Precipitation Process in Al-Cu-Mg Alloys Microalloyed
with Si
C.R.HUTCHINSON and S.P.RINGER
Microalloying additions of Si are known to increa significantly the respon to age hardening of
2xxx ries Al-Cu-Mg alloys,and commercial alloys such as2618are bad on this effect.Previous
work has attributed this effect to a refined dispersion of SЈor S pha(Al2CuMg)precipitates.This
work reports the results of a detailed microstructural characterization,employing transmission electron
microscopy–bad techniques,on the effects of Si additions to a ba Al-2.5Cu-1.5Mg(wt pct)alloy.
It was found that the peak hardness microstructure contains a fine and uniform dispersion of Si-
modified Guinier-Preston-Bagaratsky(GPB)zones.The zones are lath shaped,posssing{100}␣
出国留学多少钱facets,elongated along͗100͘␣directions and contain Si.The S pha was also obrved at peak
hardness,although it is concluded that the precipitates do not contribute significantly to hardening
due to their coar dispersion,which aris from their heterogeneous nucleation on the quenched-in
美剧汉尼拔defect structure.Overaging was associated with the replacement of the zones by the S pha through
a process involving dissolution and reprecipitation together with heterogeneous nucleation of S at the
zones.The precipitation ofЈ(Al2Cu)and(Al5Cu6Mg2)pha was also obrved in alloys containing
Ն0.5wt pct Si.It is demonstrated that the total solute content of the alloy has a major influence on
the precipitation reactions during aging.
I.INTRODUCTION by Silcock,[10]who showed that they posss a rod-shaped
morphology extended along the cube directions of the␣A LLOYS of the Al-Cu-Mg system form an important
matrix.Recent three-dimensional atom probe(3DAP)work part of the2xxx ries alloys and are widely ud in structural
by Reich et al.[6]indicates that they are compod of Cu aerospace applications.Moreover,this system provides the
and Mg atoms and that their formation is preceded by the basis for the development of many other important Al alloys.
formation of more diffu Cu-Mg coclusters.Despite reports An important example includes Si-modified alloys having
of the SЉpha by Bagaryatsky,[8,9]Cuisat et al.,[11]Zahra et compositions that lie in the(␣ϩS)region of the experimen-
al.,[12,13]and Ratchev et al.,[5]it is noteworthy that Silcock,[10] tally derived Al-Cu-Mg pha diagram propod by Brook,[1]
Wilson and Partridge,[7]Jena et al.,[14]and Ringer et al.[3,4] which have application in skin ctions and rivet components
were unable to confirm the prence of the SЉpha.The of supersonic jet aircraft.Alloy2618is bad on Al-3Cu-
SЉand SЈphas are propod to be precursors to the equilib-3Mg-0.5Si(wt pct)and was originally developed for the
rium S pha(Al2CuMg,Cmcm,aϭ0.400nm,bϭ0.923 Concorde superstructure.Although additions of Si are asso-
nm,and cϭ0.714nm).[15]There is now thought to be little ciated with improved tensile and creep strengths,details of
difference between the SЈand S phas,[3,6,16]which occur the preci mechanism by which Si enhances hardening and
as lath-shaped precipitates on{210}␣planes elongated along refines microstructure remain unclear.Renewed interest in
͗100͘␣.High resolution transmission electron microscopy this system was stimulated by studies of the rapid hardening
(HRTEM)[17]and microbeam electron diffraction that occurs in ba Al-Cu-Mg alloys[2–6]and the obrvation
(MBED)[18]have revealed that the habit plane of the lath-that this effect is enhanced in Si-bearing alloys.[2,7]
shaped S-pha precipitates is(001)s,oriented such that The precipitation quence obrved during elevated tem-
(100)s//{100}␣and[010]s//͗012͘␣.It is noteworthy that slight perature aging of Al-Cu-Mg alloys in the(␣ϩS)region
rotations from the preceding orientation relationship have depends on the Cu/Mg ratio.For a Cu/Mg weight ratio of
been reported[18]and that the distribution and morphology 2.2,Bagaryatsky[8,9]reported the following precipitation
of the S pha do vary.In a study of an Al-2.5Cu-1.2Mg(wt quence.
pct)alloy aged artificially at190ЊC,Wilson and Partridge[7] Supersaturated solid solution→GPB found that the SЈ(S)precipitates nucleated heterogeneously
on quenched-in dislocation loops and helices,forming corru-zones→SЉ→SЈ→S.
gated sheets on{210}␣planes.In subquent work,Gupta The Guinier-Preston(GP)zone structures in the alloys et al.[16]examined an alloy with a similar Cu/Mg ratio but were designated as Guinier-Preston-Bagaratsky(GPB)zones lower total solute content.They found that aging of an Al-
1.53Cu-0.79Mg(wt pct)alloy at190ЊC resulted in rod-
shaped SЈ(S)precipitates and concluded that the total soluterunning
content of the alloy influences the precipitate morphology.
C.R.HUTCHINSON,formerly student at Monash University,is post-
graduate student,Department of Materials Science and Engineering,Uni-There have been veral studies on the effects of microal-versity of Virginia,Charlottesville,V A22903,USA.S.P.RINGER,formerly loying with Si on precipitation process in Al-Cu-Mg Associate
Professor of the Department of Materials Engineering at Monash alloys.Vietz and Polmear[2]obrved an enhanced respon University,is now Director of the Electron Microscope Unit at the Univer-
to age hardening at elevated temperatures in Al-2.5Cu-sity of Sydney,NSW,2006,Australia.
Manuscript submitted January17,2000. 1.5Mg(wt pct)with additions of0.25(wt pct)Si.It was
Table I.Alloys Ud in the Prent Study
found that the prence of Si raid the level of hardening
over the entire time scale.Times to peak hardness were
Alloy
unaltered and it was propod that Si affected only the first
Designation Composition(Wt Pct)Composition(At.Pct) stage of hardening.In a study of the effects of0.25(wt pct)
1Al-2.5Cu-1.5Mg Al-1.1Cu-1.7Mg
Si additions on age hardening of Al-2.5Cu-1.2Mg(wt pct),
2Al-2.5Cu-1.5Mg-0.1Si Al-1.1Cu-1.7Mg-0.1Si Wilson et al.[19]showed that the room temperature age-
3Al-2.5Cu-1.5Mg-0.25Si Al-1.1Cu-1.7Mg-0.25Si hardening respon was delayed and the elevated tempera-4Al-2.5Cu-1.5Mg-0.5Si Al-1.1Cu-1.7Mg-0.5Si
ture respon enhanced by the Si additions.It was reported
that the rate of formation of GPB zones at room temperature
was less in the Si-bearing alloy and that the dispersion of
with Si to Al-Cu-Mg alloys may not only depend on the SЈ(S)precipitates formed at elevated temperatures was finer
Cu/Mg ratio but also on the total solute content of the alloy. and more uniform.The depresd room temperature
In summary,there are a number of uncertainties in the respon to age hardening of the Si-containing alloy was
microstructural evolution in Al-Cu-Mg-Si alloys.First,the interpreted as a preferred interaction between Si atoms and
results of Wilson and co-workers and Suzuki et al.suggest vacancies.Wilson[20]found that the prence of0.24wt pct
an apparent conflict in the identity of which precipitates are Si in solid solution caud the quenched-in dislocation loops
stimulated by Si additions to alloys with similar Cu/Mg and helices to be smaller and fewer than in the Si free ternary
ratios and total solute contents.Second,it remains to confirm alloy.This was consistent with the obrvations by Weatherly
using high resolution techniques whether the effect of Si and Nicholson[21,22](reported by Wilson et al.[19])on an Al-
additions is to refine the distribution of the S pha,as 2.7Cu-1.35Mg-0.2Si(wt pct)alloy.Wilson et al.attributed
reported by Wilson and co-workers.[7,19,20]Furthermore,the the refined distribution of SЈ(S)to the influence of Si on
potential mechanism underlying this refinement remains to the defect structure and the formation of GPB zones.First,
be clarified.Similarly,various precipitates reported by Wil-the reduced number of dislocation loops and helices in the
son et al.and Weatherly and Nicholson[21,22]in alloys con-as-quenched(AQ)Si-containing alloys provided fewer het-
taining0.25wt.pct Si and by Suzuki et al.and Gupta et erogeneous nucleation sites for SЈ(S).Second,the incread
al.in the alloys containing0.5wt pct Si or greater remain stability of GPB zones was attributed to their modification unidentified.Finally,the work of Wilson and Partridge[7] by Si,which was propod to inhi
bit the nucleation and in ternary Al-Cu-Mg alloys suggested that the morphology growth of SЈ.Weatherly and Nicholson[22]found that Si had adopted by SЈ(S)and the precipitation quence depend on little effect on the length of SЈprecipitates but greatly the total solute content of the alloy.It remains to establish reduced the cross-ctional area in an Al-2.7Cu-1.35Mg-whether this is also the ca for the Si-containing alloys and, 0.2Si(wt pct)alloy aged at260ЊC.if so,if it can account for the differences in the experimental In contrast with the obrvations of Wilson and co-work-obrvations reported so far.This article address the ers,[7,19,20]Suzuki et al.[23]suggests an entirely different pre-questions through direct characterization using TEM-bad cipitation quence,which results in the formation of SЈ(S),techniques on a t of alloys containing systematic additions Ј(Mg2Si),and an unknown X pha in the alloy Al-2Cu-of Si to a ba Al-2.5Cu-1.5Mg wt pct.
0.9Mg-0.25Si(wt pct).Furthermore,Suzuki et al.propod
that the addition of0.5wt pct Si promotes the precipitation
II.EXPERIMENTAL PROCEDURE
ofЈ(Al2Cu),X,and Q(Al5Cu2Mg8Si6)and suppress the
formation of SЈ(S)andЈ.More recently,Gupta et al.[24,25]Systematic additions of0.1,0.25,and0.5wt pct Si were and Jena et al.[26]studied the effect of0.23,0.49,0.76,and made to a ba alloy composition of Al-2.5Cu-1.5Mg(wt 1.03wt pct additions of Si to Al-1.52Cu-0.75Mg(wt pct).pct)(Table1).The alloys were cast into book molds,homog-The alloys contain a lower total solute content than the
enized,and scalped.The samples were solution treated in
a salt bath at525ЊCϮ2ЊC for1hour followed by immediate alloys studied by Suzuki et al.and by Wilson and co-work-
quenching into cold water.In the ca of elevated tempera-ers.[7,19,20]This work showed that additions greater than0.23
ture aging,the samples were immediately placed into a salt wt pct resulted in the formation of the insoluble compound
bath at200ЊCϮ1ЊC.Samples undergoing room tempera-Q(Al5Cu2Mg8Si6).The large differences in the reported
ture aging were stored at a temperature ofϳ20ЊC.The precipitation quence between additions of0.23and0.5wt
age-hardening respon of the alloys was monitored by pct Si to the Al-Cu-Mg alloys were interpreted in terms of
Vickers hardness measurements(VHN)using a5-kg load. the effect of the formation of Q pha on the composition of
The microstructural changes in the alloys were examined the matrix.On the basis of differential scanning calorimetry,
using scanning electron microscopy(SEM),conventional Gupta et al.[25]also concluded that Si was bound preferen-
transmission electron microscopy(CTEM),and HRTEM. tially with GPB zones.The precipitation of SЈ(S)was con-
梵语翻译
Samples for SEM were prepared by standard metallographic firmed by transmission electron microscopy(TEM)and the
techniques and examined using a JEOL*JSM-840A scan-effects of0.23wt pct Si on the precipitation quence were
in agreement with the obrvations of Wilson and co-work-*JEOL is a trademark of Japan Electron Optics Ltd.,Tokoyo.
ers.[7,19,20]In a related study,Chaturvedi et al.,[27]using the
same alloy as Gupta luded that the prence of
ning electron microscope operating at20kV.Energy disper-
sive X-ray spectroscopy(EDXS)was conducted in the SEM 0.23wt pct Si in Al-1.52Cu-0.74Mg(wt pct)does not alter
using a TRACOR-NORTHERN**microanalysis system the quence,structure,or distribution of the precipitating
pha,but reduces the activation energy for GPB zone forma-
**TRACOR-NORTHERN is a trademark of Noran Instruments,Inc., tion.The studies of Gupta and co-workers,Wilson et al.,Middleton,WI.
and Suzuki et al.suggest that the effect of microalloying
寒假英语周记Fig.1—Electron backscattered images of(a)Al-2.5Cu-1.5Mg-0.1Si,(b)Al-2.5Cu-1.5Mg-0.25Si,and(c)Al-2.5Cu-1.5Mg-0.5Si(wt pct)in the AQ condition. An energy dispersive X-ray spectrographic trace from an insoluble particle in the alloy containing0.5(wt pct)Si is provided in(d).
Table II.Volume Fraction of Insoluble Mg2Si Particles(at operating at8kV.The choice of8kV was made on the
the95Pct Confidence Level)
basis of Monte Carlo simulations of the interaction volume
of the electron probe to ensure that chemical information
V olume Fraction was obtained exclusively from the precipitates and not from V olume Fraction after Aging at
the underlying matrix.Thin foils for TEM were prepared in AQ200ЊC for7 by standard techniques and examined using either a PHIL-Alloy(Wt Pct)Condition Days
IPS†CM-20microscope or a JEOL JEM-
Al-2.5Cu-1.5Mg-0.1Si0.22Ϯ0.15pct0.30Ϯ0.18pct
Al-2.5Cu-1.5Mg-0.25Si0.44Ϯ0.17pct0.97Ϯ0.31pct †PHILIPS is a trademark of Philips Electronic Instruments Corp.Mah-
Al-2.5Cu-1.5Mg-0.5Si0.81Ϯ0.25pct 1.05Ϯ0.30pct wah,NJ.
2000FXII both operating at200kV.Examination of the
AQ specimens was performed within1hour of quenching.
III.RESULTS
Furthermore,to facilitate meaningful qualitative compari-
sons between the defect structures in the as-quenched and
A.AQ Microstructures
naturally aged microstructures,considerable care was taken
to obtain reprentative micrographs away from the edge of Examination of the AQ microstructures by SEM indicated the foil.Two-beam conditions were ud to obtain thickness
that the ternary Al-2.5Cu-1.5Mg(wt pct)alloy was single fringes so that images could be recorded from regions of pha;however,the Si-containing alloys contained insoluble
constituents(Figure1).The density of the constituents constant thickness.The HRTEM was performed on a JEOL
JEM-4000EX operating at400kV and energy-dispersive X-was determined by standard point counting techniques,[28]
and the respective volume fractions at the95pct confidence ray spectroscopy(EDXS)analysis in the TEM was carried
out using a JEOL JEM-2010F operating at200kV equipped level are summarized in Table II.The volume fraction of
wanbangconstituents was found to increa with Si content.
with a field emission gun and an Oxford EDXS system.
Fig.2—BF transmission electron micrographs recorded near the͗001͘␣orientation from the AQ microstructure of the(a)ba Al-2.5Cu-1.5Mg,(b)Al-2.5Cu-1.5Mg-0.1Si,(c)Al-2.5Cu-1.5Mg-0.25Si,and(d)Al-2.5Cu-1.5Mg-0.5Si(wt pct)alloys.
sikuIn SEM backscatter mode,the constituents image darker 3.Each of the alloys exhibited similarly shaped curves,with than the surrounding matrix,indicating a lower average an initial period of constant hardness from the AQ state atomic weight.Figure1(d)is a reprentative EDXS spec-before rising to a plateau within200hours.The aging trum obtained from the constituents and rves to indicate respon of the microalloyed specimens falls into a band that they consist exclusively of Si and Mg.The dark nature that exhibits a delayed respon to hardening and a lower of the constituents and the absorption of Mg,which would hardness than the ternary alloy at all times monitored. need to be corrected for in a quantitative analysis of the
composition of the constituents,indicates that the are 2.Microstructural examination
Mg-rich constituents.On the basis of the obrvations,Figure4provides a ries of BF TEM micrographs the constituents were tentatively identified ded near the͗001͘␣zone axis for each alloy after natural Occasionally,white constituents were also obrved.The aging for400days.The ternary alloy(Figure4(a))shows were found to be pure Si.a large increa in dislocation loop density,and growth of Figure2shows bright-field(BF)TEM micrographs the loops is evident when their size is compared to that recorded near the͗001͘␣zone axis of each alloy in the AQ obrved in the AQ condition.Following careful examina-condition.The BF image of the ternary alloy(
Figure2(a))tion,we were unable to discern diffraction effects in the shows the formation of both dislocation loops and helices corresponding SAED patterns other than tho arising from and there was no evidence of precipitation.In contrast,dislo-the matrix or surface oxide films.Similarly,contrast from cation loops were generally abnt from the microstructure precipitates was not obrved in the BF images.
of the Si-containing alloys,which contained dislocation heli-The BF TEM images of the Si-containing quaternary ces.Again,there was no evidence of fine-scale precipitation.alloys show the growth of dislocation helices and,unlike The AQ hardness for each alloy is summarized in Table III.the ternary alloy,a general abnce of dislocation loops It is en that the hardness of the Si-containing alloys in the from the microstructures.The growth of helical dislocations AQ condition decread with increasing Si content.The appears to be along͗110͘
␣directions(Figures2(b)and(c)); lected area electron diffraction(SAED)patterns recorded however,growth of helices in͗001͘
␣directions was also for each alloy in the AQ condition(not reproduced here)obrved with increasing frequency in alloys with higher Si
only exhibited reflections from t(Figure4(d)).Wilson et al.[19,20]also obrved
growth of dislocations along matrix͗100͘␣directions and B.Naturally Aged Alloys speculated that it may be due to pinning of the screw disloca-
tion by Si atoms along its length.There was no precipitation 1.Hardening
apparent in the BF images of the quaternary Si-containing The hardness-time curves for each alloy following aging
at room temperature(natural aging)are provided in Figure alloys.
Table III.Changes in Hardness(VHN)during Aging at200؇C
Alloy1Alloy2Alloy3Alloy4 Wt Pct Al-2.5Cu-1.5Mg Al-2.5Cu-1.5Mg-0.1Si Al-2.5Cu-1.5Mg-0.25Si Al-2.5Cu-1.5Mg-0.5Si AQ hardness61.35755.854.5versus是什么意思
2min hardness88.2106.4107.2104.5
2min hardness-AQ hardness26.949.451.450
Peak hardness116.2134.5145146
captioned
Peak hardness-AQ hardness54.977.589.291.5
2min hardness/peak hardness0.760.790.740.72
(2min-AQ)/(Peak-AQ)0.490.640.580.55
“2min”denotes the hardness after aging for2min at200ЊC and“Peak”denotes peak hardness at200ЊC.
which incread with Si content.This is indicated in the
SAED patterns,where streaking through{010}␣positions
in͗001͘␣directions increas in intensity with increasing Si
content.The effects are typical of the shape effect from
GPB zones elongated along͗001͘␣directions,and the BF
images in Figures6through8show the variants parallel to
the electron beam,in the end-on orientation.Finally,it is
noted that the underaged microstructures retained a den
distribution of the quenched-in defects,although the contrast
conditions in the micrographs prented in Figures6(a),7(a),
and8(a)were lected to emphasize the fine scale
precipitation.
b.Peak hardness(10hours at200ЊC for Al-2.5Cu-
1.5Mg-0.1Si;14hours at200ЊC for Al-
2.5Cu-1.5Mg-
0.25Si;and20h at200ЊC for Al-2.5Cu-1.5Mg-0.5Si)
The peak hardness microstructures for each of the Si-Fig.3—Room temperature(natural aging)hardness-time curves for the
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containing alloys is dominated by a very fine and uniform Al-2.5Cu-1.5Mg-(Si)(wt pct)alloys studied.
distribution of lath-shaped Si-modified GPB zones.The fine
scale precipitation has been termed“Si-modified GPB C.Aging at200ЊC zones”so as to emphasize their obvious similarity to the
GPB zones that form in ternary Al-Cu-Mg alloys.Although 1.Hardening
compositional and morphological differences between the Figure5is a hardness-time curve containing data for each
precipitate phas are reported subquently,the do not of the alloys after aging at200ЊC.All alloys exhibited two
em to warrant the introduction of a unique or new nomen-stages of hardening,the first of which was complete within
clature.For example,when viewed at higher magnifications, conds of exposure at the aging temperature.Each of the
as provided in the int micrographs,the zones appeared Si-containing alloys exhibits an enhanced age-hardening
to exhibit{001}␣facets,which is distinct from the almost respon,and it is particularly noteworthy that the hardness
equiaxed cross ction reported for GPB zons in Al-Cu-Mg increment associated with the initial rapid hardening was
alloys.[3–10]Heterogeneous precipitation of the S pha is almost double that obrved in the ternary alloy.The cond
also apparent on dislocation helices,formed from the stage of hardening for the Si-containing alloys involved a
quenched-in defect structure.In the vicinity of the S-pha relatively steady ri to peak hardness followed by overag-
precipitates,regions free of precipitation of the Si-modified ing.The peak hardness incread with Si content as did the
GPB zones were obrved.Similar obrvations were rate of decline in hardness associated with overaging.The
recently reported for the ba ternary Al-2.5Cu-1.5Mg wt changes in hardness associated with each of the alloys at
pct alloy,[6]and this is interpreted as indirect evidence for 200ЊC are summarized in Table III.
a preferred solute-dislocation interaction,which precedes
2.Microstructural examination formation of the Si-modified GPB zones.The SAED patterns Following recent work on the clustering,precipitation,exhibited very sharp and obvious streaking in͗001͘
␣direc-and hardening process in the ternary alloy,[3–6]a detailed tions through the{010}
␣positions in reciprocal space.The microstructural examination was made for each of the Si-are characteristic of well-defined precipitates elongated in
containing alloys.Figures6through8are BF TEM micro-͗001͘
␣directions in the matrix.
graphs recorded near the͗001͘␣zone axis and include the A more detailed examination of the peak hardness micro-corresponding SAED patterns,respectively,in(a)under-structures was undertaken using HRTEM,and Figures9and aged,(b)peak-hardness,and(c)overaged(7days)10feature results from the Al-2.5Cu-1.5Mg-0.25Si(wt pct) conditions.alloy,which were typical for all the Si-containing alloys
studied.Figure9(a)is an HRTEM micrograph recorded with a.Underaged(5minutes at200ЊC)
the electron beam parallel to͗001͘␣and demonstrates the The BF TEM micrographs for each of the Si-containing
alloys show signs of very fine precipitation,the density of lack of periodic order in the Si-modified GPB zones that
