MATERIALS CHARACTERIZATION 44:277–284 (2000)© Elsevier Science Inc., 2000. All rights reserved. 1044-5803/00/$–see front matter655 Avenue of the Americas, New York, NY 10010 PII S1044-5803(99)00067-4

277

E

ffect of Cold Work and Trace Rare-Earth Additions on the Aging Behavior of Al–Cr AlloysContaining Zirconium

M. K. Banerjee and S. Datta

Department of Metallurgy, Bengal Engineering College (Deemed University), Howrah, India

Precipitation behavior during aging of a cold-worked aluminium chromium alloy contain-ing zirconium has been investigated by electron microscopy transmission. The effect of tracerare-earth additions on the aging behavior of Al–Cr–Zr alloys has been studied using micro-hardeness testing and metallographic analysis. A detailed microstructural analysis revealedthat the age hardening in these alloys is mainly due to the formation of precipitates on dislo-cations. It was further noted that cold working prior to aging greatly accelerated the processof age hardening, and a two-stage hardening has been observed in most of the alloys with orwithout cold work. Two types of precipitates, viz CrAl

7

and Cr

2

A

11

, formed at different ag-

ing temperatures, have been identified. © Elsevier Science, Inc., 2000. All rights reserved.

INTRODUCTION

In their study of the age-hardening behav-ior of Al–Cr alloys, Nagahama and Miki [1]showed that cold working prior to agingaccelerates the ageing kinetics. It was pro-posed earlier that if an alloy exhibits en-hanced precipitation when it is cold workedprior to aging, trace element additions willlikely influence the aging behavior of thealloy [1–3]. There are also reports wheretrace additions enhance precipitation dur-ing aging even though the prior cold workis ineffective in this regard [4–6]. It has alsobeen observed that the addition of siliconhas considerable influence on the aging be-havior of binary Al–Cr alloys [7]. The rareearths are reported elsewhere to have ex-erted considerable effect on the age-hard-ening behavior of Cu–Cr alloy [8]. Fromthese earlier observations it appears thatthe addition of rare-earth elements in Al–Cr alloys can have a significant effect on theaging behavior. Hence the purpose of thepresent investigation is to study the effect

of rare earth additons on the aging behav-ior of Al-Cr alloys containing zirconium.The addition of trace elements has a verysignificant effect on to the precipitation be-havior of a number of age-hardenable alu-minium alloys [9–11], and may affect the ki-netics of G.P. zone formation [12]. AlthoughG.P. zones are not observed to form in bi-nary Al–Cr alloys, the rare-earth additionmay promote the formation of G.P. zones inthese alloys. Thus, it may be of technical in-terest to study this effect in context of theearlier observation that silver stimulates GPzone formation in Al–Mg alloys [11].

The effect of cold work on the rare-earth–modified Al–Cr alloys is not well docu-mented in the literature. So, to gain aclearer understanding about the mecha-nism of age hardening in the above alloys,the present work was performed with therare-earth–treated alloys by way of agingthem before and after cold working, andtracing their aging behavior by microhard-ness measurements and transmission elec-tron microscopy (TEM).

278

M. K. Banerjee and S. Datta

EXPERIMENTAL METHODS

Aluminium–Chromium alloys were pre-pared in a resistance heating electric fur-nace by melting electrolytically pure alu-minium under suitable flux cover. Therequisite quantity of chemically pure chro-mium and zirconium powders were com-pacted with pure aluminium powder.These compacts were added to the moltenaluminium bath. The alloy was degassedprior to solidification and remelted to en-sure homogeneity. In some melts, mischmetal and yttrium were added by a plung-ing technique adopted to the remelted liq-uid alloy. Finally, the alloys were poured inpreheated 25mm square molds of cast iron.The cast alloys were hot rolled at 350

8

C to afinal thickness of 6mm. The alloys subse-quently solution annealed at 600

8

C for 5 hand then were quenched into iced brine.Some specimens of each alloy were coldrolled to a final reduction in thickness of50%. The as-quenched alloys (both withand without cold deformation) were iso-chronally aged for 45 and 90 min, respec-tively. Microhardness tests using a Knoopindentor were performed for all aged sam-ples in a Microduramet 400 attached to aPolyver Met optical microscope. Specimensfor TEM analysis were prepared by con-ventional jet-polishing techniques using a30% HNO

3

–70% CH

3

OH solution at

2

10

8

C.Electron-transparent specimens were ex-amined by a JEOL 200 transmission elec-tron microscope operated at 200kV. Tensiletests were also performed on the unde-formed alloys in an Instron testing machineat a strain rate of 6

3

10

2

4

/s (see Table 1).

RESULTS

EFFECT OF RARE-EARTH ADDITIONS ON THE AGING RESPONSE OF THE ALLOYS

Figure 1 shows the effect of rare earths onthe hardness of the alloys when aged iso-chronally. A two-stage age-hardening effectis noticed in the base Al–Cr–Zr alloys, withreversion taking place at 400

8

C. The peakhardeness in the first stage occurs at 300

8

C,followed by a fall in hardeness to its mini-mum value at 400

8

C. Beyond this tempera-ture, the hardness value further rises notice-ably. When the base alloy is modified with0.08% misch metal (alloy 2), the first peak-hardness temperature is shifted to a highervalue—400

8

C. The reversion phenomenonoccurs at about 450

8

C. Following the rever-sion, the hardness further rises in the secondstage on hardening. However, an increase inthe amount of misch metal from 0.08 to0.15% (alloy 3) does not result in any signifi-cant change in the aging behavior of the al-loy. It also appears that the additions ofmisch metal significantly improves the levelof age hardening and retards softening inthe first stage of age hardening. In the yt-trium-treated alloy (alloy 4), although theextent of age hardening is increased quiteconsiderably, the reversion effect is not ob-served, and the age hardening occurs in asingular stage. The peak hardness tempera-ture (330

8

C) is found to be slightly higherthan the first peak temperature of the basealloy. The combined addition of misch metaland yttrium has caused a delayed softening,with the accompanying reduction it the ex-tent of reversion. From these observations itappears that yttrium characteristically tends

Table 1

Chemical Composition of the Alloys (wt. %)

Alloy Cu Fe Mg Si Ti Cr Zr Trace addition

1 0.12 0.20 0.002 0.12 0.004 0.68 0.34 —2 0.16 0.20 0.002 0.13 0.004 0.66 0.31 0.08 MM3 0.17 0.22 0.003 0.19 0.004 0.65 0.30 0.15 MM4 0.16 0.19 0.002 0.15 0.004 0.67 0.30 0.10 Y5 0.16 0.21 0.002 0.17 0.005 0.68 0.30 0.08 MM

and 0.08 Y

Aging Behavior of Al–Cr Alloys

279

to supress precipitate dissolution, and mischmetal delays the attainment of peak hard-ness, thereby helping to retain the aginghardness up to a higher temperature. The in-dividual effects of yttrium and misch metal,in their combined addition to the Al–Cr–Zralloy, is significant, and the maximum hard-ness of this alloy is found to be intermediateto those of yttrium and misch metal-treatedalloys.

EFFECT OF COLD WORKING

Cold working the solutionized alloys underinvestigation prior to aging accelerates agehardening, presumably by providing extranucleation sites at the dislocations (Fig. 2).The extent of age hardening in all the alloys(with or without rare earths) is very highcompared to the undeformed alloys. Notonly is the first peak-hardness temperaturelowered, it also remains relatively constant,irrespective of the alloy composition. Incomparison to undeformed material, the

temperature at which reversion takes placeis also reduced. Interestingly, while thelevel of first stage hardening is almost thesame for most alloy compositions, it is quitedifferent in the second stage. Both mischmetal and yttrium have increased the sec-ond peak hardness value when added tothe Al–Cr–Zr alloy, although yttrium is lesseffective in this regard. The nature of theseaging curves is indicative of the fact thatthe precipitates formed during the firststage are dissolved above 250

8

C, and addi-tional precipitation takes place to constituteanother peak-aging hardness at 400

8

C. Theprecipitation phenomenon during the sec-ond stage is amply influenced by trace rare-earth additions, whereas the same in thefirst stage is relatively insensitive to rare-earth treatment.

MICROSTRUCTURES

The TEMs show that the base alloy containsboth large prismatic and fine needle-like

FIG. 1. Variation of hardness with aging temperature for Al–Cr alloys with trace addition.

280

M. K. Banerjee and S. Datta

precipitates when aged at a higher temper-ature—500

8

C (Fig. 3). When the base alloyis treated with misch metal (alloy 3), nu-merous globular precipitates of varyingsizes are also observed in the microstruc-tures (Fig. 4). This is in contrast with themicrostructure of alloy 4 (containing yt-trium), which shows considerable precipi-tation at the dislocations (Fig. 5).

The as-quenched microstructure, with asuperimposed SADP at the top right-handcorner, shows a dislocation network with afew small precipitates that have formed onthese dislocations (Fig. 5). The SAD patternshows only matrix reflections with no obvi-ous precipitate reflections. Also, no evi-dence of G.P. zone formation was obtainedin these Al–Cr alloys.

When the above alloys are cold worked,the number of nucleation sites is consider-ably increased. Upon aging at 250

8

C (wherethe first peak occurs) one can see fine pre-cipitates forming on dislocation loops and

tangles (Fig. 6). The SAD pattern from thisalloy aged at 250

8

C essentially shows a 001matrix reflection with irregular diffractionreflections from the precipitates (Fig. 7).The diffraction data suggest that the pre-cipitates are consistent with the CrAl

7

phase with monoclinic crystal structure asreported elsewhere [13]. This is verified inFig. 8, which shows [201] monocline reflec-tion in the SAD pattern of a coarse precipi-tate formed due to overaging. When agedat 300

8

C, needle-shaped precipitates arealso observed in the microstructure (Fig. 9).

DISCUSSION

EFFECT OF TRACE ELEMENTS

Based on the microstructural observationsin this study, G.P. zones do not appear toform in these alloys. It is known that theformation of G.P. zones in most aluminium

FIG. 2. Variation of hardness with aging temperature for cold-worked Al–Cr alloys with trace addition.

Aging Behavior of Al–Cr Alloys

281

alloys is dependent on the solute atoms–vacancies interaction. In this investigation,it was observed from the micrograph of theas-quenched alloy (Fig. 6) that a few largeintermetallics are present. This is due tostrong interaction between chromium andaluminium. These intermetallics did notdissolve during the prolonged solutioniz-ing treatment. Therefore, their incoherentinterfaces act as a sink for the annihilationof excess vacancies. Also, it is reported, thatthe transition metals like chromium, man-ganese, etc., preferentially combine withthe vacancies [14]. Thus, in the quenchedAl–Cr alloys the excess vacancies are notavailable to promote the small-scale diffu-sion of solute atoms. As a result, G.P. zoneswould not be formed in these alloys.

Many trace additions in a number of alu-minium alloys have reportedly retardedthe G.P. zone formation by way of binding

the excess vacancies [12]. Although furtherretardation of G.P. zone formation by theaddition of rare earths in the present seriesof alloys is not conclusive, it is quite clearthat the rare-earth additions have not beeneffective here to stimulate the zone forma-tion in Al–Cr alloys that otherwise do notform G.P. zones. This is in contrast to theaddition of silver in Al–Mg alloys whereG.P. zone formation is promoted due totrace addition [4].

In Al–Cr–Zr alloys, age hardening is ob-served to occur in two stages, and twotypes of precipitates, viz. globular and nee-dle shaped, are formed at different temper-ature regions. The needle-shaped precipi-tates are normally formed at elevatedtemperatures. Due to the inherent quenchsensitivity of the binary Al–Cr alloy, the ex-tent of age hardening is rather low.

FIG. 3. TEM of alloy 1 (aged at 5008C) showing rodand needle-shaped precipitates.

FIG. 4. Same alloy (from a different area) showingcoarser needle-like as well as globular precipitates.

FIG. 5. TEM of alloy 4 (cold worked and aged at4508C) showing precipitates on dislocations.

FIG. 6. TEM of alloy 4 (cold worked and aged at 2508C)showing fine precipitates on dislocations. Slip bandsare also visible.

282

M. K. Banerjee and S. Datta

During hot working, Al

3

Zr particles areformed at the dislocations [15] in these al-loys, thereby retarding recrystallization.The formation of Al

3

Zr stimulates the for-mation of chromium aluminides, althoughit is not conclusive if Al

3

Zr has acted as het-erogeneous nucleation sites for precipita-tion or has formed any ternary intermetal-lics. At higher aging temperatures (above350

8

C), the precipitates dissolve and thenfurther new precipitation of Cr

2

Al

11

, char-acteristic of high temperature aging, willoccur in the second stage of age hardening.

The shift of the hardness peak to a higherlevel of aged hardness in the misch metal-treated alloy is ascribed to the enhancednucleation of precipitates, that is, contin-ued precipitation in a finer form and to agreater extent. A possible increase in super-saturation of chromium due to misch metal

addition may also be one of the reasons fora higher degree of age hardening in this al-loy. On the contrary, the yttrium-bearingalloy exhibits a single-stage age hardeningdue to precipitation mainly at dislocations(Fig. 5) Yttrium may possibly interact withthe quenched-in vacancies, not allowingthem to be easily annihilated at the matrix/precipitate interface. It has been reportedelsewhere that rare-earths elements like yt-trium may bring about a greater dispersionof chromium atoms within a liquid solventof a lower melting point [8]. Thus, yttriumreduces the tendency of premature forma-tion of large CrAl

7

intermetallics. As a re-sult, vacancy annihilation is retarded. Theevidence of preferential precipitation ondislocations in the alloy suggests that theremay be mobile yttrium–vacancy–chromiumatom complexes as reported earlier [9].These complexes are transported onto thenearby dislocations and ultimately formprecipitates. The combined addition of yt-trium and misch metal has shown thatmisch metal and yttrium do not produceany synergestic effect in Al–Cr alloys.

EFFECT OF COLD WORKING

The results on cold-worked alloys showvery high level of age hardening at earlierpeak temperature, which indicates that fineprecipitates in large amounts are formed inthese alloys. The electron micrograph of thecold-worked alloy aged at 250

8

C shows thehigh dislocation content with extremelyfine precipitates (Fig. 6).

FIG. 7. SADP of Fig. 6 showing a regular matrix pat-tern with irregularly arranged diffraction spots fromprecipitates.

FIG. 8. SADP of a precipitate of Fig. 6.

FIG. 9. TEM of alloy 4 (cold worked and aged at3008C) showing formation of needle-like precipitates.

Aging Behavior of Al–Cr Alloys

283

Thus, age hardening in the Al–Cr alloy iscontrolled by precipitate nucleation. Thetrace additions could improve the stitua-tion to some extent, and hence, improve theaging response. Cold working prior to ag-ing produces a high dislocation content.This increases the number of nucleationsites, and curtails the diffusion path for theslow-moving chromium atoms. Moreover,extensive cold working at the quenchingtemperature (i.e., RT) also generates largenumber of vacancies that do not have anyaccess to annihilations. This considerablyhelps in the diffusion of chromuim to nu-cleation sites, that is, dislocations. This iswhy the aging curve rises very steeply tothe peak hardness value that occurs at alower temperature, viz. 250

8

C. The nearconstancy of the peak hardness valuesagain establishes that besides the precipi-tate size and volume fraction, the interpar-ticle distance is also lessened, and one ob-tains a high aging response in this alloy. Italso proves that the first-stage age harden-ing in this alloy system with or withouttrace additions is mainly controlled by theheterogeneous precipitation at dislocationwithout regard to the effect of trace addi-tions. Beyond the first peak the rapid dropin hardness value is due to both the morerapid dissolution of the existing fine pre-cipitates and the slow appearance of thesecondary precipitates. This additional pre-cipitation cannot occur easily because ofthe annihilation of dislocations when agingis performed at temperatures considerablyabove the recrystallization temperature.However, in the second stage of hardening,one would observe the dependence of peakhardness on to the trace additions. When thecold-worked alloys are aged at high temper-atures, the dislocations are annihilated, andthe number of available nucleation sites de-creases. Therefore, trace elements appear toaffect nucleation in different ways, so differ-ent levels of age hardening are observed. Be-cause yttrium is more effective in stabilizingthe CrAl

7

precipitates formed in the firststage of age hardening, its effect in the sec-ond-stage hardening is less pronouncedthan misch metal.

CONCLUSIONS

In summary, the following conclusionshave been made: (1) age hardening in Al–Cr–Zr alloys with or without trace additionis due to the precipitation at dislocation; (2)addition of rare earths stimulates the nucle-ation of the precipitates. Misch metal andyttrium act differently in this regard; (3)cold working prior to aging greatly acceler-ates age hardening in these alloys; (4) Al–Cr–Zr alloys with or without rare earths ex-hibit a two-stage age hardening; and (5)two types of precipitates, viz. CrA

17

andCr

2

Al

11

, are formed in these alloys as afunction of aging temperature.

The author is greatly indebted to the Board ofResearch in Nuclear Sciences, Department ofAtomic Energy, Govt. of India, for its supportin carrying out this work, through sanction No.34/15/89-G.

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Received May 1998; accepted April 1999.