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Solidification microstructures in the rapidly solidified powder of high alloyed V–Cr tool steel
Journal of Materials Processing Technology 157–158 (2004) 729–734
Solidification microstructures in the rapidly solidified powder of high alloyed V–Cr tool steel M. Kusya,∗ , L. Caplovica , P. Grgaca , A. Vyrostkovab a
Faculty of Materials Science and Technology, Slovak University of Technology in Bratislava, Bottova 24, Trnava, Slovak Republic b Institute of Materials Research, Slovak Academy of Sciences, Wattsonova 47, Koˇsice, Slovak Republic
Abstract The article deals with the solidification microstructures of the rapidly solidified powder of 3% C–3% Cr–12% V (wt.%) hypereutectic iron alloy Ch3F12 (C3Cr3V12). Five different solidification microstructures were classified in powder particles: microstructure with eutectic spherulites, microstructure with eutectic colonies without primary carbides, microstructure with the primary carbides in the centers of eutectic colonies, microstructure with star-like carbides and microstructure with dispersed globular carbides. Light microscopy, scanning electron microscopy and transmission electron microscopy were used for experimental observations. X-ray diffraction analysis proved the presence of M4 C3 carbide, austenite and indicated the presence of M7 C3 carbides. The chemical composition of primary carbide phase was determined by energy-dispersive X-ray analysis. © 2004 Elsevier B.V. All rights reserved. Keywords: Hypereutectic alloy; Rapid solidification; Solidification microstructures; Powder metallurgy
1. Introduction The tool steels of ledeburite type belonging to the group of metallic materials exploited the gas atomisation as a means, leading to the enhancement of their technical parameters. The iron based alloy Ch3F12 characterised by the presence of carbides of eutectic as well as primary origin can be categorised to this group of materials [1–3]. The cooling rates during melt atomisation achieve values of order up to 106 K s−1 [4]. Owing to the rapid quenching of droplets in the atomisation process, the solidification of developing microvolumes is carried out at the non-equilibrium conditions and consequently the changes in the character of solidification microstructure occur [5–10]. Microstructures and phase composition of classical tool steels of ledeburite type prepared by gas atomisation process have been studied and reported in several works [11–16]. Relatively less attention has been devoted to the analysis of the rapidly solidified (RS) powders from new tool materials with the increased content of vanadium and carbon [17–19]. The analysis of primary structures of RS particles ∗
Corresponding author. E-mail address: [email protected] (P. Grgac).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.07.141
was accomplished for powders from hypereutectic alloys on ferrous base [20]. The main aim of this article is to analyse in detail the solidification microstructures in the particles of rapidly solidified powder of hypereutectic alloy C3Cr3V12 in the base stage, i.e. after atomisation.
2. Material and experimental methods Rapidly solidified powder marked as C3Cr3V12 with the chemical composition 3% C–3% Cr–12% V (wt.%) belongs to the iron based hypereutectic alloys. The powder produced by nitrogen gas atomisation contains of globular particles of different sizes. RS powder was divided into 13 standard size fractions in the range from 40 to 400 m. For microstructural analysis, RS powder particles were bounded by electrolytically deposited nickel, metallographically prepared using standard methods and NITAL etched. Methods and experimental techniques of light microscopy (LM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were applied. The analysis of minimum 100 particles from each size fraction was performed to evaluate
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Fig. 1. X-ray diffraction pattern of RS powder C3Cr3V12 in the base state.
the relative proportion of the basic types of solidification microstructures in the analysed powder. Transmission electron microscopy and energy-dispersive X-ray analyses were performed on the single-stage carbon replicas. X-ray spectrometric analysis with Bragg–Brentano semifocusation geometry was carried out using Cr K␣ radiation.
3. Results of experiments and disscusion
Fig. 3. The presence of different types of solidification microstructures, light microscopy.
Based on the analysis of primary microstructures by scanning electron microscopy, five main types of solidification microstructures were identified in the particles of RS powder. From the morphological point of view, the characteristic solidification microstructures of RS particles can be described as follows:
The phase analysis of RS powder of C3Cr3V12 alloy was performed by X-ray diffraction analysis. The diffraction pattern shown in the Fig. 1 was obtained from granullometric fraction 125 / 160 m. Diffraction pattern contains two dominant peaks corresponding the diffractions on the (1 1 1) and (2 0 0) austenite planes. The main constituent of the rapidly solidified powder was found to be the metastable austenite. The presence of carbide phases of M4 C3 type was detected by X-ray analysis as well. Diffraction peaks indicate also the presence of M7 C3 carbides. RS powder particles of C3Cr3V12 tool alloy had after the nitrogen gas atomisation globular morphology as it can be seen in the Fig. 2. Light microscopy of RS particles revealed the presence of different structural modifications on metallographical sections (Fig. 3), which evoked the need of more detailed analysis of solidification microstructures from the entire granulometric spectrum applying the SEM and TEM.
1. Microstructure with the disperse globular carbides—is defined as a structure with globular carbide particles evenly dispersed in the matrix (Fig. 4). The percentage population of this microstructure in the single granulometric fractions is illustrated in Fig. 5. 2. Microstructure with star-like primary carbides—is characterised by the presence of isolated carbides with the morphologically variable multi-arm star-like formations (Fig. 6). Fig. 7 shows the microstructure population in the entire size fractions. 3. Microstructure with the primary carbides in the centers of eutectic colonies—this structure contains star-like carbides surrounded by eutectic whereby the primary carbide phase intergrowths continually to the carbide skeleton of eutectic colony (Fig. 8). The microstructure population in each size fraction is depicted by histogram in Fig. 9. 4. Microstructure with eutectic colonies without primary carbides—colony with the characteristic features of sectorial growth, in the center of eutectic colony the primary carbide was not developed (Fig. 10). The percentage pop-
Fig. 2. Spherical morphology of RS powder particles of C3Cr3V12 alloy, SEM.
Fig. 4. Microstructure with disperse globular carbides, SEM.
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Fig. 8. Microstructure with the primary carbide in the center of eutectic colony, SEM. Fig. 5. Histogram of percentage population of the microstructure with disperse globular carbides.
Fig. 9. Histogram of percentage population of the microstructure with the primary carbides in the center of eutectic colonies. Fig. 6. Microstructure with star-like primary carbides, SEM.
ulation of this microstructure in the single granulometric fractions is shown by histogram in Fig. 11. 5. Microstructure with eutectic spherulites—microstructure without primary carbides is characterised by a very fine lamellar morphology of eutectic formations with spherical shape, features of sectorial growth of the eutectic colony are not present (Fig. 12). Histogram in Fig. 13 indicates the microstructure population in single granulometric fractions. Fig. 10. Microstructure with eutectic colonies without primary carbides, SEM.
Fig. 7. Histogram of percentage population of the microstructure with starlike primary carbides.
Fig. 11. Histogram of percentage population of the microstructure without primary carbides in the eutectic colonies.
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Fig. 12. Microstructure with eutectic spherulites, SEM.
Fig. 15. Microstructural selection map of RS particles of different size.
Fig. 13. Histogram of percentage population of the microstructure with eutectic spherulites.
RS particles with superior globular formations morphologically isolated from the surrounding microstructure were identified in each analysed fraction as well Fig. 14. The dependence of their amount on the size fraction of RS powder was not distinguished. The chemical composition of globular formations was determined by the energy-dispersive X-ray analysis. The average chemical composition of metallic compound of these structural formations was 96 wt.% V, 2 wt.% Cr and 2 wt.% Fe. The map of solidification microstructure population shown in the Fig. 15 results from the evaluation of the single characteristic microstructures occurence in the RS particles of different sizes. Only two structural variants were found in the entire spectrum of granulometric fractions: microstruc-
Fig. 14. Microstructure of RS particle with globular formations, LM.
ture with dispersed globular carbides and microstructure with star-like carbides. Another two variants, microstructure with primary carbides in the centers of the eutectic colonies and eutectic colonies without primary carbides, were observed only in the RS particles less than 125 m in diameter. Eutectic spherulites were identified in the particles under 71 m. TEM of single-stage carbon replicas from RS particles enables to obtain the more detailed information about the morphological variants of characteristic microstructures and their phase composition. The electron diffraction of the singlestage carbon replica of microstructure with dispersed globular carbide particles proves that the RS particles contain of M4 C3 carbide phase of V4 C3 type (Fig. 16). The average chemical composition of metallic compound of dispersed globular carbides moved about the level of 93 wt.% V, 5 wt.% Cr and 2 wt.% Fe. Star-like formations of primary carbides (structural variant 2) were subjected to the electron diffraction as well. As it follows from the Fig. 17, the star-like formations were again formed by the phase of V4 C3 type. The average chemical composition of metallic compound measured on the star-like carbides was 90 wt.% V, 7 wt.% Cr and 3 wt.% Fe.
Fig. 16. Microstructure with dispersed globular carbides. TEM, selective electron diffraction of carbides.
M. Kusy et al. / Journal of Materials Processing Technology 157–158 (2004) 729–734
Fig. 17. Microstructure with star-like primary carbides. TEM, selective electron diffraction of carbides.
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Fig. 20. Microstructure with eutectic spherulites. TEM, selective electron diffraction on the extracted particles.
In the Fig. 19, the eutectic colony without primary carbide can be seen. The carbide skeleton of the colony is formed by the V4 C3 phase as it was confirmed by electron diffraction. Eutectic spherulites represented the finest microstructural formation in the RS particles (Fig. 20). The presence of the V4 C3 carbide phase was affirmed in this case as well. Fig. 18. Microstructure with the primary carbide in the center of eutectic colony. TEM, selective electron diffraction of particles extracted from the eutectic colony.
The microstructure of RS particle with the primary carbide in the center of eutectic colony can be seen in detail in Fig. 18, together with the electron diffraction of particles extracted from eutectic colony around the primary carbide. In this case, the presence of carbide phase V4 C3 was confirmed as well. The average chemical composition of the primary carbide in the center of eutectic colony was measured at the level of 90 wt.% V, 7 wt.% Cr and 3 wt.% Fe.
Fig. 19. Microstructure with eutectic colony without primary carbide. TEM, selective electron diffraction of extracted particles of eutectic carbides.
4. Discussion In the atomisation, very different undercoolings and therefore a variety of microstructures can develop in the droplets of the melt with the same diameter [1,7,10,21]. Five main types of solidification microstructures were observed in the particles of RS powder of the C3Cr3V12 iron based alloy: microstructure with dispersed globular carbides, microstructure with star-like carbides, microstructure with primary carbides in the centers of the eutectic colonies, eutectic colonies without primary carbides and eutectic spherulites. The quantitative results are shown in the Fig. 15 where the population of the main structural variants in the fraction is plotted as a function of particle size. According to present study, microstructure with dispersed globular carbides and the microstructure with star-like carbides represent the dominant morphological variants in RS particles. In general, the cooling and solidification of the undercooled droplets proceed in the following stages [7,10]: quenching of the melt, nucleation, rapid crystal growth during recalescence (quasiadiabatic crystal growth during recalescence) and quasi-isothermal solidification during the period following the recalescence. On the basis of quantitative population of the single microstructural variants in the entire granulometric spectrum of RS powder and thermal history of solidified droplets it can be concluded, that from the point of view of the recales-
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cence effect the resulting microstructures in the RS particles can be divided into two groups. RS particles in which structures with eutectic spherulites and eutectic colonies were developed during solidification belong to the first group. It can be supposed that the solidification of these particles started under relatively high melt undercoolings and no structural changes induced by recalescence were accomplished during solidification. The second group of microstructures in RS particles is comprised by so-called heterogeneous structural variants developed in two stages. Probably, in the primary stage of solidification of RS particles the eutectic colonies were developed. Then, in the postrecalescence stage, the carbide ribs of eutectic colonies were transformed to the globular and quasi-globular carbide phases.
5. Conclusion The microstructural analysis of RS powder particles detected the presence of five main variants of solidification microstructures: microstructure with eutectic spherulites, microstructure with eutectic colonies without primary carbides, microstructure with primary carbides in a center of the eutectic colonies, microstructure with star-like carbides and microstructure with dispersed globular carbides. The main constituent of the rapidly solidified particles was found to be metastable austenite. Carbide phase V4 C3 represents the major solidification carbide phase and the compound of eutectic colonies.
Acknowledgement The research has been supported by VEGA MS SR and SAV within the projects no. 1/7339/20 and 1/6264/99.
References [1] P. Grgac, Associated Professor Graduation Work, MtF Slovak University of Technology, Trnava, 1991. [2] P. Jurci, Ph.D. Thesis, MtF Slovak University of Technology, Trnava, 1996. [3] P. Jurci, In: Technologia 99, 1999, SjF STU Bratislava, 334. [4] M. Behulova, Ph.D. Thesis, SjF Slovak University of Technology, Bratislava, 1996. [5] P. Jurci, D. Vojtech, P. Stolar, Z. Metallkd. 88 (9) (1997) 733. [6] R. Goetzinger, M. Barth, D.M. Herlach, Acta Mater. 46 (5) (1998) 1647. [7] A. Norman, K. Eckler, A. Zambon, F. G¨artner, S.A. Moir, E. Ramous, D.M. Herlach, A.L. Greer, Acta Mater. 46 (10) (1998) 3355. [8] S.C. Gill, W. Kurz, Acta Mater. 43 (1) (1994) 139. [9] S.A. Moir, K. Eckler, D.M. Herlach, Acta Mater. 46 (11) (1998) 4029. [10] M. Behulova, R. Moravc´ık, M. Kusy, L. Caplovic, P. Grgac, L. Stancek, Mater. Sci. Eng. A 304–306 (2001) 540. [11] E. Horn, Pract. Met. 13 (1976) 221. [12] H.F. Fischmeister, A.D. Ozersky, L. Olson, Powder Metall. 25 (1982) 1. [13] G.G. Mukchin, L.P. Korotkova, Metalloved. Term. Obrab. Met. (10) (1982) 8. [14] A.D. Ozersky, H.F. Fischmeister, L. Olson, G.A. Panova, Metalloved. Term. Obrab. Met. 3 (1984) 19. [15] A.P. Gulyayev, L.P. Segienko, E.P. Tolkatcheva, Metalloved. Term. Obrab. Met. (5) (1985) 37. [16] T.A. Tchernishova, L.K. Bolotova, A.P. Gulyayev, L.P. Sergienko, Metalloved. Term. Obrab. Met. 9 (1987) 17. [17] A.N. Popandopulo, L.N. Gerastchenko, Z.S. Bystrova, G.P. Anastasiadi, Poroshkovaya Metallurgiya (12) (1976) 5. [18] A.N. Popandopulo, L.N. Gerastchenko, Poroshkovaya Metallurgiya (11) (1976) 62. [19] L.V. Karabasova, L.A. Savina, N.R. Sayfutdinova, M.A. Shtremel, Izv. Vyss. Uc. Zav.—Tchernaja Metallurgia 1 (1984) 94. [20] P. Grgac, Ph.D. Thesis, Slovak University of Technology, Bratislava, 1986. [21] A. Zambon, B. Badan, K. Eckler, F. G¨artner, A.F. Norman, A.L. Greer, D.M. Herlach, E. Ramous, Acta Mater. 46 (13) (1998) 4657.
Solidification microstructures in the rapidly solidified powder of high alloyed V–Cr tool steel M. Kusya,∗ , L. Caplovica , P. Grgaca , A. Vyrostkovab a
Faculty of Materials Science and Technology, Slovak University of Technology in Bratislava, Bottova 24, Trnava, Slovak Republic b Institute of Materials Research, Slovak Academy of Sciences, Wattsonova 47, Koˇsice, Slovak Republic
Abstract The article deals with the solidification microstructures of the rapidly solidified powder of 3% C–3% Cr–12% V (wt.%) hypereutectic iron alloy Ch3F12 (C3Cr3V12). Five different solidification microstructures were classified in powder particles: microstructure with eutectic spherulites, microstructure with eutectic colonies without primary carbides, microstructure with the primary carbides in the centers of eutectic colonies, microstructure with star-like carbides and microstructure with dispersed globular carbides. Light microscopy, scanning electron microscopy and transmission electron microscopy were used for experimental observations. X-ray diffraction analysis proved the presence of M4 C3 carbide, austenite and indicated the presence of M7 C3 carbides. The chemical composition of primary carbide phase was determined by energy-dispersive X-ray analysis. © 2004 Elsevier B.V. All rights reserved. Keywords: Hypereutectic alloy; Rapid solidification; Solidification microstructures; Powder metallurgy
1. Introduction The tool steels of ledeburite type belonging to the group of metallic materials exploited the gas atomisation as a means, leading to the enhancement of their technical parameters. The iron based alloy Ch3F12 characterised by the presence of carbides of eutectic as well as primary origin can be categorised to this group of materials [1–3]. The cooling rates during melt atomisation achieve values of order up to 106 K s−1 [4]. Owing to the rapid quenching of droplets in the atomisation process, the solidification of developing microvolumes is carried out at the non-equilibrium conditions and consequently the changes in the character of solidification microstructure occur [5–10]. Microstructures and phase composition of classical tool steels of ledeburite type prepared by gas atomisation process have been studied and reported in several works [11–16]. Relatively less attention has been devoted to the analysis of the rapidly solidified (RS) powders from new tool materials with the increased content of vanadium and carbon [17–19]. The analysis of primary structures of RS particles ∗
Corresponding author. E-mail address: [email protected] (P. Grgac).
0924-0136/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2004.07.141
was accomplished for powders from hypereutectic alloys on ferrous base [20]. The main aim of this article is to analyse in detail the solidification microstructures in the particles of rapidly solidified powder of hypereutectic alloy C3Cr3V12 in the base stage, i.e. after atomisation.
2. Material and experimental methods Rapidly solidified powder marked as C3Cr3V12 with the chemical composition 3% C–3% Cr–12% V (wt.%) belongs to the iron based hypereutectic alloys. The powder produced by nitrogen gas atomisation contains of globular particles of different sizes. RS powder was divided into 13 standard size fractions in the range from 40 to 400 m. For microstructural analysis, RS powder particles were bounded by electrolytically deposited nickel, metallographically prepared using standard methods and NITAL etched. Methods and experimental techniques of light microscopy (LM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were applied. The analysis of minimum 100 particles from each size fraction was performed to evaluate
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Fig. 1. X-ray diffraction pattern of RS powder C3Cr3V12 in the base state.
the relative proportion of the basic types of solidification microstructures in the analysed powder. Transmission electron microscopy and energy-dispersive X-ray analyses were performed on the single-stage carbon replicas. X-ray spectrometric analysis with Bragg–Brentano semifocusation geometry was carried out using Cr K␣ radiation.
3. Results of experiments and disscusion
Fig. 3. The presence of different types of solidification microstructures, light microscopy.
Based on the analysis of primary microstructures by scanning electron microscopy, five main types of solidification microstructures were identified in the particles of RS powder. From the morphological point of view, the characteristic solidification microstructures of RS particles can be described as follows:
The phase analysis of RS powder of C3Cr3V12 alloy was performed by X-ray diffraction analysis. The diffraction pattern shown in the Fig. 1 was obtained from granullometric fraction 125 / 160 m. Diffraction pattern contains two dominant peaks corresponding the diffractions on the (1 1 1) and (2 0 0) austenite planes. The main constituent of the rapidly solidified powder was found to be the metastable austenite. The presence of carbide phases of M4 C3 type was detected by X-ray analysis as well. Diffraction peaks indicate also the presence of M7 C3 carbides. RS powder particles of C3Cr3V12 tool alloy had after the nitrogen gas atomisation globular morphology as it can be seen in the Fig. 2. Light microscopy of RS particles revealed the presence of different structural modifications on metallographical sections (Fig. 3), which evoked the need of more detailed analysis of solidification microstructures from the entire granulometric spectrum applying the SEM and TEM.
1. Microstructure with the disperse globular carbides—is defined as a structure with globular carbide particles evenly dispersed in the matrix (Fig. 4). The percentage population of this microstructure in the single granulometric fractions is illustrated in Fig. 5. 2. Microstructure with star-like primary carbides—is characterised by the presence of isolated carbides with the morphologically variable multi-arm star-like formations (Fig. 6). Fig. 7 shows the microstructure population in the entire size fractions. 3. Microstructure with the primary carbides in the centers of eutectic colonies—this structure contains star-like carbides surrounded by eutectic whereby the primary carbide phase intergrowths continually to the carbide skeleton of eutectic colony (Fig. 8). The microstructure population in each size fraction is depicted by histogram in Fig. 9. 4. Microstructure with eutectic colonies without primary carbides—colony with the characteristic features of sectorial growth, in the center of eutectic colony the primary carbide was not developed (Fig. 10). The percentage pop-
Fig. 2. Spherical morphology of RS powder particles of C3Cr3V12 alloy, SEM.
Fig. 4. Microstructure with disperse globular carbides, SEM.
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Fig. 8. Microstructure with the primary carbide in the center of eutectic colony, SEM. Fig. 5. Histogram of percentage population of the microstructure with disperse globular carbides.
Fig. 9. Histogram of percentage population of the microstructure with the primary carbides in the center of eutectic colonies. Fig. 6. Microstructure with star-like primary carbides, SEM.
ulation of this microstructure in the single granulometric fractions is shown by histogram in Fig. 11. 5. Microstructure with eutectic spherulites—microstructure without primary carbides is characterised by a very fine lamellar morphology of eutectic formations with spherical shape, features of sectorial growth of the eutectic colony are not present (Fig. 12). Histogram in Fig. 13 indicates the microstructure population in single granulometric fractions. Fig. 10. Microstructure with eutectic colonies without primary carbides, SEM.
Fig. 7. Histogram of percentage population of the microstructure with starlike primary carbides.
Fig. 11. Histogram of percentage population of the microstructure without primary carbides in the eutectic colonies.
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Fig. 12. Microstructure with eutectic spherulites, SEM.
Fig. 15. Microstructural selection map of RS particles of different size.
Fig. 13. Histogram of percentage population of the microstructure with eutectic spherulites.
RS particles with superior globular formations morphologically isolated from the surrounding microstructure were identified in each analysed fraction as well Fig. 14. The dependence of their amount on the size fraction of RS powder was not distinguished. The chemical composition of globular formations was determined by the energy-dispersive X-ray analysis. The average chemical composition of metallic compound of these structural formations was 96 wt.% V, 2 wt.% Cr and 2 wt.% Fe. The map of solidification microstructure population shown in the Fig. 15 results from the evaluation of the single characteristic microstructures occurence in the RS particles of different sizes. Only two structural variants were found in the entire spectrum of granulometric fractions: microstruc-
Fig. 14. Microstructure of RS particle with globular formations, LM.
ture with dispersed globular carbides and microstructure with star-like carbides. Another two variants, microstructure with primary carbides in the centers of the eutectic colonies and eutectic colonies without primary carbides, were observed only in the RS particles less than 125 m in diameter. Eutectic spherulites were identified in the particles under 71 m. TEM of single-stage carbon replicas from RS particles enables to obtain the more detailed information about the morphological variants of characteristic microstructures and their phase composition. The electron diffraction of the singlestage carbon replica of microstructure with dispersed globular carbide particles proves that the RS particles contain of M4 C3 carbide phase of V4 C3 type (Fig. 16). The average chemical composition of metallic compound of dispersed globular carbides moved about the level of 93 wt.% V, 5 wt.% Cr and 2 wt.% Fe. Star-like formations of primary carbides (structural variant 2) were subjected to the electron diffraction as well. As it follows from the Fig. 17, the star-like formations were again formed by the phase of V4 C3 type. The average chemical composition of metallic compound measured on the star-like carbides was 90 wt.% V, 7 wt.% Cr and 3 wt.% Fe.
Fig. 16. Microstructure with dispersed globular carbides. TEM, selective electron diffraction of carbides.
M. Kusy et al. / Journal of Materials Processing Technology 157–158 (2004) 729–734
Fig. 17. Microstructure with star-like primary carbides. TEM, selective electron diffraction of carbides.
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Fig. 20. Microstructure with eutectic spherulites. TEM, selective electron diffraction on the extracted particles.
In the Fig. 19, the eutectic colony without primary carbide can be seen. The carbide skeleton of the colony is formed by the V4 C3 phase as it was confirmed by electron diffraction. Eutectic spherulites represented the finest microstructural formation in the RS particles (Fig. 20). The presence of the V4 C3 carbide phase was affirmed in this case as well. Fig. 18. Microstructure with the primary carbide in the center of eutectic colony. TEM, selective electron diffraction of particles extracted from the eutectic colony.
The microstructure of RS particle with the primary carbide in the center of eutectic colony can be seen in detail in Fig. 18, together with the electron diffraction of particles extracted from eutectic colony around the primary carbide. In this case, the presence of carbide phase V4 C3 was confirmed as well. The average chemical composition of the primary carbide in the center of eutectic colony was measured at the level of 90 wt.% V, 7 wt.% Cr and 3 wt.% Fe.
Fig. 19. Microstructure with eutectic colony without primary carbide. TEM, selective electron diffraction of extracted particles of eutectic carbides.
4. Discussion In the atomisation, very different undercoolings and therefore a variety of microstructures can develop in the droplets of the melt with the same diameter [1,7,10,21]. Five main types of solidification microstructures were observed in the particles of RS powder of the C3Cr3V12 iron based alloy: microstructure with dispersed globular carbides, microstructure with star-like carbides, microstructure with primary carbides in the centers of the eutectic colonies, eutectic colonies without primary carbides and eutectic spherulites. The quantitative results are shown in the Fig. 15 where the population of the main structural variants in the fraction is plotted as a function of particle size. According to present study, microstructure with dispersed globular carbides and the microstructure with star-like carbides represent the dominant morphological variants in RS particles. In general, the cooling and solidification of the undercooled droplets proceed in the following stages [7,10]: quenching of the melt, nucleation, rapid crystal growth during recalescence (quasiadiabatic crystal growth during recalescence) and quasi-isothermal solidification during the period following the recalescence. On the basis of quantitative population of the single microstructural variants in the entire granulometric spectrum of RS powder and thermal history of solidified droplets it can be concluded, that from the point of view of the recales-
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cence effect the resulting microstructures in the RS particles can be divided into two groups. RS particles in which structures with eutectic spherulites and eutectic colonies were developed during solidification belong to the first group. It can be supposed that the solidification of these particles started under relatively high melt undercoolings and no structural changes induced by recalescence were accomplished during solidification. The second group of microstructures in RS particles is comprised by so-called heterogeneous structural variants developed in two stages. Probably, in the primary stage of solidification of RS particles the eutectic colonies were developed. Then, in the postrecalescence stage, the carbide ribs of eutectic colonies were transformed to the globular and quasi-globular carbide phases.
5. Conclusion The microstructural analysis of RS powder particles detected the presence of five main variants of solidification microstructures: microstructure with eutectic spherulites, microstructure with eutectic colonies without primary carbides, microstructure with primary carbides in a center of the eutectic colonies, microstructure with star-like carbides and microstructure with dispersed globular carbides. The main constituent of the rapidly solidified particles was found to be metastable austenite. Carbide phase V4 C3 represents the major solidification carbide phase and the compound of eutectic colonies.
Acknowledgement The research has been supported by VEGA MS SR and SAV within the projects no. 1/7339/20 and 1/6264/99.
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