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Vapour quenching of copper-zirconium alloys
Materials Science and Engineering, 30 (1977) 219 - 222
219
© Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands
Vapour Quenching of Copper-Zirconium Alloys
MICHAEL SCOTT
University of Sussex, School of Engineering and Applied Sciences, Brighton, Sussex BN1 9QT (Gt. Britain) (Received May 11, 1977)
SUMMARY
Co-sputtering of copper-zirconium alloys containing approximately 40 at.% zirconium produces an amorphous phase. Thermal evaporation of the same alloys, however, gives a structure idential with the 'transformed amorphous' phase previously reported to form during the decomposition of a splat-quenched Cu6oZr4o glass.
c o m p o n e n t atoms have large size differences [1]. Consequently, Cu-Zr was chosen as a suitable system for a comparative study. It is well established that a glassy phase can be formed b y splat-quenching alloys within about 10 at.% of the composition Cu6oZr4o; the structure and crystallisation behaviour of these alloys have been examined in detail [9, 10]. In this paper the preliminary results of attempts to prepare similar phases b y vapour quenching are described.
INTRODUCTION EXPERIMENTAL
Amorphous metallic phases have been produced by b o t h vapour to solid [1] and liquid to solid quenching techniques [2]. The two processes, however, are intrinsically different since, in an undercooled liquid metal, the crystallisation rate (and therefore the cooling rate necessary to form a glass) is controlled by bulk diffusion, whereas during the atom b y atom growth of a vapour-deposited film the time required for arrangement to a crystalline array is governed b y surface diffusion [3]. Comparison of the products of the two methods is limited b y lack of suitable experimental data [4] and a programme was therefore set up to fill this gap. Two types of alloy are likely to form a non-crystalline phase on rapid solidification of the liquid (splat-quenching); those containing about 80 at.% noble or transition metal with the balance metalloid (e.g., Pds0Siuo [5], FeaoPlsC7 [6] ) and those based on early-late transition metal combinations such as Ni-Nb [7] and Cu-Zr [8]. This latter group of alloys is particularly interesting since it also satisfies the t w o criteria most c o m m o n l y required for the formation of an amorphous phase b y vapour deposition -- the glass-forming range is near the equiatomic composition and the
Two vapour quenching techniques were used: thermal co-evaporation and r.f. sputtering. In the former case, copper and zirconium were evaporated at <10 6 Torr from independent sources; the copper from a resistanceheated m o l y b d e n u m boat and the zirconium b y electron-beam heating. Quartz crystal monitors were used to measure and control the evaporation rates of the two species and, hence, the composition of the foils. Deposition rates of up to 1 nm s ~1 were achieved onto synthetic sapphires coated with either collodion or sodium metaphosphate and cooled to 90 K. The films were removed for electron microscopy by floating them from the sapphires in either amylacetate or distilled water. The sputtering apparatus has been described elsewhere [3]. High purity (99.99%) argon, purified b y a getter in the sputtering chamber, was used as a sputtering gas and sectored sheets of 99.9% Cu and Zr were used as the target materials. Typical sputtering conditions were: argon pressure 10 -2 Tort, power ~0.5 kW, axial magnetic field ~20 Oe, giving sputtering rates of 0.1 - 0.2 n m s -1. Freshly cleaved crystals of NaC1 a b o u t 1 mm thick were used as substrates and were
220
bonded with silver dag to a copper holder which was, in turn, cooled b y liquid nitrogen. The holder was rotated at about 60 rpm during sputtering to ensure homogeneity of the deposited films. The films were removed b y dissolving the NaC1 substrates in water. In b o t h cases, films between 40 and 80 nm were found to be suitable for transmission electron microscopy at 100 kV.
RESULTS
1 pm
1 pm
Fig. 1. (a) Microstructure of thermally evaporated Cu60Zr4o film. (b) Specimen showing growth of large crystals. (c) Diffraction pattern of (a) showing the 'transformed amorphous' structure.
Thermally evaporated films were prepared between 40 and 70 at.% Zr. In all cases bright field electron microscopy revealed a featureless matrix {Fig. l(a)) which, at higher magnifications, appeared slightly mottled. In a few cases the matrix contained up to about 10% b y volume of precipitates whose morphology varied from small spheres to well defined angular crystals (Fig. l(b)). On first inspection the matrix was thought to be amorphous but the associated diffraction pattern (Fig. l(c)) revealed that this was not the case. The pattern contained a broad inner halo equivalent to a lattice spacing of between 2.7 and 2.8 h together with several sharper outer rings. The intensities of these outer rings varied between specimens from cases where only the first was visible to those where up to five could be detected. In specimens where large crystals were present single spots were superimposed on these rings. More success in producing an amorphous structure was achieved with the co-sputtering technique, despite the fact that sputtered atoms have a higher energy on arrival at the substrate and are therefore likely to diffuse farther [11]. Films produced from a target consisting of 25% copper b y area gave a typically amorphous diffraction pattern (Fig. 2(b)) consisting of one bright halo together with a second (and occasionally a third) which was weaker and more diffuse. The lattice spacings associated with these two haloes (1.75 - 2.10 A and 0.95 - 1.35 A, respectively) were consistent with those obtained from splat-quenched Cu-Zr specimens. The bright field micrographs were featureless at low magnifications and showed the typical mottled contrast at higher magnifications (Fig. 2(a)). From published data on the sputtering rates of copper and zirconium
221
O.2pm
Fig. 2. (a) Microstructure of sputtered film, nominally Cu60Zr40. (b) Diffraction pattern of (a) showing amorphous structure.
[12] it was estimated that these films contained between 40 and 50 at.% Zr. Films of the same composition deposited at room temperature contained many small crystals within an amorphous matrix, whereas those of widely different compositions gave polycrystalline structures. Full details of these will be given in a subsequent publication.
DISCUSSION
The amorphous phase obtained b y sputtering appears identical with those produced b y splat-quenching, confirming that it is possible to produce amorphous phases in the same alloy b y the liquid to solid and vapour to
solid routes. This result agrees with Mizoguch et al. [13] who have also recently reported an amorphous phase in sputtered Cu-Zr films. No known stable or metastable phase in the Cu-Zr system nor any oxide of Cu or Zr could account for the diffraction pattern of the thermally evaporated films. However, if the first broad halo was ignored the remaining rings could arise from an f.c.c, structure with lattice parameter 3.55 +- 0.05 A. An identical structure was reported b y Vitek et al. [10] as a decomposition product of the amorphous Cu6oZr4o obtained b y splatquenching. For want of a better term these authors describe the microstructure as 'transformed amorphous' but were unable to offer a conclusive explanation of its structure or mode of formation. The lattice parameter of the f.c.c, c o m p o n e n t (3.55 A) agrees, within experimental error, with that of copper {3.61 A) and it is tempting to presume that the sharp rings in the diffraction pattern arise from Cu-rich clusters. The remaining matrix would then be Zr-rich with respect to the nominal alloy composition and it is relevant to note that the lattice spacing equivalent to the inner halo (2.75 £) coincides with the first reflection expected from h.c.p. ~-Zr {1010) [13]. The most plausible explanation of the 'transformed amorphous' structure, therefore, is that it consists of Cu-rich clusters in an amorphous or microcrystalline Zr-rich matrix. Vitek et al., in fact, showed that the dark field image formed from the outer (f.c.c.) rings contained small diffracting regions 30 - 50 £ in size, consistent with this model. The 'transformed amorphous' structure observed in the thermally evaporated films could have arisen in two ways: either it was the as
222 al temperature for formation of an amorphous phase. Measurements of the temperature of the sapphire surface during deposition showed that it may rise by as much as 100 K above its initial value, presumably as a result of the radiant heat emitted from the evaporation sources, in particular the electron gun. During sputtering, however, less heat was evolved and the substrate temperature remained close to that of liquid nitrogen, sufficient to allow the formation of a noncrystalline phase.
CONCLUSIONS
It has been shown t h a t by sputtering alloys containing approximately 60 at.% Cu and 40 at.% Zr it is possible to produce an amorphous phase. Thermal evaporation of the same alloys, however, led to the formation of a structure identical with that which forms as the first stage of annealing splat-quenched amorphous Cu-Zr alloys.
ACKNOWLEDGEMENTS
The thermal evaporation work was carried out at the University of Pennsylvania, Laboratory for Research on the Structure of Matter, and was supported by the National Science Foundation. I am grateful to Robert Maddin for suggesting the project and to William Romanow for experimental assistance. The sputtering was done at the University of
Sussex, School o f Engineering and Applied Sciences, with help from Brian Cantor. This part o f the project was supported by the Science Research Council.
REFERENCES 1 S. Mader, J. Vac. Sci. Technol., 2 (1965) 35. 2 T. Masumoto and R. Maddin, Mater. Sci. Eng., 19 (1975) 1. 3 B. Cantor and R. W. Cahn, in N. J. Grant and B. C. Giessen (eds.),Proc. 2nd Int. Conf. Rapidly Quenched Metals, MIT, 1975, Section I, M I T Press, Cambridge, Mass., 1976, p. 59. 4 M. G. Scott and R. Maddin, in N. J. Grant and B. C. Giessen (eds.), Proc. 2nd Int. Conf. Rapidly Quenched Metals, MIT, 1975, Section I, M I T Press, Cambridge, Mass., 1976, p. 249. 5 P. Duwez, R. Willens and R. C. Crewdson, J. Appl. Phys., 36 (1965) 2267. 6 S. C. H. Lin and P. Duwez, Phys. Status Solidi, 34 (1969) 469. 7 R. C. Ruhl, B. C. Giessen, M. Cohen and N. J. Grant, Acta Metall., 15 (1967) 1693. 8 R. Ray, B. C. Giessen and N. J. Grant, Scr. Metall., 2 (1968) 357. 9 A. Revcolevschi and N. J. Grant, Metall. Trans.,
3 (1972) 1545. 10 J. M. Vitek, J. B. VanderSande and N. J. Grant, Acta Metall., 23 (1975) 165. 11 K. L. Chopra, J. Appl. Phys., 37 (1966) 3405. 12 G. Carter and J. Colligon, Ion Bombardment of Solids, Heinemann, London, 1968, p. 310. 13 T. Mizoguchi et al., 2nd Int. Conf. Amorphous Magnetism, Troy, New York, 1976 to be published. 14 ASTM Powder Diffraction File 5°0665. 15 H. A. Davies and J. B. Hull, in N. J. Grant and B. C. Giessen (eds.), Proc. 2nd Int. Conf. Rapidly Quenched Metals, MIT, 1975, Section II, Mater. Sci. Eng., 23 (1976) 193.
219
© Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands
Vapour Quenching of Copper-Zirconium Alloys
MICHAEL SCOTT
University of Sussex, School of Engineering and Applied Sciences, Brighton, Sussex BN1 9QT (Gt. Britain) (Received May 11, 1977)
SUMMARY
Co-sputtering of copper-zirconium alloys containing approximately 40 at.% zirconium produces an amorphous phase. Thermal evaporation of the same alloys, however, gives a structure idential with the 'transformed amorphous' phase previously reported to form during the decomposition of a splat-quenched Cu6oZr4o glass.
c o m p o n e n t atoms have large size differences [1]. Consequently, Cu-Zr was chosen as a suitable system for a comparative study. It is well established that a glassy phase can be formed b y splat-quenching alloys within about 10 at.% of the composition Cu6oZr4o; the structure and crystallisation behaviour of these alloys have been examined in detail [9, 10]. In this paper the preliminary results of attempts to prepare similar phases b y vapour quenching are described.
INTRODUCTION EXPERIMENTAL
Amorphous metallic phases have been produced by b o t h vapour to solid [1] and liquid to solid quenching techniques [2]. The two processes, however, are intrinsically different since, in an undercooled liquid metal, the crystallisation rate (and therefore the cooling rate necessary to form a glass) is controlled by bulk diffusion, whereas during the atom b y atom growth of a vapour-deposited film the time required for arrangement to a crystalline array is governed b y surface diffusion [3]. Comparison of the products of the two methods is limited b y lack of suitable experimental data [4] and a programme was therefore set up to fill this gap. Two types of alloy are likely to form a non-crystalline phase on rapid solidification of the liquid (splat-quenching); those containing about 80 at.% noble or transition metal with the balance metalloid (e.g., Pds0Siuo [5], FeaoPlsC7 [6] ) and those based on early-late transition metal combinations such as Ni-Nb [7] and Cu-Zr [8]. This latter group of alloys is particularly interesting since it also satisfies the t w o criteria most c o m m o n l y required for the formation of an amorphous phase b y vapour deposition -- the glass-forming range is near the equiatomic composition and the
Two vapour quenching techniques were used: thermal co-evaporation and r.f. sputtering. In the former case, copper and zirconium were evaporated at <10 6 Torr from independent sources; the copper from a resistanceheated m o l y b d e n u m boat and the zirconium b y electron-beam heating. Quartz crystal monitors were used to measure and control the evaporation rates of the two species and, hence, the composition of the foils. Deposition rates of up to 1 nm s ~1 were achieved onto synthetic sapphires coated with either collodion or sodium metaphosphate and cooled to 90 K. The films were removed for electron microscopy by floating them from the sapphires in either amylacetate or distilled water. The sputtering apparatus has been described elsewhere [3]. High purity (99.99%) argon, purified b y a getter in the sputtering chamber, was used as a sputtering gas and sectored sheets of 99.9% Cu and Zr were used as the target materials. Typical sputtering conditions were: argon pressure 10 -2 Tort, power ~0.5 kW, axial magnetic field ~20 Oe, giving sputtering rates of 0.1 - 0.2 n m s -1. Freshly cleaved crystals of NaC1 a b o u t 1 mm thick were used as substrates and were
220
bonded with silver dag to a copper holder which was, in turn, cooled b y liquid nitrogen. The holder was rotated at about 60 rpm during sputtering to ensure homogeneity of the deposited films. The films were removed b y dissolving the NaC1 substrates in water. In b o t h cases, films between 40 and 80 nm were found to be suitable for transmission electron microscopy at 100 kV.
RESULTS
1 pm
1 pm
Fig. 1. (a) Microstructure of thermally evaporated Cu60Zr4o film. (b) Specimen showing growth of large crystals. (c) Diffraction pattern of (a) showing the 'transformed amorphous' structure.
Thermally evaporated films were prepared between 40 and 70 at.% Zr. In all cases bright field electron microscopy revealed a featureless matrix {Fig. l(a)) which, at higher magnifications, appeared slightly mottled. In a few cases the matrix contained up to about 10% b y volume of precipitates whose morphology varied from small spheres to well defined angular crystals (Fig. l(b)). On first inspection the matrix was thought to be amorphous but the associated diffraction pattern (Fig. l(c)) revealed that this was not the case. The pattern contained a broad inner halo equivalent to a lattice spacing of between 2.7 and 2.8 h together with several sharper outer rings. The intensities of these outer rings varied between specimens from cases where only the first was visible to those where up to five could be detected. In specimens where large crystals were present single spots were superimposed on these rings. More success in producing an amorphous structure was achieved with the co-sputtering technique, despite the fact that sputtered atoms have a higher energy on arrival at the substrate and are therefore likely to diffuse farther [11]. Films produced from a target consisting of 25% copper b y area gave a typically amorphous diffraction pattern (Fig. 2(b)) consisting of one bright halo together with a second (and occasionally a third) which was weaker and more diffuse. The lattice spacings associated with these two haloes (1.75 - 2.10 A and 0.95 - 1.35 A, respectively) were consistent with those obtained from splat-quenched Cu-Zr specimens. The bright field micrographs were featureless at low magnifications and showed the typical mottled contrast at higher magnifications (Fig. 2(a)). From published data on the sputtering rates of copper and zirconium
221
O.2pm
Fig. 2. (a) Microstructure of sputtered film, nominally Cu60Zr40. (b) Diffraction pattern of (a) showing amorphous structure.
[12] it was estimated that these films contained between 40 and 50 at.% Zr. Films of the same composition deposited at room temperature contained many small crystals within an amorphous matrix, whereas those of widely different compositions gave polycrystalline structures. Full details of these will be given in a subsequent publication.
DISCUSSION
The amorphous phase obtained b y sputtering appears identical with those produced b y splat-quenching, confirming that it is possible to produce amorphous phases in the same alloy b y the liquid to solid and vapour to
solid routes. This result agrees with Mizoguch et al. [13] who have also recently reported an amorphous phase in sputtered Cu-Zr films. No known stable or metastable phase in the Cu-Zr system nor any oxide of Cu or Zr could account for the diffraction pattern of the thermally evaporated films. However, if the first broad halo was ignored the remaining rings could arise from an f.c.c, structure with lattice parameter 3.55 +- 0.05 A. An identical structure was reported b y Vitek et al. [10] as a decomposition product of the amorphous Cu6oZr4o obtained b y splatquenching. For want of a better term these authors describe the microstructure as 'transformed amorphous' but were unable to offer a conclusive explanation of its structure or mode of formation. The lattice parameter of the f.c.c, c o m p o n e n t (3.55 A) agrees, within experimental error, with that of copper {3.61 A) and it is tempting to presume that the sharp rings in the diffraction pattern arise from Cu-rich clusters. The remaining matrix would then be Zr-rich with respect to the nominal alloy composition and it is relevant to note that the lattice spacing equivalent to the inner halo (2.75 £) coincides with the first reflection expected from h.c.p. ~-Zr {1010) [13]. The most plausible explanation of the 'transformed amorphous' structure, therefore, is that it consists of Cu-rich clusters in an amorphous or microcrystalline Zr-rich matrix. Vitek et al., in fact, showed that the dark field image formed from the outer (f.c.c.) rings contained small diffracting regions 30 - 50 £ in size, consistent with this model. The 'transformed amorphous' structure observed in the thermally evaporated films could have arisen in two ways: either it was the as
222 al temperature for formation of an amorphous phase. Measurements of the temperature of the sapphire surface during deposition showed that it may rise by as much as 100 K above its initial value, presumably as a result of the radiant heat emitted from the evaporation sources, in particular the electron gun. During sputtering, however, less heat was evolved and the substrate temperature remained close to that of liquid nitrogen, sufficient to allow the formation of a noncrystalline phase.
CONCLUSIONS
It has been shown t h a t by sputtering alloys containing approximately 60 at.% Cu and 40 at.% Zr it is possible to produce an amorphous phase. Thermal evaporation of the same alloys, however, led to the formation of a structure identical with that which forms as the first stage of annealing splat-quenched amorphous Cu-Zr alloys.
ACKNOWLEDGEMENTS
The thermal evaporation work was carried out at the University of Pennsylvania, Laboratory for Research on the Structure of Matter, and was supported by the National Science Foundation. I am grateful to Robert Maddin for suggesting the project and to William Romanow for experimental assistance. The sputtering was done at the University of
Sussex, School o f Engineering and Applied Sciences, with help from Brian Cantor. This part o f the project was supported by the Science Research Council.
REFERENCES 1 S. Mader, J. Vac. Sci. Technol., 2 (1965) 35. 2 T. Masumoto and R. Maddin, Mater. Sci. Eng., 19 (1975) 1. 3 B. Cantor and R. W. Cahn, in N. J. Grant and B. C. Giessen (eds.),Proc. 2nd Int. Conf. Rapidly Quenched Metals, MIT, 1975, Section I, M I T Press, Cambridge, Mass., 1976, p. 59. 4 M. G. Scott and R. Maddin, in N. J. Grant and B. C. Giessen (eds.), Proc. 2nd Int. Conf. Rapidly Quenched Metals, MIT, 1975, Section I, M I T Press, Cambridge, Mass., 1976, p. 249. 5 P. Duwez, R. Willens and R. C. Crewdson, J. Appl. Phys., 36 (1965) 2267. 6 S. C. H. Lin and P. Duwez, Phys. Status Solidi, 34 (1969) 469. 7 R. C. Ruhl, B. C. Giessen, M. Cohen and N. J. Grant, Acta Metall., 15 (1967) 1693. 8 R. Ray, B. C. Giessen and N. J. Grant, Scr. Metall., 2 (1968) 357. 9 A. Revcolevschi and N. J. Grant, Metall. Trans.,
3 (1972) 1545. 10 J. M. Vitek, J. B. VanderSande and N. J. Grant, Acta Metall., 23 (1975) 165. 11 K. L. Chopra, J. Appl. Phys., 37 (1966) 3405. 12 G. Carter and J. Colligon, Ion Bombardment of Solids, Heinemann, London, 1968, p. 310. 13 T. Mizoguchi et al., 2nd Int. Conf. Amorphous Magnetism, Troy, New York, 1976 to be published. 14 ASTM Powder Diffraction File 5°0665. 15 H. A. Davies and J. B. Hull, in N. J. Grant and B. C. Giessen (eds.), Proc. 2nd Int. Conf. Rapidly Quenched Metals, MIT, 1975, Section II, Mater. Sci. Eng., 23 (1976) 193.