Co-based alloys

Figure 10 presents the Curie temperature (T ) vs the TM-content (x) for Co- and Fe-based biaary alloys. Alloying rare-earth elements with small amounts of transition metals (x < 0.2) leads to a decrease ia Curie temperature. This is particularly obvious ia the Gd—Co system where it corresponds to a nonmagnetic dilution similar to that of Cu (41,42). This iadicates that TM atoms experience no exchange coupling unless they are surrounded by a minimum number j of other TM atoms. The critical number is j = 5 for Fe and j = 7 for Co. The steep iacrease of for Co-based alloys with x about 0.7 is based on this effect. 

None of the biaary compounds with this composition is well matched to the needs of MO recording. Gd—Fe has too high a Curie temperature and has an in-plane anisotropy. Tp is too low for binary alloys such as Tb—Fe and Dy—Fe. Co-based alloys which exhibit a close to room temperature have... 

Methanol tolerance is a very important property of Pd-M alloys. In particular, methanol tolerance was demonstrated for PdsFe/C and for Pd-Co based alloys [Mustain et al., 2007 Raghuveer et al., 2005 Shao et al., 2006c Zhang L et al., 2007]. The high ORR activity in the presence of a high concentration of methanol indicates that the Pd-Co and Pd-Fe electrocatalysts are not active for methanol oxidation. 

Deposition and patterning of the bottom magnetic pole follow. The pole is usually electroplated with a through-photoresist window frame mask to a thickness level of 2 to 4 fim. Note that whereas the magnetic pole is made into a pancake shape to increase the head efficiency, it is the narrow p>ole tip s dimension that determines the narrow track width. As stated, the widely used Co-based alloy magnetic poles are elec-trodeposited (wet process). Nanocrystalline FeN-based alloys are sputter-deposited in a vacuum chamber (dry process). 

An important application is the aluminizing of air foils of gas-turbine engines made of high temperature Ni- or Co-base alloys. The aluminizing can be carried out either in a pack process or in an out-of-the-pack process. 

The combination of the above factors has rendered the nanocrystalline solution competitive, not only with amorphous Co-based alloys, but also with classical crystalline alloys and ferrites. The consequence is a steadily increasing level of applications in magnetic cores for ground fault interrupters, common mode chokes and high frequency transformers. Fig. 14 shows some typical examples. The worldwide production rate meanwhile approaches an estimated 1000 tons/year, and the trend is increasing. The only drawback of the nanocrystalline material appears to be the embrittlement that occurs upon crystallization, which requires final shape annealing and, thus, restricts application mainly to toroidally wound cores. 

In this paper we present some results concerning the kinetics of high pressure migration of liquid cobalt and Co-base alloys into a diamond powder vs temperature as well as the migration effect on diamond powder compaction. 

The group of iron-based alloys are of minor importance. The composition ranges for Ni- and Co-based alloys (cast and wrought) are summarized in Table 8.4. It can be seen that the carbon content is in the range of 0.25-1.0% for cast Co-based alloys and 0.05-0.4% for hot woriced. 

Co-base alloys with 25-30% Cr, 5-7% Mo and low amounts of other metals such as Ni, Mn, Fe, Si are called Co-Cr alloys, while those with about 20% Cr, 10% Ni and up to 15% ungsten (W) are called wrong Co-Cr alloys. 

During several decades, stainless steel was the most frequently used alloy for joint replacements. At present. Co-base alloys have taken first place, and about 70% of all orthopedic implants are made from Co-Cr alloys. During the past 20 years, titanium and its alloys have become more important due to their bone-like elasticity and their excellent biological behavior. 

Tvmgsten additions have not been observed to cause catastrophic oxidation of Ni- and Co-base alloys, perhaps because of the higher melting temperatures of the tungsten oxides as compared with molybdenum oxides, e.g., Tnp = 1745 Kfor WO3. However, W additions have been observed to induce some scale breakdown on Ni-Cr alloys. Additions of W to Co-Cr alloys appear to be beneficial in decreasing the transient oxidation period and promoting the formation of a continuous Cr203 layer. 

Refractory metals are used as carbide formers (vanadium) in alloys that contain insufficient chromium to form a protective layer of Cr203 (M0O3 or WO3 in Mo or W containing alloys) or as solution strengthening elements in Co-based alloys (Mo or W) [8]. 

El-Dahshan ME, Whittle DP and Stringer J, Effect of presulfidation on the oxidation behavior of Co-based alloys. Part I. Presulfidation at sulfur partial pressures above the dissociation pressure of cobalt sulfide. 

The superalloys are typically Ni, Ni-Fe, and Co based alloys with Cr, Ti, W, A1 additions. They were originally used for high temperature applications (over 810 °C) or in severe corrosive media. Superalloys can be distinguished from high alloyed steels (see definition in Section 2.1.3). Since iron is not the major compound (as defined in [201, CEN, 2000]), they are considered non-ferrous materials. The casting of superalloys may occur in certain investment casting foundries, as well as partly in foundries which specialise in high alloyed steel qualities. 

Some superalloys, particularly Ni-Fe and Co based alloys, are directly melted in electric furnaces by classical methods usually applicable to stainless steels. However for Ni and special Ni-Fe superalloys, vacuum induction melting is required in order to reduce the content of interstitial gases (O, H, N) to a very low level. This enables foundries to achieve high and controlled contents of oxidisable elements such as Ti or Al. 

Ceramic Cutting Tools, Fig. 9 Relative chemical resistance of various tool materials, mostly against Fe or Co based alloys... 

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