Proceedings of the 9th International Conference on Aluminium Alloys (2004)
Edited by J.F. Nie, A.J. Morton and B.C. Muddle© Institute of Materials Engineering Australasia Ltd666

Formation Mechanisms of Precipitate Free Zones in Age-Hardenable
Al Alloys
S. Hirosawa, Y. Oguri, T. Ogura, T. Sato
Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, O-okayama, Meguro-ku,
Tokyo 152-8552, Japan.
Keywords: Precipitate free zone, Al-Cu(-Sn) alloy, Al-Zn-Mg(-Ag) alloy, Microalloying elements,Microstructure, Computer simulation

Abstract
The formation mechanism of precipitate free zones (PFZ) has been clarified for Al-Cu(-Sn)
and Al-Zn-Mg(-Ag) alloys using transmission electron microscopy (TEM), energy
dispersive X-ray analysis (EDX) and a Monte Carlo computer simulation. The vacancy
depletion mechanism was found to work predominantly in the early stage of aging if thealloys are quickly quenched and aged at moderate temperatures. The simulation model
taking into account the effects of vacancy sink well reproduced the experimental results,suggesting that the initial microstructures before aging are quite important to control themicrostructure of the vicinity of grain boundaries.

1. Introduction
Precipitate free zones (PFZ) are often observed adjacent to grain boundaries of agehardenable Al alloys. Two theories are widely accepted to explain the formation
mechanism of PFZ by taking into account either solute depletion [1] or vacancy depletion[2, 3] in the vicinity of grain boundaries. However, it is still unclear which mechanism ofthem works predominantly in an alloy subjected to a particular heat treatment.
Experimental approaches to verify the solute depletion mechanism seem to be easier
because microchemical analysis such as energy dispersive X-ray (EDX) analysis can beavailable nowadays. On the other hand, the vacancy depletion is hardly detected throughany experimental techniques because PFZ is a quite narrow microstructure of nanometerscale compared with abundant transgranular regions. Note that information from positronannihilation measurement is averaged over the specimen, resulting in theundistinguishable estimation of vacancy concentration within PFZ from that inside grains.
Therefore, our developed Monte Carlo computer simulation [4-6] becomes extremely
effective in predicting the atomistic behavior of vacancies during the formation of PFZ.
In this work, the formation mechanism of PFZ has been clarified for age-hardenable Al
alloys such as Al-Cu and Al-Zn-Mg alloys using transmission electron microscopy (TEM)
and EDX. A Monte Carlo simulation model was established to mimic the vacancy depletionmechanism by following the actual procedure of heat treatments. The reason why some
additional elements could decrease the width of PFZ was also considered based on theinteratomic interactions utilized in the simulation model.

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The characteristic features of individual elements well explained not only which formation
mechanism of PFZ predominantly works in an Al alloy but also why the modification ofPFZ could be achieved by the addition of some microalloying elements.

2. Experimental and Simulation Model
The chemical compositions of the investigated alloys are Al-4.0Cu (-0.05Sn) and Al-4.9Zn1.8Mg (-0.3Ag) (in mass%). The cold-rolled sheets were solution-treated at 793 and 743K
for 3.6ks, followed by iced-water quenching, respectively. The aging treatment was carriedout at 433K for various aging times. TEM observation and EDX analysis was performed
using a JEOL 3010 transmission electron microscope at an accelerating voltage of 300kV.
A simulation model for the vacancy depletion mechanism was set up on a rigid threedimensional fcc lattice with 50×50×100 unit cells (total lattice sites: one million) under theperiodic boundary condition. As initial simulation microstructures in three regions sitting insequence from a grain boundary, all the lattice sites were occupied at random by Al andconstituent atoms and vacancies with the same composition as that of the experimentallyinvestigated alloy. These microstructures correspond to those under the solution-treatment
condition and subsequently experienced the actual procedure of heat treatments; i.e.
quenching to 273K and aging at 433K, resulting in the different distributions of solute-richclusters in the three regions. Note that in this simulation model vacancy concentrations
were changed depending on the distance from the grain boundary in order to take into
account the effects of vacancy sink during quenching. The actual procedure of diffusion ofatoms and the derivation method of simulation parameters are described in [4-6].

3. Results and Discussion
3.1 Formation Mechanisms of PFZ
Figure 1(a) and 2(a) illustrate TEM micrographs showing the existence of PFZ along grainboundaries of the Al-Cu and Al-Zn-Mg alloys aged at 433K for about 10ks. The
transgranular precipitates have been identified to be GP(1) zones for the Al-Cu alloy and η’for the Al-Zn-Mg alloy, respectively. The corresponding EDX profiles across the grainboundaries revealed that solute concentrations are highly maintained even within PFZ inthe under-aged condition. Similar supersaturation of solute atoms has been reported forAl-Zn-Mg [7, 8] and Al-Mg [9] alloys, suggesting that the vacancy depletion mechanism
mainly works for the formation of PFZ in the earlier stage of aging. It should be also notedfrom Figure 1(a) and Figure 2(a) that the number density of precipitates gradually
decreases towards grain boundaries with a slightly larger size than that within grains.
On the other hand, totally different trends were observed in the over-aged condition
although the precipitates inside grains changed to GP(2) zones (and scarcely θ’-Al2Cu)
and η-MgZn2 (Figure 1(b) and Figure 2(b)). The continuous decrease in soluteconcentrations was observed not only within PFZ but also even outside PFZ, leading to a
proposal that there is a newly defined region of ‘transition area’ between PFZ and grain [10,11]. Such depletion of solute atoms is obviously attributed to the growth of grain boundary
precipitates, indicating that the solute depletion mechanism controls the PFZ formation in
the later stage of aging.

668This change of formation mechanisms well corresponds to the variation in width of PFZ
during aging. It is obvious from Figure 3 that the average width of PFZ in the Al-Cu and AlZn-Mg alloys monotonously decreases in the earlier stage of aging and then gradually
increases with aging time. Similar results have been observed for Al-6Zn-1.2Mg (in mass%)
alloys aged at 413-433K [12]. From the experimental results, therefore, PFZ was found tobe formed by the combination of the two mechanisms, regardless of alloy system, if thealloys are quickly quenched and aged at moderate temperatures. Note that high aging
temperatures and prolonged aging times only increase the width of PFZ due to theformation of a larger amount of grain boundary precipitates [13, 14].

Figure 1: TEM micrographs and EDX analysis results around grain boundaries of the Al-Cu alloys aged at
433K for 7.2ks (a) and 90ks (b). The same set is illustrated in (c) for the Sn-added Al-Cu alloy aged at 433K
for 7.2ks.

Figure 2: TEM micrographs and EDX analysis results around grain boundaries of the Al-Zn-Mg alloys aged
at 433K for 10.8ks (a) and 259.2ks (b). The same set is illustrated in (c) for the Ag-added Al-Zn-Mg alloy
aged at 433K for 10.8ks.

3.2 Simulation results
Figure 4 illustrates the simulated evolution of Cu clusters in the three regions sitting insequence from a grain boundary of the Al-1.74mol%Cu (i.e. 4.0mass%Cu) alloy aged at

669473K. Remember that the experimentally investigated Al-Cu alloy exhibited more
remarkable decrease in width of PFZ than the Al-Zn-Mg alloy (Figure 3), resulting in aselection as a better example of the vacancy depletion mechanism. As a result ofquenching procedure, the different configurations of Cu atoms were found to be obtainedin the as-quenched (A.Q.) condition depending on the distance from the grain boundary;i.e. nearly supersaturated solid solution of Cu in region A as opposed to the accelerated
formation of fine Cu clusters in region C. This difference of microstructures before aging
was produced by a lower vacancy concentration in region A, where the effects of vacancy
sink were taken into account, and by a larger number of ‘excess vacancies’ in region Cwith the same concentration as the equilibrium one at 793K.

Figure 3: Changes in average width of PFZ in the Al-Cu(-Sn) and Al-Zn-Mg(-Ag) alloys aged at 433K.

On the subsequent aging procedure, furthermore, quite interesting microstructural change
was observed in our simulation model in good agreement with the experimental results; i.e.
the continuous decrease in width of PFZ and the sparse formation of larger precipitates at
the PFZ boundaries (Figs.1-3). It is clearly seen in Figure 4 that regions A and B were
initially within PFZ at 1×108MCS, but after prolonged simulation steps slightly larger Cuclusters formed in the two regions with a lower number density than that in region C. This
simulation result strongly indicates that Cu clusters with larger sizes than a critical one
could not dissolve easily during aging treatment, resulting in the denser formation ofsmaller clusters in region C. This is quantitatively estimated in Figure 5, where theaggregate consisting of more than 40 Cu atoms was counted as a Cu cluster because
disk-like GP zones in Al-Cu alloys could be visible by TEM when over about 2nm indiameter. Note that the following equation was utilized to correlate the number of Cu
atoms within a spherical cluster formed in our simulation model, n, to the diameter of platelike GP zones in Al-Cu alloys, D;
D = (2n / π)1/2aAl ,(1)where aAl is the lattice constant of pure Al (i.e. 0.404nm). It is obvious from Figure 5 thatthe suppressed formation of Cu clusters in the A.Q. condition is significantly attributed tothe sparse distribution of precipitates in region A after prolonged aging times. Therefore,
the utilized simulation model seems to reproduce well the formation of PFZ based on thevacancy depletion mechanism. Note that only modelling based on the solute depletionmechanism has been developed with a suggestion that the vacancy also plays animportant role in giving rise to PFZ [15, 16].
3.3 Effects of microalloying elements
The effects of microalloying elements on the formation of PFZ are remarkably observed inFigure 1(c) and Figure 2(c). It is evident that the addition of Sn to Al-Cu alloys and the670

addition of Ag to Al-Zn-Mg alloys reduced the width of PFZ with highly maintained soluteconcentrations up to closer regions to grain boundaries. The average widths of PFZ in theinvestigated alloys are quantitatively compared in Figure 3, where the microalloying
additions were confirmed to modify significantly the microstructure of the vicinity of grainboundaries, suggesting that the elongation could be improved without decreasing
strengths [11]. Note that similar modification of PFZ has been observed in Sc-added Al-Li[13] and (Ag or Cu)-added Al-Mg-Si [17] alloys.

Figure 4: Simulated evolution of Cu clusters in the three regions sitting in sequence from a grain boundary of
the Al-1.74mol%Cu (i.e. 4.0mass%Cu) alloy aged at 473K. Only Cu atoms are depicted by black dots forbetter visibility.

Figure 5: Temporal changes of the number density of Cu clusters in the two regions A and C of Figure 4 for
the Al-1.74mol%Cu alloy simulated at 473K.

To understand the reason why the width of PFZ decreases in the Sn-added Al-Cu and Agadded Al-Zn-Mg alloys, our proposed ordering parameters between elements, Vi-j [4-6] ,were utilized as a measure of how much an i-j pair interacts in Al. Note that, as a rule ofthumb, a smaller value of Vi-j corresponds to the stronger interaction of i-j, resulting in the671

formation of co-clusters consisting of those elements. In the Sn-added Al-Cu alloy, forexample, a smaller value of VSn-vacancy (-9.5kJ/mol) than VCu-vacancy (0.86kJ/mol) suggeststhat Sn atoms preferentially trap vacancies, resulting in not only the prevention of vacancy
sink during quenching but also the less different distribution of Cu atoms regardless of thedistance from grain boundaries. Remember that the similar microstructures in the A.Q.
condition will produce the similar distributions of Cu clusters at prolonged simulation steps.
In the Ag-added Al-Zn-Mg alloy, in contrast, smaller values of VAg-Zn (1.6kJ/mol) and VAg-Mg(1.8kJ/mol) will result in the prevention of solute depletion of Zn and Mg due to theformation of Zn-Mg-Ag clusters. In fact, Meloney et al. [18] reported by using threedimensional atom probe (3DAP) that Ag promotes co-clustering of Ag and Zn atoms,
followed by Zn-Mg-Ag rich clusters which accelerate the nucleation of the intermediate η’precipitates. Therefore, the systematic prediction of the atomistic behavior of microalloying
elements based on our uniquely estimated Vi-j seems to be sufficiently useful to control themicrostructure of the vicinity of grain boundaries. Note that our previous papers [5, 6, 19]summarize the actually estimated Vi-j for a variety of i-j pairs in the form of ‘orderingparameter map’, allowing effectively behaving microalloying elements in Al alloys to beproposed.
4. Conclusions
In this work, a computer simulation model was established to mimic the vacancy depletionmechanism, which was experimentally clarified as the predominant mechanism for thePFZ formation of Al-Cu(-Sn) and Al-Zn-Mg(-Ag) alloys aged at moderate temperatures forshorter times. This simulation model well reproduced the experimental results; i.e. thecontinuous decrease in width of PFZ and the sparse formation of larger precipitates at thePFZ boundaries. It was firstly revealed in this work that the different microstructures
produced by quenching procedure are significantly attributed to the resultant distribution ofprecipitates after prolonged aging times. Such atomistic behaviors of vacancy, constituent
and microalloying atoms were well explained in terms of our estimated orderingparameters.
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