Evolution of Grain Boundary Precipitates in Al 7075 Upon Aging and Correlation with Stress Corrosion...

Evolution of Grain Boundary Precipitates in Al 7075 Upon Agingand Correlation with Stress Corrosion Cracking Behavior

RAMASIS GOSWAMI, STANLEY LYNCH, N.J. HENRY HOLROYD,STEVEN P. KNIGHT, and RONALD L. HOLTZ

Transmission electron microscopy (TEM) was employed to investigate the microchemistry andmicrostructure of grain boundary precipitates in Al 7075 aged at room temperature for severalhours, at 393 K (120 �C) for 12 hours (under aged), at peak aged (T651) and over aged (T73)conditions. High resolution TEM analysis of precipitates at grain boundaries and fine probeenergy dispersive spectrometry showed that the grain boundary precipitates at peak and overaged conditions are hexagonal g phase with stoichiometry Mg(CuxZn1�x)2. Considerableincrease in Cu content in the grain boundary g in the over aged condition compared to the peakaged condition was observed. The average Cu content in the over aged condition was found tobe 20 at. pct. The higher Cu content of the precipitate is associated with a lower stress corrosioncracking plateau velocity.

DOI: 10.1007/s11661-012-1413-0� The Minerals, Metals & Materials Society and ASM International 2012

I. INTRODUCTION

ALUMINUM 7XXX series alloys have been exten-sively used for structural components in military as wellas civilian aircraft.[1] However stress corrosion cracking(SCC) of the higher strength tempers in salt spray/saltfog environments continues to be a problem for 7XXXseries alloys,[2,3] particularly with respect to maintenanceof aging aircraft.[4] It is generally accepted that in saltwater environment SCC susceptibility is the result ofanodic dissolution of grain boundary precipitates whichsupports the generation of hydrogen through cathodicreaction, which in turn is associated with the embrittle-ment in Al 7XXX alloys.[5–12]

Different aging treatments have been employed toimprove the resistance to SCC of 7XXX aluminumalloys. Simple overaging treatments[13] and more sophis-ticated retrogression and re-aging (RRA)[14] are knownto considerably reduce the SCC velocities and increasethe SCC thresholds (K1SCC) in Al-Zn-Mg alloyscontaining Cu. The overaging treatments reduce thestrength of the materials by 10 to 15 pct,[3,15,16] whileRRA treatments maintain the strength close to the peakaged (PA) condition,[16,17] or even can result inimproved strength in some cases.[18]

The improvement of SCC due to aging treatments inthese alloys has been attributed to various factors,[5–9]

such as (i) increasing grain-boundary precipitate size,

spacing or volume fraction and increasing precipitatefree zone (PFZ) width (ii) changes of microchemistry ofgrain boundary precipitates, and (iii) decreasing slipplanarity.[8] There are still unresolved questions aboutwhich factors are important in controlling the SCCbehavior, partly because the various microstructuralfeatures are not independently controllable.It has been proposed that the SCC plateau velocity of

Al-Zn-Mg-Cu alloys[19] in aqueous environmentdecreases with increasing Cu content, as a function ofaging time. It was argued that increasing Cu content ofthe grain boundary precipitates with aging makes theprecipitate more noble.[19] Cooling rates during quench-ing from solution treated temperature have been shownto affect the SCC plateau velocity in works by Knightet al.[5,20] on a 7079 alloy. They argued that microchem-istry, particularly the content of Cu of the grainboundary precipitate may determine alloy susceptibilityto SCC in salt water environment.Several attempts[7] have been made to correlate the

increase in grain boundary precipitate size and spacingwith higher resistance to SCC. Recent observations oncomparing 7075 and 7079 under similar aging condi-tions showed that the plateau velocity for 7079 Al canbe four order of magnitude higher than that of7075 Al.[5,20] However, the grain boundary microstruc-ture, precipitate size and spacing for two alloys atsimilar over aging condition are very similar. There areother observations that for two different 7XXX seriesalloys at peak aged condition, the plateau velocity issignificantly different for similar grain boundary precip-itate size and spacing[7] or an alloy with different ageingconditions can have similar SCC velocities but differentgrain-boundary precipitate size and spacing.[7] Theseexperimental findings suggest that the increase in grainboundary precipitate size and spacing alone cannot beresponsible for the SCC resistance in 7XXX seriesalloys, particularly in 7075 Al alloy.

RAMASIS GOSWAMI and RONALD L. HOLTZ, Scientists, arewith the Multifunctional Materials Branch, Naval Research Labora-tory, Code 6351, Washington, DC. Contact e-mail: [email protected] STANLEY LYNCH, Scientist, is with the DefenceScience and Technology Organisation, Melbourne, VIC, Australia.N.J. HENRY HOLROYD, Consultant, is from Riverside, CA 92506.STEVEN P. KNIGHT, Scientist, is with the RMIT University,Melbourne, Australia.

Manuscript submitted February 16, 2012.Article published online September 28, 2012

1268—VOLUME 44A, MARCH 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A

The copper content of grain-boundary precipitates in7075 has been reported to increase with over agingrelative to the peak-aged condition.[21,22] Anotherstudy[23] also showed that increasing overall coppercontent of various peak-aged 7XXX series aluminumalloys tends to increase the amount of copper at theexpense of the zinc content in the grain boundary gprecipitate. These reports suggest that changes of themicrochemistry of the grain boundary precipitates playsa role in the changes of SCC resistance.

As the grain boundary cohesion/decohesion behavioris affected by the solute segregation in Al-Mg[24] andAl-Li[25] systems, it also is important to investigate theextent of solute segregation in 7075 Al as a function ofaging. It was predicted based on sublimation enthalpiesthat Zn and Mg might increase grain boundary embrit-tlement while Cu might increase grain boundary cohe-sion in an Al-alloy.[26] The extent of segregation of Zn,

Mg and Cu in 7XXX alloys during aging is unclear.Attempts have been made to correlate the grain bound-ary cohesion behavior in 7XXX alloys with aging withsegregation, based on results obtained from transmis-sion electron microscopic (TEM) studies, although usingconventional large probe size.[21,22,27] In a more recentTEM study,[28] the zinc and copper contents in the PFZand grain-boundaries were measured in 7075 alloy forthe peak-aged, over aged, and RRA conditions. In thepeak-aged condition, there was a depletion of both zincand copper in the PFZ relative to the matrix. Overagingresulted in further depletion in copper only, while thedistribution of zinc varied very little. Segregation at thegrain-boundary of a 7150 alloy[29] also has beenreported in the peak-aged, over aged, and RRA condi-tions; and in Al-Mg-Zn alloys.[30] Grain-boundary levelsof magnesium were found to be twice as high in the overaged and RRA-treated conditions as in the peak-aged

Fig. 1—(a) A bright-field TEM image showing grain boundary precipitates in naturally aged condition. (b) A HADDF image in naturallyaged condition. (c) A HAADF image after aging at 393 K (120 �C) for 12 hours. (d) A bright-field TEM image at peak (T651) aged condition.(e) A bright-field TEM image at over (T73) aged condition.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 44A, MARCH 2013—1269

condition. Furthermore, overaging beyond peak-strength reduced both zinc and copper by 50 and70 pct, respectively, while RRA decreased both zinc andcopper by ~90 pct relative to the peak-aged sample.

Thus, it is of interest to systematically investigate theeffect of aging on the Cu content of grain boundaryprecipitates of 7075 Al alloy, as the anodic dissolutionrate depends on the amount of Cu in the grain boundaryprecipitates. The present study examines the correlationof the composition of grain boundary precipitates,particularly the content of Cu, and the grain boundarysegregation with the susceptibility to SCC in 7075 Alalloys as a function of aging.

II. EXPERIMENTAL

Thin plates (about 1 mm thick) of commercial 7075 Al(T651) were solution treated at 753 K (480 �C) for 1 hourand quenched in cold water. Samples were then aged atroom temperature (naturally aged) for several hours, andat 393 K (120 �C) for up to 18 hours. Some samples wereleft in the as-received peak aged (T651). Over agedsamples were prepared from another commercial platereceived in T73 condition. The overall compositions ofthe T651 and T73 plates, measured by scanning electronmicroscopy (SEM) energy dispersive X-ray spectroscopy(EDS), were very similar. The ratios of Zn:Mg:Cu for thetwo plates were virtually identical, 3.8:1.7:1 and 3.7:1.7:1,for the T651 and T73 material, respectively.

Philips CM-30 and JEOL 2200 analytical TEM oper-ating at 300 and 200 keV, respectively, were used tocharacterize the microstructure of 7075 Al as a functionof aging. TEM samples were prepared by initiallypolishing arc-cut disk samples mechanically and finallyby thinning in an ion mill with a gun voltage of 5 kV, acurrent of 5 mA, and a sputtering angle of 10 deg at lowtemperatures. The fine probe EDS was employed todetermine the distribution of Mg, Zn, Cu and Al at the

grain boundaries, as well as within the matrix grains andprecipitates. Further compositional information wasobtained with high-angle annular dark field (HAADF)imaging with the scanning TEM (STEM) mode. Alsoknown as Z-contrast imaging, with this imagingmode,[31] the brighter regions correspond to heavieratoms, as the scattering cross-section is approximatelyproportional to Z2. Precipitates, for example, enrichedwith Zn atoms in 7XXX series Al alloys will appearbright as compared to the matrix in HAADF imagingmode. SEM was used to study fracture surfaces in shorttransverse (ST) orientation. Resistivity of samples as afunction of aging was measured with a four point probemethod at room temperature.

III. RESULTS

A. General Microstructure Evolution

The general aging behavior in 7075 Al alloy will firstbe described briefly, as considerable work has beenreported in the literature on microstructures of 7XXXalloys at peak and over aged conditions.[32,33] Our resultsare consistent with the general trend that grain boundaryprecipitation occurs with aging, and the precipitate sizeand spacing increase with aging. The microstructureevolution as a function of aging in 7075 Al is demon-strated by Figures 1(a) through (e). In naturally agedcondition (Figure 1(a)), grain boundaries were observedto be decorated by nanocrystalline precipitates with size5 to 10 nm. Figures 1(b) and (c) are the HAADF imagesshowing the extremely fine and discrete clusters of brightdots at the grain boundary and within the grain atnaturally aged and aged at 393 K (120 �C) for 12 hours,respectively. The precipitates at the grain boundary areclosely spaced in these conditions. Extremely fine pre-cipitates (Figures 1(b) and (c)), mostly GP zones, can beobserved within the grain, suggesting such clusters aremostly Zn rich. At peak aged condition, the precipitatesat grain boundaries are much coarser, 50 to 100 nm(Figure 1(d)). The precipitate spacing for peak agedcondition has also increased considerably as comparedto the under aged [12 hours at 393 K (120 �C)] andnaturally aged samples. Upon further aging (T73), theprecipitates size and spacing (Figure 1(e)) were found toincrease to some extent as compared to that of thesample at peak aged (T651) condition.In under aged condition, in addition to grain boundary

precipitate, precipitation occurs within the grain(Figures 1(c) and 2).[34] High resolution TEM (HRTEM)shows that these precipitates, approximately 2 to5 nm, form with a truncated octahedron morphology(Figure 2), bounded by {111} and {001} matrix planes.Such precipitates form along {111} planes of the matrix.No extra spots due to these precipitates (the inset ofFigure 3) at the fast Fourier transform (FFT) could beobserved from number of these precipitates and thematrix. This suggests that these Zn-rich clusters are FCC.They are most likely GP (II) zones[35–40] with truncatedoctahedron morphology. Such GP zones formed along{111} planes grow with continued aging to form rod or

Fig. 2—A high resolution TEM (HRTEM) image showing the GPzones for sample aged at 393 K (120 �C) for 12 hours. The FFTincluding several GP zones is given in the inset.


platelet like g phase[41–43] with an orientation relationship(OR) with the matrix given by (111)a || (0001)g and[1-10]a || [10-10]g, which is consistent with one of thepreviously reported ORs.[36] Figure 3(a) is a bright-fieldTEM image showing a number of rod/platelet like phase.Such precipitates are associated with the parallel Moirefringes (Figures 3(a) and (b)), since they are embedded inthe foil. In order to avoidMoire fringes, and to increase theimage clarity, the foil was thinned to a great extent so thatsomeof the platelets are not fully embedded. TheHRTEMimage from one such rod/platelet like phase close to the[011] zone of the matrix is given in Figure 3(c). ThecorrespondingFFTwas given as an inset. The inverse FFT(IFFT) image formed including the matrix and eta spots isshown for clarity (Figure 3(d)). Analysis of the HRTEMimage along with FFT shows that the rod/platelet like

phase is g phase, which is hexagonal (space group: P63/mmc) with lattice parameters, a = 5.21 A, c = 8.6 A.[36]

The total number of g precipitate variants is four in theobserved OR, which can be obtained by dividing the orderthepoint group symmetryof thematrix (m3m)by theorderof the intersection point group symmetry (�3m) in thisOR.[44] In addition to rod/platelet type g phase, the GPzones (Figure 4) and other nanocrystalline particles areobserved inpeakaged (T651) condition.However,g¢phasecould not be observed at this aging condition.

B. Grain Boundary Precipitate Structure andMicrochemistry

As clearly the SCC in 7075 is intergranular innature[13] attention was focused on the microstructural

Fig. 3—(a) A bright-field TEM image showing rod/platelet like precipitates with the grain in peak aged (T651) condition. As the precipitates areembedded, Moire fringes are observed. (b) A HRTEM image of one such rod/platelet type precipitate in the matrix showing the Moire fringes.(c) A HRTEM image showing the platelet in the matrix without Moire fringe. (d) The inverse fast Fourier transform (IFFT) image obtainedfrom the area indicated by a square box in (c). The FFT showing the orientation relation with the matrix is given as an inset.


features close to grain boundaries, such as the structureand composition of the grain boundary precipitate, andthe segregation of elements at the grain boundary. Thesemicrostructural features at the grain boundaries stronglyinfluence the electrochemical reactions at the crack tip,that in some instances control crack growth rate.[7] Inpeak and over aged condition, TEM studies showedthat the grain boundary precipitates are similar to theg phase (MgZn2) with considerable amount of Cu in it.We looked at a number of grain boundary precipitates

(Figure 5(a)) to study the structure and composition.Figure 5(b) is a HRTEM image of one such grainboundary precipitate close to the [0001] zone. The FFTobtained from this precipitate showed the {10-10} typespots with d-spacing ~4.5 A. The {10-10} lattice planes arealso shown in Figure 5(b).We occasionally observe Cr-richdispersoids at grain boundaries in addition to g phase.Compositionmaps ofMg,Al,CuandZnwere obtained

across the grain boundary containing several precipitates.Composition maps and the corresponding HAADFimages of the under aged, peak aged and over agedconditions was shown (Figures 6(a), (b), 7(a), (b), and8(a), (b)), respectively. Lines scans (Figures 6(c), 7(c), and8(c)) of Mg, Cu and Zn were extracted from the mapsshowing the relative proportions of the element at grainboundary precipitate. The precipitate composition wasobtained in a spot mode. The composition of precipitateas a function of aging condition is given in Figure 9,showing significant increase in Cu as a function of aging.However, the amount of Zn decreases considerably withaging, and theMg increases by small amount. At peak agecondition the average content of Mg, Cu, and Zn is 45.2,15.1, and 39.7 at. pct, respectively. The average Mg, Cu,and Zn at over aged condition is 40.0, 20.00 and40.00 at. pct, respectively. From the HRTEM and com-position analysis, it can be concluded the precipitatestoichiometry is close to Mg(CuxZn1�x)2 and structure issimilar to the g phase. There is a possibility of the presenceof some amount ofAl at the Zn site. However, it could notbe assessed quantitatively as the precipitates are sur-rounded by Al-matrix.

C. Grain Boundary Segregation

In the present work, the grain boundary compositionbetween precipitates was measured on a number ofboundaries using fine probe EDS with 1 nm probe size

Fig. 4—A HRTEM image close to the [0-11] zone showing GP zonesby arrows at peak aged (T651) condition. A number of these zoneswith platelet type morphology form on {111} planes. A few of theGP zones are spherical.

Fig. 5—(a) A bright-field TEM image at peak aged condition showing a grain boundary with several precipitates. (b) A HRTEM image of onesuch precipitate showing the crystal structure of the precipitate conforms to hexagonal MgZn2.


in STEM-HAADF mode on carefully thinned areas ofthe sample. The very fine probe size and very thin samplesensures that the EDS signals are only from the grainboundary. Results on segregation are shown presented inFigures 10 and 11 for peak and over aged samples,respectively. The segregation in naturally aged and underaged conditions could not be measured, as the precipi-tates are too closely spaced even for the 1 nm probe size.No apparent segregation of Zn, Mg and Cu at peak andover aged condition could be observed. In fact, a smallamount of depletion of all solute elements were observedfor peak and over aged conditions. This suggests that thegrain boundary decohesion/cohesion behavior is notaffected by the solute with aging for this alloy. Thedepletion of solute at grain boundaries is consistent withthe mechanism of growth of grain boundary allot-riomorphs, which has been put forwarded by Aaronand Aaronson.[45] They argued that precipitates at grainboundaries grow by the collector plate mechanism wheresolute atoms from the grain interior are collected to thegrain boundary and then diffuse quickly along the grainboundary to precipitates by grain boundary diffusion. Atlow homologous temperature, for example at 393 K

(120 �C), the grain boundary diffusion will be moreeffective compared to the volume diffusion. No accumu-lation of solute atoms is expected at grain boundaries as itquickly diffuses to precipitates, and the growth will takeplace mostly by the grain boundary diffusion.

D. Grain Boundary Precipitate Dissolution in Saltwater

It is generally accepted that crack growth in Al alloysunder stress in an aggressive environment, particularly indistilled water or water vapor atmosphere, involves seriesof events, such as production of hydrogen at the cracktip, diffusion of hydrogen ahead of the crack tip anddecohesion of grain boundary due to hydrogen.[5] In saltwater environment, the anodic dissolution of grainboundary precipitates at the crack tip is also an impor-tant factor to control the growth rate. To study thedissolution behavior of the g-phase, TEM samples wereimmersed into a brine solution containing 3.5 pct NaClat room temperature, and subsequent TEM observationswere carried out after immersing the sample for around180 to 300 seconds. It was observed that grain bound-aries with g/g¢ phase containing negligibly small amount

Fig. 6—(a) A HAADF image containing grain boundary precipitate for sample aged at 393 K (120 �C) for 12 h. (b) The elemental maps of Mg,Al, Cu and Zn. (c) A line scan across the grain boundary from the region marked by thick arrow in (a).


of Cu were completely dissolved after 300 seconds in thesalt water in naturally aged condition (Figures 12(a) and(b)). However, in peak aged condition the precipitates atthe grain boundary did not dissolve completely for thesame immersion duration (Figure 12(c)). Our TEMobservations showed that in the peak aged condition,the Cu content of the g phase, 15.1 at. pct, is considerablyhigher as compared to the naturally aged condition.The dissolution rate can be directly correlated withthe electrochemical potential of the intermetallicMg(CuxZn1�x)2 compound,[7] which has been observedto increase with the increase in Cu content. This decreasesthe driving force for anodic dissolution with respect tomatrix with aging, which suggests the rate of dissolutiondecreases with the increase of Cu content. In over agedcondition, TEM results show that the average Cu contentin grain boundary precipitate is 20 at. pct. Thus thedissolution rate at the over aged condition will besignificantly lower as the driving force decreases further.

IV. DISCUSSION

The present results demonstrate clearly that the Cucontent of the Mg(CuxZn1�x)2 precipitates at grain

boundaries increases by around 25 pct (Figure 9) in theover aged condition (T73) compared with the peak agedcondition (T651). In the under aged [393 K (120 �C) for12 hours] as well as in the naturally aged condition, theCu content in the precipitates at grain boundaries isnegligibly small. Although the composition, particularlyCu and Zn of the grain boundary precipitates haschanged considerably, particularly in peak and over agedcondition as compared to the under aged condi-tion [393 K (120 �C) for 12 hours], the structure ofthe Mg(CuxZn1�x)2 precipitates remains the same(Figure 5) asMgZn2 with aging. The Cu atoms substitutethe Zn atoms of the Mg(CuxZn1�x)2 intermetallic com-pound, and thus the increase in Cu content of theMg(CuxZn1�x)2 intermetallic compound results in de-crease in Zn to maintain the stoichiometry. The stabilityof the Mg(CuxZn1�x)2 intermetallic compound increaseswith the addition of Cu mostly as a result of increase inconfigurational entropy. The Cu content will most likelyincrease until the Cu and Zn ratio is 1:1 in Mg(CuxZn1�x)2 with prolonged aging.Electrochemical open circuit potential (OCP) mea-

surements have been studied by Knight et al.,[5,6,20] onover aged Al 7075 and 7079 alloys to better understandrelationships between grain boundary composition and

(a)

(c)

(b)

Fig. 7—(a) A HAADF image containing grain boundary precipitate for peak aged sample. (b) The elemental maps of Mg, Al, Cu and Zn. (c) Aline scan across the grain boundary from the region marked by thick arrow in (a).


electrochemical reactions in salt water environment. TheSCC plateau velocity for these alloys showed a verystrong dependence on OCP, dropping more than fourorders of magnitude for a change of OCP from �1250 to�1000 mV. This change of OCP is related to a change ofCu concentration in the grain boundary precipitatesfrom at 1 to 10 at. pct. Thus, this result showed thatSCC plateau velocity is strongly correlated to Cu

concentration of the grain boundary precipitates. Thoseresults are in good agreement with the observations ofSarkar et al.[19] that the plateau velocity decreases by anorder of magnitude with increasing overaging and theCu content of the Al-Zn-Mg-Cu alloys increases fromzero to 1.6 wt pct. Recent observations by Ramgopalet al.[46] on the polarization curves of Mg(CuxZn1�x)2intermetallic compound with different amount of Cushowed that higher concentration (17 to 27 at. pct) ofCu shifts the OCP curves considerably to more noblevalues, around �1100 to �1200 mV, relative to MgZn2,with OCP of �1400 to �1500 mV. This certainlysuggests that the rate of dissolution of the precipitatesin the 7075 alloy should decrease as the Cu content ofthe grain boundary precipitate in the 7075 increases.The present TEM results confirm that the mean Cucontent for over aged sample is 20 at. pct, confirmingthat the precipitate will be much more noble relative toMgZn2, thus explains the high resistance to SCC of theover aged condition.The present measurements of grain-boundary precip-

itate compositions at peak and over aged condition(Figure 9) are somewhat different than the earlierexperimental observations by Knight et al.[5] and Menget al.,[23] although the Zn:Mg:Cu ratios of our samples,3.8:1.7:1 and the Knight et al. material, 3.8:1.6:1 andMeng et al. material 4.0:1.8:1, are all very similar. The

(a)

(c)

(b)

Fig. 8—(a) A HAADF image containing grain boundary precipitate for over aged sample. (b) The elemental maps of Mg, Al, Cu and Zn.(c) A line scan across the grain boundary from the region marked by thick arrow in (a).

Fig. 9—The average composition of Zn, Mg and Cu of the grainboundary precipitate as a function of aging condition, NA (naturallyaged), T651 (peak aged) and T73 (over aged).


differences in precipitate compositions could in fact arisefrom different overaging temperatures, and probe sizeand mode of EDS measurements as discussed below.

Grain boundary segregation of Mg, Zn and Cu in Al7XXX series alloys has been reported by number ofauthors[21,22,27–30]; however, this is not observed in thepresent study. Ambiguities can often arise in the analysisof grain boundary microchemistry when very fine scaleprecipitation is involved, accompanied by poor diffrac-tion contrast for small precipitates at grain boundaries.The previously reported segregation measurements, wenote, have been made with a relatively larger probe sizein regular (bright-field imaging) TEM mode. In thismode, fine precipitates present cannot be efficientlydetected because of strong diffraction contrast from thematrix. To accurately measure the compositions alonggrain boundaries in the present work, we instead usedvery fine probe (probe size 1 nm) EDS measurements inthe STEM-HAADF mode. The HAADF imaging modeis more accurate method than standard TEM tomeasure grain boundary precipitates, as the diffractioncontrast of the matrix is minimized. Furthermore, theTEM foil must be very thin and the grain boundaryorientated very close to edge-on to minimize thecontribution from the surrounding matrix when

measuring the composition of the part of grain bound-ary between precipitates.

V. SUMMARY AND CONCLUSIONS

TEM was employed to investigate the microchemistryand microstructure of grain boundary precipitates in Al7075 aged naturally at room temperature for severalhours, at 393 K (120 �C) for 5, 12 and 18 hours, and forpeak aged (T651) and over aged (T73) conditions. Forthe under aged conditions at 393 K (120 �C) for up to12 hours, high density of nanocrystalline Zn-rich pre-cipitates were observed at grain boundaries. These couldbe either g¢ or g phase. Fine probe EDS results with aprobe size of 1 nm showed that the amount of Cu in theprecipitates at this stage is negligible. However, consid-erable increase in Cu content of the precipitates wasobserved for peak aged and over aged conditions.HRTEM analysis of precipitates at grain boundaries forthese conditions showed that the crystal structureconforms to hexagonal g phase with stoichiometry.Mg(CuxZn1�x)2. For the peak and over aged condi-

tions the average Cu content, x, is 15 and 20 at. pct,respectively.

(a)

(c)

(b)

Mg

Cu

Zn

Fig. 10—(a) A HAADF image containing grain boundary precipitate for peak aged sample. (b) The elemental maps of Mg, Al, Cu and Zn.(c) A line scan across the grain boundary from the region between the precipitates marked by thick arrow in (a) showing that the depletion ofall element as compared to the matrix. The black arrow in (c) shows the approximate position of grain boundary.


(a)

(c)

(b)

Fig. 11—(a) A HAADF image containing grain boundary precipitate for over aged sample. (b) The elemental maps of Mg, Al, Cu and Zn. (c) Aline scan across the grain boundary from the region between the precipitates marked by thick arrow in (a) showing that the depletion of allelement as compared to the matrix. The black arrow in (c) shows the approximate position of grain boundary.

Fig. 12—(a) Typical grain boundary precipitates in the naturally aged condition before the immersion in salt water. (b) Grain boundaries show-ing the complete dissolution of precipitates after 300 s exposure to salt water in naturally aged condition. (c) Grain boundary showing the pre-cipitates after 300 s exposure to salt water in peak aged condition.


The observed increase in Cu content of the precipi-tates with aging correlates with the increasing resistanceto intergranular SCC. The electrochemical potential ofthe intermetallic Mg(CuxZn1�x)2 compound increaseswith the increase in Cu content. This decreases thedriving force for anodic dissolution with respect tomatrix, particularly for the over aged condition wherethe Cu level is highest. As the dissolution rate decreaseswith increasing Cu content, the alloy becomes lesssusceptible to SCC.

In this study, very fine probe EDS in the STEM-HAADF imaging mode from the regions betweenprecipitates along a number of grain boundaries showedthe absence of segregation of Zn, Mg and Cu for peakaged or over aged conditions.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the Office ofNaval Research, program officers Dr. L. Kabacoff andDr. A. K. Vasudevan for funding this work.

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Evolution of Grain Boundary Precipitates in Al 7075 Upon Aging and Correlation with Stress Corrosion...

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