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Effects of 0.1 and 0.2 wt.% aluminium addition to zinc on the interdiffusion between zinc and iron at 400°C
Materials Science and Engineering A251 (1998) 87 – 93
Effects of 0.1 and 0.2 wt.% aluminium addition to zinc on the interdiffusion between zinc and iron at 400°C Syahbuddin a, P.R. Munroe a,*, C.S. Laksmi b, B. Gleeson c a
School of Materials Science and Engineering, The Uni6ersity of New South Wales, Sydney, NSW 2052, Australia b Pasminco Research Centre, Boolaroo, NSW 2284, Australia c Materials Science and Engineering Department, Iowa State Uni6ersity, Ames, IA 50011, USA Received 7 January 1998; received in revised form 23 March 1998
Abstract This paper reports the interdiffusion behaviour between iron and zinc in Fe/Zn, Fe/Zn – 0.1 wt.% Al, and Fe/Zn – 0.2 wt.% Al couples annealed at 400°C for times ranging from 1 to 30 h. Under steady-state growth conditions, the interdiffusion zone in the Fe/Zn and Fe/Zn–Al alloy couples consisted of intermetallic layers of the Fe – Zn phases G-Fe3Zn10, dk-FeZn7, dp-FeZn10, and z-FeZn13. The intermetallic layers grew at a more rapid rate in the Fe/Zn – Al alloy couples than in the Fe/Zn couple. The thickening kinetics of the total interdiffusion zone in all couples were parabolic for annealing times greater than 3 h. The compact d structure (dk) appeared at the G/dp interface of the Fe/Zn couples after about 7 h annealing; whereas, the same structure in either of the Fe/Zn–Al alloy couples was not observed until after about 13 h annealing. Enrichment of aluminium was detected at the dp/z and z/h-zinc interfaces in the Fe/Zn–Al couples. The extent of this enrichment was highest in the Fe/Zn – 0.2 wt.% Al couple. The aluminium enrichment is believed to have contributed to the increase in the thickening kinetics of the intermetallic layers in the Fe/Zn–Al alloy couples owing to a negative thermodynamic interaction between aluminium and zinc. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Interdiffusion; Multilayered growth; Aluminium effect; Fe – Zn intermetallic compounds
1. Introduction Galvannealing in the automotive industry has become an important process in the fabrication of ductile, zinc-rich coatings on steels. In the galvannealing process, the steel is first immersed in a zinc-alloy bath at about 450°C for up to 5 s, and then given a post-coating heat treatment at temperatures in the range from 400 to 600°C for 10 – 20 s. The heat treatment causes the zinc in the coating to interdiffuse with the substrate iron to form several Fe – Zn intermediate phases, such as G-Fe3Zn10, G1-Fe5Zn21, dk-FeZn7, dp-FeZn10, and z-FeZn13. Most studies on the galvannealing process have been concerned with the morphology of the Fe– Zn intermetallic layers [1 – 3]. Little attention has been given to the kinetics of the interdiffusion between the
* Corresponding author. Tel.: + 61 2 93854425; fax: + 61 2 93856400. 0921-5093/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0921-5093(98)00641-8
steel and the zinc coating during the annealing stage. Moreover, the zinc alloys commonly used for hot-dip galvanizing typically contain of a small concentration (0.1–0.2 wt.%) of aluminium, which is added to inhibit intermetallic formation. The trace amount of aluminium has a significant effect on the interdiffusion behaviour between iron and zinc, but the effects of these aluminium additions on the growth on Fe–Zn intermetallics are poorly understood. In particular, there has been a dearth of systematic diffusion studies on the Fe/Zn–Al alloy system. The purpose of this study was to characterise the solid-state interdiffusion between pure iron and pure zinc and zinc alloys containing 0.1 and 0.2 wt.% Al. The interdiffusion temperature was 400°C and annealing times varied from 1 to 30 h. This work is part of a larger study on the ‘aluminium effect’ in Fe-Zn alloys. The present paper reports the results for solid/solid interdiffusion. As will be reported in subsequent papers, the ‘aluminium effect’ occurs in both solid/solid and solid/liquid couples, but solid/solid diffusion couples
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Fig. 1. Optical micrographs comparing the interdiffusion zones formed in the Fe/Zn and Fe/Zn – Al alloy couples after annealing at 400°C for 7 and 30 h.
offer the advantage of being easier to study. Long-term interdiffusion was studied in order to obtain steadystate behaviour and, hence, allow for the determination of the interdiffusion coefficients via the Boltzmann– Matano technique; short-term interdiffusion behaviour is non-ideal. The chemical interactions inferred from the results will be useful in interpreting liquid/solid interdiffusion, relevant to a galvannealing process.
2. Experimental procedures The materials used for the diffusion-couple experiments were pure iron (99.98%) obtained from Aldrich, Australia, and pure zinc and zinc alloys, containing either 0.1 or 0.2 wt.% Al, obtained from Pasminco Research Centre, Boolaroo, Australia. The different materials were first cut into 2× 7 ×7 mm coupons and then polished down to a 1 mm finish. A diffusion couple was made by clamping an iron coupon to a coupon of pure zinc, Zn–0.1 wt.% Al or Zn – 0.2 wt.% Al. The diffusion anneals were carried out in a horizontal, resistance-tube furnace under flowing argon gas at 400°C for times ranging from 1 to 30 h. At the end of a given diffusion anneal, the diffusion couple was quenched into water in order to preserve its high-tem-
perature microstructure. The diffusion couple was then mounted in an epoxy resin and cross-sectionally polished through to a 1 mm diamond finish. The polished cross-sections were etched in a solution of 0.1% nital and 0.1% picral in order to reveal the microstructure of the Fe–Zn intermetallic layers. The microstructure, thickness and composition profiles in the intermetallic layers formed in the interdiffusion zone were examined using optical microscopy, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). All EPMA measurements were conducted on unetched cross-sections. The standards used for the EPMA were pure Al, Zn, and Fe.
3. Results and discussion
3.1. Interdiffusion zone microstructure Isothermal annealing at 400°C for 1–30 h of the Fe/Zn and Fe/Zn–Al alloy couples caused the iron to interdiffuse with the zinc and resulted in the formation of Fe–Zn intermediate phases. It was found that iron was the principal diffusing species, with the Fe–Zn intermediate phases growing into the zinc or the Zn–Al alloys. In general, for a short annealing of 1 h, the
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Fig. 2. The d/z interface in the Fe/Zn – 0.2 wt.% Al couples after annealing at 400°C for 15 h.
interdiffusion zone consisted of a very thin layer (: 3 mm) of G phase in contact with the iron, an intermediate layer ( : 25 mm) of dp phase, and a thick outer layer ( :22 mm) of z phase in contact with the zinc or Zn–Al alloys. The dp layer, which exhibits a palisade structure, contained a large number of cracks running perpendicular to the original couple interface. Such crack formation is a general characteristic of the dp structure formed during hot-dip galvanizing [4]. A comparison between the different couples showed that the dp/z and z/h interfaces were considerably more planar in the Fe/Zn couples than in the Fe/Zn – Al alloy couples. As shown in Fig. 1, the G, dp and z layers continuously composed the interdiffusion zone of the Fe/Zn– Al alloy couples even after 7 h annealing. In the case of the Fe/Zn couple, however, a compact d (dk) layer developed between the G and dp layers at some time
Fig. 3. Interdiffusion zone thickening kinetics in the Fe/Zn and Fe/Zn – Al alloy couples after annealing at 400°C for 1–30 h.
between 5 and 7 h. The dk phase is distinguished from the dp phase because these two forms of d differ in composition, morphology, and atomic arrangement (i.e. degree of order); although they both have an hexagonal close packed crystal structure [5]. The delayed formation of the dk layer observed in this study was also found by Onishi et al. [6] in a diffusion study of Fe/electrodeposited Zn couples at 240–410°C. These authors found the dk structure to form at the G/dp interface after about 7 h annealing at 380°C. Formation of the dk layer in the Fe/Zn–Al couples did not occur until after about 13 h annealing, indicating that the presence of aluminium in the zinc inhibits the formation of dk structure at the G/dp interface. As reported by Hong et al. [7], the dk structure has a superlattice which is three times larger than that of the dp structure. It is very likely that the transformation from the dp to dk structure was slower in the Fe/Zn–Al couples on account of the aluminium possibly decreasing the stability of the more ordered dk, thereby decreasing the driving force for dp dk transformation. Further annealing for 30 h caused the dp/z interface to become more planar in the Fe/Zn couples, but non-planar in the Fe/Zn–Al alloy couples. As shown in Fig. 2, the dp/z ‘interface’ in the Fe/Zn–0.2 wt.% Al couple consists of an intermixed zone of dp and z. The variation in the dp/z interface morphologies in the different diffusion couples is ostensibly a consequence of the aluminium providing an extra degree of thermodynamic freedom, thus allowing for the stable growth of a two-phase interdiffusion zone. Moreover, the G:dk + dp:z thickness proportions were not the same for the two Fe/Zn–Al alloy diffusion couples, with the proportion being about 2:71:27 for the Fe/Zn–0.1 wt.% Al couple and 2:79:19 for the Fe/Zn–0.2 wt.% Al couple. This result indicates the notable influence that a trace amount of aluminium has on the transport rates in the different Fe–Zn intermediate phases.
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Fig. 4. Fe – Zn intermetallic thickening in (a) the Fe/Zn, (b) Fe/Zn – 0.1 wt.% Al, and (c) Fe/Zn – 0.2 wt.% Al alloy couples during annealing at 400°C for 1 – 30 h.
P.R. Munroe et al. / Materials Science and Engineering A251 (1998) 87–93 Table 1 Steady-state (t\3 h) parabolic rate constants for the thickening kinetics of the dp+dk and z layers and the total interdifusion zone Couple
Fe/Zn Fe/Zn – 0.1 wt% Al Fe/Zn – 0.2 wt% Al
kp (mm2 s−1) Total Interdiff. zone dp+dk Layer
z Layer
0.61 0.86
0.22 0.42
0.08 0.06
0.93
0.58
0.03
3.2. Growth kinetics of the intermetallic layers The thickening kinetics of the interdiffusion zones in the three couples are shown in Fig. 3. The interdiffusion zone in the Fe/Zn couple thickened according to the parabolic rate law, thus indicating diffusion-controlled growth kinetics. By contrast, the thickening kinetics of the interdiffusion zones in the Fe/Zn–Al couples were linear for the first 1 – 3 h annealing and then parabolic for 3 – 30 h annealing. Moreover, it is seen in Fig. 3 that the thickening kinetics of the interdiffusion zones in the Fe/Zn – Al couples increased significantly, and were higher than the kinetics found in the Fe/Zn couple. Fig. 4 shows the thickening behaviour of the various Fe–Zn intermetallic layers in the interdiffusion zones of the three different couples as a function of annealing time. It is seen that after 1 h annealing, the proportion of z layer formed was large in each of the couples, ranging from 53% in the Fe/Zn couple down to 37– 42% in the Fe/Zn – Al couples. With continued annealing, however, the proportion of z decreased owing to the predominance of the dp +dk layers. In the case of the Fe/Zn couple, dp was the thickest layer for annealing times of 3–30 h, beyond which dk was the thickest layer. dp remained the thickest layer in the Fe/Zn–0.2
Fig. 5. The concentration profiles of iron in the interdiffusion zone of the Fe – Zn couple after annealing at 400°C for 1, 7, 13 and 30 h.
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wt.% Al couple for annealing times of 3–30 h. Similar behaviour was also found in the Fe/Zn–0.1 wt.% Al couple except after 30 h annealing, where dk was then the thickest layer. The proportion of z layer decreased with increasing aluminium addition in the zinc. Table 1 summarises the steady-state, thickening rate constants of the total interdiffusion zone, together with the dk + dp and z layers in the Fe/Zn and Fe/Zn–Al alloy diffusion couples. Steady-state was taken to be annealing times from 3 to 30 h, within which parabolic kinetics were observed. The interdiffusion zone thickening kinetics in each of the couples were dominated by the dk + dp layer during these annealing times. In contrast to the parabolic, steady-state behaviour, the thickening kinetics of the dk + dp and z layers were linear for about the first 1–3 h of interdiffusion. It is seen in Table 1 that the parabolic rate constants for the dk +dp layer increased with increasing aluminium content. By contrast, the parabolic rate constants for the z layers in the Fe/Zn–Al alloy couples were lower than that in the Fe/Zn couple. Thus, the increased thickening rate of the interdiffusion zone in the Fe/Zn–Al couples is attributable to an increase in the thickening rate of the dp + dk layer with increasing aluminium addition.
3.3. Concentration profiles The effect of diffusion-annealing time on the iron concentration profile in the interdiffusion zone of the Fe/Zn couple is shown in Fig. 5. The different Fe–Zn intermediate phases are identified in this figure on the basis of composition. It is seen that after 1 h annealing the iron concentration at the apparent G/dp interface was approximately 9 wt.%, while the presence of the dk layer after 7, 13 and 30 h annealing resulted in an iron concentration at the G/dk interface of about 12 wt.%. The iron content in the dp layer at the dp/z interface was about 7.5 wt.%, irrespective of the annealing time, whereas the dk layer at the dk/dp interface contained about 10 wt.% Fe after annealing times greater than 1 h. These interfacial compositions are in general agreement with the Fe–Zn equilibrium phase diagram in which dp and dk exist in distinct phase fields [8]. In this phase diagram, the iron concentration ranges from about 7 to 8.5 wt.% in dp at 400°C and from about 10 to 12 wt.% in dk. The observed discontinuity in the iron concentration profile at the dk/dp interface is also in agreement with the phase diagram, which predicts the iron concentration difference between the dk and dp phases to be about 1.5 wt.%. It is therefore concluded that the dp and dk are indeed distinct phases and that the dp and dk layers were at local equilibrium for the longer annealing times of 7–30 h. The situation was different after 1 h annealing, however, where the dp layer was apparently at local equilibrium only at the dp/z interface, and not at the G/dp interface. There was
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Table 2 Interdiffusion coefficients in the z and d layers formed in the Fe/Zn, Fe/Zn – 0.1 wt% Al and Fe/Zn – 0.2 wt% Al couples annealed at 400°C for 30 h Phase
Fe (wt.)
D×10−13 (m2 s−1) Fe/Zn
Fe/Zn – 0.1 wt% Al
Fe/Zn – 0.2 wt% Al
Liter. 2.6 [10] 14.9 [11]
z
6.5 6.6
11.7 11.9
20.6 21.4
22.8 23.4
dp
7.7 7.9
— 5.2
16.6 16.8
44.5 45.2
dp/dk
8.2 10.0
0.7 0.8
0.9 0.9
1.5 1.6
dk
10.7 11.4
6.8 7.0
8.3 8.3
10.1 10.2
a comparatively greater proportion of the dp structure at the short annealing time, which may be attributed to a higher diffusivity in this structure due to either compositional or structural effects. Since the boundary compositions in the various layers were constant with the longer annealing times, thus indicating steady-state conditions, the interdiffusion coefficient could be determined using the Boltzman– Matano technique [9]. Table 2 summarises the interdiffusion coefficients measured in the dk, dp and z layers of the Fe/Zn and Fe/Zn – Al alloy couples as a function of composition for the annealing time of 30 h. The interdiffusion coefficients in the G layer could not be accurately determined due to the limited thickness of this layer. Further, and as a first approximation, the cross-term effects of aluminium in the Fe/Zn –Al couples were ignored (i.e. the systems were treated as binaries). It is seen in Table 2 that the interdiffusion coefficients in the z layer (Dz ) were essentially independent of the composition. The average value of Dz in the Fe/Zn couple was found to be about 11.8×10 − 13 m2
Fig. 6. Concentration profile of aluminium in the Fe–Zn intermediate phases of the Fe/Zn –0.1 wt.% Al diffusion couple after annealing at 400°C for 30 h.
15.0 [12]
s − 1, while in the Fe/Zn–Al alloy couples the average is almost a factor of two higher at 22.1× 10 − 13 m2 s − 1. Values of Dz previously reported in the literature [10,11] are also presented in Table 2 for comparison. Gelling et al. [10] studied the interdiffusion between electrodeposited zinc and iron at 400°C and found Dz to be 2.6 × 10 − 13 m2 s − 1, while Onishi et al. [11], in a study of the d/Zn pseudo-binary and Fe–Zn binary couples found Dz to be about 14.9 × 10 − 13 m2 s − 1. The present results for the Fe/Zn couple are seen to be in reasonable agreement with that of Onishi et al. In the case of the dp and dk layers formed in the present study, it is seen that the interdiffusion coefficients were very dependent on composition (i.e. structure). In general, the D values for dp and dk phases determined in the Fe/Zn couples in this study are about 50% lower than the Dd reported by Sequera et al. [12] in an Fe/Zn interdiffusion study at 400°C. These authors determined Dd using Kidson’s [13] multilayer growth theory in which the diffusion coefficients are assumed to be independent of composition. It is clear from the present results, however, that such an assumption is incorrect. The interdiffusion coefficients in the dk structure (Ddk) of the all couples were essentially independent of iron concentration. As shown in Table 2, the Ddk values increased by about a factor 1.2 with every 0.1 wt.% Al addition to the zinc. However, the most significant influence of aluminium on interdiffusion was found in the dp structure. In this structure, the Ddp value of about 44.9× 10 − 13 m2 s − 1 determined in the Fe/Zn– 0.2 wt.% Al couple is about a factor of 2.7 and 8.8 greater than the Ddp values found in the Fe/Zn–0.1 wt.% Al and Fe/Zn couples, respectively. Moreover, it was found that aluminium enrichment of up to 2.5 wt.% occurred at the G/dp and z/h-zinc interfaces of the Fe/Zn–Al alloy couples. Fig. 6 shows an example of this aluminium enrichment for the case of the Fe/Zn– 0.1 wt.% Al couple annealed for 30 h. The extent of the
P.R. Munroe et al. / Materials Science and Engineering A251 (1998) 87–93
aluminium enrichment was greatest in the Fe/Zn–0.2 wt.% Al couple. The processes which result in this form of enrichment are at present, not fully understood. Notwithstanding, however, it is very possible that aluminium enrichment at the dp/z and z/h-zinc interfaces was partly responsible for the higher interdiffusion coefficients in the dp and z layers of the Fe/Zn–Al alloy couples as compared to the Fe/Zn couple. For instance, if the thermodynamic interaction parameter between iron and aluminium is negative, the aluminium enrichment shown in Fig. 6 would have the effect of increasing the iron activity gradients across the dp and z layers and, hence, increasing the apparent interdiffusion coefficients measured assuming a binary system. Evidence that aluminium has a strong thermodynamic interaction in the Fe–Zn system is the fact that an apparently stable two-phase zone of dp and z developed in the Fe/ Zn–Al couples. Such a zone could only be stable during diffusion-controlled growth if the trace aluminium acted as a component in the system, thus giving an additional thermodynamic degree of freedom under isothermal and isobaric conditions (i.e. F = number components− number phases=3− 2= 1). If strong thermodynamic interaction between iron and aluminium does exist, the flux of iron in a given layer must be expressed as JFe = − DFeFe
o¯CFe o¯C −DFeAl Al o¯x o¯x
(1)
where the first and second terms on the right hand side correspond to the diffusivity of iron in its own concentration gradient (#CFe/#x) and the aluminium concentration gradient (#CAl/#x), respectively. DFeFe is referred to as main-interdiffusion coefficient and is always positive, whereas DFeAl is the cross-coefficient and may be either positive or negative. Thus, on the basis of the concentration profiles shown in Fig. 5, −DFeFe (#CFe/#x) is positive. Further, since aluminium increases the thickening kinetics of the dp and z layers in the Fe/Zn – Al couples (Fig. 4), it can be inferred that the flux of iron is increased in the presence of aluminium and, hence, − DFeAl (#CAl/#x) is also positive. Given that #CAl/#x is positive (Fig. 6), this would mean that DFeAl is negative. A negative cross-coefficient corresponds to a negative interaction parameter [14], which is consistent with the above analysis. Determination of DFeAl values was not conducted in this study; however, such a determination would be very difficult experimentally due to the small dk, dp, and z phase fields in the Fe–Zn–Al system at 400°C [15].
.
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4. Conclusions The addition of 0.1 and 0.2 wt.% Al to zinc had the following effects on the Fe–Zn interdiffusion behaviour at 400°C and diffusion times of 1–30 h: 1. Aluminium delayed the formation of the dk structure at the G/dp interface; 2. Aluminium caused the dp/z interface to be more non-planar; and 3. Aluminium increased the extent of interdiffusion in the dp and z layers, possibly due to the enrichment of aluminium at the dp/z and z/h-zinc interfaces and a negative thermodynamic interaction between iron and aluminium.
Acknowledgements Sincere thanks are extended to the management of Pasminco Limited and Pasminco Research Centre, Australia, for their financial support of this investigation and for permission to publish this paper. The assistance of Dr J.B. See and Mr. Scott Cook, formerly of Pasminco Research Centre, is also gratefully acknowledged.
References [1] M. Sagiyama, J. Inagaki, M. Morita, NKK Tech. Rev. 63 (1991) 38. [2] H. Kawaguci, Y. Hirose, in: Proceedings of the TMS Symposium on the Physical Metallurgy of Zinc Coated Steels: Processing, Structure and Properties, TMS Annual Meeting, San Francisco, 1994, p. 31. [3] C.E. Jordan, A.R. Marder, Metal. Mater. Trans. A 25 (1994) 937. [4] A. Borhan-Tavakoli, Z. Metallkd. 75 (1984) 436. [5] P.J. Gelling, E. Willem de Bree, G. Gierman, Z. Metallkd. 70 (1979) 312. [6] M. Onishi, Y. Wakamatsu, H. Miura, Trans. Jpn. Inst. Metals 15 (1974) 331. [7] M.H. Hong, H. Saka, Scripta Materialia 36 (1997) 1423. [8] M.A. Ghoniem, K. Lohberg, Metallurgica 26 (1972) 1026. [9] R.J. Borg, G.J. Dienes, An Introduction to Solid State Diffusion, Academic Press, San Diego, California, 1988. [10] P.J. Gelling, E. Willem de Bree, G. Gierman, Z. Metallkd 70 (1979) 315. [11] M. Onishi, Y. Wakamatsu, J. Jpn. Inst. Metals 37 (1973) 1279 – 1283. [12] C.A.C. Sequeira, A.M.G. Pacheco, C.M.G.S. Nunes, Surf. Eng. 3 (1987) 247. [13] G.V. Kidson, J. Nucl. Mater. 3 (1961) 21. [14] C.C. Brown, J.S. Kirkardy, Trans. TMS-AIME 230 (1964) 222. [15] W. Koster, T. Godecke, in: Proceedings of the 9th International Conference. Hot-dip Galvanizing, 1971, p. 128.
Effects of 0.1 and 0.2 wt.% aluminium addition to zinc on the interdiffusion between zinc and iron at 400°C Syahbuddin a, P.R. Munroe a,*, C.S. Laksmi b, B. Gleeson c a
School of Materials Science and Engineering, The Uni6ersity of New South Wales, Sydney, NSW 2052, Australia b Pasminco Research Centre, Boolaroo, NSW 2284, Australia c Materials Science and Engineering Department, Iowa State Uni6ersity, Ames, IA 50011, USA Received 7 January 1998; received in revised form 23 March 1998
Abstract This paper reports the interdiffusion behaviour between iron and zinc in Fe/Zn, Fe/Zn – 0.1 wt.% Al, and Fe/Zn – 0.2 wt.% Al couples annealed at 400°C for times ranging from 1 to 30 h. Under steady-state growth conditions, the interdiffusion zone in the Fe/Zn and Fe/Zn–Al alloy couples consisted of intermetallic layers of the Fe – Zn phases G-Fe3Zn10, dk-FeZn7, dp-FeZn10, and z-FeZn13. The intermetallic layers grew at a more rapid rate in the Fe/Zn – Al alloy couples than in the Fe/Zn couple. The thickening kinetics of the total interdiffusion zone in all couples were parabolic for annealing times greater than 3 h. The compact d structure (dk) appeared at the G/dp interface of the Fe/Zn couples after about 7 h annealing; whereas, the same structure in either of the Fe/Zn–Al alloy couples was not observed until after about 13 h annealing. Enrichment of aluminium was detected at the dp/z and z/h-zinc interfaces in the Fe/Zn–Al couples. The extent of this enrichment was highest in the Fe/Zn – 0.2 wt.% Al couple. The aluminium enrichment is believed to have contributed to the increase in the thickening kinetics of the intermetallic layers in the Fe/Zn–Al alloy couples owing to a negative thermodynamic interaction between aluminium and zinc. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Interdiffusion; Multilayered growth; Aluminium effect; Fe – Zn intermetallic compounds
1. Introduction Galvannealing in the automotive industry has become an important process in the fabrication of ductile, zinc-rich coatings on steels. In the galvannealing process, the steel is first immersed in a zinc-alloy bath at about 450°C for up to 5 s, and then given a post-coating heat treatment at temperatures in the range from 400 to 600°C for 10 – 20 s. The heat treatment causes the zinc in the coating to interdiffuse with the substrate iron to form several Fe – Zn intermediate phases, such as G-Fe3Zn10, G1-Fe5Zn21, dk-FeZn7, dp-FeZn10, and z-FeZn13. Most studies on the galvannealing process have been concerned with the morphology of the Fe– Zn intermetallic layers [1 – 3]. Little attention has been given to the kinetics of the interdiffusion between the
* Corresponding author. Tel.: + 61 2 93854425; fax: + 61 2 93856400. 0921-5093/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0921-5093(98)00641-8
steel and the zinc coating during the annealing stage. Moreover, the zinc alloys commonly used for hot-dip galvanizing typically contain of a small concentration (0.1–0.2 wt.%) of aluminium, which is added to inhibit intermetallic formation. The trace amount of aluminium has a significant effect on the interdiffusion behaviour between iron and zinc, but the effects of these aluminium additions on the growth on Fe–Zn intermetallics are poorly understood. In particular, there has been a dearth of systematic diffusion studies on the Fe/Zn–Al alloy system. The purpose of this study was to characterise the solid-state interdiffusion between pure iron and pure zinc and zinc alloys containing 0.1 and 0.2 wt.% Al. The interdiffusion temperature was 400°C and annealing times varied from 1 to 30 h. This work is part of a larger study on the ‘aluminium effect’ in Fe-Zn alloys. The present paper reports the results for solid/solid interdiffusion. As will be reported in subsequent papers, the ‘aluminium effect’ occurs in both solid/solid and solid/liquid couples, but solid/solid diffusion couples
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P.R. Munroe et al. / Materials Science and Engineering A251 (1998) 87–93
Fig. 1. Optical micrographs comparing the interdiffusion zones formed in the Fe/Zn and Fe/Zn – Al alloy couples after annealing at 400°C for 7 and 30 h.
offer the advantage of being easier to study. Long-term interdiffusion was studied in order to obtain steadystate behaviour and, hence, allow for the determination of the interdiffusion coefficients via the Boltzmann– Matano technique; short-term interdiffusion behaviour is non-ideal. The chemical interactions inferred from the results will be useful in interpreting liquid/solid interdiffusion, relevant to a galvannealing process.
2. Experimental procedures The materials used for the diffusion-couple experiments were pure iron (99.98%) obtained from Aldrich, Australia, and pure zinc and zinc alloys, containing either 0.1 or 0.2 wt.% Al, obtained from Pasminco Research Centre, Boolaroo, Australia. The different materials were first cut into 2× 7 ×7 mm coupons and then polished down to a 1 mm finish. A diffusion couple was made by clamping an iron coupon to a coupon of pure zinc, Zn–0.1 wt.% Al or Zn – 0.2 wt.% Al. The diffusion anneals were carried out in a horizontal, resistance-tube furnace under flowing argon gas at 400°C for times ranging from 1 to 30 h. At the end of a given diffusion anneal, the diffusion couple was quenched into water in order to preserve its high-tem-
perature microstructure. The diffusion couple was then mounted in an epoxy resin and cross-sectionally polished through to a 1 mm diamond finish. The polished cross-sections were etched in a solution of 0.1% nital and 0.1% picral in order to reveal the microstructure of the Fe–Zn intermetallic layers. The microstructure, thickness and composition profiles in the intermetallic layers formed in the interdiffusion zone were examined using optical microscopy, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). All EPMA measurements were conducted on unetched cross-sections. The standards used for the EPMA were pure Al, Zn, and Fe.
3. Results and discussion
3.1. Interdiffusion zone microstructure Isothermal annealing at 400°C for 1–30 h of the Fe/Zn and Fe/Zn–Al alloy couples caused the iron to interdiffuse with the zinc and resulted in the formation of Fe–Zn intermediate phases. It was found that iron was the principal diffusing species, with the Fe–Zn intermediate phases growing into the zinc or the Zn–Al alloys. In general, for a short annealing of 1 h, the
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Fig. 2. The d/z interface in the Fe/Zn – 0.2 wt.% Al couples after annealing at 400°C for 15 h.
interdiffusion zone consisted of a very thin layer (: 3 mm) of G phase in contact with the iron, an intermediate layer ( : 25 mm) of dp phase, and a thick outer layer ( :22 mm) of z phase in contact with the zinc or Zn–Al alloys. The dp layer, which exhibits a palisade structure, contained a large number of cracks running perpendicular to the original couple interface. Such crack formation is a general characteristic of the dp structure formed during hot-dip galvanizing [4]. A comparison between the different couples showed that the dp/z and z/h interfaces were considerably more planar in the Fe/Zn couples than in the Fe/Zn – Al alloy couples. As shown in Fig. 1, the G, dp and z layers continuously composed the interdiffusion zone of the Fe/Zn– Al alloy couples even after 7 h annealing. In the case of the Fe/Zn couple, however, a compact d (dk) layer developed between the G and dp layers at some time
Fig. 3. Interdiffusion zone thickening kinetics in the Fe/Zn and Fe/Zn – Al alloy couples after annealing at 400°C for 1–30 h.
between 5 and 7 h. The dk phase is distinguished from the dp phase because these two forms of d differ in composition, morphology, and atomic arrangement (i.e. degree of order); although they both have an hexagonal close packed crystal structure [5]. The delayed formation of the dk layer observed in this study was also found by Onishi et al. [6] in a diffusion study of Fe/electrodeposited Zn couples at 240–410°C. These authors found the dk structure to form at the G/dp interface after about 7 h annealing at 380°C. Formation of the dk layer in the Fe/Zn–Al couples did not occur until after about 13 h annealing, indicating that the presence of aluminium in the zinc inhibits the formation of dk structure at the G/dp interface. As reported by Hong et al. [7], the dk structure has a superlattice which is three times larger than that of the dp structure. It is very likely that the transformation from the dp to dk structure was slower in the Fe/Zn–Al couples on account of the aluminium possibly decreasing the stability of the more ordered dk, thereby decreasing the driving force for dp dk transformation. Further annealing for 30 h caused the dp/z interface to become more planar in the Fe/Zn couples, but non-planar in the Fe/Zn–Al alloy couples. As shown in Fig. 2, the dp/z ‘interface’ in the Fe/Zn–0.2 wt.% Al couple consists of an intermixed zone of dp and z. The variation in the dp/z interface morphologies in the different diffusion couples is ostensibly a consequence of the aluminium providing an extra degree of thermodynamic freedom, thus allowing for the stable growth of a two-phase interdiffusion zone. Moreover, the G:dk + dp:z thickness proportions were not the same for the two Fe/Zn–Al alloy diffusion couples, with the proportion being about 2:71:27 for the Fe/Zn–0.1 wt.% Al couple and 2:79:19 for the Fe/Zn–0.2 wt.% Al couple. This result indicates the notable influence that a trace amount of aluminium has on the transport rates in the different Fe–Zn intermediate phases.
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Fig. 4. Fe – Zn intermetallic thickening in (a) the Fe/Zn, (b) Fe/Zn – 0.1 wt.% Al, and (c) Fe/Zn – 0.2 wt.% Al alloy couples during annealing at 400°C for 1 – 30 h.
P.R. Munroe et al. / Materials Science and Engineering A251 (1998) 87–93 Table 1 Steady-state (t\3 h) parabolic rate constants for the thickening kinetics of the dp+dk and z layers and the total interdifusion zone Couple
Fe/Zn Fe/Zn – 0.1 wt% Al Fe/Zn – 0.2 wt% Al
kp (mm2 s−1) Total Interdiff. zone dp+dk Layer
z Layer
0.61 0.86
0.22 0.42
0.08 0.06
0.93
0.58
0.03
3.2. Growth kinetics of the intermetallic layers The thickening kinetics of the interdiffusion zones in the three couples are shown in Fig. 3. The interdiffusion zone in the Fe/Zn couple thickened according to the parabolic rate law, thus indicating diffusion-controlled growth kinetics. By contrast, the thickening kinetics of the interdiffusion zones in the Fe/Zn–Al couples were linear for the first 1 – 3 h annealing and then parabolic for 3 – 30 h annealing. Moreover, it is seen in Fig. 3 that the thickening kinetics of the interdiffusion zones in the Fe/Zn – Al couples increased significantly, and were higher than the kinetics found in the Fe/Zn couple. Fig. 4 shows the thickening behaviour of the various Fe–Zn intermetallic layers in the interdiffusion zones of the three different couples as a function of annealing time. It is seen that after 1 h annealing, the proportion of z layer formed was large in each of the couples, ranging from 53% in the Fe/Zn couple down to 37– 42% in the Fe/Zn – Al couples. With continued annealing, however, the proportion of z decreased owing to the predominance of the dp +dk layers. In the case of the Fe/Zn couple, dp was the thickest layer for annealing times of 3–30 h, beyond which dk was the thickest layer. dp remained the thickest layer in the Fe/Zn–0.2
Fig. 5. The concentration profiles of iron in the interdiffusion zone of the Fe – Zn couple after annealing at 400°C for 1, 7, 13 and 30 h.
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wt.% Al couple for annealing times of 3–30 h. Similar behaviour was also found in the Fe/Zn–0.1 wt.% Al couple except after 30 h annealing, where dk was then the thickest layer. The proportion of z layer decreased with increasing aluminium addition in the zinc. Table 1 summarises the steady-state, thickening rate constants of the total interdiffusion zone, together with the dk + dp and z layers in the Fe/Zn and Fe/Zn–Al alloy diffusion couples. Steady-state was taken to be annealing times from 3 to 30 h, within which parabolic kinetics were observed. The interdiffusion zone thickening kinetics in each of the couples were dominated by the dk + dp layer during these annealing times. In contrast to the parabolic, steady-state behaviour, the thickening kinetics of the dk + dp and z layers were linear for about the first 1–3 h of interdiffusion. It is seen in Table 1 that the parabolic rate constants for the dk +dp layer increased with increasing aluminium content. By contrast, the parabolic rate constants for the z layers in the Fe/Zn–Al alloy couples were lower than that in the Fe/Zn couple. Thus, the increased thickening rate of the interdiffusion zone in the Fe/Zn–Al couples is attributable to an increase in the thickening rate of the dp + dk layer with increasing aluminium addition.
3.3. Concentration profiles The effect of diffusion-annealing time on the iron concentration profile in the interdiffusion zone of the Fe/Zn couple is shown in Fig. 5. The different Fe–Zn intermediate phases are identified in this figure on the basis of composition. It is seen that after 1 h annealing the iron concentration at the apparent G/dp interface was approximately 9 wt.%, while the presence of the dk layer after 7, 13 and 30 h annealing resulted in an iron concentration at the G/dk interface of about 12 wt.%. The iron content in the dp layer at the dp/z interface was about 7.5 wt.%, irrespective of the annealing time, whereas the dk layer at the dk/dp interface contained about 10 wt.% Fe after annealing times greater than 1 h. These interfacial compositions are in general agreement with the Fe–Zn equilibrium phase diagram in which dp and dk exist in distinct phase fields [8]. In this phase diagram, the iron concentration ranges from about 7 to 8.5 wt.% in dp at 400°C and from about 10 to 12 wt.% in dk. The observed discontinuity in the iron concentration profile at the dk/dp interface is also in agreement with the phase diagram, which predicts the iron concentration difference between the dk and dp phases to be about 1.5 wt.%. It is therefore concluded that the dp and dk are indeed distinct phases and that the dp and dk layers were at local equilibrium for the longer annealing times of 7–30 h. The situation was different after 1 h annealing, however, where the dp layer was apparently at local equilibrium only at the dp/z interface, and not at the G/dp interface. There was
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Table 2 Interdiffusion coefficients in the z and d layers formed in the Fe/Zn, Fe/Zn – 0.1 wt% Al and Fe/Zn – 0.2 wt% Al couples annealed at 400°C for 30 h Phase
Fe (wt.)
D×10−13 (m2 s−1) Fe/Zn
Fe/Zn – 0.1 wt% Al
Fe/Zn – 0.2 wt% Al
Liter. 2.6 [10] 14.9 [11]
z
6.5 6.6
11.7 11.9
20.6 21.4
22.8 23.4
dp
7.7 7.9
— 5.2
16.6 16.8
44.5 45.2
dp/dk
8.2 10.0
0.7 0.8
0.9 0.9
1.5 1.6
dk
10.7 11.4
6.8 7.0
8.3 8.3
10.1 10.2
a comparatively greater proportion of the dp structure at the short annealing time, which may be attributed to a higher diffusivity in this structure due to either compositional or structural effects. Since the boundary compositions in the various layers were constant with the longer annealing times, thus indicating steady-state conditions, the interdiffusion coefficient could be determined using the Boltzman– Matano technique [9]. Table 2 summarises the interdiffusion coefficients measured in the dk, dp and z layers of the Fe/Zn and Fe/Zn – Al alloy couples as a function of composition for the annealing time of 30 h. The interdiffusion coefficients in the G layer could not be accurately determined due to the limited thickness of this layer. Further, and as a first approximation, the cross-term effects of aluminium in the Fe/Zn –Al couples were ignored (i.e. the systems were treated as binaries). It is seen in Table 2 that the interdiffusion coefficients in the z layer (Dz ) were essentially independent of the composition. The average value of Dz in the Fe/Zn couple was found to be about 11.8×10 − 13 m2
Fig. 6. Concentration profile of aluminium in the Fe–Zn intermediate phases of the Fe/Zn –0.1 wt.% Al diffusion couple after annealing at 400°C for 30 h.
15.0 [12]
s − 1, while in the Fe/Zn–Al alloy couples the average is almost a factor of two higher at 22.1× 10 − 13 m2 s − 1. Values of Dz previously reported in the literature [10,11] are also presented in Table 2 for comparison. Gelling et al. [10] studied the interdiffusion between electrodeposited zinc and iron at 400°C and found Dz to be 2.6 × 10 − 13 m2 s − 1, while Onishi et al. [11], in a study of the d/Zn pseudo-binary and Fe–Zn binary couples found Dz to be about 14.9 × 10 − 13 m2 s − 1. The present results for the Fe/Zn couple are seen to be in reasonable agreement with that of Onishi et al. In the case of the dp and dk layers formed in the present study, it is seen that the interdiffusion coefficients were very dependent on composition (i.e. structure). In general, the D values for dp and dk phases determined in the Fe/Zn couples in this study are about 50% lower than the Dd reported by Sequera et al. [12] in an Fe/Zn interdiffusion study at 400°C. These authors determined Dd using Kidson’s [13] multilayer growth theory in which the diffusion coefficients are assumed to be independent of composition. It is clear from the present results, however, that such an assumption is incorrect. The interdiffusion coefficients in the dk structure (Ddk) of the all couples were essentially independent of iron concentration. As shown in Table 2, the Ddk values increased by about a factor 1.2 with every 0.1 wt.% Al addition to the zinc. However, the most significant influence of aluminium on interdiffusion was found in the dp structure. In this structure, the Ddp value of about 44.9× 10 − 13 m2 s − 1 determined in the Fe/Zn– 0.2 wt.% Al couple is about a factor of 2.7 and 8.8 greater than the Ddp values found in the Fe/Zn–0.1 wt.% Al and Fe/Zn couples, respectively. Moreover, it was found that aluminium enrichment of up to 2.5 wt.% occurred at the G/dp and z/h-zinc interfaces of the Fe/Zn–Al alloy couples. Fig. 6 shows an example of this aluminium enrichment for the case of the Fe/Zn– 0.1 wt.% Al couple annealed for 30 h. The extent of the
P.R. Munroe et al. / Materials Science and Engineering A251 (1998) 87–93
aluminium enrichment was greatest in the Fe/Zn–0.2 wt.% Al couple. The processes which result in this form of enrichment are at present, not fully understood. Notwithstanding, however, it is very possible that aluminium enrichment at the dp/z and z/h-zinc interfaces was partly responsible for the higher interdiffusion coefficients in the dp and z layers of the Fe/Zn–Al alloy couples as compared to the Fe/Zn couple. For instance, if the thermodynamic interaction parameter between iron and aluminium is negative, the aluminium enrichment shown in Fig. 6 would have the effect of increasing the iron activity gradients across the dp and z layers and, hence, increasing the apparent interdiffusion coefficients measured assuming a binary system. Evidence that aluminium has a strong thermodynamic interaction in the Fe–Zn system is the fact that an apparently stable two-phase zone of dp and z developed in the Fe/ Zn–Al couples. Such a zone could only be stable during diffusion-controlled growth if the trace aluminium acted as a component in the system, thus giving an additional thermodynamic degree of freedom under isothermal and isobaric conditions (i.e. F = number components− number phases=3− 2= 1). If strong thermodynamic interaction between iron and aluminium does exist, the flux of iron in a given layer must be expressed as JFe = − DFeFe
o¯CFe o¯C −DFeAl Al o¯x o¯x
(1)
where the first and second terms on the right hand side correspond to the diffusivity of iron in its own concentration gradient (#CFe/#x) and the aluminium concentration gradient (#CAl/#x), respectively. DFeFe is referred to as main-interdiffusion coefficient and is always positive, whereas DFeAl is the cross-coefficient and may be either positive or negative. Thus, on the basis of the concentration profiles shown in Fig. 5, −DFeFe (#CFe/#x) is positive. Further, since aluminium increases the thickening kinetics of the dp and z layers in the Fe/Zn – Al couples (Fig. 4), it can be inferred that the flux of iron is increased in the presence of aluminium and, hence, − DFeAl (#CAl/#x) is also positive. Given that #CAl/#x is positive (Fig. 6), this would mean that DFeAl is negative. A negative cross-coefficient corresponds to a negative interaction parameter [14], which is consistent with the above analysis. Determination of DFeAl values was not conducted in this study; however, such a determination would be very difficult experimentally due to the small dk, dp, and z phase fields in the Fe–Zn–Al system at 400°C [15].
.
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4. Conclusions The addition of 0.1 and 0.2 wt.% Al to zinc had the following effects on the Fe–Zn interdiffusion behaviour at 400°C and diffusion times of 1–30 h: 1. Aluminium delayed the formation of the dk structure at the G/dp interface; 2. Aluminium caused the dp/z interface to be more non-planar; and 3. Aluminium increased the extent of interdiffusion in the dp and z layers, possibly due to the enrichment of aluminium at the dp/z and z/h-zinc interfaces and a negative thermodynamic interaction between iron and aluminium.
Acknowledgements Sincere thanks are extended to the management of Pasminco Limited and Pasminco Research Centre, Australia, for their financial support of this investigation and for permission to publish this paper. The assistance of Dr J.B. See and Mr. Scott Cook, formerly of Pasminco Research Centre, is also gratefully acknowledged.
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