The phenomenon of internal friction (IF) of Al-16 wt% Ag and Al-16 wt% Ag-0.28 wt% Fe alloys aged at 428 and 523 K was studied at different testing temperatures using the free-decay method. The wire specimens were investigated for IF recovery under two different conditions of excitation. In the first, the specimens were continuously excited and in the other, they were subjected to normal condition of restrictive excitation. Under certain conditions of heat treatment and mode of excitation, an anomalous behaviour of IF recovery has been observed. TEM investigations confirmed that addition of Fe to Al-Ag alloy accelerates the precipitation of metastable phases. The mean values of the excitation energy of both alloys were found to be equal to that quoted for precipitate-dislocation interactions.Proučavali smo pojavu unutarnjeg trenja (UT) u slitinama Al-16 wt% Ag i Al-16 wt% Ag-0.28 wt% Fe starenim na 428 i 523 K, metodom slobodnih oscilacija na nizu mjernih temperatura. UT uzoraka žice ispitivali smo dvama načinima uzbude. U prvom s neprekidnom uzbudom, a u drugom smo uzorke podvrgavali normalnim uvjetima ograničene uzbude. U nekim uvjetima toplinske obrade i načina uzbude, opazili smo anomalan oporavak UT. Ispitivanja pomoću TEM potvrdila su da dodavanje Fe slitini Al-Ag ubrzava precipitaciju metastabilnih faza. Našli smo da je srednja energija uzbudne energije obje slitine jednaka onoj koja se navodi za interakcije dislokacija s precipitatima

Graiss, G.

Mahmoud, A.

English

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ISSN1330–0008CODENFIZAE4EFFECT OF MECHANICAL EXCITATION ON THE OVER-RECOVERY OFINTERNAL FRICTION OF Al-16 Wt% Ag AND Al-16 Wt% Ag-0.28 Wt% FeALLOYSG. GRAISS and M. A. MAHMOUDPhysics Department, Faculty of Education, Aim-Shams University, Cairo, EgyptReceived 3 July 2000; Accepted 6 November 2000The phenomenon of internal friction (IF) of Al-16 wt% Ag and Al-16 wt% Ag-0.28wt% Fe alloys aged at 428 and 523 K was studied at different testing temperaturesusing the free-decay method. The wire specimens were investigated for IF recov-ery under two different conditions of excitation. In the first, the specimens werecontinuously excited and in the other, they were subjected to normal conditionof restrictive excitation. Under certain conditions of heat treatment and mode ofexcitation, an anomalous behaviour of IF recovery has been observed. TEM inves-tigations confirmed that addition of Fe to Al-Ag alloy accelerates the precipitationof metastable phases. The mean values of the excitation energy of both alloys werefound to be equal to that quoted for precipitate-dislocation interactions.PACS numbers: 62.40.+1, 61.72.Ff UDC 539.67Keywords: internal friction of alloys, Al-16 wt% Ag and Al-16 wt% Ag-0.28 wt% Fe alloys,aging at 428 and 523 K, free-decay method, anomalous internal friction recovery1. IntroductionThe basic sequence of decomposition in Al-Ag alloy has been found as α (fcc) →spherical Guinier-Preston (G. P.) zones (tetrahedral) → metastable γ′-phase (hcp)→ equilibrium γ-phase (hcp) [1]. The experimental evidence for the formation ofan ordered η-state of G. P. zones below 443 K and a disordered ε-state abovethis temperature has been reported by many authors [2–4]. Positron trapping ratemethod showed that the coarsening of G. P. zones takes place simultaneously withthe formation and growth of γ′-precipitates during isothermal aging above 473 K[5,6].Internal friction (IF) measurements serve to supply much useful informationconcerning the internal structure of Al-Ag alloy, including mobile dislocations den-sity and the precipitation phases which takes place during the aging process. TheFIZIKA A (Zagreb) 9 (2000) 3, 137–146 137graiss and mahmoud: effect of mechanical excitation on . . .transient increase of internal friction (IF), Q−1 (Q−1 is defined as the tangent ofthe phase angle by which the strain lags behind the stress), observed in metals afterplastic deformation is known to decrease more or less rapidly as a function of time.Such a phenomenon is known as the recovery of IF [7]. According to Granato andLücke model [8], the increase of IF after plastic deformation is due to the creationof a large number of new, freely-moving dislocations. The subsequent disappear-ance of the effect with time (i.e., recovery of IF) is ascribed either to the pinning ofdislocations by second-phase particles, or rearrangements of dislocation networks,resulting in an overall decrease in the dislocation damping.The recovery of IF after deformation is usually determined by the free-decaymethod, i.e. by forcing the specimen to vibrate and measuring the decrement ofamplitude of vibration as a function of time. Reviewing the literature on previouswork on the recovery of IF using the torsion pendulum [9–12], one can recognize,as a matter of experimental procedure, that the time intervals during which nomeasurements of IF, Q−11 take place, the specimen is normally kept steady. Thismode of excitation is considered as a restrictive excitation (RE). The followingquestion arises: if the test specimen is forced to vibrate, within the appropriatelimits of strain amplitude, during the whole time of decay (i.e. during both intervalsof measurement and between the measurements), how such a continuous excitation(CE) affects the recovery behaviour. The present work is aimed at studying theeffect of such CE on the recovery of IF of Al–16 wt% Ag and Al–16 wt% Ag–0.28wt% Fe alloys.2. Experimental procedure and techniqueTwo alloys, each containing Al–16 wt% Ag, were made by melting silver ofpurity 99.999% with either pure Al (99.999%) (alloy A), or with Al containing 0.28wt% Fe (alloy B). Each alloy was molten under vacuum in a high-purity graphitecrucible. After annealing at 823 K for four days (for homogenization), the ingotswere swagged and cold drawn into wires 0.35 mm in diameter for IF measurements,and rolled into strips 0.25 mm thich for electron-microscopy investigation. Chemicalanalysis revealed that the specimen composition is very close to the weighted-incomposition. A solution treatment was carried out by heating the specimens at823 K for two hours, followed by rapid quenching in cold water at 273 K andimmediately aged at 428 and 523 K for one hour. The rate of quenching was about104 K/s. The values Q−1 of IF were determined using a torsion pendulum of lowfrequency (about 1 Hz) with electromagnetic excitation and optical readout. Theuniaxial tensional deformation ε = ∆`/` (where ∆` is the increase of length of thespecimen and ` is its original length) was carried out “in situ” at room temperaturefor all specimens and was equal 0.04. After deformation, the specimens were setat various testing temperatures of 298, 323, 343, 363 and 383 K, and the recoverybehaviour under CE and RE was studied. All values of Q−1 were taken underconstant strain amplitude equal to 2.1× 10−4. Preliminary experiments have beendone which proved that IF doesn’t depend on the strain amplitude within the valueapplied. The accuracy of temperature measurements was about ±1 K. The sheet138 FIZIKA A (Zagreb) 9 (2000) 3, 137–146graiss and mahmoud: effect of mechanical excitation on . . .specimens for electron microscopy measurements were prepared by electropolishingusing a twin jet machine. The electrolyte was 10% perchloric acid and 90% ethanolat 273 K. The voltage and current values were around 13 to 19 V and 8 to 14 mA,respectively. Microstructure was examined using a JOEL-100 transmission electronmicroscope operating at 100 kV.3. Results and observationsBefore applying the tensile deformation to the heat-treated specimens, the valueof IF of each specimen was measured first. That value is denoted by Q−1u (i.e. Q−1corresponding to the undeformed state). Immediately after deformation, a largeincrease in IF value was observed. This value is denoted by Q−10 (Q−1 correspondingto the time t = 0). The recovery behaviour was indicated by the value Q−1t thatwas determined at time t. It was influenced by the mode of excitation (RE or CE)and by the heat treatment.Figure 1 shows the recovery of IF as a relation between the normalized IFS =Q−1t − Q−1uQ−10 − Q−1uand time. From the figure, it is clear that:1) The recovery behaviour of both alloys can be divided into three stages: (i)an initial short stage (stage I) which predominates within the first 4 minutes,(ii) an intermediate stage (stage II) which starts after stage I and extends toabout 9 minutes, and finally (iii) a slow saturation stage which takes placeafter more than 10 minutes.2) Aging the specimens at 428 or 523 K for one hour resulted in a decrease inQ−11 to the extent that the value of S became negative, which means thatunder certain conditions of heat treatment and mode of excitation, the testspecimen can be brought to a condition that its IF during a stage of recoveryperiod is less than its IF in the undeformed state. This anomalous behaviourof IF recovery is considered as an “over-recovery effect”.3) For the same alloy and the same heat treatment, the rate of decrease of S forthe CE specimens is higher compared with that of RE specimens.4) Under the same condition of heat treatment and mode of excitation, the rateof decrease of S is appreciably lower for alloy B than in alloy A.5) Under the same conditions of testing temperature and mode of excitation, Svalue for alloys aged at 523 K is higher than for those aged at 428 K.We are interested only in the first and second stages of recovery.FIZIKA A (Zagreb) 9 (2000) 3, 137–146 139graiss and mahmoud: effect of mechanical excitation on . . .(C.E) (R.E)T     K383363343323289tAlloy AAlloy B0 10 20 30 0 10 20 30Time  (min)S-0.2-0.10.00.10.20.30.40.50.60.70.80.91.0-0.2-0.10.00.10.20.30.40.50.60.70.80.91.0Fig. 1a. Normalized internal friction (S) as a function of time of deformed speci-mens aged at 428 K. The applied testing temperatures, type of alloys and mode ofexcitation are indicated.140 FIZIKA A (Zagreb) 9 (2000) 3, 137–146graiss and mahmoud: effect of mechanical excitation on . . .(C.E) (R.E) T     K383363343323289tAlloy AAlloy B0 10 20 30 0 10 20 30Time  (min)S0.00.10.20.30.40.50.60.70.80.91.0-0.10.00.10.20.30.40.50.60.70.80.91.0Fig. 1b. Normalized internal friction (S) as a function of time of deformed speci-mens aged at 523 K. The applied testing temperatures, type of alloys and mode ofexcitation are indicated.Figures 2 and 3 show the precipitation of G. P. zones in specimens aged at 428K for one hour for alloys A and B, respectively. It is clear that the sizes of G. P.zones in alloy B (with Fe addition) are coarser than in alloy A. Figure 4 shows theformation of G. P. zones and some plates of γ′-precipitates in alloy A aged at 523K for one hour.FIZIKA A (Zagreb) 9 (2000) 3, 137–146 141graiss and mahmoud: effect of mechanical excitation on . . .140 000 x 140 000 xFig. 2. Thin-film micrograph of Al–16 wt% Ag specimen heated at 428 K for 1hour, showing G. P. zones.Fig. 3 (right). Thin-film micrograph of Al–16 wt% Ag–0.28 wt% Fe specimen heatedat 428 K for 1 hour, showing G. P. of zones of larger sizes.25 000 xFig. 4. Thin-film micrograph of Al–16 wt% Ag specimen heated at 523 K for 1hour, showing G. P. zones and some plates of γ′-precipitates.4. DiscussionIt is well known that IF and its recovery are influenced by the presence of so-lute atoms and second-phase particles [8–12]. In the present work, the metastablephases, the G. P. zones and γ′-precipitates (see Figs. 2, 3 and 4) are considered assecond-phase particles, whereas they can act as major barriers or pinners for movingdislocations. Before applying the tensile deformation to the heat-treated specimen,the interaction between the pre-existing dislocations and metastable phases (as wellas other IF sources, like damping of dislocations due to the point-defect diffusion,142 FIZIKA A (Zagreb) 9 (2000) 3, 137–146graiss and mahmoud: effect of mechanical excitation on . . .grain-boundary relaxation, thermoeleasticity etc.) gives a value of IF equal to Q−1u(Q−1 corresponding to the undeformed state). Immediately after deformation, thefreshly created dislocations are vibrating with the full network loop length, causingthe highest value of IF, Q−10 (Q−1 corresponding to the time t = 0). The timedependence of Q−1t (and consequently S) after straining, as shown in Fig. 1, canbe explained by the rate of interaction of metastable phases with dislocations. Ac-cording to Chambers and Smoluchowski [13], the number of second-phase particlesper unit volume, which act as major barriers of dislocation motion, proceeds bya relatively higher rate immediately after deformation (rapid stage of recovery).As a result, the average dislocation loop length, Lp, and consequently S are bothreduced by a higher rate. In the intermediate stage of recovery, due to the relativelylower rate of interaction between metastable phases and dislocations, a relativelylower rate of reduction of Lp results in a lower rate of recovery. Finally, a slowsaturation stage is reached corresponding to a minimum value of Lp.Due to the tensional deformation, a large number of introduced dislocationsmove amongst the metastable precipitates that are known to block the dislocationmovement very effectively through the pinning process [14–17]. These pinning dis-locations “per se” may act to hinder the motion of other, unpinned dislocations tothe extent that Q−1t of a lower value than Q−1u could be obtained. This might ac-count for the observed negative value of S (over-recovery effect). The disappearanceof this effect is explained in the following discussion.The lower values of S of the CE specimens, as compared to the correspondingvalues of the RE specimens, may be related to the fact that an additional elastic en-ergy, associated with the continuous vibration, would be stored in the CE specimens[18]. Such energy is expected to increase the rate of interactions between metastablephases and moving dislocations, i.e. enhancing the process of rearrangement of dis-locations to form a more stable configuration of networks through a redistributionof a large number of pinning agents (G. P. zones and γ′-precipitates) on the dislo-cation segment. Thus the redistribution of dislocation loop lengths will be in favourof diminishing the effective loop lengths, Lp. By increasing the testing temperature,which leads to a decrease of S values, provided an explanation that the unpinningprocess is a thermally activated mechanism.The lower values of S for the specimens aged at 428 K, as compared with thespecimens aged at 523 K, is attributed to an intense interaction between the η-states of G. P. zones and dislocations. Aging specimens at 523 K for one hour leadsto partial dissolution and coalescence of G. P. zones and a formation of a smallernumber of coherent γ′-precipitates (see Fig. 4). G. P. zones of larger sizes and γ′-precipitates are known to be less effective as bariers for moving dislocations [16,17].As a result, a lower rate of recovery (i.e. a higher value of S) is expected.A close inspection of many electron micrographs aged at 428 K revealed thatG. P. zones in alloy B were present in smaller numbers and had a larger size ascompared to alloy A (compare Fig. 2 with Fig. 3). The reason is that in alloy B (Al-Ag-Fe), due to the binding between vacancies and Fe solutes, the vacancy-impurityclusters are likely to be formed [19]. These clusters might act as nucleation sitesfor a large number of small zones. Thus a large number of zones would be obtainedFIZIKA A (Zagreb) 9 (2000) 3, 137–146 143graiss and mahmoud: effect of mechanical excitation on . . .immediately after a minimum time of aging. In the course of aging time, the growthof zones at the expense of their number occured, leading to the existence of a smallernumber of large-sized zones and γ′-precipitates (see Fig. 3). Such an effect reflecteditself in higher values of S for alloy B as compared to the corresponding values foralloy A, since coarse G. P. zones and γ′-precipitates are less effective as barriersfor motion of dislocations, as mentioned above. So, one can conclude that thepresence of Fe plays a significant role in coarsening of G. P. zones and accelerationof formation of γ′-precipitates.To elucidate the mechanism governing the recovery process, one can assumethat it is controlled by the reaction-rate equationt exp(− EkT1) = const. ,where E is the activation energy per mol, T1 the testing temperature and k theBoltzmann constant. By making cuts at different values of S in Fig. 1 and obtainingthe corresponding values of ln t, a relation between these values and 1000/T1 couldbe drawn as shown in Fig. 5. The energy activating the process was calculated fromthe slopes of the straight lines obtained. The results are shown in Table 1.Table 1. Values of activation energy in kJ/mol. The accuracy of the values isapproximately ±0.2 kJ/mol.Stage Mode of Ta = 428 K Ta = 523 Ktype excitation Alloy A Alloy B Alloy A Alloy BFirst (CE) 12 10 10 10stage (RE) 12 15 11 10Second (CE) 12 15 11 10stage (RE) 18 19 15 14From Table 1, it is observed that:1) The slight differences in the values of activation energies between the contin-uous and restricted modes of excitation may be due to the additional elasticenergy associated with the continuous vibration, as mentioned above.2) The values of activation energy for both alloys are about the same. Thisindicates that the IF recovery is due to the same mechanism. The smallvalues of the activation energies indicate that the recovery process is not dueto diffusion [7]. It is more likely due to the precipitate-dislocation interactions[7,10,17].144 FIZIKA A (Zagreb) 9 (2000) 3, 137–146graiss and mahmoud: effect of mechanical excitation on . . .Fig. 5. Arrhenius plot of ln t as a function of 103/T for restrictively (×) andcontinuously (•) excited specimens. Aging temperatures, type of alloys and stageare indicated.5. ConclusionAn anomalous behaviour of IF recovery has been observed in Al-Ag alloy thathad been aged at temperatures of 428 and 523 K to form G. P. zones and γ′-precipitates. Under certain conditions of heat treatment and mode of excitation, thetest specimen can be brought to a condition that its IF during a stage of recoveryperiod is less than its IF in the undeformed state. This anomalous behaviour isconsidered as an “over-recovery” effect.References[1] T. Kanadani, T. Matsushima, N. Hosokawa, T. Kanee and A. Sakaibara, phys. statussol. (a) 124 (1991) k97.[2] G. Dlubek, G. Wendrock and K. Pavelzyk, phys. status sol. (a) 140 (1993) 311.[3] G. Bischof, V. Groger, G. Krexner and R. M. Nieminen, J. Phys.: Condensed Matter8 (1996) 7523.[4] A. Malik, B. Schonfeld and G. Kostorz, Z. Metalle 88 (1997) 625.[5] V. Shchegoleva, Fiz. Metallov i Metallovedenie 55 (1983) 59.[6] Ph. A. Dubey, B. Schonfeld and G. Kostorz, Acta Metall. 39 (1991) 1116.FIZIKA A (Zagreb) 9 (2000) 3, 137–146 145graiss and mahmoud: effect of mechanical excitation on . . .[7] G. Graiss, G. Saad and L. N. Makar, Solid State Communication 70 (1988) 375.[8] A. V. Granato and K. Lücke, J. Appl. Phys. 27 (1956) 583, 798.[9] R. Monzen, K. Suzuki, A. Sato and T. Mori, Acta Metall. 31 (1983) 519.[10] G. Graiss and G. Saad, phys. status sol. (a) 92 (1985) 489.[11] G. Graiss, G. Saad and A. Fawzy, phys. status sol. (a) 110 (1988) 481.[12] G. Graiss, G. Saad and L. N. Makar, J. Mater. Science 25 (1990) 1033.[13] R. H. Chambers and R. Smoluchowski, Phys. Rev. 117 (1960) 725.[14] T. Kanadani and A. Umaga, phys. status sol. (a) 148 (1995) k23.[15] T. Kanadani and A. Umaga, phys. status sol. (a) 151 (1995) k29.[16] G. H. Deaf, S. B. Yossef, M. A. Mahmoud, G. Graiss and M. A. Kenawy, phys. statussol. (a) 158 (1996) 471.[17] G. H. Deaf, S. B. Yossef and M. A. Mahmoud, phys. status sol. (a) 168 (1998) 389.[18] M. A. Kenawy, G. Graiss, G. Saad and A. Fawzy, phys. status sol. (a) 107 (1988) 171.[19] M. Doyama, J. Nucl. Mater. 70 (1978) 350.UČINAK MEHANIČKE UZBUDE NA PREKO-OPORAVAK UNUTARNJEGTRENJA LEGURA Al-16 Wt% Ag I Al-16 Wt% Ag-0.28 Wt% FeProučavali smo pojavu unutarnjeg trenja (UT) u slitinama Al-16 wt% Ag iAl-16 wt% Ag-0.28 wt% Fe starenim na 428 i 523 K, metodom slobodnih oscilacijana nizu mjernih temperatura. UT uzoraka žice ispitivali smo dvama načinimauzbude. U prvom s neprekidnom uzbudom, a u drugom smo uzorke podvrgavali nor-malnim uvjetima ograničene uzbude. U nekim uvjetima toplinske obrade i načinauzbude, opazili smo anomalan oporavak UT. Ispitivanja pomoću TEM potvrdila suda dodavanje Fe slitini Al-Ag ubrzava precipitaciju metastabilnih faza. Našli smoda je srednja energija uzbudne energije obje slitine jednaka onoj koja se navodi zainterakcije dislokacija s precipitatima.146 FIZIKA A (Zagreb) 9 (2000) 3, 137–146

Učinak mehaničke uzbude na preko-oporavak unutarnjeg trenja legura Al-16 Wt% Ag i Al-16 Wt% Ag-0.28 Wt% Fe

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