Boundary on the Reversion of Age-Hardened Al-15mass % Zn Alloy

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