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Acla metal/. Vol. 34, No. 9, pp. 1847-1851, 1986 oooi-6160/86 $3.00 + 0.00Printed in Great Britain Pergamon Journals Ltd

EFFECT OF PREDEFORMATION ON THETHERMOPOWER OF AlCu ALLOY

F. ABD EL-SALAMI and S. A. KHAIRYPhysics Department, Faculty of Education, Ain Shams University, Cairo and Physics Department,

Faculty of Science, Cairo University. Cairo, Egypt(Received 12 Mu! 1985)

Abstract-Thermoelectric power (TEP) measurements for Al4 wt% Cu alloy samples in the form of wires0.5 mm diameter quenched then deformed torsionally to different degrees were carried out after isothermalannealing in the temperature range 383423 K. Precipitation of excess solute Cu atoms, as traced throughTEP measurements was enhanced by predeformation and attained different stable sizes depending on theannealing temperatures as revealed by the observed final stable TEP values. The energy activatingprecipitation process (Q) was found to be deformation dependent. The time exponent 1.06 as calculatedfrom the volume fraction of Cu atoms precipitated at a certain time t to form Guinier-Preston (GP) zoneswas found to be independent on predeformation over all the investigated temperature range and consistedwith Hams theory for precipitation on dislocations. The results were interpreted on the basis of thedominating and predeformation and the different contributions of the solute atoms when dispersed in thematrix or included in the formed GP zones to the measured TEP.Rbum&Nous avons effectut des mesures du pouvoir thermo-klectrique (PTE) sur des &chantillons dunalliage Al_4%Cu (en poids) en forme de fils de 0,5 mm de diamttre trem@s puis d&form&s en torsion Bdiffirents degrks, apres un recuit isotherme entre 383 et 423 K. La prkcipitation datomes de cuivre enexcts, suivie par des mesures de PTE, &tait favorisCe par la prCdCformation et atteignait diffirentes taillesstables selon les tempkratures de recuit, comme lont montre les valeurs finalis stables. Nous avons trouvCque ltnergie dactivation (Q) du phtnomtne de prCcipitation dCpendait de la diformation. Nous avonstrouvt. que Iexposant du temps I,06 calcult g partir de la fraction volumique datomes de cuivre prtcipitks$ un temps t pour former des zones de Guinier et Preston (GP) &tait indtpendant de la pr&dtformationpour tout le domaine de tempkratures ttudik et quil Ctait compatible avec la thkorie de Hams de laprkcipitation sur les dislocations. Nous avons interpr&C ces rtsultats B partir des micanismes de diflusiondominants, du rBle des difauts rtticulaires induits par tempe et predeformation et des diff&rentescontributions des atomes de sol& au PTE measurk, quand ces atomes sont dispersCs dans la matrice oucontenus dans les zones GP.Zusammenfassung-An 0,5 mm Diinnen Drlhten der Legierung AI-4 Gew.-% Cu, die abgeschreckt unddanach unterschiedlich stark in Torsion verformt worden waren, wurde die thermoelektrische Kraft nachisothermem Ausheilen im temperaturbereich zwischen 383 und 423 K gemessen. Die Ausscheidung deriiberschiissigen Cu-Atome, ermittelt mit der thermoelektrischen Messung, Wurde durch die Vor-verformung verstlrkt; je nach Auscheiltemperature ergaben sich unterschiedlicht stabile GrGBen derAusscheidungen, wie die beobachteten Endwerte der thermoelektrischen Kraft anzeigen. Die Aktivierung-senergie des Ausscheidungprozesses hing von der Verformung ab. Der Zeitexponent 1,06, berechnet ausdem Volumanteil der bei einer gewissen Zeit t in Guinier-Preston-Zonen ausgeschiedenen Cu-Atome, warim gesamten untersuchten Temperaturbereich unabhingig von der Vorverformung und entsprach derTheorie von ham Fiir die Ausscheidung an Versetzungen. Die Ergebnisse werden auf der Grundlage derdominierenden Streuprozesse, der Rolle der durch die Abschreckung und die Vorverformung eingefiihrtenDefekte und der unterschiedlichen Beitrige der Fremdatome, ob verteilt in der Matrix oder enthalten inden Guinier-Preston-Zonen, gedeutet.

INTRODUCTION

When a metal is isothermal at temperature T, elec-tron collisions with phonons or with static defects inthe lattice such as impurities or physical imper-fections such as dislocations will tend to restore theelectrons to overall thermodynamic equilibrium. Butwhen a temperature gradient is present electrondiffusion and phonon-drag effect take place. It wasindicated [l] that, thermoelectric power (TEP) mea-surements were consistent with a phonon-drag com-ponent which is appreciable at room temperature andpersists up to 700 K. Although the diffusion S, and

phonon-drag S, components of TEP were supposedto be linearly [2] dependent on temperature, yet it wasfound [3] that, the diffusion component of the TEPof Al cannot be represented completely by a termlinear in temperature. Nielsen and Tylor [4] derivedan expression for the diffusion TEP of dilute alloywhich predicts effects that in the past have beenascribed solely to phonon-drag. Possibly theNielsen-Taylor effect and phonon-drag are com-plementary in many alloys [5].

It was concluded [6] that, TEP is potentially auseful probe for the study of precipitation phenom-

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Measurements on Al alloy systems [3] emphasizedthat the TEP is extremely sensitive to the presence ofconstituent elements in solid solution and the an-nealed state is considerably different from that in thecold-worked condition. On the basis of the simplemodel of dilute alloy TEP proposed by Gold et al. [7]which assumes different independent scattering mech-anisms, the TEP is given by

s =1 Pisi l CPtI ,

where S, and pi are the TEP and the resistivity of theith scattering mechanism.

The dependence of TEP on structural variations istherefore valid through the structure dependence ofelectrical resistivity. For Al4 wt% Cu alloy thechange of electrical resistivity during isothermalaging may has the form [8]

AP =P -((PT+~,&) (2)where pT is the contribution from lattice vibrations,pi is the residual resistivity and m, is the alloycomposition.The present work is an attempt to throw lights onthe structural variations through the time dependenceof the TEP for AlA wt% Cu alloy aged at different

1848 ABD EL-SALAM and KHAIRY: THERMOPOWER OF AlCutemperatures. Also, it is aimed at studying the effectof predeformation on the aging process of this alloy.

EXPERIMENTAL

The investigated Al4 wt% Cu alloy in the form ofwires 0.5 mm diameter is composed from high purity(99.999) Al and Cu. The TEP was determined by themeasurements of the temperature dependence of theSeebeck potential from a thermocouple composedfrom the alloy and Pt as a suitable reference elementin the presence of temperature gradient.

Measurements of temperature were carried outwith Pt and Pt-10% Rh thermocouples. The emfsfrom all thermocouples were determined with a pre-cision of 0.1 PV using a direct current PYE precisiondecade potentiometer.

The samples were solution heat treated at 813 Kfor 1 h then quenched in water at room temperature.Immediately after quenched the specimens were tor-sionally deformed to different degrees determined bythe dimensionless quantity ND/ L with N the numberof turns twist, D and L the diameter and length of thesample respectively. The deformed samples were agedat different temperatures and the TEP values corre-sponding to different annealing times were obtained.

Ti me, ! (m n)

NO/ LEO. 0

17. 2

. u. 16. 4$ 17. 2F

19. 0 tq 423

Fig. 1. Time dependence of the TEP of Al4 wt% Cu alloy samples deformed to different ND/L valuesthen aged at the indicated temperatures.

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ABD EL-SALAM and KHAIRY THERMOPOWER OF AlCu 1849

Fig. 2. TEP dependence of predeformed Al-4 wt% Cu alloysamples.tRESULTS lo/ 2.6

1000/T CK-1igure 1 shows the time dependence of the TEP Sfor Al4 wt% Cu alloy samples predeformed todifferent degrees after quenching then isothermallyannealed in the temperature range 383-423 K. Ingeneral, TEP increased and reached stable valuesdepending on both the annealing temperature and thedegree of predeformation. The irregular contributionof predeformation to the TEP of quenched samplesbefore aging is shown in Fig. 2.The predeformation dependence of the final TEPstable values is given in Fig. 3. Figure 4 shows theequivalent times and temperatures required to reacha certain value of the undeformed samples as deducedfrom Fig. 1, from which the activation energy wasobtained, In a similar manner, the activation energiesfor the predeformed states were determined. Prede-formation dependence of the energy activating theisothermal annealing process is shown in Fig. 5.The kinetics of the annealing process was analysedusing the equation [9]

1 --w =exp(-pt)K (3)where w is the fraction of the solute precipitated attime t, K is the time exponent constant and p isconstant. Equation (3) when rearranged giveslog In [l/( 1 - o)] = K log t + K log /I. (4)Using Fig. 1 and plotting the relation betweenlog ln[I/( 1 - u)] vs log t, straight lines were obtained

K423413403393383

17 I I I I0 0.06 0.12 0.18 0.24

AWL

Fig. 3. Final stable values of TEP dependence of the degreeof predeformation for Al-4 wt% Cu alloy samples aged at

_ 16.2 _.---ND/L= 0.0r E=0.35 ev

Fig. 4. Equivalent times and temperatures for undeformedsamples.

with siopes equal to the time exponent K. Figure 6(a)gives K values of samples deformed with differentdegrees then annealed at 383 K. Calculations showedthat K values do not deviate significantly from theirmean. For the other temperatures, curves like thatof 383 K were found to have the same features ofFig. 6(a).

For each degree of predeformation, the meanvalues of K for all temperatures are given in Fig. 6(b).It is clear that the mean values of K are independenton the degree of predeformation and amounted to thevalue 1.06.

0.1 I I I I I0 0.06 012 0.16 0.24

ND/iFig. 5. Predcfo~ation dependence of the activation energyQ.

Fig. 6. (a) Time exponent K for alloy samples predeformedto different degrees then annealed at 110C. (b) Timethe indicated temperatures. exponent mean values K for samples aged at different^ .^oegrees 01 preoetormatton.

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1850 ABD EL-SALAM and KHAIRY: THERMOPOWER OF AlCuDISCUSSION

TEP as scattering dependent phenomenon could berelated proportionally to electrical resistivity pthrough equation (1). Electrical resistivity is given bysumming the values associated with scattering byphonons, single solute atoms and Guinier-Preston(GP) zones as [8]

P =PT+(~ -w )mp:+nG(D) (5)where m is the solute concentration in the matrix, nis the number of zones per unit volume, and G(D) isthe contribution to the resistivity from one zone withan average diameter of D. Accordingly, the variationsof TEP will behave like those of resistivity as wasobserved [lo] for lead.

In supersaturated solid solution of Al-base alloys[1 I] metastable, solute rich GP zones are formed inpreference to a stable phase. It was found [S] that, GPzones formed during isothermal aging inAl4 wt% Cu alloy increased the resistivity of thealloy and that the contribution to resistivity from asolute atom in the zone is larger than one in thematrix. Accordingly, the TEP of the aged alloy isexpected to be larger than that of the quenched state(solid solution) in which Cu atoms are homoge-neously distributed in the matrix. This explains theincrease in the measured values of TEP in Fig. 1 byisothermal aging of the quenched samples.

According to HEW theory [12], the resistivitymeasured during GP zones formation is treated as aparallel circuit because the Fermi surface of pure Aland its dilute alloys has been recognized [8] to be anincomplete sphere divided into two parts-with andwithout Bragg scattering. In terms of conductivitytwo partial conductivities exist. As precipitation pro-ceeds one partial conductivity increases because ofdecreasing solute concentration in the matrix. Theother conductivity decreases because the Bragg scat-tering increases as the GP zones grow in size. Thesynthesis of both conductivities produces a minimum.Accordingly, the resistivity and in turn the TEP showmaxima which is clear from the stable values given inFig. 1, as obtained during the growth of GP zones,with levels described by the time evolution of theentire microstructure at the different investigatedtemperatures.It was found [13] that, predeformation activatedthe formation process of zones and precipitates andincreased the rate of rise [141of resistivity in the earlystages of aging and produced a more rapid fall-off.The defects created by predeformation increase thescattering centres leading to the overall observedincrease in the TEP of the deformed samples beforeaging as shown in Fig. 2. The decrease in the numberof scattering centres caused by partial annihilation ofpredeformation induced defects might explain theobserved decrease in the TEP corresponding tohigher degrees of predeformation as clear in Fig. 2.For longer annealing times corresponding to themaximum number of precipitated solute atoms in the

zones, the values of TEP in Fig. 3 seem to dependmainly on the interaction between the existing defectsand solute Cu atoms. In Fig. 3, the expected tem-perature dependence of TEP took place but prede-formation caused irregular behaviour for the finalstable values of TEP. Small degrees of prede-formation accelerated precipitation and increased theTEP to maximum value. Increasing predeformationprobably increased the annihilation of vacancieswhich reduced the number of solute atoms migratingto the zones and consequently lowered the TEP to theobserved minima for all aging temperatures. Theanomally observed in Fig. 3 for TEP curve at 403 Kmight be due to the following. This temperature403 K was found [15] to be more suitably for theformation of the first kind of GP zones in Al-Cualloy.

Before 403 K more solute Cu atoms exist in thematrix because the number of formed zones is lessthan that at 403 K. While after this temperature theformation of the second type of GP zones takes placeon the expense of the first formed GP zones [16]. Thedissolution of the first zones increases the concen-tration of the solute atoms in the matrix and lowersthe corresponding TEP.The activation energies for the annealing process asgiven in Fig. 4 and its relation to the degree ofpredeformation in Fig. 5 can be explained as follows.Vacancies retained by quenching might formvacancy-copper pairs which migrate behind dis-locations leading to the formation of copper richzones. This view agrees with the calculated activationenergy 0.35 eV for undeformed samples, which iseffectively equivalent to the difference between theenergy for vacancy migration [17] in Al (0.55 eV)minus the binding energy for a vacancy-copper pairreported [161 as 0.2 eV. Increasing predeformationaccelerated the whole precipitation process [131 andlowered the activation energy to about 0.2 eV. Thisvalue suggests an increased probability of divacancyformation [18] by predeformation. The migration ofthese divacancies with 0.2eV, which is nearly theenergy needed for divacancy [19] migration in Al,seems to play essential role in the formation of zones.

Further increase in predeformation increased thedislocation density and lowered the density of mobiledislocations. The less mobile intities therefore, neededhigher energies (Fig. 5) to complete the annealingprocess.The mean values of the time exponent given in Fig.6(A) shows the independence of the annealing mech-anism on deformation. The mean value 1.06 for thetime exponent given in Fig. 6(B) for all the degreesof predeformation agrees well with the value of unitypredicted by Ham [20] for precipitation on long thincylinders, such as dislocations.Acknow l edgements-The authors are indebted to ProfessorDr R. Kamel, Faculty of Science, Cairo University, for hiskind help and encouragement during this work.

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3.4.5.6.1.8.9.

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