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Precipitation of metastable ϵ-phase in a hypereutectic zinc-aluminium alloy containing copper
Acta metall, mater. Vol. 39, No. 10, pp. 2235-2242, 1991 Printed in Great Britain. All rights reserved
0956-7151/91 $3.00+ 0.00 Copyright © 1991PergamonPress pie
PRECIPITATION OF METASTABLE e-PHASE IN A HYPEREUTECTIC Z I N C - A L U M I N I U M ALLOY C O N T A I N I N G COPPER M. D U R M A N and S. M U R P H Y Aston University, The Aston Triangle, Birmingham 134 7ET, England (Received 18 March 1991) Almtract--Copper additions are known to increase the strength of zinc-aluminium based alloys, but the mechanism responsible has not been established. The structure of pressure-diecast alloy ZA8 containing copper has been examined by transmission electron microscopy and direct evidence for the formation of a transitional copper-rich phase has been obtained. During both the eutectic solidification and the solid-state transformation of the high-temperature fl phase, copper had become concentrated in the zinc-rich product (q), and on cooling was rejected from solid solution as a dense precipitation of particles 80 nm in diameter and 2-3 nm in thickness. The precipitate was identified by electron diffraction as the metastable cph E-phase (CuZn4), and its orientation relationship with the matrix and habit plane were determined. The asymmetrical orientation relationship adopted was shown to reduce elastic strain between the two cph structures of different c/a ratios. The dense precipitation of ~-phase was unaffected by ageing for five years at ambient temperatures, and is thought to be responsible for the high strength of zinc-based alloys containing copper. Rtsma6--On salt que des additions de cuivre augmentent la r6sistance m6canique des alliages ;i base de zinc-aluminium, mais le m6canisme responsable de ce ph6nom6ne n'est pas connu. O n examine la structure d'un alliage ZA8 moul6 en coquine sous pression contenant du cuivre, par microscopic 61ectronique en transmission et on met en 6vidence directement la formation d'une phase de transition fiche en cuivre. Pendant la solidification eutectique et la transformation ~i r6tat solide de la phase fl de hante temp6rature, le cnivre se concentre dans le produit r/riche en Zn, et pendant le refroidissement, il est rejet6 hers de la solution solide sous la forme d'une pr6cipitation dense de particules de 80 nm de diam6tre et de 2-3 nm d'6palsseur. Le pr6cipit6, identifi6 par diffraction 61ectronique, est la phase m6tastable ~ hc (CuZn() et on d6termine sa relation d'orientation avec la matrice et son plan d'accolement. On montre que la relation d'orientation asym6trique adopt6e nktuit la dfformation 61astique entre les deux structures hc ayant des rapports c/a diff6rents. La pr6cipitation dense de la phase e n'est pas affect6e par un vieillissement de cinq ans ~i la temp6rature ambiante, et on pense qu'elle est responsable de la forte r6sistance m6canique des alliages ~ base de zinc contenant du cuivre. Zmammmfamug--Kupferzugaben erh6hcn die Festigkeit von Legicrungen auf Zink-Aluminium-Basis; der zugeh6rige Mechanismus ist jedoch noch nicht bekannt. Die Struktur der Legierung ZA8 mit Kupferzugabe nach DruckguB wird im Eiektronenmikroskop untersucht; Hinweise ergeben sich, nach denen sich fibergangsweise eine kupferreiche Phase bildet. Wghrend der entektischen Erstarrung trod der Festk6rperumwandlung der Hochtemperaturphase fl konzentriert sich Kupfer in dem zinkreichen Produkt r/, W~hrend des Abkfihlens wird das Kupfer yon dem Mischkristall abgewiesen und bildet eine dichte Population yon Teilchen mit Durchmesser yon 80 nm und der Dicke yon 2-3 rim. Nach der elektronenmikroskopischen Analyse bilden diese Ausscheidungen der metastabile hex. ~-Phase (CuZn(); deren Orientierungsbeziehungund Habitebene werden bestimmt. Es wird gezeigt, dal3 die auftretende asymmetrische Orientierungsbeziehung die elastische Verzerrung zwischen den beiden hex. Strukturen mit unterschiedlichem c/a-Verh~ltnis verringert. Die dichte Ausscheidungspopulation der ~-Phase wird dutch Auslagern bei Raumtemperatur iiber ffinf Jahre nicht beeinfluBt; sie wird als Ursache ffir die hohe Festigkeit der Legierungen auf Zink-Basis rnit Kupfergehalt angesehen.
1. INTRODUCTION Copper has long been known to increase the strength [1-3] and creep-resistance [4-7] of zinc-based alloys, but at the expense of reduced elongation [2, 3] and in alloys with over 1% Cu, an increased susceptibility to dimensional instability on ageing [8-10]. The commercial alloys are based on the AI/Cu/Zn system [11-14], and Fig. I is a vertical section through the ternary phase diagram at a constant 5% Cu [14]
which indicates that at near-ambient temperatures the stable phases in zinc-rich alloys are ,, (f.c.c. aluminium-rich solid solution with about 10% Zn), ~/ (cph zinc-rich solid-solution with low aluminium and copper contents) and T ' (rhombohedral intermetallic phase with 58% AI, 30% Cu and 12% Zn) [14, 15]. However at temperatures above about 280°C the stable, copper-rich phase in most commercial alloys is the cph epilson phase CuZn( containing approximat©ly 16% Cu and 1% AI.
2235
2236
DURMAN and MURPHY: ~-PHASE IN Zn-A1 380 - -
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Zn (wt%) Fig. I. Vertical section through the AI-Cu-Zn ternary phase diagram at 5% Cu, showing phases present under equilibrium conditions at low temperatures, from [14]. Despite the importance of copper as an alloying constituent in high-strength commercial alloys, the structural changes induced by copper have not been established, although electron-probe microanalysis of cast alloys using an ED analytical system showed that copper was concentrated in the interdendritic, zinc-rich regions, and in alloys such as ZA27, with 2.5% Cu, as-cast alloys contained large, interdendritic E-phase particles [16, 17], which were gradually replaced by the stable, copper-rich T' phase on long-term ageing [17]. X-ray diffraction and electron microscopy work on experimental dilute Zn/Cu alloys showed that small plate-like zones were formed in the zinc matrix on quench-ageing, but electron diffraction patterns failed to show the presence of a second phase [18]. Other X-ray work on Zn/AI/Cu alloys has shown that E-phase is formed in quenched alloys on ageing at ambient or moderately elevated temperatures [19, 20], but no microstructural work has been carried out to determine how or where this phase was formed. In this work an examination of the fine-scale metallurgical structure of a typical, commercial lowcopper, zinc-based alloy was carded out to establish the nature and location of the copper-rich phases present, and thus to determine the role of copper in improving the mechanical properties of such alloys. 2. E X P E R I M E N T A L
Zinc alloy diecastings made from commercial ZA8 alloy were supplied by Pasminco (Europe) Ltd. and were used for the major part of this study. These had
aged naturally for a period in excess of five years after casting and were thus typical of material in service. Analysis by atomic absorption spectroscopy gave the alloy composition: Zn-8.1% AI; 0.024% Mg;
1.06% Cu; <0.01% Fe.
The castings were typically thin-walled components approximately 250 mm long by 50 mm wide by 2.18 mm thick, with additional shallow ribs and bosses, and the metallography was carded out on samples cut from the thinner parts of the casting. Supplementary work was carded out on a similar alloy freshly cast, to examine the effects of long-term ageing. A general study of the metallurgical structure was made on conventionally ground and polished samples in the SEM using backscattered electron imaging to produce atomic number contrast. TEM work was carried out on thin foil samples prepared using a disc-jet method with a perchloric acid-ethanol based electrolyte held at - 2 0 ° C using a potential of 25 V. The thin foils were examined using Jeol and Philips transmission electron microscopes, most of the work being carded out in a Philips EM400T instrument equipped with STEM and a quantitative EDS analytical system, at 100 keV. Conventional TEM was used to produce high-resolution images and selected-area diffraction patterns (SADP). For chemical analysis by EDS, the STEM mode of operation was used to obtain a highly-focussed beam with a minimum diameter of 10 nm, but in order to obtain
DURMAN and MURPHY: E-PHASE IN Zn-A1
2237
overall average analyses of the small phases present, the beam was scanned over a short line raster set within the selected phase. The intensities of the characteristic X-ray peaks were obtained by counting for live times of between 200 and 500 s, using interpolated background subtraction. Corrections to the measured intensities were made for preferential absorption of X-rays in the 8/zm Be window of the detector, but strictly semi-quantitative anlayses were computed from the measured peak intensities using atomic number corrections only, since the chemically heterogeneous structure of the cast material produced large local variations in foil thickness which made full ZAF corrections impracticable. 3. RESULTS ZA8 is an hypereutectic alloy in the Zn/AI system, and solidifies to form primary fl dendrites in a lamellar eutectic matrix of fl and r/plates, where r/is the cph zinc-rich solid solution, and fl is a phase with fcc crystal structure containing up to 80% Zn. fl phase is unstable at temperatures below about 275°C, and on cooling after casting decomposes eutectoidally to form t/phase and ct, the f.c.c, aluminiumrich solid solution containing some zinc. Under the high cooling rates associated with pressurediecasting, extremely fine structures were developed which may only be resolved in the SEM or TEM. Figure 2 shows a general view of the microstructure of the casting using atomic number contrast in the SEM. The rounded areas in this micrograph are small, prior-fl primaries, about 4-5 #m in diameter, with very limited development of dendritic form, plus larger, more dendritic bodies. They had decomposed eutectoidally on cooling to form fine lamellar or particulate products. The eutectic matrix had solidified as a particulate or lamellar mixture of fl in ~/, but the fl had decomposed into ~t -t- r/on cooling, the newly-formed
Fig. 2. SEM micrograph in atomic number contrast, showing general structure of pressure-diecast ZA8, consisting of numerous small, prior-fl dendrites, and an interdendritic eutectic made up of an r/ matrix containing rows of ~t particles derived from the low-temperature transformation of the fl lamellae. AM39/l~-D
Fig. 3. Low magnification CTEM micrograph, showing transformed, rounded, former fl dendrites, and polycrystalline ~/matrix containing embedded 0tand fine precipitates. r/joining the r/matrix and the ~t remaining as strings of particles or narrow plates. Figure 3 is a bright-field (BF) conventional transmission electron micrograph of a similar area to that of Fig. 2 and shows the decomposition structures of the primary and eutectic fl phases at higher resolution, and a precipitated phase within the zinc (r/) matrix. Figure 4 is a BF CTEM micrograph of the eutectic region taken at higher magnification, and shows the r/phase, formed partly by eutectic solidification and partly by fl phase decomposition, occupied by large numbers of uniformly-distributed precipitate partides. At least two families of the precipitates were present in the r/ matrix, each with its own growth direction or habit plane. The similar BF micrograph of Fig. 5 was from a dendritic region in which the r/ had been formed entirely from decomposition of t, and showed similar precipitates. Other examinations confirmed that all the ~/ phase regions contained this second phase precipitate. The ct phase showed no similar precipitate, although the small rounded particles visible in some ct grains were identified as an f.c.c, metastable ctm phase. This work is reported elsewhere.
Fig. 4. BF micrograph of eutectic region showing dense, uniformly-distributed precipitate of E-phase in the ~/matrix.
2238
DURMAN and MURPHY: ~-PHASE IN Zn-A1
Fig. 5. BF micrograph of dendritic region showing that the r/from the solid-state transformation: fl = ct + ~/also conrained a dense precipitate of ~-phase. _
Chemical analyses of the ~/ matrix using EDS showed that the aluminium content of this phase was small, about 0.5%, but the copper content (including the fine precipitate), measured in different areas of both the eutectic and primary regions, gave a consistent mean value of 3.4 + 0.8% from all the analyses. Similar analyses of the ~t and ~tm phases gave mean copper contents of 2.0% and 1.9% respectively. Since the average copper content of the alloy as a whole was 1% these copper analyses were too high. This is most probably due to the development of a copper-rich surface film on the foil surfaces during electropolishing since both aluminium-copper and zinc--copper alloys are known to suffer from this problem, but the different analytical results suggest that copper was preferentially segregated into the r/ products of both the eutectic and eutectoid transformations on cooling. In view of the known low solubility of copper in ~/ at ambient temperatures [22], these results suggested that the fine precipitates in the ~/were a copper-rich phase which had formed on cooling. To identify this phase, a large number of CTEM selected area diffraction patterns (SADP) from the eutectic r/phase were examined for subsidiary reflections, but only (0110)~ and <1~10)~ zones showed minor reflections which could not be associated with epitaxial surface oxide. Figure 6(a) shows an example of an (0110), zone with two sets of streaked reflections from the precipitate. The indexing of this diffraction pattern [Fig. 6(b)] showed that all three zones were cph, but the c/a ratio of the subsidiary crystals differed from that of the matrix, and the {0001} planes were not parallel. The c/a ratio of the r/ matrix corresponded closely to that of zinc with 1% dissolved copper [23], and thus a calibration factor was obtained from the zinc reflections which allowed the interplanar spacings of the subsidiary phase to be determined. This phase was then identified as the cph E-phase (CuZn4), with a = 0.274 nm, c = 0.429 nm, and c/a = 1.567.
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The two sets of e-phase reflections indicated that two variants of the orientation relationship had been adopted by the precipitates. Figure 7(a) shows a BF micrograph of another area of eutectic matrix, and Figure 7(b) a portion of the corresponding SADP with the image of the diffraction aperture enclosing two precipitate {0002}, reflections which were used to produce the CDF image of Fig. 7(c). Both families of e-phase precipitates which had contributed to the SADP were illuminated in Fig. 7(c), and the size of precipitates was estimated as about 80 nm in diameter and 2-3 um thick. SADPs from (l~10)n zones, such as that shown in Fig. 8(a) and (b) also showed that two families of E-phase precipitates had contributed to the diffraction reflections, both with the same zone as the matrix approximately parallel to the beam, but having adopted different variants of the orientation relationship. The angular relationships between the E-phase reflections and the matrix reflections were used to determine the precise orientation relationship
DURMAN and MURPHY:
E-PHASE IN Zn-A1
2239
to the <2II3>, zone, i.e. they lie perpendicular to the (2II7), plane. 4. DISCUSSION
At about the eutectic temperature, r/can dissolve about 2.7% Cu, but this decreases to about 0.3% at near ambient temperatures [21]. The great density of E-phase precipitation from the r/matrix in the ZA8 alloy indicated that the r/ phase had been highly saturated with copper at the eutectic temperature, a conclusion supported by the approximate EDS analyses. On cooling, the excess had been rejected from solid solution, the large numbers of small precipitates indicating easy nucleation of the second phase but little subsequent growth. Since both matrix and precipitate had the same cph crystal structure and differed only in that E-phase in equilibrium with r/ contains about 16% Cu [14], and has a reduced c/a relative to zinc, nucleation of the E-phase particles would be expected to proceed very easily. This was also indicated by early work [18] showing that the substantial hardness changes which accompanied the formation of platelets in quenched-aged Zn-1% Cu alloys were essentially completed after 2.5 h at room
Fig. 7. (a) BF micrograph showing the t/matrix with E-phase precipitates. (b) Portion of the SADP from (a) showing the two {0002}~ reflections selected for CDF image. (c) CDF image from the {0002}~, two families of E-phase precipitates illuminated within the ~/matrix.
between these phases. Figure 9 is a stereographic projection of the poles of the two cph crystals. Here the (0001) projection of the E-phase was rotated 4.5 ° about the loT10] and superimposed on a standard (0001) projection of the r/ phase. The angular relationships between the E-phase reflections and those of the matrix were consistent with the orientation relationship indicated by Fig. 9 for all zones examined. Two sets of parallel planes were selected to define the orientation relationship: (0110)~ // (01 i0),
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2240
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temperature. Supplementary CTEM studies carried out on a new ZA8 diecasting produced 7 days before examination also demonstrated that a high density of E-phase particles similar to that in the long term aged samples was already present in the t/ phase shortly after the casting operation. The observed asymmetrical orientation relationship may be a result of coherency stresses developed during growth of the E-phase. To examine the possible effects of coherency, the interplanar spacings of the ~/matrix and ~-phase planes were calculated, and are listed with their differences in Table 1. The lattice parameters of ~/ with 1% Cu [22] were used in the calculation. Table 1 shows that the differences in interplanar spacings were small for planes which are set parallel by the observed orientation relationship, whereas that between the (0001) planes is quite large. If a symmetrical orientation were to be maintained
between the two phases, by allowing planes in the (0001) zones to remain parallel, these eventually would all have the same positive mismatch of 2.62% and planes close to the (0001) would have large negative mismatches. The observed oftenTable 1. Interplanar spacings of ~/and E phases t/(1% Cu) E-phase Difference (%)
(hkil) 0001 10].O 10]'1 10].2 10]'3 11'~0 11'~1 1112 11~3 1210 1211 1212
4.9020 2.3123 2.0913 1.6819 1.3344 1.3350 1.2880 1.1724 1.0338 0.8740 0.8604 0.8232
4.2940 2.3729 2.0769 1.5920 1.2256 1.3700 1.3052 1.1549 0.9897 0.8969 0.8779 0.8276
-- 12.40 +2.62 -0.69 -5.34 -8.15 +2.62 +1.33 -1.49 --4.26 +2.62 +2.03 +0.53
DURMAN and MURPHY: ~-PHASE IN Zn-AI tation relationship indicates that coherency was maintained between one only of the six possible sets of planes belonging to (2TI3) zones by an appropriate rotation of the E-phase lattice from the symmetrical orientation. The lattice mismatch between the planes in this zone varies from +2.62% to - 1 . 4 9 % , with most planes close to zero, which would allow the precipitates to remain semi-coherent in the common plane and thus able to grow easily in that plane due to the low mismatch. Thus the precipitates should become plate-like and their habit plane should be the common plane which is near (2iT7)~. Confirmation of this prediction was obtained by examining the direction of streaking in the SADPs. If the e-plate was thin in the direction normal to the (2TI7)~, the reciprocal lattice points would be extended in that direction, so that in the diffraction patterns from the e-phase, streaking of the reflections would be observed lying in a direction which is the projection of that direction on the plane of the diffraction pattern. The diffraction pattern of Fig. 6 is from a ( 0 i 1 0 ) zone, and the streaking direction is approximately parallel to the (2TT7)~ plane normal, hence the observed streaks are consistent with the suggested habit plane. It was also observed that streaked E-phase reflections in the diffraction pattern from the (1~10)~ zone of Fig. 8 were parallel to the (1012I), plane normal, which lies between the (2]'I7)~ and the zone axis (1~10)~, again supporting the conclusion that the (2ii7)~ was the habit plane of the e-phase. Copper is known to improve both strength and creep resistance of Z n - A I - C u alloys, but the identification of a dense dispersion of very small, metastable, copper-rich e-phase particles in the r/matrix, strongly indicates that the improvements are due to the effects of this precipitate on the strength of the t/ phase, which is the major phase in the zinc-rich commercial alloys. Despite the fact that e-phase is unstable, and must eventually be replaced by the equilibrium phase T', e-phase was present in large quantities in both freshly made and long-term aged castings, and no diffraction evidence was found for the equilibrium T' phase after a prolonged period of natural ageing. The T' phase contains 12% Zn, 58% Cu and 30% A1 [15], so since the e-phase was found to be dispersed in r/, which contains very little dissolved aluminium, conversion of e to T' must involve either migration of aluminium into the r/, or re-solution of e and diffusion of copper through the ~/and the s-phase. The similar crystal structures and small lattice misfits between e-phase and the zinc matrix, combined with the long diffusion paths are considered to be the reasons for the long-term metastability of the e-phase in ZA8. 5. CONCLUSIONS 1. Copper was preferentially concentrated in the zinc-rich matrix phase (r/) of pressure-diecast ZA8 alloy.
2241
2. On cooling, excess copper had been rejected from solid solution in the ~/in the form of a dense precipitation of small particles of the metastable E-phase about 80 nm in diameter and 2-3 nm thick, which was still present after ageing for five years after casting. 3. An asymmetrical orientation relationship between the cph E-phase precipitates and the cph ~/ matrix was identified, and is described by the following parallelisms: (0110)d/(0110), (~112)~//(~ 112), but since the c/a ratios of the two crystals differ substantially [0001], is 4.5 ° from [0001]n. 4. The zone containing the mutually parallel planes of both crystals was ( 2 I I 3 ) , perpendicular to the (2IT7), plane, which was identified as the habit plane of the e-phase platelets. At least two families of precipitates were present in each r/grain, adopting different variants of the same orientation relationship and thus different habit planes of the same form. 5. The fine dispersion of metastable e-phase particles in the r/matrix, together with their long-term persistence, is considered to be largely responsible for the known improvement in mechanical properties brought about by the addition of copper to zincbased alloys. Acknowledgements--This work was carried out in the
Department of Mechanical & Production Engineering at Aston University, and thanks are due to the Head of Department, Dr J. E. T. Penny for the provision of facilities. The majority of the TEM work was carried out using the instruments at the University of Birmingham whose help in the provision of high-resolution TEM facilities is gratefully acknowledged. The authors are also grateful for the financial support of the Republic of Turkey for one of the authors and the unstinting assistance and generous support of Pasminco (Europe) Ltd in furthering this work. REFERENCES
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DURMAN and MURPHY:
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E-PHASE IN Zn-A1
18. J. M. Schultz and H. T. Shore, Trans. metall. Soc. A.LM.E. 242, 1381 (1968). 19. R. Ciach, J. Krol and K. Wegrzyn-Tasior, Bull. Acad. Pol. Sci. 17, 371 (1969). 20. G. Wendrock, B. Major, H. Lfffler, K. Grabianowska and R. Ciach, Cryst. Res. Tech. 16, 837 (1981). 21. M. Hansen and K. Anderko, Constitution of Binary Alloys, 2nd edn. McGraw-Hill, New York (1958). 22. W. B. Pearson, A Handbook of the Lattice Spacings and Structures of Metals and Alloys, Vol. 2. Pergamon Press, Oxford (1967).
0956-7151/91 $3.00+ 0.00 Copyright © 1991PergamonPress pie
PRECIPITATION OF METASTABLE e-PHASE IN A HYPEREUTECTIC Z I N C - A L U M I N I U M ALLOY C O N T A I N I N G COPPER M. D U R M A N and S. M U R P H Y Aston University, The Aston Triangle, Birmingham 134 7ET, England (Received 18 March 1991) Almtract--Copper additions are known to increase the strength of zinc-aluminium based alloys, but the mechanism responsible has not been established. The structure of pressure-diecast alloy ZA8 containing copper has been examined by transmission electron microscopy and direct evidence for the formation of a transitional copper-rich phase has been obtained. During both the eutectic solidification and the solid-state transformation of the high-temperature fl phase, copper had become concentrated in the zinc-rich product (q), and on cooling was rejected from solid solution as a dense precipitation of particles 80 nm in diameter and 2-3 nm in thickness. The precipitate was identified by electron diffraction as the metastable cph E-phase (CuZn4), and its orientation relationship with the matrix and habit plane were determined. The asymmetrical orientation relationship adopted was shown to reduce elastic strain between the two cph structures of different c/a ratios. The dense precipitation of ~-phase was unaffected by ageing for five years at ambient temperatures, and is thought to be responsible for the high strength of zinc-based alloys containing copper. Rtsma6--On salt que des additions de cuivre augmentent la r6sistance m6canique des alliages ;i base de zinc-aluminium, mais le m6canisme responsable de ce ph6nom6ne n'est pas connu. O n examine la structure d'un alliage ZA8 moul6 en coquine sous pression contenant du cuivre, par microscopic 61ectronique en transmission et on met en 6vidence directement la formation d'une phase de transition fiche en cuivre. Pendant la solidification eutectique et la transformation ~i r6tat solide de la phase fl de hante temp6rature, le cnivre se concentre dans le produit r/riche en Zn, et pendant le refroidissement, il est rejet6 hers de la solution solide sous la forme d'une pr6cipitation dense de particules de 80 nm de diam6tre et de 2-3 nm d'6palsseur. Le pr6cipit6, identifi6 par diffraction 61ectronique, est la phase m6tastable ~ hc (CuZn() et on d6termine sa relation d'orientation avec la matrice et son plan d'accolement. On montre que la relation d'orientation asym6trique adopt6e nktuit la dfformation 61astique entre les deux structures hc ayant des rapports c/a diff6rents. La pr6cipitation dense de la phase e n'est pas affect6e par un vieillissement de cinq ans ~i la temp6rature ambiante, et on pense qu'elle est responsable de la forte r6sistance m6canique des alliages ~ base de zinc contenant du cuivre. Zmammmfamug--Kupferzugaben erh6hcn die Festigkeit von Legicrungen auf Zink-Aluminium-Basis; der zugeh6rige Mechanismus ist jedoch noch nicht bekannt. Die Struktur der Legierung ZA8 mit Kupferzugabe nach DruckguB wird im Eiektronenmikroskop untersucht; Hinweise ergeben sich, nach denen sich fibergangsweise eine kupferreiche Phase bildet. Wghrend der entektischen Erstarrung trod der Festk6rperumwandlung der Hochtemperaturphase fl konzentriert sich Kupfer in dem zinkreichen Produkt r/, W~hrend des Abkfihlens wird das Kupfer yon dem Mischkristall abgewiesen und bildet eine dichte Population yon Teilchen mit Durchmesser yon 80 nm und der Dicke yon 2-3 rim. Nach der elektronenmikroskopischen Analyse bilden diese Ausscheidungen der metastabile hex. ~-Phase (CuZn(); deren Orientierungsbeziehungund Habitebene werden bestimmt. Es wird gezeigt, dal3 die auftretende asymmetrische Orientierungsbeziehung die elastische Verzerrung zwischen den beiden hex. Strukturen mit unterschiedlichem c/a-Verh~ltnis verringert. Die dichte Ausscheidungspopulation der ~-Phase wird dutch Auslagern bei Raumtemperatur iiber ffinf Jahre nicht beeinfluBt; sie wird als Ursache ffir die hohe Festigkeit der Legierungen auf Zink-Basis rnit Kupfergehalt angesehen.
1. INTRODUCTION Copper has long been known to increase the strength [1-3] and creep-resistance [4-7] of zinc-based alloys, but at the expense of reduced elongation [2, 3] and in alloys with over 1% Cu, an increased susceptibility to dimensional instability on ageing [8-10]. The commercial alloys are based on the AI/Cu/Zn system [11-14], and Fig. I is a vertical section through the ternary phase diagram at a constant 5% Cu [14]
which indicates that at near-ambient temperatures the stable phases in zinc-rich alloys are ,, (f.c.c. aluminium-rich solid solution with about 10% Zn), ~/ (cph zinc-rich solid-solution with low aluminium and copper contents) and T ' (rhombohedral intermetallic phase with 58% AI, 30% Cu and 12% Zn) [14, 15]. However at temperatures above about 280°C the stable, copper-rich phase in most commercial alloys is the cph epilson phase CuZn( containing approximat©ly 16% Cu and 1% AI.
2235
2236
DURMAN and MURPHY: ~-PHASE IN Zn-A1 380 - -
340
_
/
ii
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p+el
I I I , p + T'+~
a
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260
220
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,If
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20
40
60
80
+ T'+
Zn (wt%) Fig. I. Vertical section through the AI-Cu-Zn ternary phase diagram at 5% Cu, showing phases present under equilibrium conditions at low temperatures, from [14]. Despite the importance of copper as an alloying constituent in high-strength commercial alloys, the structural changes induced by copper have not been established, although electron-probe microanalysis of cast alloys using an ED analytical system showed that copper was concentrated in the interdendritic, zinc-rich regions, and in alloys such as ZA27, with 2.5% Cu, as-cast alloys contained large, interdendritic E-phase particles [16, 17], which were gradually replaced by the stable, copper-rich T' phase on long-term ageing [17]. X-ray diffraction and electron microscopy work on experimental dilute Zn/Cu alloys showed that small plate-like zones were formed in the zinc matrix on quench-ageing, but electron diffraction patterns failed to show the presence of a second phase [18]. Other X-ray work on Zn/AI/Cu alloys has shown that E-phase is formed in quenched alloys on ageing at ambient or moderately elevated temperatures [19, 20], but no microstructural work has been carried out to determine how or where this phase was formed. In this work an examination of the fine-scale metallurgical structure of a typical, commercial lowcopper, zinc-based alloy was carded out to establish the nature and location of the copper-rich phases present, and thus to determine the role of copper in improving the mechanical properties of such alloys. 2. E X P E R I M E N T A L
Zinc alloy diecastings made from commercial ZA8 alloy were supplied by Pasminco (Europe) Ltd. and were used for the major part of this study. These had
aged naturally for a period in excess of five years after casting and were thus typical of material in service. Analysis by atomic absorption spectroscopy gave the alloy composition: Zn-8.1% AI; 0.024% Mg;
1.06% Cu; <0.01% Fe.
The castings were typically thin-walled components approximately 250 mm long by 50 mm wide by 2.18 mm thick, with additional shallow ribs and bosses, and the metallography was carded out on samples cut from the thinner parts of the casting. Supplementary work was carded out on a similar alloy freshly cast, to examine the effects of long-term ageing. A general study of the metallurgical structure was made on conventionally ground and polished samples in the SEM using backscattered electron imaging to produce atomic number contrast. TEM work was carried out on thin foil samples prepared using a disc-jet method with a perchloric acid-ethanol based electrolyte held at - 2 0 ° C using a potential of 25 V. The thin foils were examined using Jeol and Philips transmission electron microscopes, most of the work being carded out in a Philips EM400T instrument equipped with STEM and a quantitative EDS analytical system, at 100 keV. Conventional TEM was used to produce high-resolution images and selected-area diffraction patterns (SADP). For chemical analysis by EDS, the STEM mode of operation was used to obtain a highly-focussed beam with a minimum diameter of 10 nm, but in order to obtain
DURMAN and MURPHY: E-PHASE IN Zn-A1
2237
overall average analyses of the small phases present, the beam was scanned over a short line raster set within the selected phase. The intensities of the characteristic X-ray peaks were obtained by counting for live times of between 200 and 500 s, using interpolated background subtraction. Corrections to the measured intensities were made for preferential absorption of X-rays in the 8/zm Be window of the detector, but strictly semi-quantitative anlayses were computed from the measured peak intensities using atomic number corrections only, since the chemically heterogeneous structure of the cast material produced large local variations in foil thickness which made full ZAF corrections impracticable. 3. RESULTS ZA8 is an hypereutectic alloy in the Zn/AI system, and solidifies to form primary fl dendrites in a lamellar eutectic matrix of fl and r/plates, where r/is the cph zinc-rich solid solution, and fl is a phase with fcc crystal structure containing up to 80% Zn. fl phase is unstable at temperatures below about 275°C, and on cooling after casting decomposes eutectoidally to form t/phase and ct, the f.c.c, aluminiumrich solid solution containing some zinc. Under the high cooling rates associated with pressurediecasting, extremely fine structures were developed which may only be resolved in the SEM or TEM. Figure 2 shows a general view of the microstructure of the casting using atomic number contrast in the SEM. The rounded areas in this micrograph are small, prior-fl primaries, about 4-5 #m in diameter, with very limited development of dendritic form, plus larger, more dendritic bodies. They had decomposed eutectoidally on cooling to form fine lamellar or particulate products. The eutectic matrix had solidified as a particulate or lamellar mixture of fl in ~/, but the fl had decomposed into ~t -t- r/on cooling, the newly-formed
Fig. 2. SEM micrograph in atomic number contrast, showing general structure of pressure-diecast ZA8, consisting of numerous small, prior-fl dendrites, and an interdendritic eutectic made up of an r/ matrix containing rows of ~t particles derived from the low-temperature transformation of the fl lamellae. AM39/l~-D
Fig. 3. Low magnification CTEM micrograph, showing transformed, rounded, former fl dendrites, and polycrystalline ~/matrix containing embedded 0tand fine precipitates. r/joining the r/matrix and the ~t remaining as strings of particles or narrow plates. Figure 3 is a bright-field (BF) conventional transmission electron micrograph of a similar area to that of Fig. 2 and shows the decomposition structures of the primary and eutectic fl phases at higher resolution, and a precipitated phase within the zinc (r/) matrix. Figure 4 is a BF CTEM micrograph of the eutectic region taken at higher magnification, and shows the r/phase, formed partly by eutectic solidification and partly by fl phase decomposition, occupied by large numbers of uniformly-distributed precipitate partides. At least two families of the precipitates were present in the r/ matrix, each with its own growth direction or habit plane. The similar BF micrograph of Fig. 5 was from a dendritic region in which the r/ had been formed entirely from decomposition of t, and showed similar precipitates. Other examinations confirmed that all the ~/ phase regions contained this second phase precipitate. The ct phase showed no similar precipitate, although the small rounded particles visible in some ct grains were identified as an f.c.c, metastable ctm phase. This work is reported elsewhere.
Fig. 4. BF micrograph of eutectic region showing dense, uniformly-distributed precipitate of E-phase in the ~/matrix.
2238
DURMAN and MURPHY: ~-PHASE IN Zn-A1
Fig. 5. BF micrograph of dendritic region showing that the r/from the solid-state transformation: fl = ct + ~/also conrained a dense precipitate of ~-phase. _
Chemical analyses of the ~/ matrix using EDS showed that the aluminium content of this phase was small, about 0.5%, but the copper content (including the fine precipitate), measured in different areas of both the eutectic and primary regions, gave a consistent mean value of 3.4 + 0.8% from all the analyses. Similar analyses of the ~t and ~tm phases gave mean copper contents of 2.0% and 1.9% respectively. Since the average copper content of the alloy as a whole was 1% these copper analyses were too high. This is most probably due to the development of a copper-rich surface film on the foil surfaces during electropolishing since both aluminium-copper and zinc--copper alloys are known to suffer from this problem, but the different analytical results suggest that copper was preferentially segregated into the r/ products of both the eutectic and eutectoid transformations on cooling. In view of the known low solubility of copper in ~/ at ambient temperatures [22], these results suggested that the fine precipitates in the ~/were a copper-rich phase which had formed on cooling. To identify this phase, a large number of CTEM selected area diffraction patterns (SADP) from the eutectic r/phase were examined for subsidiary reflections, but only (0110)~ and <1~10)~ zones showed minor reflections which could not be associated with epitaxial surface oxide. Figure 6(a) shows an example of an (0110), zone with two sets of streaked reflections from the precipitate. The indexing of this diffraction pattern [Fig. 6(b)] showed that all three zones were cph, but the c/a ratio of the subsidiary crystals differed from that of the matrix, and the {0001} planes were not parallel. The c/a ratio of the r/ matrix corresponded closely to that of zinc with 1% dissolved copper [23], and thus a calibration factor was obtained from the zinc reflections which allowed the interplanar spacings of the subsidiary phase to be determined. This phase was then identified as the cph E-phase (CuZn4), with a = 0.274 nm, c = 0.429 nm, and c/a = 1.567.
,, \
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',\
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Fig. 6. (a) SADP from eutectic ~/ in ZA8, consisting of a (0110), zone and two sets of streaked precipitate reflections. (b) Indexing of (a), streaked diffraction patterns are from two ( 0 I I 0 ) , zones, only one set is labelled for clarity.
The two sets of e-phase reflections indicated that two variants of the orientation relationship had been adopted by the precipitates. Figure 7(a) shows a BF micrograph of another area of eutectic matrix, and Figure 7(b) a portion of the corresponding SADP with the image of the diffraction aperture enclosing two precipitate {0002}, reflections which were used to produce the CDF image of Fig. 7(c). Both families of e-phase precipitates which had contributed to the SADP were illuminated in Fig. 7(c), and the size of precipitates was estimated as about 80 nm in diameter and 2-3 um thick. SADPs from (l~10)n zones, such as that shown in Fig. 8(a) and (b) also showed that two families of E-phase precipitates had contributed to the diffraction reflections, both with the same zone as the matrix approximately parallel to the beam, but having adopted different variants of the orientation relationship. The angular relationships between the E-phase reflections and the matrix reflections were used to determine the precise orientation relationship
DURMAN and MURPHY:
E-PHASE IN Zn-A1
2239
to the <2II3>, zone, i.e. they lie perpendicular to the (2II7), plane. 4. DISCUSSION
At about the eutectic temperature, r/can dissolve about 2.7% Cu, but this decreases to about 0.3% at near ambient temperatures [21]. The great density of E-phase precipitation from the r/matrix in the ZA8 alloy indicated that the r/ phase had been highly saturated with copper at the eutectic temperature, a conclusion supported by the approximate EDS analyses. On cooling, the excess had been rejected from solid solution, the large numbers of small precipitates indicating easy nucleation of the second phase but little subsequent growth. Since both matrix and precipitate had the same cph crystal structure and differed only in that E-phase in equilibrium with r/ contains about 16% Cu [14], and has a reduced c/a relative to zinc, nucleation of the E-phase particles would be expected to proceed very easily. This was also indicated by early work [18] showing that the substantial hardness changes which accompanied the formation of platelets in quenched-aged Zn-1% Cu alloys were essentially completed after 2.5 h at room
Fig. 7. (a) BF micrograph showing the t/matrix with E-phase precipitates. (b) Portion of the SADP from (a) showing the two {0002}~ reflections selected for CDF image. (c) CDF image from the {0002}~, two families of E-phase precipitates illuminated within the ~/matrix.
between these phases. Figure 9 is a stereographic projection of the poles of the two cph crystals. Here the (0001) projection of the E-phase was rotated 4.5 ° about the loT10] and superimposed on a standard (0001) projection of the r/ phase. The angular relationships between the E-phase reflections and those of the matrix were consistent with the orientation relationship indicated by Fig. 9 for all zones examined. Two sets of parallel planes were selected to define the orientation relationship: (0110)~ // (01 i0),
(b)
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2240
D U R M A N and M U R P H Y :
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temperature. Supplementary CTEM studies carried out on a new ZA8 diecasting produced 7 days before examination also demonstrated that a high density of E-phase particles similar to that in the long term aged samples was already present in the t/ phase shortly after the casting operation. The observed asymmetrical orientation relationship may be a result of coherency stresses developed during growth of the E-phase. To examine the possible effects of coherency, the interplanar spacings of the ~/matrix and ~-phase planes were calculated, and are listed with their differences in Table 1. The lattice parameters of ~/ with 1% Cu [22] were used in the calculation. Table 1 shows that the differences in interplanar spacings were small for planes which are set parallel by the observed orientation relationship, whereas that between the (0001) planes is quite large. If a symmetrical orientation were to be maintained
between the two phases, by allowing planes in the (0001) zones to remain parallel, these eventually would all have the same positive mismatch of 2.62% and planes close to the (0001) would have large negative mismatches. The observed oftenTable 1. Interplanar spacings of ~/and E phases t/(1% Cu) E-phase Difference (%)
(hkil) 0001 10].O 10]'1 10].2 10]'3 11'~0 11'~1 1112 11~3 1210 1211 1212
4.9020 2.3123 2.0913 1.6819 1.3344 1.3350 1.2880 1.1724 1.0338 0.8740 0.8604 0.8232
4.2940 2.3729 2.0769 1.5920 1.2256 1.3700 1.3052 1.1549 0.9897 0.8969 0.8779 0.8276
-- 12.40 +2.62 -0.69 -5.34 -8.15 +2.62 +1.33 -1.49 --4.26 +2.62 +2.03 +0.53
DURMAN and MURPHY: ~-PHASE IN Zn-AI tation relationship indicates that coherency was maintained between one only of the six possible sets of planes belonging to (2TI3) zones by an appropriate rotation of the E-phase lattice from the symmetrical orientation. The lattice mismatch between the planes in this zone varies from +2.62% to - 1 . 4 9 % , with most planes close to zero, which would allow the precipitates to remain semi-coherent in the common plane and thus able to grow easily in that plane due to the low mismatch. Thus the precipitates should become plate-like and their habit plane should be the common plane which is near (2iT7)~. Confirmation of this prediction was obtained by examining the direction of streaking in the SADPs. If the e-plate was thin in the direction normal to the (2TI7)~, the reciprocal lattice points would be extended in that direction, so that in the diffraction patterns from the e-phase, streaking of the reflections would be observed lying in a direction which is the projection of that direction on the plane of the diffraction pattern. The diffraction pattern of Fig. 6 is from a ( 0 i 1 0 ) zone, and the streaking direction is approximately parallel to the (2TT7)~ plane normal, hence the observed streaks are consistent with the suggested habit plane. It was also observed that streaked E-phase reflections in the diffraction pattern from the (1~10)~ zone of Fig. 8 were parallel to the (1012I), plane normal, which lies between the (2]'I7)~ and the zone axis (1~10)~, again supporting the conclusion that the (2ii7)~ was the habit plane of the e-phase. Copper is known to improve both strength and creep resistance of Z n - A I - C u alloys, but the identification of a dense dispersion of very small, metastable, copper-rich e-phase particles in the r/matrix, strongly indicates that the improvements are due to the effects of this precipitate on the strength of the t/ phase, which is the major phase in the zinc-rich commercial alloys. Despite the fact that e-phase is unstable, and must eventually be replaced by the equilibrium phase T', e-phase was present in large quantities in both freshly made and long-term aged castings, and no diffraction evidence was found for the equilibrium T' phase after a prolonged period of natural ageing. The T' phase contains 12% Zn, 58% Cu and 30% A1 [15], so since the e-phase was found to be dispersed in r/, which contains very little dissolved aluminium, conversion of e to T' must involve either migration of aluminium into the r/, or re-solution of e and diffusion of copper through the ~/and the s-phase. The similar crystal structures and small lattice misfits between e-phase and the zinc matrix, combined with the long diffusion paths are considered to be the reasons for the long-term metastability of the e-phase in ZA8. 5. CONCLUSIONS 1. Copper was preferentially concentrated in the zinc-rich matrix phase (r/) of pressure-diecast ZA8 alloy.
2241
2. On cooling, excess copper had been rejected from solid solution in the ~/in the form of a dense precipitation of small particles of the metastable E-phase about 80 nm in diameter and 2-3 nm thick, which was still present after ageing for five years after casting. 3. An asymmetrical orientation relationship between the cph E-phase precipitates and the cph ~/ matrix was identified, and is described by the following parallelisms: (0110)d/(0110), (~112)~//(~ 112), but since the c/a ratios of the two crystals differ substantially [0001], is 4.5 ° from [0001]n. 4. The zone containing the mutually parallel planes of both crystals was ( 2 I I 3 ) , perpendicular to the (2IT7), plane, which was identified as the habit plane of the e-phase platelets. At least two families of precipitates were present in each r/grain, adopting different variants of the same orientation relationship and thus different habit planes of the same form. 5. The fine dispersion of metastable e-phase particles in the r/matrix, together with their long-term persistence, is considered to be largely responsible for the known improvement in mechanical properties brought about by the addition of copper to zincbased alloys. Acknowledgements--This work was carried out in the
Department of Mechanical & Production Engineering at Aston University, and thanks are due to the Head of Department, Dr J. E. T. Penny for the provision of facilities. The majority of the TEM work was carried out using the instruments at the University of Birmingham whose help in the provision of high-resolution TEM facilities is gratefully acknowledged. The authors are also grateful for the financial support of the Republic of Turkey for one of the authors and the unstinting assistance and generous support of Pasminco (Europe) Ltd in furthering this work. REFERENCES
1. M. E. Houghton and M. T. Murray, Metals Forum 6, 211 (1984). 2. E. Oervais, H. Levert and M. Bess, Trans. Am. Foundr. Soc., 88, 183 (1980). 3. C. A. Loong, Proc. 25th CIM Conf., p. 157, Toronto, Canada 0986). 4. H. Naziri and R. Pearce, Inst. J. Mech. Sci. 12, 513 (1970). 5. K. Nuttall, J. Inst. Metals 101, 329 (1973). 6. L. E. Mulvania, R. D. Weltzin and P. A. Talcott, Metals Engng Q. 14, 55 (1974). 7. T. Savaskan and S. Murphy, Z. Metallic. 74, 76 (1983). 8. S. Murphy, N. Mykura and Y. H. Zhu, Proc. 24th CIM Conf., Paper 7.1, Vancouver, Canada (1985). 9. N. Mykura, S. Murphy and Y. H. Zhu, Mater. Res. Soc. Proc. 21, 841 (1984). 10. N. Mykura, Y. K. Zhu and S. Murphy, Can. Metall. Q. 25, (2) 151 (1986).
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DURMAN and MURPHY:
11. W. Koster and K. Moeller, Z. Metallk. 33, 278 (1941). 12. E. Gebhardt, Z. Metallk. 34, 208 (1942). 13. H. H. Amdt and K. Moeller, Z. Metallic. 51, 596 (1960). 14. S. Murphy, Z. Metallk. 71, 96 (1980). 15. S. Murphy, Metals Sci. 9, 163 (1975). 16. R. J. Barnhurst and J. C. Farge, Proc. 25th CIM Conf., p. 85, Toronto, Canada (1986). 17. Y. H. Zhu, B. Yah and W. Huang, Proc. 25th CIM Conf., p. 23, Toronto, Canada (1986).
E-PHASE IN Zn-A1
18. J. M. Schultz and H. T. Shore, Trans. metall. Soc. A.LM.E. 242, 1381 (1968). 19. R. Ciach, J. Krol and K. Wegrzyn-Tasior, Bull. Acad. Pol. Sci. 17, 371 (1969). 20. G. Wendrock, B. Major, H. Lfffler, K. Grabianowska and R. Ciach, Cryst. Res. Tech. 16, 837 (1981). 21. M. Hansen and K. Anderko, Constitution of Binary Alloys, 2nd edn. McGraw-Hill, New York (1958). 22. W. B. Pearson, A Handbook of the Lattice Spacings and Structures of Metals and Alloys, Vol. 2. Pergamon Press, Oxford (1967).