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Intermetallics growth at Alα-brass interfaces
~cro mrrall. Vol. 33. NO. 1, pp. 97-104. Printed in Great Britain.
ooo1-6160/8s 53.00 +o.oo
1985
Pcrgamon Press Ltd
INTERMETALLICS GROWTH INTERFACES
AT Al/a -BRASS
R. S. TIMSIT Alcan Intemationai
Ltd,
Kingston
laboratories,
P.O. Box 8400, Kingston, Ontario K7L 424, Canada
(Received IO February 1984) Abstract-The
growth rate of the interdiffusion layer formed in “bulk” Al/cl-brass diffusion couples has been measured at temperatures ranging from 150 to 450°C. The layer consists of four stacked bands of intermetallic compounds. The chemical composition and crystal structure of the compounds were determined by electron probe analysis and X-ray diffraction, respectively. The structure of the band imm~iately adjacent to the At is that of CuAI,. The structure of the next two bands is associated with that of the Cq,*,Ab,s phase. The dominant structure of the band adjacent to the brass is that of an hexagonal phase of the Cu-Zn system. The activation energies associated with intetmetallics growth are reported.
ReSum&-Nous avons mesur6 la vitesse de croissance de la couche d’interdiffusion fonnie dans des couples “massifs” de diffusion aIuminium~iaiton-~ pour des temp&atures comprises entre 150 et 450°C. Cette couche est constitt& de quatre bandes empilbes de compos6s inte~~talliqu~. Nous avons d&tern&& lastructure cristailine et la composition chimique de ces composk par analyse ii. la sonde tlectronique et par diffraction de rayons X. La structure de la bande immidiatement adjacente li I’aluminium est celle de CuAI,. La structure des deux bandes suivantes est associCe ii celle de la phase CU~,~,A&,.,~. La structure dominante de la bande adjacente au laiton est celle d’une phase hexagonale du syst&ne Cu-Zn. Nous ptisentons les valeurs des energies d’activation associ&es H la croissance des compos&s intermCtalliques. Z~rn~nr~ung-Die Wa~stumsg~wind~gkeit von S.chichten, di_e sich be! der Interdiffusion in m~roskopischen Metallpaaren Aluminium/a-M~ing bilden, wurde im Temperaturbereich zwischen 150 und 450°C gemessen. Die sich bildende Zwischenschicht besteht aus einem Stapel von vier intermetallischen BHndern. Die Kristallstmktur wurde mit Riintgenbeugung und die chemische Zusammensetzung wurde mit Elektronenmikroanalyse bestimmt. Das Band an der Aluminiumseite hat CuAl,-Struktur. Die beiden nLhsten Biinder hiingen mit der Cu,,, Ab.,9-Phase zusammen. Die dominierende Struktur des Bandes auf der Messingseite entspricht einer hexagonalen Phase des Cu-Zn-Systems. Aktivierungsener-
gien fir das Wachstum diescr inte~etalli~hen
Strukturen werden angegeben.
peratures ranging from 150 to 450°C. The effect of the reliability of intermetallics growth on aluminium/brass electrical contacts will be examined in a later publication.
INTRODUCTION
The formation of brittle intermetallics at the interface separating two dissimilar materials can be deleterious to the mechanical stability of the interface [1,2]. This effect has given rise to considerable concern in the microelectronics industry because it is detrimental to the long-term reliability of thin-film electrical contacts [3]. Recently the presence of intermetallics in so-called “bulk” electrical interfaces such as those found in m~hanicaliy-wired electrical junctions has been linked to the overheating of certain types of electrical connections (41. It has been stipulated that the contact resistance of these connections rises to unacceptable levels independently of the mechanical integrity of the interface in part because the intermetallics formed are relatively poor electrical conductors and grow sufficiently thick to hinder electrical flow directly. The present investigation examines intermetallics growth at bulk Al/brass interfaces because of the possible relevance of this growth to the performance of many electrical junctions wired with aluminium. The paper focuses on the growth kinetics, chemical composition and crystal structure of intermetallics formed in bulk diffusion couples at temA.M. 33/l--G
EXPERIMENTAL The materials used in the investigation were AAl350Al and an a-brass with the nominal composition by wt% Cu 70/Zn 30. The measured compositions listed in Table 1 are very similar to the compositons of electrical conductor alloys used commercially. Al/brass couples suitable for interdiffusion studies were produced by roll-bonding. The best couples were obtained by rolling the combination of a brass sheet 1.6 mm in thickness with an Al sheet 0.5 mm thick through a 50% reduction pass. The surfaces were pickled in a 50% nitric acid solution and were wire-brushed prior to rolling. Diffusion couples were obtained by cutting coupons approx. 1 x 1 cm* from the composite material. Diffusion was allowed to proceed for selected time intervals at each of several temperatures, ranging from 150 to 450°C in an Ar atmosphere. The work was carried out with an electric furnace capable of 91
TIMSIT:
98
INTERMETALLICS
GROWTH AT Al/a-BRASS
diffraction of material obtained by micro-extraction from each of the intermetallic layers.
‘Table 1. Composition of materials in wt% Element Al Si Mn Fe Ni cu zn Pb Others
BKISS 0.001 0.001 0.025 0.001 69.0 I 30.88 0.001 0.084
INTERFACES
AA1350 Al
RESULTS
99.6 0.01 0.26 0.02 0.001 0.001 0.02
Composition and crystal structure
maintaining temperatures to better than 1°C of a set point. Cross-sections of the diffusion couples after heat treatment were examined both optically and with a scanning electron microscope (SEM) and concentraton profiles for Al, Cu and Zn across the Al/brass interface were obtained using electron probe analysis. From these analyses the growth rates and compositions of several intermetallic bands were measured. The crystallographic structure of the intermetallic phases were identified by X-ray powder
Over the entire temperature range investigated, the interdiffusion layer consists of four major bands. These are shown in the SEM micrographs of Fig. I which illustrate the layered structure of cross-sections of the diffusion couples after exposure to different temperatures. Band A adjoining the Al is light grey. Bands B and C are both dark grey and band D which adjoins the brass is bright orange. This last band is always the widest. Sub-bands of varying tones of grey are generally observable within the four major intermetallic layers. These sub-bands either delineate transition regions between two major intermetallic layers or arise as a result of a small but sharp discontinuity in the elemental composition within the major intermetallics. A sub-band can be clearly seen within layer C in Fig. l(a) and in the lower end of layer B in
BRASS
D C
-9 A
Al
(a)
BRASS
D C B A
(b) Fig. I. Micrograph of interdiffusion layer: (a) after 765 h at 250°C; (b) after 6 h at 400°C.
TIMSIT: 0
Al
4
cu zn
I
INTERMETALLKS
GROWTH AT Al/a-BRASS
250°C A
BC
0
100
0
10
20
40
30
Distance
(pm)
Fig. 2. Representative element concentration anneafing at 250°C.
profife after
o Al a cu
99
INTERFACES
Fig. i(b). All layers are made up of large columnar grains. it is emphasized that the interdiffusion layer did not grow uniformly across the roll-bonded interface, especially below 3WC, but occurred in discrete regions, These discontinuities have been observed by other workers [S] and are attributed to the presence of obstructive residuat oxide films at the originat interface. Examples of element concentration profiles obtained by electron probe analysis are presented in Figs 2-4. The spatial resolution of the technique (typically 2rm) preluded an analysis on sampIes annealed below 250°C because the interdiffusion widths obtained under these conditions were never su~cient~y large. One of the interesting observations which emerges from the analyses is the relatively low Zn concentration in all the bands except layer D. The ranges of composition of the major phases in each of the four bands at all the temperatures investigated are shown in Tabfe 2. Layer A consists of an intermetaltic phase of approximate composition Al&, Cu 54, 350%
a Zn
50
0
Distance Fig. 3. Representative element ~~ntration
100 (yn)
I50
profile after annealing at 350°C.
o Al 450 *c 100
80
0 0
50
100 Distance fprn f
150
Fig. 4. Representative element concentration profile afier annealing at 450°C.
200
loo
TIMSIT:
INTERMETALLICS
GROWTH
AT Al/a-BRASS
INTERFACES
Table 2. Summary of results relating lo composition, crystal structure and growth rates of intermetallic bands Composition range fwt%) Zn
Crystul structure
(cm% 1)
Activation energy (kcal mot-‘)
-1
CuAll
5.6 x IO-’
21.9
64-69 6%70
2-4 8-10
Cub,, Al,,, Cuw Alw
4.2 x IO-’ 8.6 x IO’
23.4 50.7
21-15 t9-15
77-75 76
2-10 5-9
Cup,, 4, Cue.61Abm traces of Cu&
3.9 x lo-’ 0.114
16.8 31.5
3-2
60-57
37-41
Largely
8.5 x IO-’
21.8
1.2 x 10’
38.4
3.4 x 10-6 4.0 6.8 x 10-s 2.0 x 10’
17.3 33.7 19.0 40.1
Temperature range (“C)
Al
Cu
250-450
46-43
SE56
B
250-300 350-450
34-21 23-20
c
250-300 350-450
D
250-300
Lpyer A
350-450
3-2
60-57
37-41
%6Zns.4 CrhJk
gm&=iab~c component at 4WC) A+B+C B+C
1SO-300 350”4.50 250-300 350-450
Zn 1 wto/,, and a narrow transition band to layer B. lines corr~ponding to inter-planar spacings of 2.51 The composition of layer B could not be established and 2.89 A coincide with diffraction lines associated reliably at temperatures below 350°C because this with the C&Al, lattice 191,the absenceof#a strong line layer was always too narrow under these conditions. expected from this structure at a d-spacing of 3.56 A However, there are indications that the Cu content of rules out the presence of this phase. The origin of layer B at temperatures lower than 350°C is of the these two weak lines (and others) in this diffraction order of 67 wt”/, with the balance largely Al. Above pattern remains unclear. It is worth noting that 3WK! the Cu content in band B is reasonably despite the suggestion in Fig. 4 that layer C is grown uniform at approx. 69 wt”/, with the Al decreasing with the ChAI; crystallographic configuration at from 23 to 20 wt%. Layer C consists of approx. 450°C since the atomic concentrations of Cu, Zn and Cu76 wt%, a maximum of lOwt% of Zn and an Al in the layer are approximately in the ratio 8: I :4, average Al content of 16 wt”/, at all the temperatures the diffraction analysis never revealed any clear indiinvestigated. Layer D contains an average of only cation of this phase at this or any other temperature. 2 wt”/, Al and an average Cu and Zn content of 59 This is in contrast with the results of investi8ations of Al/Cu and Al/brass interdiffusion at more elevated and 39 wto/ respectively, also at all temperatures. The X-ray powder diffraction analysis did not temperatures [2, 10, 111 where the C&Al, phase was always yield wholly unambiguous information on observed. crystallographic structure. This was due, in part, to The dominant structure of layer D was clearly an inabihty to extract from a selected band a sample identified as that of the hexagonal phase wrrespondrange 3842 wt”/, Zn [12] in of material which was always free of metal from ing to the com~sition immediateIy adjacent layers. The analysis identified the Cu-Zn system. Samples annealed below 300°C the crystal structure of layer A clearly as that of showed, in addition, traces of the C~.a,Zn,,,s strucCuAl, [6] as shown in Fig. 5(a). This is consistent with ture [13] whereas specimens heated at 400°C and the chemical composition yielded by the electron above revealed the presence of an appreciable CuZn probe analysis. The structures of layers B and C were component [13]. An example of the digraction patless clearly defined. Material extracted from these tern obtained in this last instance is shown in layers usuaily showed evidence of some CuAlr, as Fig. 5(c), where the lines originating both from the indicated in Fig. 5(b), but this was interpreted as hexagonal Cu,,Zn,,, and the CuZn phases are clearly contamination from band A. The lines correspondin visible. These results and corresponding data obto interplanar spacings of 2.044, 2.047 and 2.052 x tained from material grown at lower temperatures are are attributed to the yz-Cy,e,Ab,, structure [7,8]. consistent with the compositions of layer D obtained The identification of this phase as the dominant from the electron probe analysis. crys~llographic component is generally consistent with the range of chemical compositions of layers B Growth rates and C yielded by electron probe analysis. It is worth The growth rate of each intermetallic band was mentioning that samples annealed at 450°C yielded, determined by measuring the average band thickness in addition, evidence of traces of the CurAl, structure after selected time intervals at each of the selected (61 in layer C. annealing temperatures. The resu!ts are shown in The origin of a few of the weak lines in Fig. 5(b) Fig. 6(a)--(f). The straight lines represent a leastcould not be established. For example, although the squares fit to the experimental data. These fits assign
TIMSIT:
INTERMETALLICS
GROWTH AT Al/a-BRASS
101
INTERFACES
I 4.30
4.30 CuA12
2.37 212 Co Al2
2.052 2.047 2.044
(a)
Cu Al .6 .4
2.12 1.83 Cuxznl-X
1.47 CuZn
I.20 CuZn
Fig. 5. Typical X-ray powder diffractographs. The lattice spacings of the planes yielding the most intense reflections are indicated in Angstrom units (a) From intermetallic band A at all temperatures. (b) From intermetallic band B and C grown at temperatures lower than 45O’C. The lines corresponding to CuAI,
were identified as originating from ~ntaminant material from band A. (c) From inte~etailic band D grown at 450°C. Electron microprobe analysis indicated that the Cu atomic concentration x varies from 0.59 to 0.63. origin a weight given by the number of experimental points. Within the scatter of the experimental data the dependence of the thickness x of each of the bands on the interdiffusion time I at a given temperature can be represented as to the
x2 = kt, where k is the interdiffusion rate constant at the selected temperature. The proportionality of layer thickness to 4 indicates that the diffusion of atoms through the intermetalli~ bands is considerably slower than diffusion across the growth interfaces 1161. The activation energy characterizing the growth of each intermetallic layer was determined on the usual assumption that the dependence of k on temperature is expressed as k = k,exp(-Q/TR), where k, is a constant, Q is the activation energy and T is the absolute temperature. The results of this analysis are shown in Fig. 7 and are summarized in Table 2. It was found that the intermetallic growth could generally not be described satisfactorily by a single activation energy over the entire temperature
range investigated but that an adequate description is possible by considering the data from the two temperature regions 150-300 and 350-45O”C separately. As shown in Table 2 the activation energies determined for tem~ratures lower than approx. 300°C are considerably smaller than those obtained for the more elevated temperatures. The lower activation energies may be indicative of inter-metallic growth by a short-circuit diffusion mechanism such as grain boundary or dissociated dislocation diffusion [ 171. The activation energies above 300°C are characteristic of bulk diffusion. This interpretation of the data is consistent with the observation in Fig. I that the density of grain boundaries in the Al-rich layers, and particularly in layer A, is larger at 250 than at 400°C and thus that short-circuit diffusion probably dominates at lower temperatures. The values of Q at elevated temperatures both for the diffusion bands (A f B f C) and for layer A are reasonably close to the recent values of 29.5 and 24.0 kcal mol-’ measured for the corresponding layers in Cu/Al couples in the same temperature range [2]. The activation energy for the growth of layer B at temperature higher than 300°C is very large and generally uncharacteristic of activation energies for
102
INTEKMETALLICS
TIMSIT:
GKOWTI-I
AT Al/a-BRASS
INTERFACES
(4 LOYW I4 -
oA
250%
OB
.
ti r--
hr:‘*l
(b) Layer oA 28
OB AC
l o * A+B+C
Y-t w
(hrs”*)
Layer 0
Fig. 6. Growth B and (A+
A
of diRi&on bands at the temperatures s&-cted in the precut invesligation. 3 + C) al 150 and 200°C; (b) 2WC; (c) 300°C; (d) 350°C; fee) 4!M*C; (f) 450°C. 103
104
TIMSIT:
INTERMETALLICS
GROWTH AT Al/a-BRASS
phases in the Cu-Al system and one is a Cu-Zn intermetallic compound containing only traces of Al. The similarity of the intermetallics with those formed at Al/Cu interfaces is restricted to the presence of the CuAl, phase in both systems. The present work has detected no clear evidence for the formation of the r&Al, crystallographic structure usually grown at Al/Cu interfaces. At temperatures exceeding 3OO”C, the activation energy for growth of the three phases rich in Al and Cu is very similar to the energy which characterizes the rate of interdiffusion in AlfCu diffusion couples. The present work suggests that intermetallic growth occurs largely by short-circuit diffusion at temperatures lower than approx. 350°C and by bulk diffusion at the more elevated temperatures. Finally, there is an indication that the characterization of growth of each intermetallic band by a separate activation energy is not necessarily appropriate.
lo-" _
_
10-l’
_
lo-‘*
_
lo-l3
-
i * N E lo-14 _ 2 F x lo-=
-
10-16 -
~c~~~w~e~ge~e~~~-~e author gratefully acknowledges the technical assistance of W. A. C. Fraser and 1. G.
30-‘? -
10-16 _
lo-'gl
1.o
INTERFACES
I
I
I
1.2
1.4
1.6
I 1.8
I 2.0
1 2.2
I 2.4
t03/T[K-‘)
Fig. 7. Arrhenius plots of the rate constants for growth of the various diffusion bands.
Akkerman during the course of the work._T’he technical support provided by the X-ray and electron fiiicroprobe analytical groups of the laboratory is also deeply appreciated. The work was supported in part by the Canadian Electrical Association under Contract No. CEA 7619. REFERENCES 1. J. A. Rayne and C. L. Bauer, Prof. 5th &&on Lunding Conf. on Wekiments, Physical Metallurgy and Failure Phenomena, p. 353. General Electric Co., Schenectady,
NY (1979). 2. E. R. Wallach, Doctoral Thesis, Cambridge Univ., This realization, along with the observation from Fig. 7 that the Arrhenius relationship provides an unsatisfactory description of the dependence of the rate constant on temperature for layer B, suggests that the growth kinetics of this layer cannot be treated in isolation and that it must be lumped with that of either band A or band C. In order to test the validity of this conjecture the goodness of fit of the Arrhenius relationship to the growth data of layers (A + B) and (B + C) was examined. The fit to the (B + C) growth was found superior and is shown in Fig. 7. The activation energy for this combined growth in the higher temperature range is now 40.1 kcal mol-’ and characteristic of a bulk diffusion process. The result suggests that characterization of the growth of layer B by an independent activation energy is inappropriate.
dilfusion
processes.
SUMMARY Inte~etaIlic growth at bulk Al/brass interfaces has been investigated at temperatures ranging from 150
to 450°C. The interdiffusion layer consists of four major intermetallics of which three correspond to
U.K. (1975). 3. F. M. d’treurle
and P. S. Ho in Thin FilmsInrerd@sion and Reactions (edited by J. M. Poate, K. M. Tu and J. M. Mayer), p. 243. Wiley. New York
( 1978). 4. D. Newbury and S. Greenwald, .I. Res. natn Bur. Stand. 85,429
(1980).
5. N. A. Cantalejos and G. Cuminsky, J. Inst. Metals 100, 20 (1972). 6. Powder I)t@kftion Fife, Publication SMA-29. international Centre for Diffraction Data, Swa~hmore, Pa (1979). 7. S. Westman, Acta Chem. &and. 19, 2369 (1965). 8. A. J. Bradley, H. J. Goldsmidt and H. Lipson, J. I~I. Mefafs 63, 149 (1938).
9. F. Weibke, 2. anorg. Chem. 220, 30.5 (1934). IO. Y. Funamizu and K. Watanabe, Trans. Japan. Inst. Mefals 12, 147 (1971). II. A. A. Ershov, T. A. Sycheva and P. F. Zasukha, Metallova Term Obrab Met 5, 19 (1977). MeraIl. 12. C. Su~aRarayana and T. R. Anantha~man, Trans. 2, 3237 (1971). 13. W. Jolley and D. Hull, J. Insf. Mefak 92, I29 (1964). 14. H. Nowotny and A. Winkels, Z. Phys. 114,455 (1939). 15. J. E. E. Baglin and J. M. Poate, in Thin FifmsInterdiction and Reucfians (edited by J. M. Poate, K. M. Tu and J. M. Mayer), p. 305, Wiley, New York (1978). 16. U. Giisele and K. N. Tu, J. appf. Phys. S&3252 (1982). 17. D. Gupta and P. S. Ho, Thin Soiid Films 72,399 (1980).
ooo1-6160/8s 53.00 +o.oo
1985
Pcrgamon Press Ltd
INTERMETALLICS GROWTH INTERFACES
AT Al/a -BRASS
R. S. TIMSIT Alcan Intemationai
Ltd,
Kingston
laboratories,
P.O. Box 8400, Kingston, Ontario K7L 424, Canada
(Received IO February 1984) Abstract-The
growth rate of the interdiffusion layer formed in “bulk” Al/cl-brass diffusion couples has been measured at temperatures ranging from 150 to 450°C. The layer consists of four stacked bands of intermetallic compounds. The chemical composition and crystal structure of the compounds were determined by electron probe analysis and X-ray diffraction, respectively. The structure of the band imm~iately adjacent to the At is that of CuAI,. The structure of the next two bands is associated with that of the Cq,*,Ab,s phase. The dominant structure of the band adjacent to the brass is that of an hexagonal phase of the Cu-Zn system. The activation energies associated with intetmetallics growth are reported.
ReSum&-Nous avons mesur6 la vitesse de croissance de la couche d’interdiffusion fonnie dans des couples “massifs” de diffusion aIuminium~iaiton-~ pour des temp&atures comprises entre 150 et 450°C. Cette couche est constitt& de quatre bandes empilbes de compos6s inte~~talliqu~. Nous avons d&tern&& lastructure cristailine et la composition chimique de ces composk par analyse ii. la sonde tlectronique et par diffraction de rayons X. La structure de la bande immidiatement adjacente li I’aluminium est celle de CuAI,. La structure des deux bandes suivantes est associCe ii celle de la phase CU~,~,A&,.,~. La structure dominante de la bande adjacente au laiton est celle d’une phase hexagonale du syst&ne Cu-Zn. Nous ptisentons les valeurs des energies d’activation associ&es H la croissance des compos&s intermCtalliques. Z~rn~nr~ung-Die Wa~stumsg~wind~gkeit von S.chichten, di_e sich be! der Interdiffusion in m~roskopischen Metallpaaren Aluminium/a-M~ing bilden, wurde im Temperaturbereich zwischen 150 und 450°C gemessen. Die sich bildende Zwischenschicht besteht aus einem Stapel von vier intermetallischen BHndern. Die Kristallstmktur wurde mit Riintgenbeugung und die chemische Zusammensetzung wurde mit Elektronenmikroanalyse bestimmt. Das Band an der Aluminiumseite hat CuAl,-Struktur. Die beiden nLhsten Biinder hiingen mit der Cu,,, Ab.,9-Phase zusammen. Die dominierende Struktur des Bandes auf der Messingseite entspricht einer hexagonalen Phase des Cu-Zn-Systems. Aktivierungsener-
gien fir das Wachstum diescr inte~etalli~hen
Strukturen werden angegeben.
peratures ranging from 150 to 450°C. The effect of the reliability of intermetallics growth on aluminium/brass electrical contacts will be examined in a later publication.
INTRODUCTION
The formation of brittle intermetallics at the interface separating two dissimilar materials can be deleterious to the mechanical stability of the interface [1,2]. This effect has given rise to considerable concern in the microelectronics industry because it is detrimental to the long-term reliability of thin-film electrical contacts [3]. Recently the presence of intermetallics in so-called “bulk” electrical interfaces such as those found in m~hanicaliy-wired electrical junctions has been linked to the overheating of certain types of electrical connections (41. It has been stipulated that the contact resistance of these connections rises to unacceptable levels independently of the mechanical integrity of the interface in part because the intermetallics formed are relatively poor electrical conductors and grow sufficiently thick to hinder electrical flow directly. The present investigation examines intermetallics growth at bulk Al/brass interfaces because of the possible relevance of this growth to the performance of many electrical junctions wired with aluminium. The paper focuses on the growth kinetics, chemical composition and crystal structure of intermetallics formed in bulk diffusion couples at temA.M. 33/l--G
EXPERIMENTAL The materials used in the investigation were AAl350Al and an a-brass with the nominal composition by wt% Cu 70/Zn 30. The measured compositions listed in Table 1 are very similar to the compositons of electrical conductor alloys used commercially. Al/brass couples suitable for interdiffusion studies were produced by roll-bonding. The best couples were obtained by rolling the combination of a brass sheet 1.6 mm in thickness with an Al sheet 0.5 mm thick through a 50% reduction pass. The surfaces were pickled in a 50% nitric acid solution and were wire-brushed prior to rolling. Diffusion couples were obtained by cutting coupons approx. 1 x 1 cm* from the composite material. Diffusion was allowed to proceed for selected time intervals at each of several temperatures, ranging from 150 to 450°C in an Ar atmosphere. The work was carried out with an electric furnace capable of 91
TIMSIT:
98
INTERMETALLICS
GROWTH AT Al/a-BRASS
diffraction of material obtained by micro-extraction from each of the intermetallic layers.
‘Table 1. Composition of materials in wt% Element Al Si Mn Fe Ni cu zn Pb Others
BKISS 0.001 0.001 0.025 0.001 69.0 I 30.88 0.001 0.084
INTERFACES
AA1350 Al
RESULTS
99.6 0.01 0.26 0.02 0.001 0.001 0.02
Composition and crystal structure
maintaining temperatures to better than 1°C of a set point. Cross-sections of the diffusion couples after heat treatment were examined both optically and with a scanning electron microscope (SEM) and concentraton profiles for Al, Cu and Zn across the Al/brass interface were obtained using electron probe analysis. From these analyses the growth rates and compositions of several intermetallic bands were measured. The crystallographic structure of the intermetallic phases were identified by X-ray powder
Over the entire temperature range investigated, the interdiffusion layer consists of four major bands. These are shown in the SEM micrographs of Fig. I which illustrate the layered structure of cross-sections of the diffusion couples after exposure to different temperatures. Band A adjoining the Al is light grey. Bands B and C are both dark grey and band D which adjoins the brass is bright orange. This last band is always the widest. Sub-bands of varying tones of grey are generally observable within the four major intermetallic layers. These sub-bands either delineate transition regions between two major intermetallic layers or arise as a result of a small but sharp discontinuity in the elemental composition within the major intermetallics. A sub-band can be clearly seen within layer C in Fig. l(a) and in the lower end of layer B in
BRASS
D C
-9 A
Al
(a)
BRASS
D C B A
(b) Fig. I. Micrograph of interdiffusion layer: (a) after 765 h at 250°C; (b) after 6 h at 400°C.
TIMSIT: 0
Al
4
cu zn
I
INTERMETALLKS
GROWTH AT Al/a-BRASS
250°C A
BC
0
100
0
10
20
40
30
Distance
(pm)
Fig. 2. Representative element concentration anneafing at 250°C.
profife after
o Al a cu
99
INTERFACES
Fig. i(b). All layers are made up of large columnar grains. it is emphasized that the interdiffusion layer did not grow uniformly across the roll-bonded interface, especially below 3WC, but occurred in discrete regions, These discontinuities have been observed by other workers [S] and are attributed to the presence of obstructive residuat oxide films at the originat interface. Examples of element concentration profiles obtained by electron probe analysis are presented in Figs 2-4. The spatial resolution of the technique (typically 2rm) preluded an analysis on sampIes annealed below 250°C because the interdiffusion widths obtained under these conditions were never su~cient~y large. One of the interesting observations which emerges from the analyses is the relatively low Zn concentration in all the bands except layer D. The ranges of composition of the major phases in each of the four bands at all the temperatures investigated are shown in Tabfe 2. Layer A consists of an intermetaltic phase of approximate composition Al&, Cu 54, 350%
a Zn
50
0
Distance Fig. 3. Representative element ~~ntration
100 (yn)
I50
profile after annealing at 350°C.
o Al 450 *c 100
80
0 0
50
100 Distance fprn f
150
Fig. 4. Representative element concentration profile afier annealing at 450°C.
200
loo
TIMSIT:
INTERMETALLICS
GROWTH
AT Al/a-BRASS
INTERFACES
Table 2. Summary of results relating lo composition, crystal structure and growth rates of intermetallic bands Composition range fwt%) Zn
Crystul structure
(cm% 1)
Activation energy (kcal mot-‘)
-1
CuAll
5.6 x IO-’
21.9
64-69 6%70
2-4 8-10
Cub,, Al,,, Cuw Alw
4.2 x IO-’ 8.6 x IO’
23.4 50.7
21-15 t9-15
77-75 76
2-10 5-9
Cup,, 4, Cue.61Abm traces of Cu&
3.9 x lo-’ 0.114
16.8 31.5
3-2
60-57
37-41
Largely
8.5 x IO-’
21.8
1.2 x 10’
38.4
3.4 x 10-6 4.0 6.8 x 10-s 2.0 x 10’
17.3 33.7 19.0 40.1
Temperature range (“C)
Al
Cu
250-450
46-43
SE56
B
250-300 350-450
34-21 23-20
c
250-300 350-450
D
250-300
Lpyer A
350-450
3-2
60-57
37-41
%6Zns.4 CrhJk
gm&=iab~c component at 4WC) A+B+C B+C
1SO-300 350”4.50 250-300 350-450
Zn 1 wto/,, and a narrow transition band to layer B. lines corr~ponding to inter-planar spacings of 2.51 The composition of layer B could not be established and 2.89 A coincide with diffraction lines associated reliably at temperatures below 350°C because this with the C&Al, lattice 191,the absenceof#a strong line layer was always too narrow under these conditions. expected from this structure at a d-spacing of 3.56 A However, there are indications that the Cu content of rules out the presence of this phase. The origin of layer B at temperatures lower than 350°C is of the these two weak lines (and others) in this diffraction order of 67 wt”/, with the balance largely Al. Above pattern remains unclear. It is worth noting that 3WK! the Cu content in band B is reasonably despite the suggestion in Fig. 4 that layer C is grown uniform at approx. 69 wt”/, with the Al decreasing with the ChAI; crystallographic configuration at from 23 to 20 wt%. Layer C consists of approx. 450°C since the atomic concentrations of Cu, Zn and Cu76 wt%, a maximum of lOwt% of Zn and an Al in the layer are approximately in the ratio 8: I :4, average Al content of 16 wt”/, at all the temperatures the diffraction analysis never revealed any clear indiinvestigated. Layer D contains an average of only cation of this phase at this or any other temperature. 2 wt”/, Al and an average Cu and Zn content of 59 This is in contrast with the results of investi8ations of Al/Cu and Al/brass interdiffusion at more elevated and 39 wto/ respectively, also at all temperatures. The X-ray powder diffraction analysis did not temperatures [2, 10, 111 where the C&Al, phase was always yield wholly unambiguous information on observed. crystallographic structure. This was due, in part, to The dominant structure of layer D was clearly an inabihty to extract from a selected band a sample identified as that of the hexagonal phase wrrespondrange 3842 wt”/, Zn [12] in of material which was always free of metal from ing to the com~sition immediateIy adjacent layers. The analysis identified the Cu-Zn system. Samples annealed below 300°C the crystal structure of layer A clearly as that of showed, in addition, traces of the C~.a,Zn,,,s strucCuAl, [6] as shown in Fig. 5(a). This is consistent with ture [13] whereas specimens heated at 400°C and the chemical composition yielded by the electron above revealed the presence of an appreciable CuZn probe analysis. The structures of layers B and C were component [13]. An example of the digraction patless clearly defined. Material extracted from these tern obtained in this last instance is shown in layers usuaily showed evidence of some CuAlr, as Fig. 5(c), where the lines originating both from the indicated in Fig. 5(b), but this was interpreted as hexagonal Cu,,Zn,,, and the CuZn phases are clearly contamination from band A. The lines correspondin visible. These results and corresponding data obto interplanar spacings of 2.044, 2.047 and 2.052 x tained from material grown at lower temperatures are are attributed to the yz-Cy,e,Ab,, structure [7,8]. consistent with the compositions of layer D obtained The identification of this phase as the dominant from the electron probe analysis. crys~llographic component is generally consistent with the range of chemical compositions of layers B Growth rates and C yielded by electron probe analysis. It is worth The growth rate of each intermetallic band was mentioning that samples annealed at 450°C yielded, determined by measuring the average band thickness in addition, evidence of traces of the CurAl, structure after selected time intervals at each of the selected (61 in layer C. annealing temperatures. The resu!ts are shown in The origin of a few of the weak lines in Fig. 5(b) Fig. 6(a)--(f). The straight lines represent a leastcould not be established. For example, although the squares fit to the experimental data. These fits assign
TIMSIT:
INTERMETALLICS
GROWTH AT Al/a-BRASS
101
INTERFACES
I 4.30
4.30 CuA12
2.37 212 Co Al2
2.052 2.047 2.044
(a)
Cu Al .6 .4
2.12 1.83 Cuxznl-X
1.47 CuZn
I.20 CuZn
Fig. 5. Typical X-ray powder diffractographs. The lattice spacings of the planes yielding the most intense reflections are indicated in Angstrom units (a) From intermetallic band A at all temperatures. (b) From intermetallic band B and C grown at temperatures lower than 45O’C. The lines corresponding to CuAI,
were identified as originating from ~ntaminant material from band A. (c) From inte~etailic band D grown at 450°C. Electron microprobe analysis indicated that the Cu atomic concentration x varies from 0.59 to 0.63. origin a weight given by the number of experimental points. Within the scatter of the experimental data the dependence of the thickness x of each of the bands on the interdiffusion time I at a given temperature can be represented as to the
x2 = kt, where k is the interdiffusion rate constant at the selected temperature. The proportionality of layer thickness to 4 indicates that the diffusion of atoms through the intermetalli~ bands is considerably slower than diffusion across the growth interfaces 1161. The activation energy characterizing the growth of each intermetallic layer was determined on the usual assumption that the dependence of k on temperature is expressed as k = k,exp(-Q/TR), where k, is a constant, Q is the activation energy and T is the absolute temperature. The results of this analysis are shown in Fig. 7 and are summarized in Table 2. It was found that the intermetallic growth could generally not be described satisfactorily by a single activation energy over the entire temperature
range investigated but that an adequate description is possible by considering the data from the two temperature regions 150-300 and 350-45O”C separately. As shown in Table 2 the activation energies determined for tem~ratures lower than approx. 300°C are considerably smaller than those obtained for the more elevated temperatures. The lower activation energies may be indicative of inter-metallic growth by a short-circuit diffusion mechanism such as grain boundary or dissociated dislocation diffusion [ 171. The activation energies above 300°C are characteristic of bulk diffusion. This interpretation of the data is consistent with the observation in Fig. I that the density of grain boundaries in the Al-rich layers, and particularly in layer A, is larger at 250 than at 400°C and thus that short-circuit diffusion probably dominates at lower temperatures. The values of Q at elevated temperatures both for the diffusion bands (A f B f C) and for layer A are reasonably close to the recent values of 29.5 and 24.0 kcal mol-’ measured for the corresponding layers in Cu/Al couples in the same temperature range [2]. The activation energy for the growth of layer B at temperature higher than 300°C is very large and generally uncharacteristic of activation energies for
102
INTEKMETALLICS
TIMSIT:
GKOWTI-I
AT Al/a-BRASS
INTERFACES
(4 LOYW I4 -
oA
250%
OB
.
ti r--
hr:‘*l
(b) Layer oA 28
OB AC
l o * A+B+C
Y-t w
(hrs”*)
Layer 0
Fig. 6. Growth B and (A+
A
of diRi&on bands at the temperatures s&-cted in the precut invesligation. 3 + C) al 150 and 200°C; (b) 2WC; (c) 300°C; (d) 350°C; fee) 4!M*C; (f) 450°C. 103
104
TIMSIT:
INTERMETALLICS
GROWTH AT Al/a-BRASS
phases in the Cu-Al system and one is a Cu-Zn intermetallic compound containing only traces of Al. The similarity of the intermetallics with those formed at Al/Cu interfaces is restricted to the presence of the CuAl, phase in both systems. The present work has detected no clear evidence for the formation of the r&Al, crystallographic structure usually grown at Al/Cu interfaces. At temperatures exceeding 3OO”C, the activation energy for growth of the three phases rich in Al and Cu is very similar to the energy which characterizes the rate of interdiffusion in AlfCu diffusion couples. The present work suggests that intermetallic growth occurs largely by short-circuit diffusion at temperatures lower than approx. 350°C and by bulk diffusion at the more elevated temperatures. Finally, there is an indication that the characterization of growth of each intermetallic band by a separate activation energy is not necessarily appropriate.
lo-" _
_
10-l’
_
lo-‘*
_
lo-l3
-
i * N E lo-14 _ 2 F x lo-=
-
10-16 -
~c~~~w~e~ge~e~~~-~e author gratefully acknowledges the technical assistance of W. A. C. Fraser and 1. G.
30-‘? -
10-16 _
lo-'gl
1.o
INTERFACES
I
I
I
1.2
1.4
1.6
I 1.8
I 2.0
1 2.2
I 2.4
t03/T[K-‘)
Fig. 7. Arrhenius plots of the rate constants for growth of the various diffusion bands.
Akkerman during the course of the work._T’he technical support provided by the X-ray and electron fiiicroprobe analytical groups of the laboratory is also deeply appreciated. The work was supported in part by the Canadian Electrical Association under Contract No. CEA 7619. REFERENCES 1. J. A. Rayne and C. L. Bauer, Prof. 5th &&on Lunding Conf. on Wekiments, Physical Metallurgy and Failure Phenomena, p. 353. General Electric Co., Schenectady,
NY (1979). 2. E. R. Wallach, Doctoral Thesis, Cambridge Univ., This realization, along with the observation from Fig. 7 that the Arrhenius relationship provides an unsatisfactory description of the dependence of the rate constant on temperature for layer B, suggests that the growth kinetics of this layer cannot be treated in isolation and that it must be lumped with that of either band A or band C. In order to test the validity of this conjecture the goodness of fit of the Arrhenius relationship to the growth data of layers (A + B) and (B + C) was examined. The fit to the (B + C) growth was found superior and is shown in Fig. 7. The activation energy for this combined growth in the higher temperature range is now 40.1 kcal mol-’ and characteristic of a bulk diffusion process. The result suggests that characterization of the growth of layer B by an independent activation energy is inappropriate.
dilfusion
processes.
SUMMARY Inte~etaIlic growth at bulk Al/brass interfaces has been investigated at temperatures ranging from 150
to 450°C. The interdiffusion layer consists of four major intermetallics of which three correspond to
U.K. (1975). 3. F. M. d’treurle
and P. S. Ho in Thin FilmsInrerd@sion and Reactions (edited by J. M. Poate, K. M. Tu and J. M. Mayer), p. 243. Wiley. New York
( 1978). 4. D. Newbury and S. Greenwald, .I. Res. natn Bur. Stand. 85,429
(1980).
5. N. A. Cantalejos and G. Cuminsky, J. Inst. Metals 100, 20 (1972). 6. Powder I)t@kftion Fife, Publication SMA-29. international Centre for Diffraction Data, Swa~hmore, Pa (1979). 7. S. Westman, Acta Chem. &and. 19, 2369 (1965). 8. A. J. Bradley, H. J. Goldsmidt and H. Lipson, J. I~I. Mefafs 63, 149 (1938).
9. F. Weibke, 2. anorg. Chem. 220, 30.5 (1934). IO. Y. Funamizu and K. Watanabe, Trans. Japan. Inst. Mefals 12, 147 (1971). II. A. A. Ershov, T. A. Sycheva and P. F. Zasukha, Metallova Term Obrab Met 5, 19 (1977). MeraIl. 12. C. Su~aRarayana and T. R. Anantha~man, Trans. 2, 3237 (1971). 13. W. Jolley and D. Hull, J. Insf. Mefak 92, I29 (1964). 14. H. Nowotny and A. Winkels, Z. Phys. 114,455 (1939). 15. J. E. E. Baglin and J. M. Poate, in Thin FifmsInterdiction and Reucfians (edited by J. M. Poate, K. M. Tu and J. M. Mayer), p. 305, Wiley, New York (1978). 16. U. Giisele and K. N. Tu, J. appf. Phys. S&3252 (1982). 17. D. Gupta and P. S. Ho, Thin Soiid Films 72,399 (1980).