Effects of Carbon Addition on Mechanical Properties and Microstructures of Ni-Free Co–Cr–W-Based Dental Alloys
We investigated the effects of carbon concentration on the microstructures and tensile properties of Ni-free Co–29Cr–9W–1Si–C (mass%) alloys used as disk materials in dental technology based on computer-aided design and computer-aided manufacturing (CAD/CAM). The alloy specimens, which contained carbon in different concentrations, were prepared by conventional casting. The precipitates changed from intermetallic compounds in the low-carbon alloys, e.g., the σ and Laves phases, to M23C6-type carbide (M: metal) with increasing bulk carbon concentration. M23C6 dramatically enhanced the 0.2 % proof stress, which then gradually increased with increasing carbon content in the alloys. The elongation-to-failure also increased with increasing carbon content. The coarse M23C6 particles formed by higher concentrations of carbon were detrimental to ductility, however, and a maximum elongation-to-failure was obtained at a carbon concentration of ~0.1 mass%. In addition, we applied hot-deformation processing to the cast-alloy specimens and revealed that compared to as-cast alloys, the hot-rolled alloys with added carbon showed an excellent combination of high strength and high ductility. The current study can thus aid in the design of biomedical, carbon-containing, Co–28Cr–9W–1Si-based alloys.
KeywordsBiomedical Co–Cr–W alloy Carbon addition Mechanical properties Microstructures Precipitation
Computer-aided design and computer-aided manufacturing (CAD/CAM) have been accepted in dentistry as advanced techniques that accelerate the production of dental restorations. Although several methods have been introduced, CAD/CAM facilitate rapid, low-cost, and precise fabrication of custom-made dental restorations for patients. In particular, CAD/CAM-based milling [1, 2, 3] produces dental restorations from block disks or pellets of ceramics, composite resins, or metallic materials. An all-ceramic system is currently a primary choice, although zirconia-based ceramic materials commonly used in restorative applications have poorer milling performance than metallic materials. In contrast, metal–ceramic systems show a good combination of aesthetics, mechanical rigidity, and machinability owing to the ceramic veneer and metallic framework [3, 4]. For example, Co–Cr alloys are suitable restorative materials because they have excellent corrosion resistance and their components are less expensive than those of conventionally used Au-based alloys.
Recently, extensive research and development have been conducted on high-strength Co–Cr-based dental alloys [5, 6, 7]. This may be partly because their higher strength basically yields higher fatigue strength, which then improves the mechanical reliability of restorations that are subjected to occlusal forces. In addition, materials used in this application should consist of small grains because chipping failure occurs in machined components with coarse grain structures, reducing the precision of the fit of a restoration. Thus, a grain-refinement process is necessary to improve the fatigue strength, mechanical reliability, and machinability. Although a high-strength Co–Cr–W-based alloy that meets ISO 22674 Type 5 (yield stresses higher than 500 MPa ) requirements has been commercialized, it is made by utilizing powder metallurgy, which is generally a high-cost process.
We have recently proposed a strategy for designing a new class of Ni-free Co–Cr–W-based alloys with excellent mechanical properties [9, 10, 11, 12, 13, 14]. By employing thermodynamic calculations, we examined the alloying elements, namely, Si and C, to modify and further strengthen the commercial Co–28Cr–9W (mass%) alloy . In particular, this review reports the effects of carbon on the relationship between the microstructures and mechanical properties of Ni-free Co–Cr–W-based alloys [9, 10, 11, 12]. In addition to systematically investigating the carbon-concentration-dependence of the phase distributions, precipitates, and tensile properties of the alloys, we carried out a preliminary evaluation of the effects of thermomechanical processing to further improve the alloys’ mechanical performance.
19.2 Effects of Carbon on Microstructural Evolution
19.2.1 Phase Diagram of Co–28Cr–9W–1Si–C System
19.2.2 Refinement of Solidification Microstructures by Carbon Addition 
The changes in solidification microstructures resulting from carbon addition in the Co–Cr–W-based alloys were investigated experimentally for a wide range of carbon concentrations . Four kinds of Co–28Cr–9W–1Si–xC (mass%) alloys, where x = 0.005–0.33, were prepared in a high-frequency induction furnace in an argon atmosphere.
19.2.3 Effect of Hot-Deformation Processing on Microstructures 
We then prepared hot-rolled Co–28Cr–9W–1Si–C alloys with carbon concentrations up to 0.33 mass% to investigate the effect of hot-deformation processing on the microstructural evolution. The cast ingots were subjected to a homogenizing heat treatment at 1,473 K for 21.6 ks (6 h) and then directly processed by multi-pass hot-caliber rolling (initial temperature: 1,473 K; φ: 15 mm → 9.6 mm), followed by water quenching.
Average γ grain sizes and fractions of ε martensite of hot-rolled Co–28Cr–9W–1Si–C alloys with different carbon concentrations 
Carbon content (mass%)
γ grain size (μm)
Fraction of ε martensite (%)
The obtained results indicate that the microstructures of the as-cast and hot-rolled, carbon-doped, Co–Cr–W-based dental alloys are in good agreement with those predicted by thermodynamic calculations. Adding carbon to this alloy system tended to stabilize the γ matrix and cause the precipitation of M23C6, effectively reducing their grain size.
Finally, we investigated the effect of carbon on room-temperature tensile properties of the as-cast and hot-rolled Co–28Cr–9W–1Si–C alloys.1 All of the stress–strain curves obtained in tensile testing for both types of alloys showed uniform elongation followed by sudden fractures without macroscopic necking [11, 12]. This type of tensile deformation is typically observed in Co–Cr–Mo-based alloys [15, 16, 17, 18].
Our previous studies [9, 19] showed that solid-solution strengthening of carbon was negligible, as theoretically predicted; therefore, the precipitates would have dominated the strengthening of the present alloys. The amount of M23C6 precipitates that formed in the alloys, which varied with their carbon concentration (Fig. 19.5f), was actually consistent with the variations in the alloys’ 0.2 % proof stress (Fig. 19.2a). Increasing the fraction of M23C6 should have increased the strength of the alloys. The higher strength of the hot-rolled alloys than that of the as-cast counterparts partly originated from the precipitation size because finer precipitates dramatically increase the strength .
The elongation-to-failure of the as-cast and hot-rolled alloy specimens also showed similarly strong dependence on the carbon concentration (Fig. 19.10b): it increased initially, peaking at ~0.1 mass% C, and then gradually decreased with further increase in carbon concentration. Similar results were reported for Co–Cr–Mo-based alloys .
Until now, C-free Co–Cr–W-based alloys have been used for dental restorations. However, the current results revealed that the high-carbon-content alloys containing a considerable amount of M23C6 still showed sufficient ductility even in the as-cast condition. Therefore, adding carbon to the alloys is a promising strategy for developing high-strength alloys that show acceptable tensile ductility. The hard Cr-rich M23C6carbide phase may deteriorate the milling properties and the corrosion resistance of the alloys, however. Thus, the optimal concentrations of carbon in the as-cast Co–28Cr–9W–1Si–C alloys were estimated to be just above 0.1 mass%. On the other hand, the hot-rolled alloys exhibited much better mechanical properties than those in the as-cast counterparts because homogeneous microstructures with fine precipitates were obtained. As the production cost is considered to be not as significant as powder metallurgy, the hot-deformation processing is a potential route to fabricate disk materials for CAD/CAM-based milling applications.
We systematically investigated the effects of carbon on the room-temperature tensile properties and microstructures of dental Co–28Cr–9W–1Si alloys in as-cast condition and after thermomechanical processing. The microstructural development and tensile properties of alloys prepared under both processing conditions showed similar dependence on the carbon concentration, although the hot-rolled alloys showed much better mechanical properties. Adding carbon suppressed the formation of the hcp ε martensite phase and stabilized the fcc γ phase. The σ phase was identified in the low-carbon-content alloys, but it was replaced by M23C6 particles when the carbon concentration was increased. Adding carbon to the alloys dramatically strengthened them, and the 0.2 % proof stress of the alloys increased with increasing carbon concentration. However, the elongation-to-failure reached a maximum when the carbon concentration was ~0.1 mass% and then remarkably decreased with increasing carbon content thereafter. Therefore, the variation in tensile properties resulting from carbon addition to the alloys originated from the precipitation of M23C6.
The casting and hot-rolling conditions are the same as those described in the previous sections.
This research was financially supported by a Grant-in-Aid for JSPS Fellows; the Supporting Industry Program from the Ministry of Economy, Trade, and Industry (METI); and the Innovative Research for Biosis–Abiosis Intelligent Interface, Japan.
- 8.ISO22674 (2006) Dentistry—Metallic materials for fixed and removable restorations and appliances. http://www.iso.org/iso/home/store/catalogue_tc/catalogue_detail.htm?csnumber=36412
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