secondary precipitation

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Acta Materialia 51 (2003) 5151–5158 www.actamat-journals.com

Secondary precipitation in Al–Zn–Mg–(Ag) alloysC.E. Macchi a, A. Somoza a, ∗ , A. Dupasquier b , I.J. Polmear c

a Instituto de Fisca de Materials Tandil, IFIMAT, Universidad Nacional del Centro de la Provincia de Buenos Aires and CICPBA, Pinto 399 B7000GHG Tandil, Argentina

b INFM and Dipartimento di Fisica, Politecnico di Milano, Piazza L. da Vinci 32, 20133 Milan, Italyc Department of Materials Engineering, Monash University, Clayton, Vic. 3168, Australia

Received 28 March 2003; accepted 27 June 2003

Abstract

Secondary ageing of age-hardenable aluminium alloys occurs at temperatures below the solvus of GP zones after apreliminary ageing at a higher temperature. The phenomenon has technological interest, as it may be included in heattreatments giving a substantial benet on the mechanical properties. In the present work, positron annihilation lifetimespectroscopy (PALS) is applied in combination with Vickers hardness measurements for an investigation on secondaryageing of Al–4wt.%Zn–3wt.%Mg– xAg, where x = 0, 0.1, 0.2, 0.3, 0.5 wt.%. Ageing regimes have been characterisedby the substantially different evolutions that are observed. The results shed light on the interplay between the formationof coherent solute aggregates (clusters or GP zones) and the precipitation of semi-coherent or incoherent precipitates,which are in competition to control the hardening effects. PALS data show that secondary ageing in the ternary Al–Zn–Mg alloys produces coherent aggregates even in the presence of a well-developed stage of semi-coherent or incoherentprecipitation that is obtained if the alloys are rst aged to peak hardness. In the presence of Ag, on the contrary, theeffects of coherent aggregation during secondary ageing are observed only if the preliminary ageing is interrupted wellbefore reaching peak hardness.© 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Aluminium alloys; Age hardening; Secondary ageing; Positron annihilation spectroscopy

1. Introduction

Alloys based on the Al–Zn–Mg system arewidely used in the aircraft industry because of theirhigh response to age hardening. The complete age-ing sequence in a wide range of ternary compo-sitions is: supersaturated solid solution → GP(Zn,

∗ Corresponding author. Tel.: +54-2293-44-2821; fax: +54-2293-44-4190.

E-mail address: [email protected] (A. Somoza).

1359-6454/$30.00 © 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.doi:10.1016/S1359-6454(03)00364-1

Mg) zones→ η (hexagonal) → η (MgZn 2 ,

hexagonal) [1]. Furthermore, in alloys with rela-tively high Mg contents, complex cubic precipitateT, (Al, Zn) 49 Mg32 , may also form. In most ternaryAl–Zn–Mg alloys, maximum hardening occurs atageing temperatures up to 130 °C when themicrostructure consists mainly of Guinier-Prestonzones (GP zones). It has been generally acceptedthat the intermediate precipitate η nucleates from,or at the sites of these zones, although recent work has suggested that this may not necessarily be so[2]. This work has shown that, for the alloy Al–


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4Zn –3Mg (all compositions in wt.%) aged at 150°C, it is the equilibrium phase T rather than η thatprecipitates and causes hardening.

Earlier work revealed that microalloyingadditions of silver ( 0.1 wt.%) had the uniqueeffect of stimulating enhanced age hardening internary and more complex Al –Zn–Mg alloy s agedat higher temperatures ( 120 –220 °C) [3–5].Indirect evidence suggested that these effectsmight arise because of a preferred interactionbetween Ag atoms (possibly in conjunction withMg atoms) and vacancies, which stimul atesnucleation of the intermediate precipitates η [5].Recently, atom probe studies on the same alloys

have indicated that early clus teri ng of Zn and Agatoms may also be signi cant [2] . Positron annihil-ation lifetime spectroscopy (PALS) has also beenapplied for the study of the microstructural evol-ution of alloys Al –4Zn –3Mg – xAg ( x = 0–0.5wt.%) following preliminary ageing at 150 °C, incorrelation with Vickers microhardness measure-ments ( H V ) [6] . It may be recalled that PALS is anexperimental technique with enhanced sensitivityto open volumes (vacancies, vacancy –solute clus-ters, mis t at precipitate –matrix interfaces). In theeld of age hardening, PALS and other techniquesin the more general family of positron annihilationspectroscopy (PAS) can be applied to the study of the vacancy –solute interactions, which are crucialto determining the kinetics of the solute aggre-gation process, and to the detec tion of semi-coher-ent or incoherent precipitation [7].

In the present work, the investigation on Al –Zn–Mg–(Ag) alloys by PALS and H V of Ref. [6] hasbeen extended to secondary ageing, i.e. thedecomposition stage that occurs at temperaturesbelow the solvus of Guinier-Preston zones after the

interruption of ageing at a temperature typical of the T6 temper. The technological interest of sec-ondary ageing stems from the improvement of mechanical properties that has been obtained foran extended range of Al-based alloys with novelmulti-stage heat treatments that includ e secon daryageing as an intermediate or nal stage [8–10] . Thesensitivity of PALS to secondary ageing was rstobserved with Al –Cu–Mg –based alloys [11,12] ;recently, PALS was used in combination with H Vand differential scanning calorimetry (DSC) for a

detailed stud y regard ing secondary ageing of Al –4Cu –0. 3Mg. [13,14] . The present work is the rstapplication of PALS to the study of secondary age-

ing in Al –Zn–Mg-based alloys.The paper is organised as follows: Section 2

gives information regarding the details of alloypreparation and of the experimental methods; Sec-tion 3 presents experimental data on ageing atroom temperature occurring as a primary stage, i.e.immediately following the solution treatment; Sec-tion 4 presents data on secondary ageing at roomtemperature, in correlation with the alloy con-ditions reached at the interruption of the primaryageing at 150 °C. In Section 5, a summary of the

main results of the work leads to more general con-siderations.

2. Experimental

The alloys studied in the present work were theternary system Al –4Zn –3Mg (alloy 4300, compo-sition in at.%: 1.7 Zn, 3.4 Mg) and the quaternarysystems 4301, 4302, 4303, 4305, derived from4300 with Ag additions from ~0.1 wt.% (~0.026at.%) to ~0.5 wt. % (~0.13 at.%). These alloyswere prepared using high-purity materials thatwere melted in graphite crucibles, degassed, chillcast and hot forged to produce 6 mm thick strips.Samples for hardness testing and PALS measure-ments (1.8 × 20 × 20 mm 3) were prepared fromthese strips by spark cutting and polishing. Allalloys were solution treated at 460 °C, quenchedinto water at RT, aged for various times in aglycerine oil bath at 150 °C, quenched again andleft to age at 20 °C (RT). The lifetime spectrometerwas a fast –fast timing coincidence system with atime resolution (FWHM) of 255 ps. A 20 µCi

source of 22

NaCl deposited on a thin Kapton foil(7.5 µm) was sandwiched between two identicalalloy specimens. After subtracting the sourcecomponent, the spectra with about 10 6 coinci-dences were satisfactor ily analysed using thePOSITRONFIT program [15] as a single decayingexpon ential ( for a discussion on this analysis, seeRefs. [7,16] ). Vickers microhardness measure-ments were performed after each positron lifetimemeasurement by using a load of 300 g. Lifetimespectra and hardness data were always taken at RT.


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Fig. 3. On the left side, effects of preliminary ageing at 150 °C for the alloy Al –4Zn –3Mg –0.3Ag (wt .%) on positron lifetime (upperpanel) and Vickers hardness (lower panel). Data points from Ref. [7] and solid curve from Ref. [5] ; open points are measured for x = 0.5 at.% Ag. The vertical line at about 8 min marks the incubation time for initial loss of coherence. On the right side, effectsof secondary ageing at RT. The horizontal lines crossing from the left- to right-side panels connect the points reached after preliminaryageing with the corresponding secondary ageing.

not modify the type of regime, although it stronglyaffects kinetics and the morphology of the precipi-tation. This is consistent with rec ent results of

HRTEM and 3D-AP experiments [2], showingthat, in the ternary alloy with the same compositionas in the present work (Zn:Mg atomic ratio 0.5), precipitation consists of the equilibrium cubicT phase, whereas in the presence of 0.1 at.% of Ag the loss of coherency occurs most probably bynucleation of the η phase from solute clusters witha Zn:Mg ratio near to 1. The ner dispersion of η particles is seen as the reason for the increasedhardness. However, it must be observed that theextreme sensitivity of PALS to any structure asso-

ciated with open volumes enables one to observean initial loss of coherency much earlier (less than10 min at 150 °C for alloys 4303 and 4305) than

the stage when η particles can be directlyobserved by el ectr on microscopy (2 h at 150 °Cfor alloy 4305 [2]).

4.2. Secondary ageing at 20 °C

The T6I4 t empe r (I = interrupted), described indetail in Ref. [10] , consists in a two-stage thermaltreatment. In the rst stage ( preliminary ageing )the sample is held at a temperature above thesolvus of GP zones (150 °C in the present


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experiment) for a short period of time relative toa full T6 temper, thereby producing an underagedalloy. Depending on the duration of the rst stage,primary hardening precipitation develops to differ-ent levels. The rst stage is then interrupted byquenching at RT, to retain a maximum supersatu-ration of residual solute and vacancies. The secondstage of the T6I4 temper is secondary ageing ,which is of particular intere st in the presen t investi-gation. The right panels of Figs. 2 and 3 show thekinetics of hardness and positron lifetimes duringthe secondary ageing process, for different dur-ations t pa of preliminary ageing. The horizontallines crossing from the left to the right panels con-

nect each secondary ageing curve with the corre-sponding point at the end of the preliminary age-ing. The time co-ordinate of this point gives theappropriate t pa value for the secondary ageingcurve. Similar data were also collected for silverconcentrations x = 0.1, 0.2, and 0.5 wt.%. From x = 0% to 0.3% all results scale smoothly; the case x = 0.5% is very similar to that for 0.3%. Theanalysis and discussion of the different evolutionsare considered below.

4.2.1. Coherent regime at RT (t pa = 0)This is just natural ageing at 20 °C, due to the

formation of coherent structures (clusters and thenGP zones), which has been discussed above. Fig.2, regarding the ternary alloy, shows that naturalageing results in a decrease of the positron lifetimefrom t 0~208 ps toward an asymptotic valuet ~202 ps, whereas there is an increase of thehardness from 5 7 VH N to about 105 VHN (inaccordance with Fig. 1 ). The comparison with theleft panel helps to note that the nal value of thehardness is above the level obt ained w ith a full T6

temper at 150 °C (85 VHN). Fig. 3 shows verysimilar behaviour of the hardness –time curve forthe quaternary alloy. However, in this case, themaximum hardness reached with natural ageing iswell below the peak hardness obtained with thetreatment at 150 °C. The positron lifetime curvefor the quaternary alloy differs from that of theternary alloy essentially because of the lower valueof t (~196 ps instead of 202 ps). The reason canbe the presence of Ag near to the vacancies trappedinside the solute clusters. The concentration of Ag

in these clusters is known to be well above theaverage concentration of the alloy [2].

4.2.2. Predominantly coherent regime (early primary ageing: t pa from 10 to 120 min for x = 0and from 2 to 10 min for x = 0.3%)

Positron lifetimes show a non-monotonic behav-iour beginning below t 0 , but still ending near tot . Similar non-monotonic trends have beenobserved for Al –Cu–Mg-based alloys, and havebeen attributed to the changing chemical compo-sition of vacancy –solute aggregates, due to a dif-ferent temperature depe ndence of the aggregationkinetics of Cu and Mg [11,12] . The same expla-

nation can be applied to the present situation, withZn playing the same role as Cu. The initial pointof each H V curve increases slightly with t pa , butthe strongest increase occurs during the secondaryageing, up to a nal point still around 105 VHN.Note that this stage includes the incubation timefor the loss of coherence (about 90 min). Thechanges in hardness and positron lifetime occur-ring after this limit can include an irreversible con-tribution from semi-coherent/incoherent particles.However, the nal values reached after secondaryageing, which are the same as without preliminaryageing, show that this contribution is negligible incomparison with that due to new coherent struc-tures formed at RT because of the residual super-saturation of solute.

4.2.3. Transition regime (intermediate primaryageing: t pa from 120 to 1000 min for x = 0)

Due to the rapid onset of precipitation in thepresence of Ag, the intermediate stage wasobserved clearly only for the ternary alloy. Thepositron lifetime in this alloy decreases monoton-

ically, indicating the formation of coherent struc-tures. However, the initial point, at the end of thepreliminary ageing, is above t 0 and the nal valueis also above t . Most probably, this is the effectof positron trapping at the mis t surfaces of semi-coherent or incoherent precipitates, which is notanymore negligible in comparison to trapping atvacancies inside coherent structures. The effect of precipitation during the preliminary ageing on the H V curves is also evident in the increase of theinitial point of H V . However, the hardness value


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reached after long secondary ageing is essentiallythe same obtained with the simple T4 temper.Clearly, the hardening effect of coherent zones for-

med from the residual supersaturation after the pre-liminary treatment is the dominant one.

4.2.4. Incoherent (semi-coherent) regime at 150°C (t pa from 360 min for x = 0 and from 60 min for x = 0.3%)

For the ternary alloy, the hardness curves reacha peak value at 85 VHN. With long ageing times,the formation of semi-coherent or incoherent pre-cipitates leaves almost no residual solute for pro-ducing additional hardening due to further coherent

aggregation. However, the small decreasing trendof the positron lifetime curves (from 218 to 215ps) demonstrates that some structural evolutionstill occurs during secondary ageing. Most prob-ably, this is due to a slight excess of solute becauseof the solubility change with the temperature thatgives a degree of supersaturation suf cient for for-ming a small concentration of new coherent zones.The changes of the positron lifetime that areobserved during the secondary ageing would thusbe due to a balance of effects between coherentand incoherent particles in competition. In thepresence of Ag, the situation appears different:during the RT stage, H V and t remain constant atvalues depending only on t pa . This means that thestrong effect of semi-coherent or incoherent par-ticles becomes rapidly dominant, even if the treat-ment at 150 °C is interrupted before peak ageing. If any coherent structure is formed during secondaryageing, this has not been detected using the charac-terisation techniques adopted in the present work.

5. Conclusions

The main results of the present work come fromthe correlation between hardness and positron life-time curves, and from the observation of effectsof secondary ageing at 20 °C that occur followingdifferent periods of preliminary ageing at 150 °C.They can be summarised as follows:

The incubation time for the loss of coherencyis unambiguously determined from positron life-

time curves taken during arti cial ageing at 150°C; it marks the separation between a coherentregime (temporary formation of solute clustersand GP zones), which gives no signi cant hard-ening at this temperature, and the onset of theincoherent regime, corresponding to a rapidincrease of H V .

Secondary ageing effects at RT are observed forthe ternary as well as for the quaternary alloys,with features depending on the duration t pa of the preliminary ageing.

For t pa below a certain critical time, which isgreater than the incubation time mentionedabove, one has the coherent regime. In this

regime, the formation of new solute clusters andGP zones becomes dominant over the smallconcentration of semi-coherent or incoherentparticles formed during the preliminary treat-ment. Further hardening occurs up to levels sig-nicantly higher than at the end of the prelimi-nary ageing.

In the coherent regime, the non-monotonictrends of the positron lifetime curves indicatethat the chemical composition of solute clustersand GP zones formed during secondary ageing

at RT is different (probably with higher Mgcontent) from the composition of the structuresformed during the preliminary ageing.

For t pa above the critical time, one can have anintermediate regime or, in a more abrupt way,pass directly to the incoherent regime, wherehardening effects become weaker or disappear.In this case, the precipitates formed during thepreliminary ageing are dominant. However, asmall decrease of the positron lifetime, observedat least for the ternary alloy, shows that limitedformation of coherent zones may still occur.This is an interesting result, since the excess sol-ute, still present after the formation of incoher-ent precipitates, might be expected to be simplyabsorbed into the existing precipitates withoutnucleating new coherent structures.

The presence of Ag, which gives a small initialreduction of the hardening rate at RT, acceler-ates the precipitation at 150 °C. It is known thatthis is due to the precipitation of nely distrib-uted η particles [2].


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From the above results, it is possible to drawsome general indication regarding secondary age-ing phenomena. It is clear that the key factor con-

trolling hardening effects is the balance betweenprecipitates formed during the preliminary ageingand the coherent aggregates (solute clusters and GPzones) nucleated during subsequent secondary age-ing. This balance depends on the level of residualsupersaturation of solute and vacancies remainingafter preliminary ageing, and the relative hardeningassociated to coherent aggregates on the one hand,and to semi-coherent or incoherent precipitates onthe other hand. In the present work, the comparisonof Al –Zn–Mg and Al –Zn–Mg –Ag was instructive

on this aspect.

Acknowledgements

A. Somoza and C. Macchi acknowledge the n-ancial support of the Consejo Nacional de Invest-igaciones Cient ı́cas y Té cnicas (PIP/BIDN°4318/97), Agencia Nacional de Promocio ´nCient ı́ca y Tecnolo´gica. (PICT N ° 0192/97),Comisió n de Investigaciones Cient ı́cas de la Pro-vincia de Buenos Aires and Secretar ı́a de Cienciay Té cnica (UNCentro), Argentina. A. Dupasquieracknowledges the nancial support of the Minis-tero dell ’Istruzione, Universita ` e Ricerca (FIRB2001).

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