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 Ltd1061

Quantitative Analysis of the Vicinity of Grain Boundaries
with Precipitate Free Zones in Al-Zn-Mg (-Ag) Alloys
T. Ogura1, S. Hirosawa 2, T. Sato2
1

Graduate student, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan
2
Dept. of Metallurgy and Ceramics Science, Tokyo Institute of Technology,O-okayama, Meguro-ku, Tokyo 152-8552, Japan

Keywords: Precipitate free zone, nanoindentation, Al-Zn-Mg (-Ag) alloy, EDX analysis, precipitate hardening,tensile test

Abstract
To correlate quantitatively the mechanical properties of Al-4.9mass%Zn-1.8massMg(0.3mass%Ag) alloys with solute distribution in the vicinity of grain boundaries,
corresponding microstructural and compositional characteristics respectively by TEM
observation and EDX analysis, tensile test and nanoindentation measurement have beenperformed. The remarkable decreases in hardness and solute concentrations wereobserved towards grain boundaries even in the regions just outside precipitate free zone(PFZ). In the Ag-added alloy, the nanoindentation hardness could be maintained up tocloser regions to grain boundaries at a level similar to that inside grains, resulting in larger
elongations to fracture. Such a modification in solute concentration in the vicinity of grainboundaries could be achieved even by two-step aging treatments, and by which 0.2% proof
stress of the ternary alloy increased without decreasing elongation.

1. Introduction
The precipitation process of Al-Zn-Mg alloy system has been extensively investigated bynumerous researchers, and the following precipitation sequence is generally accepted [1].
Super saturated solid solution α→ GP zone → η’→η(MgZn2)→T((Al,Zn)48Mg32)(1)
The intermediateη’ phase mainly contributes to the age hardening of the alloy due to thefiner dispersion inside grains in the aging process. However, TEM observation has shownthat PFZ is also formed around grain boundaries, regardless of transgranular precipitation
being taking place. This suggests that the nucleation of precipitates is affected to someextent in the vicinity of grain boundaries. Several researchers have tried to explain theformation mechanism of PFZ mainly by two theories. One is the vacancy depletion theory,
which takes into account the depleted vacancy concentration smaller than a certain criticalvacancy concentration required for nucleation [2, 3], and the other is the solute depletion
theory taking into account the depleted solute concentrations near grain boundaries,
resulting in the decrease in supersaturation for precipitation [4]. In addition, the PFZ has
been believed to affect greatly on the mechanical properties of alloys. Thomas et al.[5]reported that microcracks are generated inside PFZ due to the preferential deformation ofPFZ, resulting in the decreased elongation to fracture. On the other hand, it was also

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reported that slip bands and stress concentration are preferentially relaxed in PFZ, resulting
in the increased strength and elongation [6]. However the definitive conclusions on theformation mechanism of PFZ and the effects on the mechanical properties have not been
established yet.
As for the modification of solute distribution in the vicinity of grain boundaries, two methods
have been applied to change the width of PFZ; i.e. (1) microalloying addition and (2)two-step aging treatment. Microalloying addition of Ag was found to exhibit both accelerated
and increased age-hardening in Al-Zn-Mg, with the smaller width of PFZ than that of theternary alloy [8]. Furthermore, it is also known that if two-step aging treatment is applied, fineprecipitates are formed even inside PFZ during the first aging [2, 7]. However, noconsideration has been given not only to the mechanical properties of the modified PFZ but
also to the formation mechanism of the PFZ in Al-Zn-Mg-Ag alloys.
In this work, the mechanical properties were quantitatively correlated with the corresponding
micro structural and compositional characteristics of solute concentration in the vicinity ofgrain boundaries in Al-4.9Zn-1.8Mg(-0.3Ag) (mass%) alloys. The nanoindentation method
was successfully utilized to estimate the mechanical properties of such quite small regions
as PFZ. Microstructure observation and EDX analysis were also performed inside grains
and near grain boundaries to compare with the results obtained by nanoindentaion andtensile tests.

2. Experimental Procedure
Two alloys studied in this work are Al-4.9Zn-1.8Mg and Al-4.9Zn-1.8Mg-0.3Ag (mass%)
alloys. The ingots were homogenized at 743K for 172.8ks, then hot- and cold-rolled to
1.3mm thick sheets. Specimens were solution-treated at 743K for 3.6ks in a salt bath
(NaNO3:KNO3=1:1), and were quenched into iced-water and kept for 60s. Aging treatments
were carried out in a silicon oil bath at 433K. The nanoindentaion measurement wasperformed around grain boundaries using ENT-1100a. Micro Vickers hardness was alsomeasured with a load of 500g for 15s. Microstructural observation and EDX analysis wereperformed using conventional TEM with an EDX system and FE-SEM. To some specimens,
furthermore, two-step aging was applied; i.e. aging at 373 or 408K for 1209.6ks after aging
at 453K for 10.8ks. Tensile test was carried out with a strain rate of 1mm / min at RT.

3. Results and Discussion
3.1 Effects of Ag Addition
The microstructural change around grain boundaries is shown in Figure 1 for the ternary andAg-added alloys aged at 433K. The precipitates inside grains were identified to be η’ and/orηfrom the corresponding diffraction patterns. PFZ is observed along grain boundaries of
the ternary alloy with a width less than 500nm. A large number of fine precipitates are
distributed homogeneously inside grains, whereas larger precipitates are formed with alower number density in the vicinity of PFZ especially in the specimens under peak and overaging conditions. Note that the size of grain boundary precipitates also gradually increases
with aging time. In Ag-added alloy, on the other hand, PFZ is observed with a width muchsmaller than that of the ternary alloy. This modification of PFZ by Ag addition agrees with the1063

result of the previous work [8]. It should be also noted that grain boundary precipitates are
not so coarsened in the Ag-added alloy compared with those of the ternary alloy.
Figure 2 (a), (b) shows backscattered electron images of the ternary alloy aged at 433K for259.2ks after nanoindentation test. The relationship between nanoindentation hardness anddistance from a grain boundary is also shown in Figure 2 (c) with the corresponding EDX
analysis results. In the ternary alloy, it is clearly seen that some of triangle indents wereloaded inside PFZ with brighter contrast along the grain boundary. Based on the direct
observation of indentation marks, the hardness inside

Figure 1: TEM images around grain boundaries in the ternary and Ag-added alloys aged at 433K for various
aging times.

loaded inside PFZ with brighter contrast along the grain boundary. Based on the direct
observation of indentation marks, the hardness inside PFZ was found to be much lower thanthose inside grains. Furthermore, it is also clear that the hardness in the regions just outside
PFZ also decreases towards the grain boundary. This tendency is quite similar to thedecreasing behavior of solute concentrations of Zn and Mg, suggesting that solute atoms
migrated to the grain boundary with regions of depleted solute concentrations anddecreased nanoindentation hardness. In the Ag-added alloy, on the other hand, thedecrease in nanoindentation hardness towards grain boundaries is much smaller than thatof the ternary alloy (Figure 2 (d)).Note that Zn and Mg are also much more remained up tocloser regions to grain boundaries in the Ag-added alloy, resulting in the same hardness as
those inside grains.
From the experimental results, it was confirmed that there is a region where precipitates arecoarsened and solute concentrations decrease compared with those of grains. Therefore,
we propose that the vicinity of grain boundaries of Al-Zn-Mg(-Ag) alloys can be separated
into three regions; i.e. “PFZ”, “transition area” and “grain”. The transition area was newly
detected in this work between grain and PFZ. Since solute concentrations in grains are kept
at levels of bulk concentrations, it is conceivable that finely dispersed precipitates greatly
contribute to the hardness. On the other hand, it is considered that in PFZ hardness is only
due to the solid solution hardening because of no precipitates. Therefore, the lower
hardness inside PFZ attributes to the decreased solute concentrations as a result of theformation of grain boundary precipitates. In transition area, furthermore, solute1064

concentrations slightly decrease and precipitates are coarsened. Therefore, the transitionarea can be defined as the region where the hardness decreases due to the less effective
solid solution hardening and precipitate hardening than those inside grains. In the Ag-added
alloys, the transition area and PFZ become much smaller because solute concentrations aremaintained up to closer regions to grain boundaries. This modification in soluteconcentration in the vicinity of grain boundaries by the addition of Ag is reasoned in refs. [9,10].

Figure 2: Backscattered electron images of nanoindentation marks around a grain boundary of the ternary
alloy aged at 433K for 259.2ks. Some of nanoindentation marks are located inside PFZ with bright contrast. (a)Low magnification, (b) high magnification. Nanoindentation hardness and solute concentration changes acrossthe grain boundary are also shown for the ternary (c) and Ag-added (d) alloys.

In order to examine the macroscopic mechanical properties, tensile test was performed forthe investigated alloys. Figure 3 shows the relationship between 0.2% proof stress andelongation of the alloys aged at 433K for various aging times. Fracture surfaces at maximum
proof stresses are also shown for the ternary (aged for 86.4ks) and Ag-added (aged for32.4ks) alloys. From the results of tensile test, elongation under the same level of proof
stress was found to be larger in the Ag-added alloy than that in the ternary alloy.
Furthermore, it was also revealed that the mode of fracture remarkably changes fromintergranular fracture to transgranular fracture by Ag addition (Figure 3 (b), (c)). This implies
that fracture occurs preferentially in the vicinity of grain boundaries with PFZ, stronglysuggesting the possibility of the fracture mechanism proposed by Thomas et al.[6].
3.2 Effects of Heat Treatment
To examine the effects of two-step aging, the ternary alloy was initially aged at 453K for
10.8ks and then aged at 373K or 408K for 1209.6ks. Figure 4 shows the microstructuralchange around grain boundaries of the single- and two-step aged alloys. In the specimens
single- or two-step aged at 408K, PFZ are observed along grain boundaries without change

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in the width (Figure 4 (a), (b)). In the specimen two-step aged at 373K, on the other hand,fine precipitates are observed within the initial PFZ formed after the first aging (Figure 4 (c),(d)). Figure 5 shows the corresponding tensile properties of the three alloys and fracturesurfaces of the two-step aged alloys. It was found that the similar improvement of
mechanical properties could be achieved by two-step aging as is the case of Ag addition(Figure 3); i.e. increased proof stress without decreasing elongation due to the change offracture mode.

Figure 3: (a) Relationship between 0.2% proof stress and elongation in the ternary and Ag-added alloys aged
at 433K. Fracture surfaces at maximum proof stress are also shown for the ternary (b) and Ag-added (c) alloys.

Figure 4: TEM images around grain boundaries in the ternary alloys single aged at 453K (a), two-step aged at
408K (b) and 373K (c) after aging at 453K. Enlarged micrograph of (c) showing fine precipitates inside PFZ ofthe first aging is also shown in (d).

4. Conclusion
To correlate quantitatively the mechanical properties of precipitate free zone (PFZ) with thecorresponding microstructural and compositional characteristics, TEM observation, EDXanalysis, tensile test and nanoindentation measurement have been performed forAl-4.9Zn-1.8Mg(-0.3Ag) (mass%) alloys. The obtained results are summarized as follows.

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1. The hardness was lower not only in PFZ but also outside PFZ than those within grains,suggesting the existence of newly proposed transition area. Therefore it was found thatthe hardness is mainly attributed to precipitation hardening in grains, solid solution
hardening in PFZ and less effective solid solution and precipitation hardenings intransition area, respectively.
2. In the Ag-added alloy, the hardness was maintained up to closer regions to grainboundaries than that in the ternary alloy. This is due to the modification of soluteconcentration in the vicinity of grain boundaries by Ag addition. Similar modification
could also be achieved by two-step aging.
3. By decreasing the width of PFZ through Ag addition and/or two-step aging treatment,
tensile properties of Al-Zn-Mg alloys could be improved quite remarkably.

Figure 5: Tensile properties of the single-aged and two-step aged alloys (a) (A: single-aged at 453K for 10.8ks,
B: two-step aged at 408K for 1209.6ks after aging at 453K and C: two-step aged at 373K for 1209.6ks afteraging at 453K). The corresponding fracture surfaces of the alloys B and C are also shown in (b) and (c),respectively.

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