Theoretical Study of the Influence of Microalloying on Sensitization of ...

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Theoretical Study of the Influence of Microalloying on Sensitization of AA5083 and Moderation of Sensitization of a Model Al-Mg-Mn Alloy via Sr Additions R.K. Gupta,‡,* R. Zhang,* C.H.J. Davies,** and N. Birbilis*

ABSTRACT

INTRODUCTION

The calculated effect of a range of quaternary alloying additions to aluminum alloy (AA)5083 (Al-4.4Mg-0.5Mn [UNS A95083]) was investigated. Attention was given to the influence of composition on the volume fraction of β-phase (Mg2Al3), and the possible formation of any additional phases (which we term g-phase herein). Alloying additions of silver, cerium, copper, lithium, neodymium, nickel, scandium, silicon, strontium, yttrium, zinc, and zirconium were studied in hypothetical additions of 0.1 wt% and 0.5 wt%, revealing that there is potential to modify the fraction of β-phase, and hence sensitization, in AA5083. Calculations indicated strontium to be the most effective addition in decreasing β-phase fraction during thermal exposure (sensitization); therefore, the influence of Sr additions were studied empirically via production of custom Al-Mg-Mn alloys. Sensitization was investigated via the nitric acid mass loss test (NAMLT), revealing that mass loss from intergranular corrosion decreased by more than 60% from the addition of 0.08 wt% Sr; however, further Sr addition (up to 0.35 wt%) subsequently led to a small decrease in mass loss. Analysis of intentionally fractured surfaces of sensitized alloys confirmed a substantial decrease in β-phase on grain boundaries due to Sr additions.

The Al-Mg (5xxx series) alloys are nominally considered a corrosion-resistant class of aluminum alloys.1-2 However, elevated temperature exposure, ranging from ~50°C to 200°C in alloys containing > ~3 wt% Mg (for example, AA5083 [UNS A95083](1) has a nominal composition of Al-4.4Mg-0.5Mn) renders them susceptible to intergranular corrosion (IGC) and stress corrosion cracking (SCC).3-7 This is attributed to the precipitation and subsequent anodic dissolution of Mg2Al3 (β-phase) at grain boundaries.6-11 Despite significant research in understanding and quantifying the IGC and SCC arising from this sensitization,11-18 comparatively limited attention has been given to the development of 5xxx alloys with improved IGC and SCC resistance. Carroll, et al., reported that addition of Zn, and a combination of Cu and Zn to Al-Mg alloys could modify the chemical composition of β-phase, and potentially also co-precipitate an AlMgZn phase enriched with Cu at grain boundaries, resulting in decreased IGC susceptibility.19-21 Unocic, et al., reported that additions of Cu and Zn supressed, but not eradicated, IGC of AA5083.22 Recent patents identify various quaternary alloying additions and report the addition of up to 0.5 wt% Cu to Al-Mg alloys can be effective in reducing IGC as measured by the nitric acid mass loss test (NAMLT).23-24 A recent study had found Nd to be highly effective in improving sensitization resistance of an Al-5Mg model alloy.25-26 The addition of low levels of Ag was also reported to decrease IGC.27 Early work by Polmear, et al.,28-29 had indicated that addition of Ag was beneficial to Al-Mg alloys gen-

KEY WORDS: 5xxx series, aluminum, CALPHAD, intergranular corrosion, sensitization Submitted for publication: August 20, 2013. Revised and accepted: November 20, 2013. Preprint available online: November 27, 2013, doi: http://dx.doi.org/10.5006/1117. ‡ Corresponding author. E-mail: [email protected]. * Department of Materials Engineering, Monash University, Clayton, Australia. ** Department of Mechanical and Aerospace Engineering, Monash University, Clayton, Australia. (1) UNS numbers are listed in Metals and Alloys in the Unified Numbering System, published by the Society of Automotive Engineers (SAE International) and cosponsored by ASTM International.

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erally, reducing IGC/SCC by utilizing the effect of Ag to permit lower Mg levels for an equivalent strength (resulting a lower susceptibility to sensitization). Given the limited work to date exploring quaternary additions to AA5083, or more generally to Al-Mg alloys, a structured wider exploration of the possibility of alloying elements to suppress IGC is warranted. Such elements, when added in small quantities, may either modify the β-phase fraction, β-phase composition, introduce an additional phase, or modify the growth kinetics of β-phase. As a prelude to a largescale experimental study, a theoretical approach investigating the effect of key elements on the equilibrium β-phase characteristics using calculated phase diagrams from equilibrium thermodynamics was used to guide empirical alloy production with a view to sensitization resistance. It is posited that in an initial sense, some important factors to potentially reduce IGC from sensitization are (in no particular order): —rendering β-phase “less anodic” by incorporating elements that are more noble than Mg —the introduction of secondary precipitates (other than β-phase) at grain boundaries to interrupt the β-phase network and potentially reduce IGC attack —alloying leading to a decrease in the volume fraction of β-phase —retarding precipitation and growth of β-phase The effect of selected alloying elements (Ag, Ce, Cu, Li, Nd, Ni, Sc, Si, Sr, Y, Zn, Zr) was studied in 0.1 wt% and 0.5 wt% additions using PANDAT† soft ware30 for a base composition of AA5083 (Al-4.4Mg0.5Mn). Calculations were carried out for a tempera ture of 100°C, which is one of the typically used artificial sensitization temperatures in experimental studies.9-18 The potential impact of the changes in alloy microstructure upon subsequent IGC performance are discussed. Theoretical analysis in this study indicated Sr to be most effective in decreasing β-phase during sensitization; therefore, effects of varying Sr additions (between the nominal range of 0.1 to 0.5 wt% corresponding to the calculated effect) on sensitization and corrosion of an Al-4Mg-0.5Mn alloy were experimentally investigated.

EXPERIMENTAL PROCEDURES Materials Al-4Mg-0.5Mn-xSr alloys (where x was determined to be 0, 0.08, 0.2, and 0.35 wt%) used in this investigation were prepared in a muffle furnace by melting an Al-4Mg-0.5Mn (wt%) master alloy, blended with pure Mg (99.9%) and an Al-2.5Sr (wt%) master alloy. Initially, the Al-4Mg-0.5Mn master alloy was melted at ~720°C, after which the appropriate amount of said alloying elements were added to the melt and † Trade name.

CORROSION—Vol. 70, No. 4

TABLE 1 Composition (wt%) of Alloys as Determined Using Inductively Coupled Plasma-Optical Emission Spectrometry

Alloy/

Composition

Mg

Mn

Fe

Si

Sr

Al

AA4 AA-0.08Sr AA-0.2Sr AA-0.35Sr

4.01 4.17 4.14 4.02

0.49 0.47 0.45 0.44

0.30 0.34 0.34 0.33

0.13 0.15 0.16 0.13

0.0 0.08 0.20 0.35

Bal. Bal. Bal. Bal.

stirred. Regular stirring of the melt was carried out over 2 h in the molten state, followed by casting into a pre-heated (to 300°C) graphite crucible. The experimental alloys were homogenized below the liquidus temperature for two days to counter any possible segregation during casting. Homogenization was followed by solution treatment at 450°C for 1 h, then a water quench (termed “as-quenched” state in this study). As-quenched alloys were cold-rolled to a 50% thickness reduction (and termed “rolled” in this study). The precise compositions of these alloys were analyzed externally by inductively coupled plasma-optical emission spectroscopy (ICP-OES) and are presented in Table 1.

Sensitization and Characterization To artificially sensitize the alloys, isothermal heat treatment was carried out on “rolled” alloys at 150°C for seven days (168 h). For this experimental study, 150°C was used to accelerate the sensitization kinetics for faster data collection. IGC susceptibility was measured in terms of mass loss per unit area (mg/cm2) adopting the NAMLT as specified in ASTM G67.31 All experiments were conducted three times, and the average values are reported with the related standard deviation. To quantify that IGC occurred following NAMLT and for verification of differences in the extent of IGC, cross sections of the alloys were examined using optical microscopy. Optical profilometry of exposed surfaces was also performed using a Veeco Wyko NT-1100†. Microstructure of the specimens was analyzed using a JEOL-7001† field emission gun-scanning electron microscope (FEG-SEM), and samples for SEM were polished to a 0.05 micron finish. Hardness of each of the alloy was measured using a Struers Duramin A300† with an applied load of 1 kg, and reported hardness values are an average of 10 hardness tests.

Fracture Surface Analysis Prior to imaging, specimens were specifically sectioned into rectangular blocks and then notched on one face to allow for a bend of >90° to occur, causing a clean fracture. Prior to fracture, specimens were placed in liquid nitrogen for an hour. The fractured surface was subsequently placed in ethanol imme-

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FIGURE 1. Effect of various alloying elements added in either 0.1 wt% (left) or 0.5 wt% (right) on the equilibrium fraction of β-phase in AA5083 (calculated at 100°C).

FIGURE 2. Effect of various alloying elements added in either 0.1 wt% (left) or 0.5 wt% (right) on the equilibrium fraction of g-phase (a term used here to define phases that are not β-phase) in AA5083 (calculated at 100°C). The calculated stoichiometry of the g-phase is overlaid on the 0.1 wt% addition chart; however, the stoichiometry remains unchanged for the 0.5 wt% addition.

diately after fracture to bring it at room temperature while avoiding condensation of water vapor from environment. Fresh fractured surfaces were analyzed using FEG-SEM. Fracture surfaces of the sensitized specimens, which were assumed to be predominately comprised of grain boundaries (SEM observation revealing intergranular fracture), were used to investigate the effect of Sr on the electrochemical properties of the grain boundaries. Red lacquer was used to mask all the surfaces of the specimen, except the fractured surface, such that only the fractured surface could be exposed to the electrolyte. Open-circuit potential (OCP) of the fracture surface in 0.1 M sodium chloride (NaCl) solution was measured using a BioLogic VMP3† potentiostat under the control of EC-Lab† software. OCP was measured with respect to the saturated calomel electrode (SCE). OCP measurements were performed four times and the representative behavior is reported in this paper.

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RESULTS CALPHAD (Calculated Phase Diagrams) Analysis In this paper, the effect of the quaternary alloying elements on microstructure of AA5083 is presented in terms of: —resultant change in β-phase fraction (Figure 1) —formation and fraction of any additional phase formed due to quaternary alloying elements, termed γ-phase (Figure 2) —change in the chemical composition of the β-phase (Figure 3) From Figure 1, we observe that some of studied alloying additions, namely, Sr, Cu, Ag, Si, Li, Ce, Zn, and Y, are effective in decreasing the β-phase fraction. For example, 0.5 wt% Sr can reduce the β-phase fraction by 70%. The efficacy of quaternary additions (considered in this study) in reducing the β-phase fraction can be classified in decreasing order as Sr >



FIGURE 3. Atomic percent of alloying (calculated at 100°C) element when added in either (left) 0.1 wt% or (right) 0.5 wt% incorporated in β-phase (i.e., extent of beta phase doping).

Cu > Ag > Si > Li > Ce > Zn > Y > Nd > Ni > Sc > Zr. Some of the alloying elements studied, i.e., Ni, Sc, and Zr, indicate a slight increase in the β-phase fraction. The presence and characteristics of any co-existent g-phase are as important as β-phase fraction because the overall corrosion performance of an alloy is largely affected by the, number, size distribution, and electrochemical characteristics of intermetallic phases.2,5-9,18,32 It is also of relevance if the g-phase forms during sensitization (i.e., co-precipitation) or whether it is present as constituent (formed during solidification); these factors are summarized in Table 2. For instance, Si (above the solubility limit) forms large Mg2Si particles (essentially insoluble during solution heat treatment), which both depletes the alloy of Mg and therefore decreases β-phase fraction,33 while also introducing an additional “anodic” particle Mg2Si.34 Cu additions to the AA5083 composition can lead to the formation of Al2CuMg (S-phase), which is known to be deleterious for corrosion.1,33 Al2MgLi formed via Li additions is also anodic in nature35 and may further enhance the IGC. The so-called g-phase (noting that this notation is used here for any phase that responds to age hardening other than the β-phase) with several of the alloying additions (specifically for Cu, Li, Sr, Ag, Ce) can co-precipitate and grow during service conditions. To determine if any of the intermetallics are also present as constituent particles, detailed experimental studies for each of the individual alloying additions in isolation would be needed to make any such validation; however, herein, we have only focused on Sr additions. Based on the thermodynamic modeling, however, precipitation of g-phase during sensitization or incorporation of Mg in the gphase (leading to less Mg available for β-phase formation) is expected to suppress IGC by reducing fraction and continuous network of β-phase, at the expense of mechanical strength. The latter is not studied here. A reported means of modifying the rate of IGC in 5xxx alloys is via modifying the chemical composi-


tion of β-phase, potentially rendering it less prone to rapid dissolution. Figure 3 shows that Zn, Li, and Cu are effective in modifying the chemical composition of β-phase. Li is not deemed to be effective in reducing IGC because it is electrochemically more active than Mg. However, there is some merit in exploring Li addition experimentally, should Li combination with Mg modify in β-phase precipitation kinetics and process.36 However, the presence of Zn and Cu in β-phase could potentially retard dissolution, and has been reported by Carroll, et al.,19-21 and Unocic, et al.22 The effect of Cu is not visible in Figure 3, since the calculated at% of Cu is relatively small (~10–10 at%). This, therefore, requires further work to understand the reports showing improved IGC resistance of Al-Mg alloys caused by Cu and Zn.19-24 The singular and potentially combined effect of the alloying additions studied on the subsequent fraction and composition of β-phase and g-phase is helpful in exploring alloys with a higher level of IGC (and possibly, SCC) resistance. Based on the present work, Sr, Zn, Ag, Cu, and Ce (and their potential combination) should be capable of potentially suppressing IGC via modifications to the predicted microstructure. Zn can modify the chemical composition of β-phase, supress β-phase formation, and co-precipitation of a g-phase during aging. Although Ni leads to a slight increase in β-phase fraction, Ni3Al precipitates during sensitization and may potentially disrupt continuous precipitation of β-phase. Sr is found to be most effective in decreasing β-phase formation; however, it also forms a large fraction of g-phase (which is present as constituent particles as well as precipitates) and therefore varying amounts of this element needs to be investigated in detail. The influence of Sr on the corrosion of commercial-purity Al as investigated in a range of Al-Sr binary alloys showed that Sr has no significant influence on the corrosion.37 The influence of Sr addition in AA5083 (or Al-Mg alloys more generally) has not been previously reported in literature. For

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TABLE 2 Summary of the Major Effect of Quaternary Alloying Additions to AA5083 Element Microstructure

Maximum Solubility in Al (wt%)

Other Comments

Ni

1) Slight increase in the β-phase fraction. 0.24 2) For 0.1 wt % Ni, Al3Ni can be dissolved during solution treatment and it re-precipitates at ~100°C. Al3Ni present as constituent for 0.5 wt% Ni. 3) Ni atoms do not incorporate in β-phase.

Zn

1) Slight decrease in β-phase fraction. 83.1 Effect of Zn on electrochemical behavior of 2) Al5Mg4Zn (g-phase) precipitates during sensitization β-phase, as well as IGC of 5xxx alloys, is for only 0.5 wt% Zn. It does not form for 0.1 wt%. reported19-21 3) Zn incorporates in β-phase.

Nd

1) No significant effect on β-phase fraction. 0.05 2) Al11Nd3 phase also forms and increases with Nd content. 3) Should not theoretically incorporate in β-phase, however, in practice may do so.

Nd was reported to significantly improve sensitization resistance of Al-5Mg alloy25-26

Sc

1) Slight increase in the β-phase fraction. 0.27 2) Al3Sc present during solidification and also some fraction precipitates during sensitization. 3) Sc does not incorporate in β-phase.

Sc is reported to improve mechanical properties without effecting corrosion performance39-40

Y

1) Slight decrease in the β-phase fraction. 0.17 2) (Al0.6Mg0.4)3Y forms during solidification as well as it precipitates. 3) Does not incorporate in β-phase.

Y is reported to retard recrystallization during mechanical processing

Zr

1) Slight increase in the β-phase fraction. 0.28 2) Al3Zr present during solidification and also some fraction can form during sensitization. 3) Does not incorporate in β-phase.

Effect of Zr on corrosion performance is not investigated in detail.

Li

1) Significant decrease in β-phase fraction. 4.2 2) Al2MgLi co-precipitates during aging as well as Li substituting for Mg atoms from β-phase. 3) Li incorporates in β-phase.

The impact on corrosion or IGC is undetermined to date35-36

Si

1) Decreases the β-phase fraction. 1.65 2) Forms Mg2Si which is present both as constituent as well as precipitates, Modifies chemical composition of Al12Mn particles. 3) Does not incorporate in β-phase.

Ce

1) Significant decrease in β-phase fraction. 0.05 2) Al13CeMg6 co-formation. 3) Does not incorporate in β-phase.

The impact on corrosion or IGC is undetermined to date

Ag

1) Decrease β-phase fraction significantly. 55.6 2) Al-Mg-Ag precipitates during sensitization. 3) Does not incorporate in β-phase.

Excess Ag additions are reported to be detrimental to corrosion1,41

Cu

1) Cu can decrease β-phase fraction significantly. 5.7 2) S phase (Al2CuMg) co-precipitates during sensitization. 3) A negligible amount of Cu (~10–10 at%) is incorporated in β-phase.

Small amount of Cu addition was reported to reduce IGC21-22

Sr

1) Potent at decreasing β-phase fraction. 0 2) Al4Sr present as constituent which transforms to Al38Mg58Sr4 phase during sensitization. 3) Does not incorporate in β-phase.

Sr was reported to have no significant influence on corrosion of Al.37 Sr is added to Al-Si alloys in trace amounts to modify eutectic phase38

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The impact on corrosion or IGC is undetermined to date

Excess Si addition is expected to lead to poor corrosion performance due to the formation of Mg2Si33-34



(a) FIGURE 4. Effect of Sr content on the predicted phase fractions (at 150°C) in an Al-4Mg-0.5Mn alloy.

the remainder of this study, we therefore focus on the AA5083+Sr system, based on the significant reduction in calculated β-phase fraction predicted to occur.

Effect of Sr Additions on Microstructure A more detailed set of calculations aimed at studying the effect of varying amounts of Sr on the phases present in AA5083 (in this case calculated as Al-4Mg-0.5Mn wt%) is presented in Figure 4. Figure 4 was calculated for a simulated equilibrium structure at 150°C, and reveals a linear decrease in β-phase fraction with increasing Sr content. The addition of 0.1 wt% Sr suggests a decreases in β-phase fraction (from 5.7% to 4.7%), which could be enough to disturb the continuous grain boundary β-phase, while an addition of 0.5 wt% Sr suggests essentially a complete suppression in β-phase fraction. The increasing Sr content leads to the formation of an Al38Mg58Sr4 phase (g-phase). The phase fraction of the Al38Mg58Sr4 phase increased linearly with Sr content (Figure 4), to a maximum of ~3.9%, which is still below the ~5.7% in the Sr-free case. The phase fraction of Al12Mn remained constant for the temperature studied. The general microstructure of sensitized alloys as investigated using SEM revealed that Sr addition altered the morphology of intermetallics (Figure 5). Intermetallics appeared to be fragmented (skeleton-like) because of the Sr addition. A higher magnification SEM image of two alloys (sensitized Al-4Mg-0.5Mn and Al-4Mg-0.5Mn-0.35Sr) as presented in the inset of Figure 5 revealed the presence of uniformly distributed particles in Sr-containing alloys, which were not present in the alloy without Sr. Figure 6 presents a high-magnification image of intermetallics and corresponding area map as collected using energydispersive x-ray spectroscopy (EDXS). Presence of an additional Sr-containing intermetallic (in addition to Al-Fe-Mn constituent type of intermetallic) is evident


(b) FIGURE 5. SEM images of sensitized (150°C/ 7 days) (a) Al-4Mg0.5Mn and (b) Al-4Mg-0.5Mn-0.35Sr alloys. High-magnification SEM image showing dispersoids is presented in the inset.

from Figure 6. Sr-containing intermetallics were not detected in the alloy with only 0.08 wt% Sr.

Effect of Sr Additions on Hardness The influence of Sr additions on hardness of the Al-4Mg-0.5Mn wt% base alloy is presented in Figure 7, which reveals no significant effect of Sr additions upon the hardness of the as-quenched alloy. This indicates that contribution of Sr in solid solution strengthening was not significant. The hardness of rolled alloys was decreased by Sr additions; however, this effect was not significant up to 0.2 wt% Sr. The addition of 0.35 wt% Sr lead to a decrease in average hardness by ~17 HV1, while the addition of 0.085 wt% Sr decreased hardness by only ~3 HV1.

Effect of Sr Additions on Sensitization Resistance The degree of sensitization (DoS), as represented by NAMLT values, was determined as a function of Sr content and is presented in Figure 8(a). Sensitization at 150°C/7 days leads to a significant increase in DoS

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FIGURE 7. Hardness of Al-4Mg-xSr alloys as a function of Sr content.

FIGURE 6. High-magniﬁcation observations and EDS element mapping analysis of the Al-5Mg-0.35Sr alloy showing Sr containing and Al-Fe-Mn type of intermetallics.

for the unmodified Al-4Mg-0.5Mn alloy (from 6 mg/cm2 to 25 mg/cm2). The addition of 0.08 wt% Sr was effective in decreasing DoS by more than 60%. Further Sr addition (up to 0.35 wt%) caused a sluggish decrease in DoS values. The DoS values for sensitized Sr-containing alloys were significantly lower than that for the alloy without Sr, which indicates elimination of sensitization due to Sr additions of >0.08 wt% Sr according to ASTM G67-04 (Figure 8). NAMLT results

(a)

were supplemented by metallographic cross sectioning after NAMLT. Representative optical microscopy images of the Al-4Mg-0.5Mn and Al-4Mg-0.5Mn-0.35Sr alloys after NAMLT are presented in the inset of the Figure 8(b), which reveals differing extents of damage. The unmodified alloy has clearly a higher depth of attack, including evidence for IGC attack. The Sr-free alloy also revealed many grains detached from the surface, whereas the surface of Sr-containing alloys was smoother.

Influence of Sr Additions on β-Phase Fast-fractured surfaces of sensitized alloys were analyzed using SEM (secondary electron mode revealing topography using a FEG-SEM) to investigate the effect of Sr on the structure of the grain boundaries. Figure 9(a) revealed the presence of uniformly distributed fine β-phase particles on the grain boundaries of the alloy without Sr. The size of these particles

(b)

FIGURE 8. NAMLT values as a function of (a) Sr content (effect of Sr addition on sensitization) and (b) β-phase fraction (inset: metallurgical cross sections of selected sensitized Al-4Mg-xSr alloys after NAMLT).

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(a)

(b) FIGURE 9. Fracture surfaces of sensitized (150°C/7 days) (a) Al-4Mg-0.5Mn and (b) Al-4Mg-0.5Mn-0.35Sr alloys. Imaging was in secondary electron mode.

FIGURE 10. EDXS line scan showing Sr-containing particles in the vicinity of the grain boundary in the Al-4Mg-0.5Mn0.35Sr alloy.

was less than Cu > Ag > Si > Li > Ce > Zn > Y > Nd > Ni > Sc > Zr



(a)

(b)

(c)

(d)

FIGURE 12. Three-dimensional surfaces of as-rolled (a) Al-4Mg-0.5Mn, (b) Al-4Mg-0.5Mn-0.1Sr, (c) Al-4Mg-0.5Mn-0.2Sr, and (d) Al-4Mg-0.5Mn-0.5Sr alloys after 7 days’ immersion in 0.1 M NaCl (followed by chemical cleaning of corrosion products).

(a)

(b)

(c)

(d)

FIGURE 13. Three-dimensional surfaces after 7 days’ immersion in 0.1 M NaCl (followed by chemical cleaning of corrosion products) of sensitized (150°C/7 days): (a) Al-4Mg-0.5Mn, (b) Al-4Mg-0.5Mn-0.1Sr, (c) Al-4Mg-0.5Mn-0.2Sr, and (d) Al4Mg-0.5Mn-0.5Sr alloys.

—co-existence of a g-phase (in the order of Cu, Li, Sr, Ag, Ce, Si, Sc, Ni) —modification of chemical composition of β-phase (Zn, Li, Cu) —potential combinations of these factors This contributes to a platform from which sensitization-resistant 5xxx alloys may be developed in the


future. One avenue that is yet to be fully exploited is that of strengthening via quaternary additions that can enable a reduction in the required level of Mg to meet target strengths. Polmear and Bainbridge28 explicitly noted this phenomenon in a provisional patent in 1963 and in related works,29 indicating that Ag could produce an appreciable hardening increment.

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FIGURE 14. Pit density as determined by optical profilometer after 7 days’ immersion in 0.1 M NaCl (followed by chemical cleaning of corrosion products) of as rolled specimens.

Such hardening effects are not obvious from Figures 1 through 3, and an attempt has been made to make some more general comments to accompany the results in Table 1. The equilibrium calculations were empirically tested for the quaternary addition with the most promise, which was determined to be Sr on the basis of β-phase reduction. Sr-containing alloys presented a remarkable improvement in sensitization resistance from addition of ≥0.08 wt% Sr, revealing sensitization in Al-Mg-Mn alloys could be eliminated by these small Sr additions. Experiments were also conducted to contribute toward a mechanistic understanding of the remarkable improvement in sensitization resistance due to Sr addition, including microstructural analysis to validate the CALPHAD-predicted decrease in β-phase fraction. Additionally, the g-phase as predicted by the CALPHAD was observed both at grain

(a)

boundaries as well as generally in micrographs of the polished specimens. OCP measurements of the fracture surfaces suggested that Sr additions contribute to a significant difference in electrochemical properties of grain boundaries, concomitant with improved IGC resistance. Grain boundaries of Sr-modified alloys were significantly ennobled by ~150 mV. In contrast, the OCP of the grain boundaries of the alloy without Sr was significantly more anodic than that of the Sr-containing alloys and initially closer to the highly negative OCP of bulk β-phase (Mg3Al3), suggesting grain boundaries were occupied by Mg2Al3. This paper presents the first study investigating influence of Sr additions on Al-4Mg-0.5Mn (i.e., AA5083), or any Al-Mg alloy, which is in itself noteworthy, and we observe that Sr is beneficial in improving sensitization resistance without enhancing pitting corrosion. A slight decrease in hardness as a result of Sr additions was observed, which was dependent upon the Sr content. This study indicates that optimized Sr additions could improve sensitization resistance without significant loss of hardness. Future work will include determination of the minimum level of Sr required to prevent sensitization, and detailed studies including transmission electron microscopy and exploration of mechanical properties to validate the notion that Sr can contribute toward the next generation of sensitization-resistant 5xxx alloys. This study presented possibilities of additions of small amounts of Sr addition to improve sensitization resistance; however, the influence of Sr additions on the number of properties, i.e., weldability, has not been investigated to date.

CONCLUSIONS v CALPHAD (calculated phase diagrams) via PANDAT† software were used to predict the effect of vari-

(b)

FIGURE 15. SEM images after constant immersion tests in 0.1 M NaCl for 7 days showing severe localized corrosion due to Fe-containing intermetallics (B) and no corrosion damage around Sr containing intermetallics (A). EDX spectra confirmed presence of Al-Sr phase.

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ous alloying elements on the fraction and composition β-phase, along with the potential formation of an additional g-phase in AA5083 alloys. The influence of calculated changes in microstructure was discussed in terms of potential for sensitization resistance. Based on the thermodynamic calculations, Sr was determined to be the most potent in decreasing the extent of sensitization, followed by the elements Cu, Ag, and Si. However, as previously reported, elements such as Nd can also decrease the extent of sensitization, and hence kinetic effects are difficult to ascertain from thermodynamic calculations alone; therefore, empirical experimental validation was pursued herein to validate the efficacy of Sr as functional sensitization-reducing element. v Custom alloy production and experimental work revealed that the addition of low levels of Sr to AA5083 (and likely more generally, the Al-Mg-Mn alloys) can lead to significant improvement in sensitization resistance. Of the Sr concentrations studied, this beneficial effect was observed when Sr was as low as 0.08 wt%. Investigation of fracture surfaces revealed that addition of Sr caused a decrease in β-phase at the grain boundaries. Electrochemical properties of the grain boundaries of sensitized and Sr-modified alloys were found to differ from those of unmodified (Sr-free) alloys. The decrease in β-phase due to Sr addition resulted in grain boundaries being less anodic and from NAMLT testing, less prone to intergranular corrosion. Sr additions were found to not adversely influence pitting corrosion. The present study has shown that there are great possibilities for eliminating sensitization of 5xxx series Al alloys via functional Sr additions.

ACKNOWLEDGMENTS Financial support from the Office of Naval Research and Office of Naval Research Global (with Airan Perez, Joseph Wells, and Liming Salvino as scientific officers) is gratefully acknowledged. The Australian Research Council is acknowledged for establishing the Centre of Excellence for Design in Light Metals. We thank I.J. Polmear for interesting discussions and the Monash Centre for Electron Microscopy. REFERENCES 1. J. Davis, Corrosion of Aluminum and Aluminum Alloys (Materials Park, OH: ASM International, 1999). 2. R.K. Gupta, N.L. Sukiman, M.K. Cavanaugh, B.R.W. Hinton, C.R. Hutchinson, N. Birbilis, Electrochim. Acta 66 (2012): p. 245. 3. E. Bumiller, “Intergranular Corrosion in AA5XXX Aluminum Alloys with Discontinuous Precipitation at the Grain Boundaries” (Ph.D. thesis, The University of Virginia, 2011). 4. R.H. Jones, D.R. Baer, H.J. Danielson, J.S. Vetrano, Metall. Mater. Trans. A 32 (2001): p. 1699. 5. G.M. Scamans, N.J.H. Holroyd, C.D.S. Tuck, Corros. Sci. 27 (1987): p. 329.


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