Structure of Liquid Aluminium-Copper Alloys

Rajendra KumarManjit Singh

1ARLIER work' done in these laboratories hasestablished the existence of copper-rich clustersin the liquid aluminium copper alloys. It was

decided to investigate the behaviour of these clustersat various temperatures in aluminium-copper alloysbecause of the great commercial and scientific interest,in age-hardening alloys. Such an understanding wouldbe of help in forming it physical picture of the earlystages of the precipitation processes. The existence ofthe clusters is thermodynamically possible as it lowersthe free energy of the system by increasing the entropyof mixing, The aluminium-rich or copper-rich clustersin liquid aluminium-copper alloys arise because of thenegative heat of solution. The density of the copper-rich clusters is greater than that of the parent liquid andthe difference in the density could be magnified severaltimes by centrifuging the molten alloy, thus causing theheavier copper-rich clusters to move away from theradius of rotation. Conditions are thus created whichdivide the liquid into aluminium-rich and copper-rich portions.

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Experimental technique

An apparatus described elsewhere' was used with themodification that the aluminium-4'0% copper alloywas kept in a thin walled copper crucible. This cruci-ble was kept inside it close fitting double walled steelcrucible, the copper crucible served to avoid the conta-mination of the aluminium copper alloys with iron.It was, however, found that some of the copper of thecrucible was taken into solution in the molten alloy,thereby increasing its average copper content up toabout 8 to 12.0 %o depending on the time of centrifuging.A flux of sodium and calcium fluorides was used toprevent surface oxidation. The alloys were centrifugedat 700 and 800'C for different times varying from I to8 hours at 120 r.p.m., the length of the radial armcarrying the crucible being 55 cm. At the end ofthe predetermined time water was allowed to rushthrough the annular space of the double walled crucibleand this served to chill the distribution of copperatoms existing in the liquid state at that instant ofcentrifuging. The solidified ingot was recovered and

Dr. Rajendra Kumar and Mr. Manjit Singh, College of Miningand Metallurgy , Banaras Hindu University, Varanasi.

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Plate I.Showing distribution of two phases near the top of the ingot.

Magnification -84X.

Place 2.

Showing distribution of two phases near the centre of the ingot.Magnification-84X.

Plate 3.Showing the demarcation in the coarsz and the fine structure.

Magnifica(ion-84X.

Plate 4.

Showing distribution of two phases near the bottom of the ingot.Magni fication 84X.

samples were taken for metall& raphic and chemicalanalvSIS.

Results and discussion

Metallographic examination repealed :i) The presence of a second phase at the grain

boundaries near the top of the ingot (alumi-nium rich cnd) plate I .

(it) The quantity of the second phase progressivelyincrca^-es towards the bottom of the ingot,Plate 2 shows the coarse cast structure near thecentre of the ingot.

(iii) The quantity of the second phase continues toincrease, but the distribution suddenly becomesfiner, The demarcation in the coarse andfine structure is shown in Plate 3. The demar-cation corresponds to a .harp increase in thecopper content of the alloy. The structureat the hottom of the crucible is shown inPlate 4.

Figs. I to 6 summarise the distribution of copperexisting after centrifuging at 7011 and 800 C for differenttimes at 120 r.p.m. It would he seen that theseparation is more effective at 800 C. In accordancewith the generally accepted notions the size of theclusters should decrease as the temperature is raised.However, the observations can he explained if you bearin mind that the heat of solution is negatively greaterat higher temperatures". The effects of correspondingchanges in viscosity have yet to he insestigated. Itwould also he seen from the figures that the coppercontent at the aluminium-rich end does not fall belowabout 6'0`% even for centrifuging up to X hours. It isobvious that the size of the clusters existing in Al-6'0%'Cu alloy is such that the centrifugal force has not beenable to overcome the forces responsible for the parti-cular cluster distribution. Further the heat of solutionis negatively greater for alloys of greater copper concent-ration, the clustering is therefore not very perfect.

Figs, 7. S and 9 show the variation in the concentra-tion of copper at a particular position with time. Thefigures show that as a result of centrifuging, copperclusters are continuously moving from top to the

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Distribution after 2 hours ' centrifuging.

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40

bottom of the ingot. As the concentration of copperdrops in the liquid a longer time is required for causinga large scale movement of copper atoms. This canbe appreciated from Fig. 9 which shows that a periodof about 7 hours is required to grow the clusters to acritical size characteristic of the speed of rotation.When the critical size is attained the centrifugal force

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and are copper rich platelets 100-500 A° in diameterand one to a few alorns distances thick and are about100 A' apart from each other. However, Jagodzinkisand Laves° have criticised the existence of the G. P.zones as outlined above on the basis that such a largeaccumulation of copper atoms would require anextremely fast rate of diffusion which is practicallyimpossible to attain. They have on the other handsuggested that the maximum copper content or theseplatelets is 10, and that the number of platelets ismuch greater than that visualised by Guinicr.

This difference of opinion in the interpretation ofX-ray data is perhaps due to the fact that whilst it ispossible to calculate the X-ray scattering that would becaused by a given configuration of atoms, it is

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overcomes the forces of viscosity, etc. and is centrifugedoutward.

Guinicr and Preston'. ". s in their classical studies onthe precipitation processes in aluminium copper alloyssuggested that the first step in the precipitation of asecond phase consists of the preferential accumulationof the copper atoms along certain crystallographicplanes. They are commonly known as G. P. zones

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not yet possible to deduce the actual atomic configura-tion from the observed X-ray scattering. The con-clusions regarding the structure of clusters are thusonly the best estimates of specialists and can seldombe regarded as conclusive.

The results obtained in this investigation have givena physical confirmation of the existence of the copper-rich clusters in the liquid state. As has been demon-strated earlier' these clusters arise because of theCu-Al bond being stronger than either the Al-Al orthe Cu-Cu bond. The strength of the bond would beeven more stronger in the solid state where the tem-peratures are comparatively lower. The super-saturatedsolid solution of copper in aluminium, therefore, con-sists of copper-rich clusters of various sizes randomlydistributed throughout the whole mass. These clustersare the potential G. P. zones and the larger of theminitially nucleate the G. P. zones, the nucleationrequiring slight, if any, diffusion of copper atoms.

Whilst it has not been possible to calculate the sizeof these clusters in the liquid state the observationshave shown that copper-rich regions do exist in thelattice of aluminium. The criticism of Jogodzinkis and

DISCUSSIONS

Dr. B. R. Nijhawan, NML: I must say that Dr. Kumarhas presented his results lucidly which should providean interesting discussion. [ would like to ask Dr. Kumaras to what are the practical implications of this studyin the sense of formulation of certain alloys whichmay be of practical use and application. Is it possibleto derive any conclusions to the effect that by suchliquid concentration of clusters of copper atoms, wecan obtain from an overall composition by centrifug-ing, certain concentration of atoms that may givecertain optimum beneficial practical properties ?Dr. Rajendra Kumar (Author) : I am thankful toDr. Nijhawan for the kind remarks he has made onthis paper and in reply I would like to say that it israther premature at this stage to discuss the practicalimplications of this paper. In a previous paper,however, we have shown that it is possible to separatelead and tin from their liquid of eutectic composition ;the concentration of Sri in the mass which was thusseparated was sufficient to give a "tin cry". Webelieve similar studies can he done on other liquidsystems and may be coupled with zone refining. Thestudies may also be useful in studying the mechanismof ageing, and we propose to investigate the effect ofvarious minor additions such as magnesium in AI-Cualloys.

Lavess is, therefore, perhaps not valid. This alsoexplains why the ageing process starts with very littleincubation period.

Acknowledgement

The authors wish to acknowledge their thanks toDr. Daya Swarup, Principal, College of Mining andMetallurgy, Banaras Hindu University, for his en-couragement and the facilities provided.

References

Kumar , R. : To be published in Trans . Ind. Inst . of Metals.

Kubaschewski , 0. and Evans , E.LL.: Metallurgical Thermo-chemistry.

Guinier, A. Compt . Rend . 206 (1938) 1641.

Guinier, A.: Nature 142 (1938) 569.

Preston , G. D. : ibid 570.

Jogodzinkis , H. and Laves , F.: Z. Metallk,40 ( 1949) 296.

Newkirk, J. B : Symp . Pption. from solid solutions,A.S.M., 1959.

Mr. D. V. Paranjpe, Tata Iron and Steel Co. Ltd.,Jamshedpur : Could the principle of centrifuging beused for a practical application, viz. separation of leadand tin from the turning of bearings containing tin-rich babbit metal and lead-rich babbit metal '? if thiscould be done, the resulting metal could be used formaking tin-rich hearings or even for producing puretin and pure lead from a mixture of the two metals inthe bearing alloys.Dr. Kumar (Author) : I think the suggestion is worthtrying in view of our earlier results on the Pb-Snsystem, where we were able to separate Pb and Sri fromtheir eutectic mixture . I have got here sonic samples ofthe tin thus separated, which you can inspect, if youso desire.Mr. R. Choubey, NML: Dr. Kumar has presentedin one of his studies a curve which shows a rise inincubation period and a sort of concentration ofCu-rich nuclei which tend to concentrate towards therim of the apparatus. I would like to knew whetherfrom this saturation state, one can guess the natureof nuclei that probably exist during the liquid statebecause near about 121,', Cu, we find some sort ofsaturation. Dr. Kumar has mentioned earlier thatdue to the copper container there is some copperpick-up and copper concentration near the rim comes

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to that higher limit. So it might he due to this causeor due to the nature of nucleii formed during liquidstate under those prevailing conditions which concen-tri.te towards the rim and give rise to that saturationlimitDr. Kumar (Author) : I would not like to say thatthere is a saturation in the liquid state because in theliquid state these alloys are known to be completelymiscible subject to the departures I have enumeratedin the paper. These departures relate to the existenceof Cu in the form of Al-Cu clusters in the parentliquid. Top limit of the Cu-concentration at thefarther end depends on the Cu-content of the liquidalloy with which we start initially. Since the sub-mission of the paper we have done certain experimentswith a view to avoid copper pick-up . we made astainless steel crucible and found that even in anAl-4%, Cu alloy the same type of distribution as des-cribed in the paper exists after centrifuging with thedifference that maximum value of Cu-concentrationwas not 12°x, but slightly less about S.Mr. N. P. S. Murtht', NML : I am interested in theexperimental technique and would like to know fromthe author : (i) What was the speed of centrifuge ?(2) Is the existence of clusters proved ? and, (3) If Cuclusters are present, the liquid should deviate froma Newtonian fluid and should show a different co-efficient of viscosity. It may even exhibit thixotropy.Hence viscosity determination may help to, at leastpartially, prove the existence of clusters.Dr. Kumar (Author) : The rotation was 120 r.m.p.and the incubation period was 5 hours. If the speedof rotation is increased, it will perhaps decrease theincubation period and the separation will be moreeffective. We have still to investigate the effect ofspeed on migration of Cu.

The existence of clusters is suggested by the negativedeparture from the Raoult's Law. The reason why wedo not say that Cu alone is migrating is that if Cu-Cubonds were stronger, the Cu will be present in theform of Cu clusters and in this case the departure fromRaoult 's Law will be positive as in the case with Phand Sri.AR. R. Seslradri, Indian Smehhi.e and Refrni,ng Co.,Bonibar : We have the problem of "inverse segrega-tion" in the case of aluminium-copper alloys. Havethe authors taken into consideration this problem ?In order to be able to study the effect of this, it couldhave been better if a concurrent study of the ingotwithout centrifuging is made and the sank is compared%kith the results obtained regardless of the ingot size.Dr. Kumar (Author) : We have not investigated therelation between "inverse segregation" and the presentproblem.Dr. V. G. Pararrjpe, Research and Control Labs., Tisco,Jamshedpur : I must compliment the author on thisinteresting set-up of experiments. I have only twosuggestions to make :

(i) It would perhaps be advantageous if thecrucible started out at the centre and wenttowards the periphery because by that youwill get it variation in speeds from 0 to a

very high value. This would he much higherthan the 1'5-2'0 fold increases obtained by theauthors. It would possibly be done by havingthe crucible spreading out on both sides sothat the centre is not affected by any centri-fugal action whatsoever. and the two sidesare definitely under it centrifugal action.

(ii) Whether this phenomenon could not perhapshe explained rather by it simple approach fromthe point of view of gravitation ? After alltwo substances which differ quite a lot inspecific gravity are dealt with and even in aliquid one can have gravitational separation.

Dr. Kumar tAuthor) : Thanks for the suggestions.Centrifuging merely helps to exaggerate the gravita-tional segregation and therefore separation is effectedin a short period of time.Dr. 1'. G. Paranlpe, Res. and Control Lob., Tisco.Jamshedpur : Normally we do not consider gravitationforces in discussing phase equilibria. But, if youreally go through expositions at (;ihhson on theheterogeneous equilibria. you will find the methods ofdealing with influences of gravitation. centrifugal forces,etc. on chemical potentiarls. Normally we can neglectgravitational potential because the substances are underthe same influence. But when one purposely sets outto create a different gravitational force in one part ofsystem compared to other, one cannot overlook thecontribution of gravitational or some of these complexcentrifugal fields to thermodynamic potentials.Dr. E. G. Ramachwulr an. .\.1 /I.: I would also liketo express my appreciation to the very able presentationof Dr. Kumar and to the very interesting subject whichhas been expounded today. I ani also interested toknow how the movement of Cu-rich clusters actuallytakes place and how much of it is due to the centri-fugal force and how much of' it is due to simple gravityntovenient. I have also a small suggestion to make toDr. Kumar, which he would perhaps examine for whatit is worth. As the mould takes up a horizontal posi-tion when rotated, and the difference in radii betweenthe top and bottom is of the orderof 5 cros only, I would suggest usinga mould of this shape.

In this mould. there Mould per-haps be no gravity effect. but onlythe centrifugal effect and the Cu-rich clusters may naturally he thenthrown to the periphery. With a mould of this typeand one with the bend opposite. it stay be possibleto explain the outward movement of Cu-rich clusters.

I also wish to comment on the implications ofDr. Kumar's statement that the existence of clusters isproved because of the departure from Raoult's Law.

I think that the existence of clusters is a postulate.Clusters have been postulated to exist, to explain thedeviations from the Raoult's Law. It is not a proof.For example, Guinier-Preston zones were postulatedto explain age-hardening. 13ut riot till low-anglescattering photographs were taken. could their existencebe taken as confirmed. In a similar fashion. I thinkDr. Kumar should eliminate the effect of gravity or of

2 4 2

erosion attack upon the copper crucible and by chemi-cal analysis prove the existence of the clusters.Mr. H. P. S. Murthy, NML : 1 think Dr . Ramachandranhas partly raised my doubt . 1 would like to have aphysical proof of their existence . Dr. Ramachandransuggested a chemical method. I am wondering whetherviscosity determination would help in this regard.A Newtonian liquid has got a particular coefficient ofviscosity and if there are certain discreet particlespresent in it, the liquid will deviate from the NewtonianLaw and its viscosity also will deviate. If these Cuclusters are present or come into existence at a particu-lar stage, then it is quite possible that the liquid maynot behave like Newtonian liquid . Can any viscosity

determinations at these temperatures help in provingor at least partially confirming the existence of clusters ?Dr. Kumar (Author) : We are building up an apparatusto determine the viscosity of the Al-Cu alloys at thesetemperatures and after we have carried out experimentalwork, we shall be in a position to discuss these pointsin more details. In the literature there seems to bepractically no experimental observations of the changesin viscosity with temperature of these alloys. We arealso trying to study the problem from the point ofview of specific heat. At this moment, we are buidingup that apparatus also. As you said, these clustersshould have some bearing on viscosity and I fullyagree with you.

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