Assessment of precipitation behavior in dental castings of a Co–Cr–Mo alloy

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Assessment of precipitation behavior in dental castings of a Co–Cr–Mo alloy Kenta Yamanaka, Manami Mori, Akihiko Chiba

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S1751-6161(15)00223-4 http://dx.doi.org/10.1016/j.jmbbm.2015.06.020 JMBBM1514

To appear in: Journal of the Mechanical Behavior of Biomedical Materials

Received date:6 May 2015 Revised date: 17 June 2015 Accepted date: 19 June 2015 Cite this article as: Kenta Yamanaka, Manami Mori, Akihiko Chiba, Assessment of precipitation behavior in dental castings of a Co–Cr–Mo alloy, Journal of the Mechanical Behavior of Biomedical Materials, http://dx.doi.org/ 10.1016/j.jmbbm.2015.06.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Assessment of precipitation behavior in dental castings of a Co–Cr–Mo alloy

Kenta Yamanakaa, *, Manami Moria,b, and Akihiko Chibaa

a

Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai

980-8577, Japan b

Department of Materials and Environmental Engineering, Sendai National College of

Technology, 48 Nodayama, Medeshima-Shiote, Natori 981-1239, Japan

*Corresponding author: Kenta Yamanaka Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Tel.: +81 22 215 2118, Fax: +81 22 215 2116 E-mail: [email protected]

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ABSTRACT

This study investigated solute portioning and precipitation in dental castings of a Co–Cr–Mo alloy and discussed their effects on alloy performance, in particular, the mechanical properties. Samples of a commercial Co–29Cr–6Mo (mass%) alloy were prepared using a dental-casting machine. The precipitates formed owing to the partitioning behaviors of the alloying elements were investigated using scanning electron microscopy, electron backscatter diffraction analysis, electron probe microanalysis, and transmission electron microscopy. The prepared samples exhibited a very coarse face-centered-cubic Ȗ-phase dendritic structure with an average grain size of a few millimeters. A large number of precipitates, which decomposed further into complex interdendritic constituents (ıand M23C6 carbide phases) were observed in the interdendritic regions rich in Cr, Mo, Si, and C. A reaction between the ı-phase and carbon is probably responsible for the carbide M23C6; however, this reaction did not occur to completion in the current case in spite of slow cooling (i.e., long exposure to elevated temperatures) in dental casting. While these precipitates result in high strength (hardness) and/or brittleness, the properties can be improved further by optimizing the alloy composition and the manufacturing process. The results of this study shed light on the significance of precipitation control in dental castings of Co–Cr–Mo alloys and should aid in the design of novel biomedical Co–Cr-based dental alloys that exhibit better performances.

Keywords: Co–Cr–Mo alloys; dental casting; microstructure; precipitate; solidification segregation; mechanical properties

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1. Introduction Co–Cr–Mo-based alloys have excellent mechanical properties and corrosion and wear resistances. As a result, these alloys have been recognized as ideal metallic biomaterials that can be subjected to severe in vivo conditions. In dentistry, removable partial denture frameworks formed with Co−Cr−Mo alloys are now extensively used (Craig et al., 2000; Powers and Sakaguchi, 2006; Wataha, 2002) because the raw material costs are much lower than those of the Au-based alloys previously used for dental restorations. Further, Co−Cr−Mo alloys exhibit better corrosion resistance than Ni–Cr-based alloys (Viennot et al., 2006). In addition, Ni can cause allergies and cancer in living organisms (Denkhaus and Salnikow, 2002) and is therefore not suitable for use as a biomaterial (Chiba et al., 1999). Hence, biocompatible Co–Cr-based cast alloys that do not contain Ni have attracted much research interest. The properties of metallic materials are known to strongly depend on their microstructure. Biomedical Co−Cr-based alloys exhibit face-centered-cubic Ȗ-phases and hexagonal-close-packed İ-phases, which remain stable as matrix phases at high temperatures (~1173 K and greater) and at room temperature, respectively. It should be noted that precipitation in the matrix phase is of critical importance in the Co−Cr alloy system. The Co−Cr binary phase diagram (Okamoto, 2003) suggests the precipitation of a hard, brittle intermetallic compound, namely a ı-phase (CoCr; space group P42/mnm) (Dickins et al., 1956); this phase also appears in Co−Cr−Mo and Co−Cr−W alloy systems, which are used

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for dental applications (Kurosu et al., 2010; Yamanaka et al., 2014d). In addition, a variety of carbides and nitrides, such as M23C6, M6C, Ș (M12C–M6C), and ʌ (M2T3X where X = C, N), among others, have been observed as precipitates in Co−Cr-based alloys of different compositions (Giacchi et al., 2011; Mineta et al., 2011; Yamanaka et al., 2013a, 2014a, 2014d). The precipitation behavior of as-cast alloys is much more complex than that of the hot-forged or heat-treated alloys used in orthopedic applications. This is because the microstructures of the as-cast alloys, which exhibit solidification segregation (a phenomenon directly related to the formation of precipitates), vary significantly with the casting conditions (e.g., the melt temperature and cooling rate). Dental castings of Co–Cr alloys are general produced using sand molds or investment materials. This is followed by cooling in air at room temperature at a rate lower than that used for the metal-mold castings conventionally used in other industries. Therefore, it is necessary to evaluate the microstructures of alloy specimens prepared by actual dental-casting techniques and not those produced using other procedures such as those employed for orthopedic applications or for producing ingots for metal working. Dental alloys are now expected to exhibit high yield stresses (e.g., higher than 500 MPa (Yoda et al., 2012)). An increase in strength basically improves the mechanical reliability of restorations, which are subjected to occlusal forces. Even though there have been several reports on the microstructures of as-cast Co–Cr alloys (Caudillo et al., 2002;

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Lee et al., 2008; Liao et al., 2012; Matkoviü et al., 2004; Ramírez et al., 2002; Saldívar-García and López, 2005; Yamanaka et al., 2014a), to the best of our knowledge, there have been few studies on the microstructural characteristics of dental-cast Co−Cr alloys and on microstructural optimization for designing high-strength cast Co−Cr alloys. An in-depth understanding of the microstructural characteristics of such alloys and, in particular, of solute partitioning during solidification segregation and the resulting precipitation will aid in the development of high-strength Co−Cr dental alloys through microstructural optimization. The aim of the present study was to investigate the microstructure of a commercial high-strength Co−Cr−Mo alloy used for dental casting, in order to establish a strategy for designing high-strength Co–Cr-based alloys. The obtained experimental results are discussed and compared with the results of thermodynamic calculations.

2. Materials and methods 2.1. Sample preparation An ingot of the Co–Cr–Mo alloy (Cobaltan, Shofu Inc., Kyoto, Japan) was melted and cast using a mold prepared with investment materials (Snow White, Shofu Inc., Kyoto, Japan). The chemical composition of the alloy is shown in Table 1; the alloy composition was essentially Co−29Cr−6Mo (mass%), with small amounts of carbon and nitrogen also present.

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An automatic dental-casting machine with a high-frequency induction heating system (Argoncaster-i, Shofu Inc., Kyoto, Japan) was used for producing the test samples. The mold was kept at room temperature before being used for casting. The dimensions of the samples used for the microstructural evaluation were the following: approximately 12 × 12 mm2 in cross section and 1 mm in thickness.

2.2. Microstructural characterization Scanning electron microscopy (SEM) and elemental mapping were performed using a field-emission electron probe microanalyzer (FE-EPMA) (JEOL JXA-8430F), which was operated at an acceleration voltage of 15 kV. Electron backscatter diffraction (EBSD) analysis was performed using an FESEM (FEI XL30S-FEG) system operated at 15 kV. The EBSD scan data were collected and analyzed using an orientation image microscopy system (TexSEM Laboratories, Inc.); a 3-ȝm step size was used for the hexagonal scan grid. In addition to the orientation analyses, kernel average misorientation (KAM) analysis, performed on the basis of the EBSD data, was used to measure the magnitude of the locally evolved plastic strain. The KAM value represents the average misorientation angle between all adjacent measurement points within a grain and is correlated to the densities of the geometrically necessary dislocations (Calcagnotto et al., 2010). In this study, the KAM values of up to five neighboring points with a maximum misorientation angle of 2° were calculated (Yamanaka et al., 2013b). The samples for the SEM, EBSD, and EPMA analyses

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were prepared by mechanical grinding and polishing using emery papers and a 0.3-ȝm alumina suspension; this was followed by polishing to a mirror-like finish using a 0.04-ȝm colloidal silica solution. In order to evaluate the crystal structures of the precipitates formed in the as-cast alloy specimens, transmission electron microscopy (TEM) was performed using a Topcon EM002B system operated at 200 kV. In addition, we investigated the precipitation chemistry and elemental distributions in the interdendritic regions using scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS) mapping. The analyses were performed using a JEOL JEM-ARM200F system with a field-emission gun; the system was operated at 200 kV. The TEM samples were prepared using a focused ion beam (FEI QUANTA 200 3D) device.

2.3. Mechanical properties Microindentation hardness tests were performed to characterize the samples mechanically. The measurements were made using an HMV Vickers microhardness tester (Shimadzu Corporation, Kyoto, Japan). A force of 9.8 N was applied for an indentation time of 10 s. The average value and its standard deviation were calculated. Uniaxial tensile tests were also performed on the as-cast specimens in accordance with the JIS1 T 6004 standard for evaluating the tensile properties of metallic materials used

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JIS: Japanese Industrial Standards 7

for dental applications. The tensile test specimens, which were 2 mm in diameter and 20 mm in gauge length, were prepared using the dental-casting technique described above. The specimens were strained to failure at a crosshead speed of 1.5 mm/min using a Shimadzu Autograph AG-5000B system.

2.4. Thermodynamic calculation The constituent phases at equilibrium were determined using the software Thermo-Calc; the obtained results were compared with the experimental results. We obtained the thermodynamic data sets used for the calculations from the TCS Steels/Fe-alloys database (Version 6).

3. Results 3.1. SEM observations and EPMA mapping Figure 1 shows an SEM backscatter electron (BSE) image of an as-cast Co−29Cr−6Mo alloy sample. Owing to the different crystallographic orientations, we could observe a highly coarse dendritic microstructure, whose grain size was as large as a few millimeters. This was probably owing to the fact that the dental casting was cooled slowly. Fine precipitates, which appear as bright particles in Fig. 1, were identified within the grains. These precipitates, whose diameters were in the range of several micrometers, were aligned parallel to each other. Since the precipitates exhibited a bright contrast, they probably had

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higher concentrations of the constituent metallic element(s) with higher atomic numbers (e.g., molybdenum). Figure 2 shows the results of the EBSD analysis of a dental-cast specimen of the Co−29Cr−6Mo alloy. The image quality (IQ) map, the inverse pole figure (IPF) map, which shows the crystallographic orientations parallel to the normal direction of the observed plane, and the phase map are shown in Figs. 2a−c, respectively. A very coarse dendritic microstructure can also be seen in Fig. 2. The microstructure consisted dominantly of Ȗ-phase (Fig. 2c). Irregular grain boundaries were also observed, while no annealing twins were present (Figs. 2a and b). High-magnification EBSD maps of the as-cast specimen are shown in Figs. 2d–f. No İ-phase was identified; therefore, the corresponding phase map is not shown here. The IQ map, shown in Fig. 2d, reveals a Ȗ-grain with periodically precipitated particles (represented in black). It can be seen that low-angle boundaries evolved within the Ȗ-grain (Fig. 2e). These low-angle boundaries are also visible in Fig. 2d, owing to the fact that the IQ values are low in such regions. Figure 2f shows the KAM map of the as-cast alloy sample. Interestingly, a higher degree of KAM (i.e., local strains) was observed around such boundaries, while the intragranular regions showed lower KAM values. Thus, it is likely that these low-angle boundaries form because of the impingement of dendrites with similar crystallographic orientations in the final solidification regions (i.e., in the interdendritic regions).

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The phenomena of solidification segregation and precipitation were examined through EPMA analysis. Figure 3a shows an SEM BSE image of the as-cast specimen. Precipitates were observed in the Ȗ-grain microstructures. The elemental maps corresponding to Fig. 3a are shown in Figs. 3b−i. The elements Cr, Mo, C, and Si were segregated into precipitates and their surrounding regions, while Co was depleted. This indicated the formation of a carbide phase. Nitrogen and sulfur were not observed; however, their concentrations in the precipitates increased. The oxygen concentration was slightly low in the precipitates compared with the surrounding Ȗ dendritic matrix.

3.2. TEM and STEM observations Figure 4a shows a TEM bright-field (BF) image of the as-cast specimen. Aggregates of the precipitates can be seen in the center of the image. The selected area diffraction (SAD) patterns, which were taken from the areas labeled (b) and (c) in Fig. 4a, are shown in Figs. 4b and c, respectively. The SAD analysis revealed the presence of two kinds of precipitates: the ı-phase and an M23C6-type carbide belonging to the space groups P42/mnm and Fm¯3m. These precipitates can also be seen in the dark-field (DF) images shown in Figs. 4d and e. Interestingly, the ı-phase had fine nanostructures within them, as shown in Fig. 4e. Figure 5 shows the results of (a) the STEM BF observations and the corresponding EDS maps of (b) Co, (c) Cr, (c) Mo, and (d) Si, which were taken from the same area as that in Fig. 4. Three phases with different chemical compositions were identified. The chemical

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compositions of these phases, as determined from the STEM-EDS analyses, are listed in Table 2. Of the three labeled areas, “Area 1” in Fig. 5a had the highest concentration of cobalt and the lowest concentration of molybdenum. Both “Area 2” and “Area 3” exhibited Mo concentrations as high as 25 mass%; however, the Co and Cr concentrations in these areas differed. Note that the interfaces between the phases were not diffuse and that the alloying elements were well segregated. Finally, Si partitioning was not clearly identified within these regions.

3.3. Mechanical properties Figure 6 shows the typical engineering stress−strain curve of the as-cast specimen. Table 3 lists the tensile properties, determined from tensile tests, together with the Vickers hardness of the as-cast specimen as measured at room temperature. The 0.2% proof stress and ultimate tensile strength were markedly high in spite of the sample being in the as-cast state. Further, the sample also exhibited an acceptable elongation-to-failure, which was higher than 10%. That is, the tensile properties of the specimen were much better than those recommended by standards such as ISO 22764 (Type 5) and JIS T 6115, among others. The as-cast alloy specimen also showed high hardness (370 Hv).

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4. Discussion As shown in Table 1, the investigated alloy contained carbon and nitrogen in sufficiently high concentrations, with both being present in amounts as high as 0.2 mass%. First, the equilibrium phases of the investigated alloy system were examined using thermodynamic calculations. Figure 7 shows a vertical section of the phase diagram of the Co–29Cr–6Mo–0.2N–xC (0 ” x ” 0.3) system, calculated using the Thermo-Calc software. The phase diagram indicated that the Ȗ-phase is stable at temperatures higher than ~1200 K; on the other hand, the room-temperature equilibrium-phase matrix consisted of the İ-phase. The intermetallic compound, namely, the ı-phase, and the Cr2N phase (hexagonal, space group: P¯31m) coexist at temperatures of 1200–1300 K. Increasing the carbon concentration retards the formation of the ı-phase; instead, the M23C6-type carbide (space group: Fm¯3m) is formed at higher carbon concentrations. The ȝ-phase (intermetallic compound, space group: R¯3m), which is rich in molybdenum, appears in the low-temperature region in Fig. 7. As shown in Fig. 2b, the matrix phase of the investigated samples was the Ȗ-phase. Further, we did not notice the İ-phase, which is usually observed in biomedical Co−Cr−Mo alloys as a martensitic phase formed after quenching from temperatures at which the Ȗ-phase is stable, plastic deformation, and an isothermal heat treatment during which the İ-phase is thermodynamically stable (< 1173 K) (Mori et al., 2010, 2014; Yamanaka et al., 2013a, 2013b, 2014b). The stabilization of the Ȗ-phase probably occurs owing to the addition of

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nitrogen. Moreover, Cr2N is known to precipitate at the nanoscale, which stabilizes the Ȗ-phase by suppressing the martensitic transformation (Yamanaka et al., 2013a, 2014b). Therefore, the Cr2N phase should have been present in the samples of the investigated alloy. The addition of carbon also stabilizes the Ȗ-phase (Yamanaka et al., 2014a, 2014c). On the other hand, we found a large number of precipitates in the interdendritic regions of the Ȗ-phase microstructure. The EPMA analysis (Fig. 3) indicated the segregation of carbon into interdendritic regions having precipitates; the concentrations of Cr, Mo, and Si were also higher in such regions. Therefore, a carbide phase, which usually has a large amount of Cr and C, was formed in the interdendritic regions. The TEM/STEM observations showed the solute distributions in detail. We could identify three kinds of phases with different concentrations of Co, Cr, and Mo. The Co-rich grains (labeled “Area 1” in Fig. 5a) are probably of the Ȗ-phase, given its chemical composition. Interestingly, two types of precipitates were observed. That corresponding to the area labeled “Area 3” was probably the ı-phase, as revealed by the SAD analysis (Fig. 4). In contrast, “Area 2” was probably M23C6-type carbide, in keeping with the results of the TEM-SAD and STEM-EDS analyses, even though the carbon distribution could not be determined, owing to limitations of the EDS technique. These results indicated that the two kinds of precipitates, that is, the intermetallic ı-phase and M23C6-type carbide, coexisted in the samples of the Co−29Cr−6Mo-based alloy prepared using a dental-casting machine.

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A Co−Cr−Mo alloy with a microstructure similar to that described above has been reported previously. Ramírez et al. (2002) prepared specimens of the alloy Co–25.5Cr–5.5Mo–0.26C under controlled solidification conditions (i.e., using controlled heating and cooling rates) and reported the precipitation behavior of the as-cast samples. Although they did not investigate the solute distributions, they did suggest that, during cooling to temperatures lower than 1423 K, the carbide M23C6 precipitates from the ı-phase as per the following reaction: ı + C ĺ M23C6. Thus, the increased carbon concentration would lead to a transition from the intermetallic ı-phase to the carbide M23C6. The present study is first report on this type of carbide-precipitation path in specimens of Co−Cr−Mo alloys prepared by dental casting; however, this reaction does not occur to completion during actual dental casting. Since the lamellar-type precipitates appear at temperatures lower than 1262 K (Ramírez et al., 2002), microstructure formation in the dental-cast specimens of the investigated alloy will likely occur at relatively high temperatures (i.e., those higher than ~1273 K). Note that the ȝ-phase, whose formation was predicted by Fig. 7, is not observed experimentally in biomedical Co−Cr−Mo alloys. In addition, nonequilibrium carbides or nitrides (Giacchi et al., 2011; Mineta et al., 2011; Yamanaka et al., 2013a, 2014a, 2014d) were not observed in this study. Whether this reaction occurs locally during solidification has not been investigated in the case of dental-cast alloy specimens, even though the precipitation behavior of metallic

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materials significantly affects their properties. Next, we discuss the relationship between the precipitation behavior of dental-cast Co−Cr alloy specimens and their mechanical properties. In a previous study, we had prepared a Co−28Cr−9W-based dental alloy with a low carbon concentration (Yamanaka et al., 2014a). The alloy had a very coarse-grained microstructure, which was similar to that shown in Fig. 1; further, the microstructure contained few precipitates. The stress−strain curve of the Co−28.5Cr−8.99W−0.04C cast alloy (without nitrogen doping) (Yamanaka et al., 2014a) is also shown in Fig. 6. The 0.2% proof stress of this Co−Cr−W-based dental alloy was much lower than that of the alloy investigated in the present study, in spite of the excellent ductility and comparable ultimate tensile strength to the present alloy. Further, the former alloy did not meet the ISO 22764 standard (Type 5, 0.2% proof stress should be higher than 500 MPa). This means that precipitates are of great importance for improving the strength of as-cast Co−Cr-based dental alloys. Both the ı-phase and the M23C6-type carbide observed in the present study were much harder than the surrounding Ȗ-matrix and thus probably strengthened the alloy. However, the volume fraction of the ı-phase is usually not very high, and the precipitation of this phase is not considered to be an effective way for increasing the alloy strength (Kurosu et al., 2007). This result is contrary to the fact that the precipitation of a fine, intergranular ı-phase in a high-volume fraction can significantly harden the corresponding alloy, such as that reported for high-entropy alloys (Tsai et al., 2013). In general, this goal is difficult to

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realize, and in the present study, the ı-phase was preferentially located in the interdendritic regions (Fig. 1). In addition, the intermetallic compound ı-phase is known to be brittle and therefore has negative impacts on the alloy properties. For instance, the formation of the ı-phase would result in insufficient ductility as well as reduced wear resistance due to two-body abrasive wear (Chen et al., 2013). Further, surface cracks caused by the brittle ı-phase may lead to contribute to crevice corrosion, although the effect of the ı-phase fraction as low as the present case is not significant on corrosion properties of the alloys (Kurosu et al., 2006). These studies indicate that the ı-phase is not considered to be an effective strengthening phase and should generally be avoided. In contrast, the carbide M23C6 is known to be a strengthening phase in biomedical Co−Cr alloys (Yamanaka et al., 2014a). This strengthening phase is the reason why the specimens of the investigated alloy exhibited such high strengths, even though their Ȗ-grain microstructure was very coarse. Therefore, the above-mentioned reaction between the ı-phase and carbon to produce the carbide M23C6 should occur to completion for realizing high-strength Co−Cr-based dental-cast alloys. However, carbide precipitation is also often detrimental to ductility (Yamanaka et al., 2014a) as well as to corrosion resistance (Bettini et al., 2011); as a result, too much carbon should not be added to the alloys. Therefore, other strengthening strategies should also be employed. Nitrogen strengthening, that is, the nanoprecipitation of chromium-containing nitrides, is one method for strengthening that maintains alloy ductility (Yamanaka et al., 2013a, 2014b).

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5. Conclusions The present study investigated solute portioning and precipitation in dental castings of a Co–Cr–Mo alloy, which have not been well characterized. The results obtained in this study can act as guidelines for tuning the microstructure of Co−Cr-based dental-cast alloys in order to increase their strength. It was found that precipitation is of critical importance for this purpose. Reducing grain (or dendrite) size is also effective in increasing the mechanical strength as well as the ductility. Further, the casting conditions should also be optimized. However, additional studies are needed to elucidate the effects of the concentrations of carbon and/or other alloying elements on the material properties, including the mechanical characteristics and the corrosion resistance, which are also important parameters for evaluating metallic biomaterials.

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Acknowledgements We thank Mr. Yasuhiro Torita, Shofu Inc., for providing the cast samples and for assistance with determining the tensile properties of the investigated alloy. We also thank Mr. Shun Ito, Dr. Makoto Nagasako, Dr. Yumiko Kodama, and Ms. Kumiko Suzuki for their technical assistance. This research was supported by a Grant-in-Aid for Young Scientists (B) (No. 26870050) from the “Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan, at the Center for Integrated Nanotechnology Support, Tohoku University; the Inter-University Cooperative Research Program; the Innovative Research for Biosis-Abiosis Intelligent Interface program of MEXT, Japan; and the Adaptable and Seamless Technology Transfer Program through Target-driven R&D (A-STEP), Japan Science and Technology Agency (JST), Japan.

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References

Bettini, E., Eriksson, T., Boström, M., Leygraf, C., Pan, J., 2011. Influence of metal carbides on dissolution behavior of biomedical CoCrMo alloy: SEM, TEM and AFM studies. Electrochim. Acta 56, 9413–9419.

Calcagnotto, M., Ponge, D., Demir, E., Raabe, D., 2010. Orientation gradients and geometrically necessary dislocations in ultrafine grained dual-phase steels studied by 2D and 3D EBSD. Mater. Sci. Eng. A 527, 2738–2746.

Caudillo, M., Herrera-Trejo, M., Castro, M.R., Ramírez, E., González, C.R., Juárez, J.I., 2002. On carbide dissolution in an as-cast ASTM F-75 alloy. J. Biomed. Mater. Res. 59, 378–385.

Chen, Y., Li, Y., Kurosu, S., Yamanaka, K., Tang, N., Koizumi, Y., Chiba, A., 2013. Effects of Sigma Phase and Carbide on the Wear Behavior of CoCrMo Alloys in Hanks’ Solution. Wear 310, 51–62.

Chiba, A., Li, X.G., Kim, M.S., 1999. High work-hardening rate and deformation twinning of Co-Ni-based superalloy at elevated temperatures. Philos. Mag. 79, 1533–1554.

Craig, R., Powers, J., Wataha, J., 2000. Dental materials: properties and manipulation, 7th ed. Mosby, St Louis.

19

Denkhaus, E., Salnikow, K., 2002. Nickel essentiality, toxicity, and carcinogenicity. Crit. Rev. Oncol. Hematol. 42, 35–56.

Dickins, G.J., Douglas, A.M.B., Taylor, W.H., 1956. The Crystal Structure of the Co−Cr ı Phase. Acta Crystallogr. 9, 297–303.

Giacchi, J. V., Morando, C.N., Fornaro, O., Palacio, H.A., 2011. Microstructural characterization of as-cast biocompatible Co−Cr−Mo alloys. Mater. Charact. 62, 53–61.

Kurosu, S., Matsumoto, H., Chiba, A., 2010. Isothermal Phase Transformation in Biomedical Co−29Cr−6Mo Alloy without Addition of Carbon or Nitrogen. Metall. Mater. Trans. A 41, 2613–2625.

Kurosu, S., Nomura, N., Chiba, A., 2006. Effect of Sigma Phase in Co−29Cr−6Mo Alloy on Corrosion Behavior in Saline Solution. Mater. Trans. 47, 1961–1964.

Kurosu, S., Nomura, N., Chiba, A., 2007. Microstructure and Mechanical Properties of Co−29Cr−6Mo Alloy Aged at 1023 K. Mater. Trans. 48, 1517–1522.

Lee, S.-H., Nomura, N., Chiba, A., 2008. Significant Improvement in Mechanical Properties of Biomedical Co−Cr−Mo Alloys with Combination of N Addition and Cr-Enrichment. Mater. Trans. 49, 260–264.

20

Liao, Y., Pourzal, R., Stemmer, P., Wimmer, M.A., Jacobs, J.J., Fischer, A., Marks, L.D., 2012. New insights into hard phases of CoCrMo metal-on-metal hip replacements. J. Mech. Behav. Biomed. Mater. 12, 39–49.

Matkoviü, T., Matkoviü, P., Malina, J., 2004. Effects of Ni and Mo on the microstructure and some other properties of Co-Cr dental alloys. J. Alloys Compd. 366, 293–297.

Mineta, S., Namba, S., Yoneda, T., Ueda, K., Narushima, T., 2011. Precipitates in As-Cast and Heat-Treated ASTM F75 Co−Cr−Mo−C Alloys Containing Si and/or Mn. Metall. Mater. Trans. A 42, 1941–1949.

Mori, M., Yamanaka, K., Chiba, A., 2014. Phase decomposition in biomedical Co–29Cr–6Mo–0.2N alloy during isothermal heat treatment at 1073 K. J. Alloys Compd. 520, 411–416.

Mori, M., Yamanaka, K., Matsumoto, H., Chiba, A., 2010. Evolution of cold-rolled microstructures of biomedical Co−Cr−Mo alloys with and without N doping. Mater. Sci. Eng. A 528, 614–621.

Okamoto, H., 2003. Co−Cr (Cobalt−Chromium). J. Phase Equilibria Diffus. 24, 377–378.

Powers, J., Sakaguchi, R., 2006. Craig’s restorative dental materials, 12th ed. Mosby, St Louis.

21

Ramírez, L.E., Castro, M., Méndez, M., Lacaze, J., Herrera, M., Lesoult, G., 2002. Precipitation path of secondary phases during solidification of the Co–25.5%Cr–5.5%Mo–0.26%C alloy. Scr. Mater. 47, 811–816.

Saldívar-García, A.J., López, H.F., 2005. Microstructural effects on the wear resistance of wrought and as-cast Co-Cr-Mo-C implant alloys. J. Biomed. Mater. Res. A 74, 269–74.

Tsai, M.-H., Yuan, H., Cheng, G., Xu, W., Jian, W.W., Chuang, M.-H., Juan, C.-C., Yeh, A.-C., Lin, S.-J., Zhu, Y., 2013. Significant hardening due to the formation of a sigma phase matrix in a high entropy alloy. Intermetallics 33, 81–86.

Viennot, S., Dalard, F., Malquarti, G., Grosgogeat, B., 2006. Combination fixed and removable prostheses using a CoCr alloy: a clinical report. J. Prosthet. Dent. 96, 100–103.

Wataha, J.C., 2002. Alloys for prosthodontic restorations. J. Prosthet. Dent. 87, 351–363.

Yamanaka, K., Mori, M., Chiba, A., 2013a. Nanoarchitectured Co−Cr−Mo orthopedic implant alloys: nitrogen-enhanced nanostructural evolution and its effect on phase stability. Acta Biomater. 9, 6259–6267.

Yamanaka, K., Mori, M., Chiba, A., 2014a. Effects of carbon concentration on microstructure and mechanical properties of as-cast nickel-free Co–28Cr–9W-based dental alloys. Mater. Sci. Eng. C 40, 127–134.

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Yamanaka, K., Mori, M., Chiba, A., 2014b. Effects of nitrogen addition on microstructure and mechanical behavior of biomedical Co−Cr−Mo alloys. J. Mech. Behav. Biomed. Mater. 29, 417–426.

Yamanaka, K., Mori, M., Chiba, A., 2014c. Influence of carbon addition on mechanical properties and microstructures of Ni-free Co–Cr–W alloys subjected to thermomechanical processing. J. Mech. Behav. Biomed. Mater. 37, 274–285.

Yamanaka, K., Mori, M., Koizumi, Y., Chiba, A., 2013b. Local strain evolution due to athermal Ȗĺİ martensitic transformation in biomedical CoCrMo alloys. J. Mech. Behav. Biomed. Mater. 32, 52–61.

Yamanaka, K., Mori, M., Kuramoto, K., Chiba, A., 2014d. Development of new Co–Cr–W-based biomedical alloys: Effects of microalloying and thermomechanical processing on microstructures and mechanical properties. Mater. Des. 55, 987–998.

Yoda, K., Suyalatu, Takaichi, A., Nomura, N., Tsutsumi, Y., Doi, H., Kurosu, S., Chiba, A., Igarashi, Y., Hanawa, T., 2012. Effects of chromium and nitrogen content on the microstructures and mechanical properties of as-cast Co−Cr−Mo alloys for dental applications. Acta Biomater. 8, 2856–2862.

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Figure and table captions Figure 1. The SEM BSE image of a specimen of the Co–29Cr–6Mo-based alloy prepared by conventional dental casting. Figure 2. (a) The IQ map, (b) IPF map, and (c) phase map of the dental-cast alloy specimen. Magnified versions of the (d) IQ, (e) IPF, and (f) corresponding KAM maps. The black and white lines in the IPF maps indicate high-angle boundaries with misorientations larger than 15° and low-angle boundaries with misorientations of 2–15°, respectively. Figure 3. The SEM BSE image and corresponding elemental maps of the dental-cast alloy specimen. Figure 4. (a) The TEM BF image and (b) TEM DF image of the interdendritic precipitates in the dental-cast alloy specimen. The corresponding SAD pattern and high-magnification DF image are shown in (c) and (d), respectively. Figure 5. (a) The STEM BF image and corresponding STEM-EDS elemental maps of (b) Co, (c) Cr, (d) Mo, and (e) Si in an interdendritic region of the dental-cast alloy specimen. Figure 6. Stress−strain curve of the dental-cast alloy specimen taken by tensile testing at room temperature. The result for the Co−28.5Cr−8.99W−0.04C cast alloy (Yamanaka et al., 2014a) is also shown for comparison. Figure 7. Vertical section of the calculated phase diagram of the Co–29Cr–6Mo–0.2N−xC (mass%, 0 ” x ” 0.3) system, obtained using Thermo-Calc software.

24

Table 1. Chemical composition of the alloy investigated in this study. Table 2. Results of the quantitative STEM-EDS analyses of the areas indicated in Fig. 5. Table 3. Vickers hardness and typical room-temperature tensile properties of a dental-cast specimen of the investigated alloy.

25

Figure

Figure 1

500 μm

Figure 2

(a)

(d)

γ (fcc)

001

111

500 μm

50 μm

(e)

(b)

(f)

(c)

2110

ε (hcp) 1010

101 0001

2˚

䂓 γ (fcc) 䂓 ε (hcp)

0˚

Figure 3

(g)

(d)

(a)

N

Mo

BSE

(h)

(e)

(b)

O

C

Co

(i)

(f)

(c)

S

Si

Cr

10 μm

high

low

(b)

BD//[110]carbide

(c)

(a)

BD//[110]σ

(b)

500 nm γ

(e)

(e)

(d)

200 nm

(c)

500 nm Figure 4

Figure 5

(a) Area 2

Area 3

Area 1

1 μm

(d)

(b)

Mo (e)

Co (c)

Si

Cr

Figure 6

Cobaltan (Present study) Co−28.5Cr−8.99W−0.04C

Figure 7

Temperature (K)

1800 1700 1600 1500 1400 1300 1200 1100 1000 900 800 0

γ + Cr2N γ + Cr2N +σ

Liquid γ + Liquid

γ + Cr2N + M23C6

ε + Cr2N + M23C6 + σ

γ + Cr2N + M23C6 + σ ε + Cr2N + σ

ε + Cr2N + M23C6 + σ + μ ε + Cr2N + M23C6 + μ

ε + Cr2N + σ + μ

ε + Cr2N + μ

0.05 0.10 0.15 0.20 0.25 0.30 Carbon concentration (mass%)

γ + ε + Cr2N + M23C6 + σ + μ

Table

Table 1.

Bal.

Co 28.9

Cr 5.91

Mo

0.01

Ni

0.04

Fe

0.18

C

0.23

N

0.26

Mn

0.49

Si

Table 2.

(mass%)

(at.%)

(mass%)

(at.%)

(mass%)

52.18

49.04

34.82

33.24

62.57

62.85

Co

31.66

26.25

48.50

40.85

32.11

28.46

Cr

16.15

24.71

16.68

25.92

5.31

8.69

Mo

Analyzed area 1 2 3

(at.%)

Table 3.

680

0.2% proof stress (MPa)

980

Ultimate tensile strength (MPa)

11

Elongationto-failure (%)

Vickers hardness Tensile properties

370 㫧 3.5

HIGHLIGHTS •

We study solute portioning and precipitation in dental castings of Co–Cr–Mo alloy.

•

This is done to determine their effects on the mechanical and corrosion properties.

•

The alloy microstructure consists primarily of a face-centered-cubic Ȗ-phase

•

Precipitates are observed in the interdendritic regions rich in Cr, Mo, Si, and C.

•

Reaction between the ı-phase and carbon is responsible for the carbide M23C6.

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