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Cobalt is applied as a base metal for a number of alloys, as an alloying element, and as a component of numerous inorganic compounds. Table 10.1 lists its major applications. Cobalt and cobalt-based materials are treated extensively in [10.1, 10.2].

Data on the electronic structure of Co and Co alloys may be found in [10.3]. Phase diagrams, crystal structures, and thermodynamic data of binary Co alloys may be found in [10.4].

Table 10.1 Applications of cobalt (after [10.1])
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1 Co-Based Alloys

Cobalt-based alloys with a carbon content in the range of 1 to 3 wt% C are widely used as wear-resistant solid materials and weld overlays. Depending on the alloy composition and heat treatment, M23C6, M6C, and MC carbides are formed. Materials with lower carbon content are mostly designed for corrosion and for heat resistance, sometimes combined with wear resistance. The metals W, Mo, and Ta are essentially added for solid solution strengthening. In a few alloys Ti and Al are added. They serve to form a coherent ordered Co3(Ti , Al) phase which precipitates and leads to strengthening by age hardening. The Cr content is generally rather high to provide oxidation and hot corrosion resistance. Table 10.2 presents a survey of Co-based alloys.

Table 10.2 Compositions of Co-based alloys
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2 Co-Based Hard-Facing Alloys and Related Materials

The behavior of Co-based wear resistant alloys is based on a coarse dispersion of hard carbide phases embedded in a tough Co-rich metallic matrix. The volume fraction of the hard carbide phase is comparatively high: e. g., at 2.4 wt% C the carbide content is 30 wt%. The carbide phases are M7C3 (Cr7C3 type) and M6C (W6C type). Table 10.3 lists characteristic properties of Co-based hard facing alloys the compositions of which are listed Table 10.1.

Table 10.3 Properties of selected Co-based hard-facing alloys
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3 Co-Based Heat-Resistant Alloys, Superalloys

Both wrought and cast Co-based heat resistant alloys, listed in Tables 10.2 and 10.4, respectively, are also referred to as Co superalloys. They are based on the face-centered cubic high-temperature phase of Co which is stabilized between room temperature and the solidus temperature by alloying with ≥ 10 wt% Ni. They are solid-solution strengthened by alloying with W, Ta and Mo. Furthermore, they are dispersion strengthened by carbides.

Differences of the high-temperature mechanical behavior of these materials are shown in terms of stress-rupture curves in Fig. 10.1.

Fig. 10.1
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Stress–rupture curves for 1000 h life of cast Co-based superalloys

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Investment-cast Co alloys are generally used for parts of complex shape such as first- and second-stage vanes and nozzles in gas turbine engines.

Table 10.4 Nominal compositions of cast cobalt-based heat-resistant alloys
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4 Co-Based Corrosion-Resistant Alloys

Compared to the heat resistant Co-based alloys, the corrosion-resistant alloys have low C concentrations and are alloyed with higher Mo contents rather than with W since Mo contributes to their corrosion and oxidation resistance. Table 10.5 shows the compositions of various Co-based corrosion-resistant alloys.

The multiphase (GlossaryTerm

MP

) alloys MP35N and MP159 combine ultra-high strength, high ductility, and corrosion resistance, including resistance to stress-corrosion cracking in the work-hardened state. The prime strengthening is based on the deformation-induced martensitic transformation of the fcc matrix phase into the hcp phase which has been termed a multiphase reaction. The multiphase microstructure provides an increased density of barriers for slip dislocations. Subsequent annealing leads to a stabilization of the two-phase structure by solute partitioning. Figures 10.2 and 10.3 show the increase in strength and decrease in ductility for alloys MP35N and Duratherm 600 with work hardening and aging.

Fig. 10.2
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Tensile properties of cold-drawn and aged MP35N

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Fig. 10.3
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Tensile properties of cold-drawn and aged Duratherm 600

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Table 10.5 Co-based corrosion resistant alloys
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5 Co-Based Surgical Implant Alloys

Co-based surgical implant alloys (Table 10.6 for compositions) are used to fabricate a variety of implant parts and devices. These are predominantly implants for hip and knee joint replacements, implants that fix bone fractures such as bone screws, staples, plates, support structures for heart valves, and dental implants. The mechanical properties (shown in Table 10.7 ) depend sensitively on the thermal and thermomechanical treatments of the materials.

Table 10.6 Compositions of Co-based surgical implant alloys
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Table 10.7 Mechanical properties of Co-based surgical implant alloys
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6 Cemented Carbides

The term cemented carbides , also called hardmetals , refers to powder-composite materials consisting of carbide particles bonded with metals or alloys. Extensive treatments are given in [10.5, 10.6]. The most common cemented carbide is WC bonded with Co. Cobalt is used as a binder since it wets the angular WC particles particularly well. Nickel is added to increase corrosion and oxidation resistance of the Co binder phase. The metals Ta, Nb, and Ti may be added to form a (W, Ta, Nb, or Ti) C solid solution carbide phase which is an additional microstructural constituent in the form of rounded particles in the so-called complex grade, multigrade, or steel-cutting grade cemented carbides. Table 10.8 lists representative materials.

Table 10.8 Compositions, microstructures and properties of representative Co-bonded cemented carbides
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