On solute segregation in AlLiCuMg alloys

Scripta METALLURGICA

Vol. 21, pp. 601-606, 1987 Printed in the U.S.A.

Pergamon Journals, Ltd. All rights reserved

ON SOLUTE SEGREGATION IN AI-Li-Cu-Mg ALLOYS R.D.K.Misra, T.V.Balasubramanian and C.R.Chakravorty Defence Metallurgical Research Laboratory PO:Kanchanbagh, Hyderabad-500 258 (India) (Received October 14, 1986) (Revised February 17, 1987) Aluminium-Lithium alloys have received considerable attention as a potential material for aerospace alloys in view of increased elastic modulus and decreased densities in comparison to other conventional aluminium alloys. These alloys offer a inherent combination of strength and toughness at lower densities. A number of studies in the past decade dealt with structure - property correlations. However, not much is known about the solute concentration changes occurring at the grain boundaries during heat treatment (I). The chemistry changes that occur in the vicinity of grain boundaries because of differences in heat treatment are quite complex in nature (2). Electron probe microanalysis of commercial aluminium alloys could only in some cases distinguish precipitate free zone (PPZ) and grain boundary precipitates from the matrix. Electron energy analysis techniques employing a small electron probe have generally provided additional information. In the case of rapidly cooled A1-7wt%Mg, Cundy et al (3) showed that solute segregation occurs at the grain boundary when the alloy is cooled from above the solvus temperature, this was attributed to the formation of small precipitates during quenching. Electron microscopy in conjunction with electron energy analysis was employed by Doig and Edington (4,5) to study compositional changes occurring at the grain boundaries of Al-Zn-Mg alloys. The above studies, however, did not provide information on the relative amounts of solute segregation at the grain boundaries. The technique of Auger electron spectroscopy has proven to be a more useful technique in the study of grain boundary chemistry. Solute segregation at grain boundaries which occurs in many systems has an important effect on material properties. The earlier studies were aimed at studying the changes in the concentration of alloying elements at the grain boundaries and relating them to stress ~ corrosion cracking (SCC) (4-9). The SSC behaviour is of serious consideration in AI-Li alloys. Recent studies (10,11) have demonstrated that AI-Li alloys are susceptible to SCC and the degree of susceptibility depends on the heat treatment (10). The analysis of anodic oxide films revealed segregation of magnesium and zinc at the grain boundaries in AI-Zn-Mg alloys (6). This segregation of magnesium and zinc was attributed to the high corrosion potential at the grain boundaries in SCC susceptible A1-Zn-Mg alloys. Qualitatively similar interpretations were made by other authors (7-9) in regard to the SCC susceptibility of aluminium-based alloys. In a previous paper (1), we reported the effect of solution treatment temperature on solute re-distribution at the grain boundaries in an Ai-Li-Cu-Mg alloy. It was observed that the solution treatment temperature affects the distribution of the solute elements at the grain boundaries (Table la; Fig. la). The extent of intergranular segregation of the solute elements depends on the solution treatment temperature for the same aging treatment. The present study is aimed at an understanding of the variation in grain boundary solute concentration with interrupted quench temperature. In an alloy quenched from a high temperature anneal and subjected to a low temperature anneal, large vacancy gradients will develop since the equilibrium vacancy concentration in most cases will have dropped several orders of magnitude during quenching (12). This

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process can also generate vacancy flows which produce solute enrichment and solute depletion around vacancy source and sinks such as free surfaces and grain boundaries. The resulting solute segregation may explain phenomena such as grain boundary hardening (13) and intergranular corrosion (14). Non-equillbrium segregation phenomena involves vacancy--solute interactions to the grain boundary under non-equilibrium conditions of rapid cooling and quench aging (15). At high solution treatment temperatures, an equilibrium concentration of vacancies is generated and distributed throughout the lattice. During the quench to a lower temperature, the concentration of vacancies becomes supersaturated. Since the grain boundary can act as a strong sink for vacancies, a concentration gradient in vacancies will develop near such interfaces and those vacancies which are within the diffusive range of the grain boundary will tend to migrate to the interface where they can be annhilated. When the solute-vacancy binding energy is larger than the thermal energy, a vacancy flux could result in solute enrichment at grain boundaries (12). Non-equillbrium segregation of solute at the grain boundaries has been observed earlier in Al-alloys wherein it was suggested that Zn, Mg, Cu and Si, having binding energies sufficiently greater than the thermal energy, segregate to grain boundaries upon quenching or during low temperature aging following a rapid quench(2). An increase of solution treatment temperature causes a higher supersaturation of vacancies and hence a larger number of vacancy-solute pairs. The chemical composition (wt.%) of AI-Li-Cu-Mg alloy selected for the study was analysed to be: 2.7Li, 1.2Cu, 0.9Mg, 0.3Si (max), 0.2Fe (max), 0.2Ni (max) and 0.1Zr. The alloy was made in our laboratory, which has undertaken a major programme in the development of A1-LI alloys. Cast ingots produced were homogenised at 773K and subsequently rolled to required dimensions. The alloy used for the present study was heat treated as indicated in Table lb. A set of samples (HT8-HT12) were solution treated at 873K for 24 h in concurrence with our earlier work(1), and subjected to Isochronal quench-lnterruption at different temperatures (T~) for I0 minutes followed by water quenching and then aging at 463K in an oi~ bath for 12 h. The time required to transfer the specimen from the furnace maintained at solution treatment temperature of 893K to the furnace kept at quench-lnteruption temperature (Ti) was about 5-8 seconds. The technique of Auger electron spectroscopy (AES) in a scanning Auger microprobe (SAM) (Physical Electronics, Inc.) was used to study the grain boundary composition as described in an earlier paper (1). The specimens for analysls were prepared by machining the heat treated specimens to flnal dimensions of 3.68 nun dia x 32.0 mm long. A circular V-notch was machined at 19.0mm from one end to facilltate fracture. The procedure adopted to determine grain boundary composition consisted of the following steps: (1) The specimens wer~ fractured at liquld nitrogen temperature ultrahigh vacuum ( < 10 "Pa) of the Auger electron spectrometer.

within

the

(2) The average fracture surface composition was obtained by large area analysls using a defocused electron beam. Information on chemical homogeneity at grain boundaries was obtained by Auger elemental images, followed by point analysis of selected grain boundary regions. Auger spectra from intergranular fracture surfaces covering an area of 200 pm x 200 ~ m w e r e obtained using a defocused electron beam. The spectra thus obtained are indicative of the average fracture surface composition of a large number of grain boundary facets (1). It indicated, as observed earller (1), that the elements Li, Cu, Mg and Si are in excess of the bulk composition as are the phases associated with them. The average fracture surface composition is probably the best criterion for comparing specimens subjected to different heat treatment Table 2b presents a synopsis of the quantltatlve data for specimens subjected to quench interruption at T4, where the indicated concentration is the average of at least three regions fro~ each fracture surface.

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The effect of solute concentration at the grain boundaries on quenching to an intermediate temperature T 4 from a constant solution treatment temperature of 873K is presented in Fi~.lb. The variation in solute concentration with interrupted quench temperature, T~ (HT8-HTI2) is qualitatlvely similar to the behaviour of samples directly quenched from solution treatment temperature, T (HTI-HT7; Table la) (Fig.la , Table 2a). However, a quantitative comparison o~ the data (Table 2, Fig.l) shows that: (a) At temperatures < 770K, the extent of solute segregation at the grain boundaries for samples (HT2-HT4) directly quenched from the solution treatment temperature, T , is similar to the values obtained for samples (HT8-HT10) for identical quencRing temperatures (i.e. Tq=Ti). (b) At temperatures > 780K,.the values of grain boundary solute concentration for samples subjected to interrupted quenching (HTII-HT12) are slightly higher in comparison to directly quenched samples (HTS-HT6) for identical quenching temperatures (Tq = Ti)The data presented in Fig. 1 can be discussed in terms of equilibrium and non-equilibrium modes of solute segregation to the grain boundaries. The non-equilibrium segregation occurs as a result of solute-pile up at a moving interface, or by solutes coupling to vacancies which are moving to grain boundaries sources or sinks during quenching (16). The non-equilibrium solute segregation arises from the flux of supersaturated vacancies to grain boundaries during the quench or afterwards during aging. In the case of equilibrium segregation solute atoms partition unequally between the two sets of site in accordance with the statistics of thermodyanamics in order to minimise the overall free energy of the system. Mclean (17) derived an equation for equilibrium solute concentration at the grain boundary by considering a statistical partitioning at equilibrium of an assembly of solute atoms between lattice and grain boundary sites. A a given temperature the equilibrium grain boundary concentration is given by: C e = A exp (Q/RT)/[I+Ac exp(Q/RT)] ...... [i] Where A is vibrational frequency factor, R is gas constant, T the absolute temperature, c the uniform concentraion of solute in the matrix, and Q an energy which may be approximately evaluated by estimating the energy of deformation caused by introduction of a sphere in a spherlcal hole in the matrix. For a solute atoms of radius r I in a site of radius r o, elastic energy, Q associa~d with the solute atoms is ~iven by (17): Q = 24 ~ K G

r 3 E 2/(3K+4G) ...... [ 2]

where K is the solute bulk modulas, G is the solvent or matrix shear modulus, r is the radius of the solute atom in the lattlce and q = ( r I - r )/r I Equation [1] predicts that for a very small uniform concentration, ther~ can'be a very high concentration of solute in the boundary, which increases as the equilibrium treatment is effected at a lower temperature. In view of the complex nature of the AI-Li-Cu-Mg alloy system, it is not appropriate at this stage to quantify various parameters associated with the equilibrium segregation phenomena, however, equation [1] helps in understanding the solute segregation behaviour. The equilibrium segregation that occurs at any solution treatment temperature is most likely to be retained at room temperature during a quench since insufficient time is allowed for equilibrium to be attained at any temperature during the quench. The data in regard to Fig. la has been discussed earlier (i) on the basis of the aforementioned equilibrium and non-equilibrium modes of segregation. Samples solution treated at a high temperature of 873K and quench interrupted at 723K, 753K and 783K (HT8-HTI0) in the equilibrium segregation regime of 723-783K (I) followed by quenching indicated solute segregation levels similar to those of samples which were not subjected to prior solution treatment at 873K, but

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~ere directly quenched from lower solution treatment temperatures of 723, 753 and 783K (HT2-HT4) (Fig.l). The segregation level for these treatments (HT8-HTI0) is found to be higher than in HT5-HT7. This observation suggests that much of the segregation in HT8-HTI0 occurred at temperatures of 723, 753 and 783K as in HT2-HT4 and is thus independent of high solution treatment temperature in this equilibrium segregation regime. In the non-equilibrium temperature regime of 783-873K (I) however, the samples solution treated at high temperature of 873K and quench interrupted at 813 and 843K (HTII-HTI2), followed by quenching, indicated segregation levels consistently greater as compared to HT5 and HT6 which were not subjected to a prior high temperature solution treatment ~ m p e r a t u r e of 873K. At a high solution treatment temperature of 873K, a large equilibrium concentration of vacancies is generated and distributed throughout the lattice, quench - interruption at intermediate temperatures of 813 and 873K leads to annealing out of the vacancies. But the fact that the measured levels of segregation in HTI1-HT12 is greater than HT5-HT6 (Fig.l) clearly exemplifies that HTII-HTI2 possess vacancies in excess of those of HT5-HT6, leading to enhanced solute segregation at the grain boundaries. These results are in agreement with the prior related work on other Al-alloys (2,8). Thus it can be concluded that the measured levels of solute segregtion of quench-interrupted samples depends to a large extent on the final temperature before quench. Acknowledgements The authors are grateful to the Aluminium-Lithium alloy development group for providing the material for this study. It is is a pleasure to thank Dr. P.Rama Rao, Director, DMRL for his continued interest in this work and permission to publish this paper. References l.R.D.K.Misra and T.V.B~asubramanian, Script Met. 19, 1177 (1985). 2.A.Joshi, C.R.Shastry and M.Levy, Met Trans. 12A, 1081 (1981). 3.S.L.Cundy, A.J.F. Metherall, M.J.Wehlar, P.N.T.Unwin and R.B.NiCholson, Proc. Roy. Soc. A307, 267 (1968). 4.P.Dolg and J.W.Edington, Met. Trans. 6A, 943 (1973). 5.P.Doig and J.W.Edington, Corrosion 31, 347 (1975). 6.I.T.Taylor and R.L.Edgar, Met. Trans. 2, 833 (1971). 7.J.B.Clarke, Acta Met. 12, 1197(1964). 8.J.A.S.Green and W.E.Montague, Corrosion 31, 209 (1975). 9.C.R.Shastry, M.Levy and A.Joshi, Corros. Sci. 21, 673 (1981). 10.L.Christdoulou, L.Struble and J.R.Pickens, Proc. 2nd Int. AI-Li Conf., Vol.2, p.5, E.A.Starke and T.H.Sanders, Eds., AIME, New York (1984). ll.P.P.Pizzo, R.P.Galvin and M.G.Nelson, Proc. 2nd Int. Conf. Vol.2, p.627, E.A.Starke and T.H.Sanders, Eds, AIME, New York (1984). 12.T.R.Anthony, Acta Met. 17, 603 (1969). 13. H.Westbrook and K.T.Aust, Acta Met. ii, 1151 (1963). 14.R.E.Hameman and K.T.Aust, Script Met. 2, 235 (1968). 15.K.T.Aust, S.J.Armjo, E.F.Koch and J.H.Westbrook, Trans. Amer. Soc. Metals 60, 360 (1967). 16.K.T.Aust, R.E.Hanneman, P.Niessen and J.H.Westbrook, Acta Met. 16, 291 (1968). 17.D.Mclean, Grain Boundaries in Metals, Clarendon Press, Oxford (1957).

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SOLUTE

Heat Treatment

SEGREGATION

605

Solution Treatment Temperature

HT 1

813K

HT HT HT HT HT HT

723K 753K 783K 813K 843K 873K

2 3 4 5 6 7

Aging Treatment held at room tempeture a f t e r quenching 12hrs at 463K 12hrs at 463K 12hrs at 463K 12hrs at 463K 12hrs at 463K 12hrs at 463K

Table la: Heat T r e a t m e n t o f directly quenched A I - L i - C u - M g alloys (1). Heat Treatment. HT HT HT HT HT

8 9 10 11 12

Solution Treatment Temperature 873K 873K 873K 873K 873K

(2#hrs) (24hrs) (2t4hrs) (24hrs) (24hrs)

+ + + + +

723K 753K 783K 813K 843K

Aging Treatment

(10 rain) (10min) (lOmin) (10rain) (10rain)

12hrs 12hrs 12hrs 12hrs 12hrs

at at at at at

463K 463K t463K 463K b~63K

Table lb: Heat Treatment o5 quench-interrupted A I - L i - C u - M g alloys

Heat T r e a t m e n t HTI HT 2 HT 3 HT 4 HT 5 HT 6 HT 7 Nominal alloy Composition

AI (1396eV)

Li (43eV)

86,2-87,5 78.7-82,7 82.2-84.6 87.6-88.3 84.2-87.1 82.0-85.1 83.1-86.2 88.8

10.7-11.4 14.1-17.2 12.8-14.6 10.2-11.3 11.8-13.2 12.5-14.0 12.8-14.7 9.8

Cu (920eV) 0.5-0.8 1.3-13 1.1-1.4 0.5-0.7 0.7-0.9 0.8-1.1 0.9-13.0 0.48

Mg (1186eV)

Si (92eV)

1.1-1.3 1.7-2.1 1.3-1.5 1.0-1.2 1.3-1.5 1.2-1.4 1.4-1.7 0.93

0"2-0"3 0"2-0"3 0,2-0"3 0-0.2 0.1-0"2 0.4 0.3-0.4 0.2

Table 2a: Average f r a c t u r e surface composition in a t o m i c % of directly quenched AI-Li-Cu-Mg alloys (1)

Heat T r e a t m e n t

AI (1396eV)

Li (4 SeV )

Cu (920eV)

Mg (1186eV)

Si (92eV)

HT HT HT HT HT

78.9-83.0 82.1-84,7 87.7-88.5 84.0-86.2 82.4-85.0

14.3-17.4 12.6-14.8 10.2-11.5 12.2-14.2 12.8-15.5

1.4-1.8 1.1-1.4 0,6-07 0.8-1.0 0.9-1.'~

1.8-2.2 1.4=1.6 1.1-1.3 1.5-1.7 1.3-1,6

0.3 0,2-0.3 0.2-0.3 0.3 0.2-03

8 9 10 11 12

Table 2b: Average f r a c t u r e surface composition in a t o m i c % oI quench i n t e r r u p t e d AI-Li-Cu-Mg alloys,

Q.

g~

0= ,,~.

~,',: ' :z ~ I== -,i i4 ,,

~ ~ g ~,.

,-~z'~~ ~:

-

I °Ln

~

= o

o

~

ATOMIC %

~'1 -" o .--" u'~

I Cu,Mg

0----,0

ATOMIC % Li

I ~'~

L

'='

~

1

Li

.-. ,.., ~ ~, Mg ATOMIC % Cu, o

ATOMIC %

o

t~

0

~