Aging of Al-lmass % Ag alloy at 273K after quenching under various conditions was studied by measurement of electrical resistivity. Scattering of the quasi-equilibrium value of resistivity (p(e)) was not random but closely related to the as-quenched value (p(o)); P(e) increased with increasing p(o)' When the quenching temperature (T(q)) was lower than or equal to 773K, the state at p(e) was controlled substantially by the concentration of quenched vacancy. On the other hand, when T(q)>823K, GP zones formed during quenching played an important role, instead of quenched vacancies, in determining the state

Kanadani, Teruto

Sakakibara, Akira

Okayama University Digital Information Repository

Effect of Quenching Condition on the Growth of GP Zones in Al-lmass % Ag Alloy

Okayama University Scientific Achievement Repository

Memoirs of the Faculty of Engineering,Okayama University,VoI.25, No.1, pp.l5-21, November 1990Effect of Quenching Condition on the Growth of GPZones in AI-lmass%Ag AlloyAkira SAKAKIBARA,* and Teruto KANADANI**(Received October 31 , 1990)SYNOPSISAging of Al-lmass%Ag alloy at 273K after quenchingunder various conditions was studied by measurement ofelectrical resistivity. Scattering of the quasi-equilibrium value of resistivity (P e ) was not randombut closely related to the as-quenched value (PO); P eincreased with increasing PO' When the quenchingtemperature (T q ) was lower than or equal to 773K, thestate at P e was controlled substantially by theconcentration of quenched vacancy. On the other hand,when Tq >823K, GP zones formed during quenching playedan important role, instead of quenched vacancies, indetermining the state.1. I NTRODUCT I ONGP zones in Al-Ag alloys have been studied extensively by manyworkers, but little attention has been paid to the conditions ofquenching by which super-saturated solid solution would be obtained.Ohta and Hashimoto(l) and Ohta et al(2) first found out the effect of* Department of Mechanical Engineering** Okayama University of Science1516 Akira SAKAKIBARA and Teruto KANADANIthe quenching conditions to the aging behavior of AI-I0mass%Zn alloyby measurement of ele~trical resistivity and investigated the effectin detail. According to their report, quasi-equilibrium state ob-tained after aging was closely related to as-quenched state; in thecase of low quenching temperature (Tq ), retained vacancy concentra-tion played a leading role, and in the case of high Tq , clustering ofZn atoms during quenching had an increasing effect.In this paper, dependence of the aging behavior of AI-lmass%Agalloy on quenching conditions is revealed by resistometry and therole of retained vacancies and the solute clusters are discussed.2. EXPERIMENTAL PROCEDURESAI-lmass%Ag (nominal composition) alloy was prepared by melting99.996% pure Al and 99.999% pure Ag in a high alumina crucible in theair. Such a dilute composition should be used because aging wouldoccur rapidly in a more concentrated alloy and the detail of theprocess could not be observed. Thus slight changes in aging behaviorinduced by the change of quenching condition was. able to be detectedonly by resistometry. Ingots, 15mm in diameter and about 150mm inlength, were homogenized at 773K for 180ks. After peeling, they werehot-forged repeatedlY at around 773K to plates of 5mm thickness. Theplates were cold rolled several times, with appropriate intermediateannealings at 773K, to strips of O.4mm thickness. Specimens, ofwhich the shape and size were the same as reported prevLously byOhta(3), were cut out of the strips.Solutionization was done by inserting the specimen between theslit made in an aluminum block placed in a furnace and holding it at773K for 3,6ks. It was furnace cooled to the quenching temperature(Tq ), held there for 3.~ks, and quenched by extracting from the slitand immersing into iced water quickly with manipulation. Aging wascarried out in an ethanol bath at 273K with an accuracy of +O.IK.Resistivity was measured by a conventional potential drop method,the specimen being immersed in the liquid nitrogen bath. The effectof the temperature of the liquid nitrogen was canceled by using adummy specimen in the same bath.3. RESULTS AND DISCUSSIONEffect of Quenching Condition on the Growth of GP Zones 17Fig.l shows aging curves of four specimens aged at 273K afterquenching from,673K. Resistivity of each specimen was calibrated bythe as-quenched res i s t i vi ty from 473K. The curves have a typi calfeature of formation of GP zones; resistivity increases from the as-quenched value (PO) to the maximum (Pm)' and decreases to thequasi-equilibrium value (P e ), Values of Po scatter a little, butPe's differ largely from each other without any relation to PO' Asingle sp~cimen was quenched and aged repeatedly under the sameconditiors, three curves of which are presented in Fig.2. This timealso PO's scatter, but very slightly. Pe's still take variousvalues, but the difference among them is smaller than that in Fig.l.It should be noted in particular that they correspond systematicallyto PO's. Large variation in Po and P e With specimen observed inFig.l may be due to the difference in .grain size and to the slightTherefore only one specimen was useddeviation in composition.afterward in this study.The difference in Po left when a single specimen was used isconsidered to be due to the various quenching conditions, perhapsslight difference in Tq and in cooling rate during quenching. Thecurve with the highest Po reaches the maximum value, Pm' the earli-es t 0 f the three and the one wi th the lowes t Po does the I a te.s t,which means that the higher Po value corresponds to the higher102tACks) -----.;>~i 5.75.6-E 5.5Iexc- 5.40..5.35.25,1 0 10-1 10°Fig.1 Aging curves at 273K for various secimens of AI-lmass%Ag alloyafter quenching from 673K.18 Akira SAKAKIBARA and Teruto KANADANI-0..5.35.2TQ=673 K5.1 Ljl---.........------I---~--- ........---':-:-'o 10-1 10° 10%tA(ks)-->~Fig.2 Aging curves at 273K for a single specimen of AI-lmass%Ag alloyquenching from 673K.concentration of retained vacancies and thus to the quicker formationof GP zones.Aging curves for a higher Tq • 773K, are shown in Fig.3. PO'sand Pe's still scatter, but the difference among them becomes small-er than that for Tq =673K in Fig.2. Even in this small difference.order of P e corresponds to that of PO; P e is higher according toPO' The relation between Po and P e was examined for various Tq'sand plotted with open circles in Fig.4. Strong dependence of P e onPo can be confirmed, but it seems that the dependence below 773K isdifferent from that above 773K.Aging curves for a still higher Tq , 823K, was shown with opencircles in Fig.5, where the maximum resistivity cannot be observeddue to rapid formation of GP zones during quenching. Therefore Pothen is considered to involve the contribution of the GP zones aswell as that of the retained vacancies. To investigate further theeffect of zone formation during quenching, other quenching methodswere employed so as to suppress it. Fig.6 shows schematically thethree methods employed in this study. Ordinary quenching is the oneso far employed that consists of quenching from Tq to T1 (273K icedwater) and then immersing into liquid nitrogen. In two-step quench-ing, the quenching into iced water was followed by the second quench-Effect of Quenching Condition on the Gruwth of GP Zones 19Is.,......EIac- 5.6Q..5.5o 10-1 100 10' 102tAC ks) ------.;>~Fig.3 Aging curves at 273K after quenching from 773K.f 5.7- 'TQ=8'13KEI~ 5.6-~5.55.4 7l3/J5.35,25,15,4 5.5 5,6 5,7 5,8 5.9PoCrQ-m)-+Fig.4 P e vs. Po plot. ordinary quencing,two-step quenching, direct quenching.20 Akira SAKAKlBARA and Teruto KANADANIing from 273K to 213K(dry ice alcohol) to pass faster the regionabove 213K where the formation of GP zone may be active due to soluteand vacancy supersaturation. Direct aging was carried out to omitthe process below the aging temperature(273K) until the first resis-tivity measurement at 60s of aging. This method was expected toeliminate largely the effect of the GP zones formed during quenchingto the aging in its very initial stage, where most quenched vacanciesannihilated and supersaturation of solute reduced. An aging curveobtained after the two-step quenching. is shown with closed circles inFig.5. In spite of larger cooling rate and hence more retainedvacancies, the obtained value of Po is lower than those for ordinaryquenching. This confirms that the PO's for ordinary quenchingcontained contribution not only of the retained vacancies but also ofthe GP zones formed during quenching. P e value became a littlelower accordingly. Relation of Po and P e for the two-step quenchingis plotted in Fig.4 with closed circles, where also the effect of theGP z~nes formed during quenching is clearly revealed. A direct agingcurve is represented by crosses in the same figure, of which P e ismuch lower than any other curves owing to the least effect of the GPzone formation during quenching. Pe's for the direct aging areplotted in Fig.4 with squares against the Po values for two-stepquenching of which the quenching condition may be similar to that ofthe direct aging. Increase of P e with Tq observed yet in the directaging may be due to the resistivity of the residual vacancies even inthe quasi-equilibrium stage.(2)In conclusion, aging of Al alloys in which GP zones form isremarkably influenced by the quenching condition. For the Al-Imass%Ag alloy quasi-equilibrium value of resistivity obtained after(a) Ordinary (b) Two-Step (c) DirectTQQuenching Quenching·tAging~ TQ Q:J11i! T1 T1 TA - ---.;.~ 1: T2llme Time TimeFig.5 Temperature change in various quenching method, schematically.Effect of Quenching Condition on the GTlJWth of GP Zones 21i 5.9-EI~ 5.8-0..5,75,6o 10-1 10° 102 1()I"(ks)--->~Fig.6 Aging curves at 273K after quenching from 823K. Open symbolsrepresent ordinary quenching, closed circles two-step quenching,and crosses direct aging.a long time of aging is closely related to the as-quenched resistivi-ty; the lower the PO' the lower the P e . When Tq<773K, P e value iscontrolled mainly by the concentration of retained vacancy. WhenTq >823K, on the other hand, the effect of retained vacancies de-creases and GP ~ones formed during quenching play an increasing roll.REFERENCES(1) M. Ohta and F. Hashimoto, J. Japan Inst. Metals, 36, 321(1972).(2) M. Ohta, T. Kanadani and A. Sakakibara, phys. staLsol. (a), 97,K193(1986) .(3) M. Ohta. J. Japan Inst. Metals, 27, 197(1963).

Effect of Quenching Condition on the Growth of GP Zones in Al-lmass % Ag Alloy

Abstract

Similar works

Full text

Available Versions

Okayama University Digital Information Repository

Okayama University Scientific Achievement Repository