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Undercooling and demixing in rapidly solidified Cu–Co alloys
Materials Science and Engineering A 449–451 (2007) 7–11
Undercooling and demixing in rapidly solidified Cu–Co alloys L. Battezzati a,∗ , S. Curiotto a,b,c , E. Johnson b , N.H. Pryds c a
b
Dipartimento di Chimica IFM e Centro di Eccellenza NIS, Universit`a di Torino, Italy Niels Bohr Institute, Nano Science Center, University of Copenhagen, Copenhagen, Denmark c Materials Research Department, Risø National Laboratory, DK-4000 Roskilde, Denmark
Received 22 August 2005; received in revised form 15 October 2005; accepted 15 February 2006
Abstract The Cu–Co system displays a metastable miscibility gap in the liquid state. A considerable amount of work has been performed to study phase separation and related microstructures showing that demixing of the liquid is followed by coagulation before dendritic solidification. Due to kinetic competition of transformation phenomena, the mechanisms have not been fully disclosed. This contribution reviews such findings with the help of a computer calculation of the phase diagram and extends the present knowledge by presenting new results obtained by rapidly solidifying various Cu–Co compositions using a wide range of cooling rates achieved by forcing the liquid into cylindric and conic moulds and by melt spinning. © 2006 Elsevier B.V. All rights reserved. Keywords: Liquid phase separation; Cu alloys; Melt spinning; Copper moulding
1. Introduction
2. Experimental details
Some Cu-based alloys, Co–Cu being a well known example, display a peritectic phase diagram with flat liquidus curve hiding a metastable miscibility gap in the liquid state which can be accessed by undercooling the melt and avoiding primary formation of a Co-rich solid solution. The alloy microstructure will depend both on the mechanism of demixing and of solidification. Actually, the literature reports experiments on electromagnetic levitation [1–3] and electron beam melting of surface layers [4,5] in the Cu–Co system showing the occurrence of a variety of microstructures formed under non-equilibrium condition. The present work discusses the relationship between demixing in the liquid state and microstructure in rapidly solidified Cu–Co alloys employing various cooling rates. A gradient in cooling rates can be achieved by quenching samples of the same alloy in suitable copper moulds and by producing ribbons with a melt spinning apparatus. Results will be compared with those obtained with samples produced by electro-magnetic levitation (EML) and drop tube processing [6,7].
Small ingots of binary Cu–Co alloys were produced by melting together suitable quantities of 99.995 purity Cu and 99.99 purity Co in a B¨uhler arc melter under Ti gettered atmosphere. The ingot had the form of a button which was sectioned and checked for its homogeneity: the ingots were fully dendritic with no sign of phase separation in the liquid state. Parts of the buttons were rapidly solidified in copper moulds in the shape of a cone or in the shape of a cylinder, to obtain a gradient of cooling rate. The mould was contained in a closed chamber evacuated several times and re-filled with inert gas (He). The alloy was induction melted in a fused silica crucible and ejected into the mould by an He overpressure. Other parts of the buttons were rapidly solidified in a laboratory made melt spinner with the alloy contained in a fused silica crucible under inert gas (He) atmosphere. The samples were then observed in cross section after appropriate preparation with a Reichert optical microscope and in a Leica SEM with Oxford EDS attachment. X-ray diffraction (XRD) patterns were collected in the θ–2θ mode with a Philips apparatus employing Co K␣ radiation. In order to enhance the chances of observing details of the microstructure, an electrochemical etching procedure has been devised to dissolve selectively either Cu or Co using conventional laboratory equipments.
∗
Corresponding author. Tel.: +39 011 6707567; fax: +39 011 6707855. E-mail address: [email protected] (L. Battezzati).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.02.246
8
L. Battezzati et al. / Materials Science and Engineering A 449–451 (2007) 7–11
Fig. 1. The Co–Cu phase diagram as optimised with a Calphad approach (full line). The dotted line gives the Curie temperature; the dashed line gives the metastable miscibility gap. Symbols represent experimental points as listed in Ref. [9].
3. Results and discussion The Co–Cu phase diagram has been re-optimised within this project to account for recent experimental data on the miscibility gap which modify substantially its location at various compositions [8]. The assessed phase diagram is shown in Fig. 1. The miscibility gap is slightly asymmetric with respect to composition with maximum occurring at an atomic fraction xCu = 0.585 and at 1556 K. The minimum difference between the liquidus and gap temperatures is about 90 K at xCu = 0.63. Details of the assessment are given elsewhere [9]. Such evaluation will be useful in assessing the minimum undercooling needed to access the miscibility gap. 3.1. Copper moulding of Cu50 Co50 The alloy was quenched in the form of a truncated cone. The diameter of the section of the tip was about 2 mm and that of the base about 5 mm. The height was 17 mm. The cone was sectioned at various heights and examined by scanning electron microscopy (SEM). Fig. 2 shows a representative image of
Fig. 2. Section of a copper moulded Cu50 Co50 cone. Microstructure at the tip.
the tip of the cone where the cooling rate has been faster. The microstructure has the following features: wide areas containing large Co-rich particles of various irregular shapes predominantly with concave boundaries, areas with fine Co-rich dendrites, a Cu-rich matrix surrounding all Co particles which is often thinwalled in between large Co particles (Fig. 2). At half height of the cone the microstructure is similar. The dendrites, however, are slightly coarser and appear to occupy a larger area fraction. Similar microstructures have been observed in the thinnest parts of a wedge shaped sample of larger size which are reported in a companion paper in which the cooling rate appropriate for the formation of this microstructure is estimated as 7 × 103 K s−1 [10]. The microstructure suggests a phase transformation path during cooling. The liquid experiences undercooling below the liquidus curve until demixing occurs. The melt follows the binodal line in the metastable phase diagram being undercooled with respect to the corresponding liquidus. The individual droplets coalesce to larger agglomerates before dendritic solidification of Co takes place within the Cu-rich liquid producing zones with finer microstructure. The two phases exchange solute both by diffusion and coagulation before the remaining liquid solidifies with a peritectic reaction as indicated by the thin-walled Cu-rich phase between the large Co-rich particles. The two zones resemble “regular” and “irregular” morphologies of Co particles found in Cu–Co samples processed by EML and solidified below the liquidus outside or within the metastable miscibility gap, respectively. The sequence of events proposed above is consistent with that suggested for cooling and solidification of Cu–Co droplets in EML experiments. 3.2. Melt spinning experiments Melt spinning of the same Co50 Cu50 alloy gave varied microstructures. The melt apparently started decomposing inside the crucible before it was ejected onto the wheel. The local composition was determined by energy-dispersive X-ray spectrometry (EDS) directly on the polished section before observation. Examples of microstructures for Cu34 Co66 are shown in Figs. 3 and 4. Fig. 3 shows almost featureless zones on the wheel
Fig. 3. Usual microstructure found in cross sections of a ribbon having Cu34 Co66 composition. Wheel side up.
L. Battezzati et al. / Materials Science and Engineering A 449–451 (2007) 7–11
9
Fig. 4. Microstructure found in a cross section of a ribbon having Cu34 Co66 composition. Wheel side up.
side and coarser crystals in the rest of the ribbon cross section. The Co-rich phase appears in the form of rounded particles with branches and internal microstructure. The latter is well evident in some larger particles, the core of which results from coagulation of smaller ones after liquid demixing. The droplets then served as sites for dendrite formation possibly because of the local temperature gradient caused by their recalescence on solidification. The large quantity of small Co-rich particles might have solidified soon after liquid demixing before extensive coagulation could take place. Apparently the cooling condition on the wheel can change markedly since Fig. 4, taken on another section, shows a different transition between microstructures. Lamellar-like crystals occur on the wheel side for about 10 m. They nucleate fine Co dendrites extending in a zone a few microns thick which ends abruptly in the middle of the ribbon. The other half of the cross section is made of irregular particles, homogeneous in size, displaying dendritic features. The coupled microstructure in the top part of Fig. 4 is not fully understood yet; it was reported earlier in an EML sample quenched on a copper chill but not explained [11]. It might originate from fine demixing and coupled growth solidification of two crystal phases, rich in Co and Cu, respectively, below the peritectic. Alternatively, liquid undercooling could be envisaged below the peritectic where there is driving force for nucleation of both fcc solid solutions and growth can proceed with coupled morphology as in a eutectic. A condition for this is that the composition be off-peritectic [12]. With the composition of this section of the ribbon, however, the temperature for spinodal decomposition, the most likely mechanism for the start of demixing [1], is higher than the peritectic temperature according to the latest evaluation [9]. So, the former should be the favoured mechanism of phase separation. After coupled growth has proceeded to some extent, the release of latent heat causes recalescence in the rest of the ribbon which then has dendrites of increasing size due to the decrease of undercooling. They were possibly originated at temperatures above the metastable miscibility gap or soon after demixing started. Cu75 Co25 ribbons were easier to prepare and have been analysed as detailed in the next paragraph. A very fine twophase microstructure is recognised on the wheel side becoming
Fig. 5. Image of the two-zone microstructure found in the Cu62 Co38 copper moulded cylinder.
Fig. 6. Deeply etched slice of the Cu62 Co38 copper moulded cylinder. The Corich particles are fully isolated from the Cu matrix.
Fig. 7. Porous zone in a Cu75 Co25 ribbon anodically etched to dissolve Co.
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L. Battezzati et al. / Materials Science and Engineering A 449–451 (2007) 7–11
Fig. 8. XRD patterns taken on the wheel and external sides of the Cu75 Co25 ribbon and the computed patterns and difference after Rietveld refinement.
coarser in the rest of the ribbon. Their appearance is close to that found in electron beam melted layers of various Cu-rich alloys [5] and in droplets produced by free fall in a drop tube [7]. The microstructure can be explained as originating from rapid solidification of finely demixed liquid particles as the one shown in Fig. 3 and at variance to that of Fig. 4. The demixing should occur in the melt pool on the quenching wheel. In fact, should phase separation occur before ejecting the melt, the two phases should have formed larger droplets and the ribbon should be inhomogeneous as for Cu34 Co66 described above. 3.3. Selective electrochemical dissolution of elements A cylinder of Cu62 Co38 composition was produced in a mould similarly to the cone. Slices were cut from the sample and used as anodes in an electrochemical cell with a phosphoric acid solution as electrolyte. The pH of the solution and the potential applied to the electrode were adjusted according to the respective Pourbaix diagrams in order to dissolve selectively either the Cu or the Co. The dissolution of the Cu-rich phase leaves Co-rich dendrites and rounded particles protruding out of the matrix. The dissolution of the Co-rich phase leaves porosity in the shape of dendrites and rounded pores. In Fig. 5 the two morphologies of Co-rich particles already described are clearly seen in a sample etched for 2 min. The anodic dissolution helps in analysing the microstructure of the Cu62 Co38 alloy which reproduced that found in the Co50 Cu50 cone. With deeper etching, images were obtained (Fig. 6) showing a volume where the matrix has been fully dissolved. Co-rich dendrites display secondary and in a few cases tertiary arms; larger irregular particles have smooth, rounded surfaces. With prolonged etching the Co-rich particles detach from the surface of the alloy and fall into the solution. Unfortunately, it has not been possible to recover them because they dissolve readily once they are not anodically protected due to the low pH of the electrolyte. Electrochemical etching was performed also with a sample of the Cu75 Co25 ribbon by dissolving the minority Co-rich phase with the aim of producing a free standing porous material. Fig. 7 shows that this is possible: the Cu-rich matrix is continuous and the pores are mostly sub-micrometric with a few of them reaching the size of a few microns.
3.4. X-ray diffraction X-ray diffraction has been employed to identify the phases in each sample and determine their lattice constant. From the data available in the literature [13], the average elemental content of the phases can be obtained. Patterns given by the Cu75 Co25 ribbon on the wheel and external side are shown in Fig. 8. Reflections of two fcc phases are seen due to the Cu-rich solid solution (higher angle reflections in the doublets) and the Co-rich solid solution (lower angle reflections in the doublets); those of the Cu-rich phase are predominant in intensity. For a proper interpretation of the patterns a Rietveld refinement [14] has been performed. The points in the figure are experimental data and the line the computed patterns. The difference of the experimental to the computed pattern is reported at the bottom of the figure. The agreement between experiment and calculation is good. Lattice constants do not differ appreciably on the two sides of the ribbon. They are: 0.3610 nm for the Cu-rich solution, and 0.3558 nm for the Co-rich solution, suggesting that the former contains 10 at.% Co and the latter 14 at.% Cu. The typical error in the lattice constant determination is ±10−4 nm implying an uncertainty in the solute content of at least 1 at.%. The solute contents of the phases in the rapidly quenched ribbon are both higher than those expected at the peritectic. The phases have retained their solute content at the moment of solidification. The high supersaturation of the Cu-rich phase is likely due to its solidification at a temperature above the peritectic, corresponding to the T0 temperature for the alloy composition (T0 is the temperature at which the free energy of the liquid phase equals that of the solid solution). Considering the free energy curves of the liquid and Cu-rich fcc phases [9], the T0 temperatures are expected to increase with increasing Co content and enter the metastable miscibility gap above the peritectic. The high Cu content in the Co-rich phase implies that solidification occurred above the peritectic and is consistent with the binary phase diagram which displays retrograde solubility. 4. Summary The microstructures of a copper moulded Cu50 Co50 cone and a Cu62 Co38 cylinder have been examined. The latter has been studied in detail after anodic etching of the Cu-rich phase. Two zones have been found, one made of coarse grains possibly
L. Battezzati et al. / Materials Science and Engineering A 449–451 (2007) 7–11
solidified soon after demixing and a second one with fine Corich dendrites solidified at a later stage from the demixed Cu-rich melt. The microstructures found in a Cu34 Co66 ribbon produced from the same master alloy as the cone is controversial. Most of it indicates solidification after phase separation and limited coarsening of droplets. An example has been found of coupled growth of the two fcc phases which could suggest eutectic-like solidification after demixing. The cross section of a melt spun Cu75 Co25 ribbon shows a fine dispersion of a Co-rich phase with an almost featureless zone on the wheel side. The microstructure is thought to originate from liquid phase separation on a fine scale followed by crystallisation of the Co-rich phase on undercooling and of the Cu-rich phase according to the position of the T0 temperature. The most likely mechanism for the formation of microstructures implies spinodal decomposition of the melt followed by coagulation to various extents according to the cooling rate imposed to the sample. In all cases there is the impending possibility of dendritic solidification of the Co-rich phase. Acknowledgement Work was supported by the European Space Agency within the project “Coolcop”, ESA-MAP Project AO 99-010.
11
References [1] I. Egry, D.M. Herlach, M. Kolbe, L. Ratke, S. Reutzel, C. Perrin, D. Chatain, Adv. Eng. Mater. 5 (2003) 819–823. [2] X.Y. Lu, M. Kolbe, D.M. Herlach (Eds.), Mater. Res. Symp. Proc., vol. 754, 2003, p. CC3.17.1. [3] M. Kolbe, C.D. Cao, X.Y. Lu, P.K. Galenko, B. Wei, D.M. Herlach, Mater. Sci. Eng. A 375–377 (2004) 520–523. [4] A. Munitz, S.P. Elder-Randall, R. Abbaschian, Met. Trans. A 23 (1992) 1817–1827. [5] A. Munitz, R. Abbaschian, J. Mater. Sci. 33 (1998) 3639–3649. [6] C.D. Cao, D.M. Herlach, M. Kolbe, G.P. G¨orler, B. Wei, Scr. Mater. 48 (2003) 5–9. [7] C.D. Cao, D.M. Herlach, B. Wei, J. Mater. Sci. Lett. 21 (2002) 341– 343. [8] C.D. Cao, G.P. G¨orler, D.M. Herlach, B. Wei, Mater. Sci. Eng. A325 (2002) 503–510. [9] M. Palumbo, S. Curiotto, L. Battezzati, Calphad 2005, Aachen, May 2005. [10] S. Curiotto, N.H. Pryds, E. Johnson, L. Battezzati, this conference. [11] A. Munitz, R. Abbaschian, Metall. Mater. Trans. A 27 (1996) 4049– 4058. [12] W. Kurz, D.J. Fischer, Fundamentals of Solidification, Trans. Tech. Pub., Aedersmanndorf, Switzerland, 1985. [13] W.B. Pearson, Handbook of Lattice Spacings and Structures of Metals and Alloys, vol. 2, Pergamon Press, Oxford, 1967. [14] L. Lutterotti, S. Matthies, H.-R. Wenk (Eds.), Proceedings of the Twelfth International Conference on Textures of Materials (ICOTOM-12), vol. 1, NRC Research Press, Ottawa, Canada, 1999, p. 1599.
Undercooling and demixing in rapidly solidified Cu–Co alloys L. Battezzati a,∗ , S. Curiotto a,b,c , E. Johnson b , N.H. Pryds c a
b
Dipartimento di Chimica IFM e Centro di Eccellenza NIS, Universit`a di Torino, Italy Niels Bohr Institute, Nano Science Center, University of Copenhagen, Copenhagen, Denmark c Materials Research Department, Risø National Laboratory, DK-4000 Roskilde, Denmark
Received 22 August 2005; received in revised form 15 October 2005; accepted 15 February 2006
Abstract The Cu–Co system displays a metastable miscibility gap in the liquid state. A considerable amount of work has been performed to study phase separation and related microstructures showing that demixing of the liquid is followed by coagulation before dendritic solidification. Due to kinetic competition of transformation phenomena, the mechanisms have not been fully disclosed. This contribution reviews such findings with the help of a computer calculation of the phase diagram and extends the present knowledge by presenting new results obtained by rapidly solidifying various Cu–Co compositions using a wide range of cooling rates achieved by forcing the liquid into cylindric and conic moulds and by melt spinning. © 2006 Elsevier B.V. All rights reserved. Keywords: Liquid phase separation; Cu alloys; Melt spinning; Copper moulding
1. Introduction
2. Experimental details
Some Cu-based alloys, Co–Cu being a well known example, display a peritectic phase diagram with flat liquidus curve hiding a metastable miscibility gap in the liquid state which can be accessed by undercooling the melt and avoiding primary formation of a Co-rich solid solution. The alloy microstructure will depend both on the mechanism of demixing and of solidification. Actually, the literature reports experiments on electromagnetic levitation [1–3] and electron beam melting of surface layers [4,5] in the Cu–Co system showing the occurrence of a variety of microstructures formed under non-equilibrium condition. The present work discusses the relationship between demixing in the liquid state and microstructure in rapidly solidified Cu–Co alloys employing various cooling rates. A gradient in cooling rates can be achieved by quenching samples of the same alloy in suitable copper moulds and by producing ribbons with a melt spinning apparatus. Results will be compared with those obtained with samples produced by electro-magnetic levitation (EML) and drop tube processing [6,7].
Small ingots of binary Cu–Co alloys were produced by melting together suitable quantities of 99.995 purity Cu and 99.99 purity Co in a B¨uhler arc melter under Ti gettered atmosphere. The ingot had the form of a button which was sectioned and checked for its homogeneity: the ingots were fully dendritic with no sign of phase separation in the liquid state. Parts of the buttons were rapidly solidified in copper moulds in the shape of a cone or in the shape of a cylinder, to obtain a gradient of cooling rate. The mould was contained in a closed chamber evacuated several times and re-filled with inert gas (He). The alloy was induction melted in a fused silica crucible and ejected into the mould by an He overpressure. Other parts of the buttons were rapidly solidified in a laboratory made melt spinner with the alloy contained in a fused silica crucible under inert gas (He) atmosphere. The samples were then observed in cross section after appropriate preparation with a Reichert optical microscope and in a Leica SEM with Oxford EDS attachment. X-ray diffraction (XRD) patterns were collected in the θ–2θ mode with a Philips apparatus employing Co K␣ radiation. In order to enhance the chances of observing details of the microstructure, an electrochemical etching procedure has been devised to dissolve selectively either Cu or Co using conventional laboratory equipments.
∗
Corresponding author. Tel.: +39 011 6707567; fax: +39 011 6707855. E-mail address: [email protected] (L. Battezzati).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.02.246
8
L. Battezzati et al. / Materials Science and Engineering A 449–451 (2007) 7–11
Fig. 1. The Co–Cu phase diagram as optimised with a Calphad approach (full line). The dotted line gives the Curie temperature; the dashed line gives the metastable miscibility gap. Symbols represent experimental points as listed in Ref. [9].
3. Results and discussion The Co–Cu phase diagram has been re-optimised within this project to account for recent experimental data on the miscibility gap which modify substantially its location at various compositions [8]. The assessed phase diagram is shown in Fig. 1. The miscibility gap is slightly asymmetric with respect to composition with maximum occurring at an atomic fraction xCu = 0.585 and at 1556 K. The minimum difference between the liquidus and gap temperatures is about 90 K at xCu = 0.63. Details of the assessment are given elsewhere [9]. Such evaluation will be useful in assessing the minimum undercooling needed to access the miscibility gap. 3.1. Copper moulding of Cu50 Co50 The alloy was quenched in the form of a truncated cone. The diameter of the section of the tip was about 2 mm and that of the base about 5 mm. The height was 17 mm. The cone was sectioned at various heights and examined by scanning electron microscopy (SEM). Fig. 2 shows a representative image of
Fig. 2. Section of a copper moulded Cu50 Co50 cone. Microstructure at the tip.
the tip of the cone where the cooling rate has been faster. The microstructure has the following features: wide areas containing large Co-rich particles of various irregular shapes predominantly with concave boundaries, areas with fine Co-rich dendrites, a Cu-rich matrix surrounding all Co particles which is often thinwalled in between large Co particles (Fig. 2). At half height of the cone the microstructure is similar. The dendrites, however, are slightly coarser and appear to occupy a larger area fraction. Similar microstructures have been observed in the thinnest parts of a wedge shaped sample of larger size which are reported in a companion paper in which the cooling rate appropriate for the formation of this microstructure is estimated as 7 × 103 K s−1 [10]. The microstructure suggests a phase transformation path during cooling. The liquid experiences undercooling below the liquidus curve until demixing occurs. The melt follows the binodal line in the metastable phase diagram being undercooled with respect to the corresponding liquidus. The individual droplets coalesce to larger agglomerates before dendritic solidification of Co takes place within the Cu-rich liquid producing zones with finer microstructure. The two phases exchange solute both by diffusion and coagulation before the remaining liquid solidifies with a peritectic reaction as indicated by the thin-walled Cu-rich phase between the large Co-rich particles. The two zones resemble “regular” and “irregular” morphologies of Co particles found in Cu–Co samples processed by EML and solidified below the liquidus outside or within the metastable miscibility gap, respectively. The sequence of events proposed above is consistent with that suggested for cooling and solidification of Cu–Co droplets in EML experiments. 3.2. Melt spinning experiments Melt spinning of the same Co50 Cu50 alloy gave varied microstructures. The melt apparently started decomposing inside the crucible before it was ejected onto the wheel. The local composition was determined by energy-dispersive X-ray spectrometry (EDS) directly on the polished section before observation. Examples of microstructures for Cu34 Co66 are shown in Figs. 3 and 4. Fig. 3 shows almost featureless zones on the wheel
Fig. 3. Usual microstructure found in cross sections of a ribbon having Cu34 Co66 composition. Wheel side up.
L. Battezzati et al. / Materials Science and Engineering A 449–451 (2007) 7–11
9
Fig. 4. Microstructure found in a cross section of a ribbon having Cu34 Co66 composition. Wheel side up.
side and coarser crystals in the rest of the ribbon cross section. The Co-rich phase appears in the form of rounded particles with branches and internal microstructure. The latter is well evident in some larger particles, the core of which results from coagulation of smaller ones after liquid demixing. The droplets then served as sites for dendrite formation possibly because of the local temperature gradient caused by their recalescence on solidification. The large quantity of small Co-rich particles might have solidified soon after liquid demixing before extensive coagulation could take place. Apparently the cooling condition on the wheel can change markedly since Fig. 4, taken on another section, shows a different transition between microstructures. Lamellar-like crystals occur on the wheel side for about 10 m. They nucleate fine Co dendrites extending in a zone a few microns thick which ends abruptly in the middle of the ribbon. The other half of the cross section is made of irregular particles, homogeneous in size, displaying dendritic features. The coupled microstructure in the top part of Fig. 4 is not fully understood yet; it was reported earlier in an EML sample quenched on a copper chill but not explained [11]. It might originate from fine demixing and coupled growth solidification of two crystal phases, rich in Co and Cu, respectively, below the peritectic. Alternatively, liquid undercooling could be envisaged below the peritectic where there is driving force for nucleation of both fcc solid solutions and growth can proceed with coupled morphology as in a eutectic. A condition for this is that the composition be off-peritectic [12]. With the composition of this section of the ribbon, however, the temperature for spinodal decomposition, the most likely mechanism for the start of demixing [1], is higher than the peritectic temperature according to the latest evaluation [9]. So, the former should be the favoured mechanism of phase separation. After coupled growth has proceeded to some extent, the release of latent heat causes recalescence in the rest of the ribbon which then has dendrites of increasing size due to the decrease of undercooling. They were possibly originated at temperatures above the metastable miscibility gap or soon after demixing started. Cu75 Co25 ribbons were easier to prepare and have been analysed as detailed in the next paragraph. A very fine twophase microstructure is recognised on the wheel side becoming
Fig. 5. Image of the two-zone microstructure found in the Cu62 Co38 copper moulded cylinder.
Fig. 6. Deeply etched slice of the Cu62 Co38 copper moulded cylinder. The Corich particles are fully isolated from the Cu matrix.
Fig. 7. Porous zone in a Cu75 Co25 ribbon anodically etched to dissolve Co.
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L. Battezzati et al. / Materials Science and Engineering A 449–451 (2007) 7–11
Fig. 8. XRD patterns taken on the wheel and external sides of the Cu75 Co25 ribbon and the computed patterns and difference after Rietveld refinement.
coarser in the rest of the ribbon. Their appearance is close to that found in electron beam melted layers of various Cu-rich alloys [5] and in droplets produced by free fall in a drop tube [7]. The microstructure can be explained as originating from rapid solidification of finely demixed liquid particles as the one shown in Fig. 3 and at variance to that of Fig. 4. The demixing should occur in the melt pool on the quenching wheel. In fact, should phase separation occur before ejecting the melt, the two phases should have formed larger droplets and the ribbon should be inhomogeneous as for Cu34 Co66 described above. 3.3. Selective electrochemical dissolution of elements A cylinder of Cu62 Co38 composition was produced in a mould similarly to the cone. Slices were cut from the sample and used as anodes in an electrochemical cell with a phosphoric acid solution as electrolyte. The pH of the solution and the potential applied to the electrode were adjusted according to the respective Pourbaix diagrams in order to dissolve selectively either the Cu or the Co. The dissolution of the Cu-rich phase leaves Co-rich dendrites and rounded particles protruding out of the matrix. The dissolution of the Co-rich phase leaves porosity in the shape of dendrites and rounded pores. In Fig. 5 the two morphologies of Co-rich particles already described are clearly seen in a sample etched for 2 min. The anodic dissolution helps in analysing the microstructure of the Cu62 Co38 alloy which reproduced that found in the Co50 Cu50 cone. With deeper etching, images were obtained (Fig. 6) showing a volume where the matrix has been fully dissolved. Co-rich dendrites display secondary and in a few cases tertiary arms; larger irregular particles have smooth, rounded surfaces. With prolonged etching the Co-rich particles detach from the surface of the alloy and fall into the solution. Unfortunately, it has not been possible to recover them because they dissolve readily once they are not anodically protected due to the low pH of the electrolyte. Electrochemical etching was performed also with a sample of the Cu75 Co25 ribbon by dissolving the minority Co-rich phase with the aim of producing a free standing porous material. Fig. 7 shows that this is possible: the Cu-rich matrix is continuous and the pores are mostly sub-micrometric with a few of them reaching the size of a few microns.
3.4. X-ray diffraction X-ray diffraction has been employed to identify the phases in each sample and determine their lattice constant. From the data available in the literature [13], the average elemental content of the phases can be obtained. Patterns given by the Cu75 Co25 ribbon on the wheel and external side are shown in Fig. 8. Reflections of two fcc phases are seen due to the Cu-rich solid solution (higher angle reflections in the doublets) and the Co-rich solid solution (lower angle reflections in the doublets); those of the Cu-rich phase are predominant in intensity. For a proper interpretation of the patterns a Rietveld refinement [14] has been performed. The points in the figure are experimental data and the line the computed patterns. The difference of the experimental to the computed pattern is reported at the bottom of the figure. The agreement between experiment and calculation is good. Lattice constants do not differ appreciably on the two sides of the ribbon. They are: 0.3610 nm for the Cu-rich solution, and 0.3558 nm for the Co-rich solution, suggesting that the former contains 10 at.% Co and the latter 14 at.% Cu. The typical error in the lattice constant determination is ±10−4 nm implying an uncertainty in the solute content of at least 1 at.%. The solute contents of the phases in the rapidly quenched ribbon are both higher than those expected at the peritectic. The phases have retained their solute content at the moment of solidification. The high supersaturation of the Cu-rich phase is likely due to its solidification at a temperature above the peritectic, corresponding to the T0 temperature for the alloy composition (T0 is the temperature at which the free energy of the liquid phase equals that of the solid solution). Considering the free energy curves of the liquid and Cu-rich fcc phases [9], the T0 temperatures are expected to increase with increasing Co content and enter the metastable miscibility gap above the peritectic. The high Cu content in the Co-rich phase implies that solidification occurred above the peritectic and is consistent with the binary phase diagram which displays retrograde solubility. 4. Summary The microstructures of a copper moulded Cu50 Co50 cone and a Cu62 Co38 cylinder have been examined. The latter has been studied in detail after anodic etching of the Cu-rich phase. Two zones have been found, one made of coarse grains possibly
L. Battezzati et al. / Materials Science and Engineering A 449–451 (2007) 7–11
solidified soon after demixing and a second one with fine Corich dendrites solidified at a later stage from the demixed Cu-rich melt. The microstructures found in a Cu34 Co66 ribbon produced from the same master alloy as the cone is controversial. Most of it indicates solidification after phase separation and limited coarsening of droplets. An example has been found of coupled growth of the two fcc phases which could suggest eutectic-like solidification after demixing. The cross section of a melt spun Cu75 Co25 ribbon shows a fine dispersion of a Co-rich phase with an almost featureless zone on the wheel side. The microstructure is thought to originate from liquid phase separation on a fine scale followed by crystallisation of the Co-rich phase on undercooling and of the Cu-rich phase according to the position of the T0 temperature. The most likely mechanism for the formation of microstructures implies spinodal decomposition of the melt followed by coagulation to various extents according to the cooling rate imposed to the sample. In all cases there is the impending possibility of dendritic solidification of the Co-rich phase. Acknowledgement Work was supported by the European Space Agency within the project “Coolcop”, ESA-MAP Project AO 99-010.
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References [1] I. Egry, D.M. Herlach, M. Kolbe, L. Ratke, S. Reutzel, C. Perrin, D. Chatain, Adv. Eng. Mater. 5 (2003) 819–823. [2] X.Y. Lu, M. Kolbe, D.M. Herlach (Eds.), Mater. Res. Symp. Proc., vol. 754, 2003, p. CC3.17.1. [3] M. Kolbe, C.D. Cao, X.Y. Lu, P.K. Galenko, B. Wei, D.M. Herlach, Mater. Sci. Eng. A 375–377 (2004) 520–523. [4] A. Munitz, S.P. Elder-Randall, R. Abbaschian, Met. Trans. A 23 (1992) 1817–1827. [5] A. Munitz, R. Abbaschian, J. Mater. Sci. 33 (1998) 3639–3649. [6] C.D. Cao, D.M. Herlach, M. Kolbe, G.P. G¨orler, B. Wei, Scr. Mater. 48 (2003) 5–9. [7] C.D. Cao, D.M. Herlach, B. Wei, J. Mater. Sci. Lett. 21 (2002) 341– 343. [8] C.D. Cao, G.P. G¨orler, D.M. Herlach, B. Wei, Mater. Sci. Eng. A325 (2002) 503–510. [9] M. Palumbo, S. Curiotto, L. Battezzati, Calphad 2005, Aachen, May 2005. [10] S. Curiotto, N.H. Pryds, E. Johnson, L. Battezzati, this conference. [11] A. Munitz, R. Abbaschian, Metall. Mater. Trans. A 27 (1996) 4049– 4058. [12] W. Kurz, D.J. Fischer, Fundamentals of Solidification, Trans. Tech. Pub., Aedersmanndorf, Switzerland, 1985. [13] W.B. Pearson, Handbook of Lattice Spacings and Structures of Metals and Alloys, vol. 2, Pergamon Press, Oxford, 1967. [14] L. Lutterotti, S. Matthies, H.-R. Wenk (Eds.), Proceedings of the Twelfth International Conference on Textures of Materials (ICOTOM-12), vol. 1, NRC Research Press, Ottawa, Canada, 1999, p. 1599.