The vast majority of commercially available cobalt alloys are air or argon melted since they are devoid of the highly reactive elements aluminium and titanium, whose presence requires more sophisticated and costly vacuum melting techniques. Silicon and manganese additions are used to enhance castibility in terms of alloy fluidity, melt deoxidation practice, and sulfur control. Vacuum melting is required to control the relatively low alloying levels of the strong monocarbide-forming reactive elements zirconium, hafnium, and titanium in contemporary alloys like MM-509. Improvements in tensile and rupture properties of more conventional alloys like X-40 have also resulted from vacuum melting due to lower interstitial levels and “cleaner” material.
Air-melted alloys, for example, typically exhibit 400 ppm oxygen and 700 ppm nitrogen, whereas vacuum-melted alloys contain less than 100 ppm of these elements. More recently, electroslag remelting (ESR) was investigated and compared to vacuum arc remelting (VAR). A slight improvement in rupture properties, especially at high stresses, was found for ESR MM-302, MM509, and X-45 compared to VAR. No significant changes in alloy microstructure or nonmetallic inclusions were noted, although chemical analysis showed a small decrease in the sulfur and phosphorus levels for ESR material.
Aluminium has been added to both wrought and cast cobalt alloys, as represented by sheet alloy S-57 and cast alloy AR-213, respectively. Additions of 5 wt. % aluminium in each of these systems are highly beneficial for oxidation and hot corrosion resistance.
Cobalt chrome alloys are oxidation resistant and corrosion resistant.
Cobalt-chromium-aluminium-yttrium coatings typify the alloys in commercial use as corrosion resistant alloys that are also applied in aerospace for turbine engine component coatings. They are strengthened by a uniform noncoherent precipitate of CoAl that generates properties similar to the carbide-strengthened alloys. CoAl tends to overage above approximately 1400 °F (760 °C); however, refractory element additions of tungsten to alloy AR215 and tantalum to S-57 stabilize the precipitate to a higher use temperature.
Titanium additions have been utilized in wrought alloys CM-7 and Jetalloy 1650 to generate a uniform coherent precipitate of ordered-FCC (Co,Ni)3Ti analogous to y’ in nickel alloys. High tensile strengths are achieved up to the temperature stability limit of this phase, that is, about 1300 °F (704 °C). However, titanium levels above about 5 wt. % produce phase instabilities that generate the HCP-Co3Ti or C0ZTi-Laves phases.
The incorporation of nitrogen in some air-melted casting alloys, either as an intentional or inadvertent addition, also has a positive although less potent strengthening effect similar to carbon through the formation of nitrides and carbonitrides. In general, these are thermodynamically less stable than the carbides and suffer degeneration reactions during service.
Boron is added to cast cobalt alloys to enhance rupture strength and ductility: however, its precise function in the microstructure is usually obscured by the carbides_ In nickel alloys boron precipitates at grain boundaries as a molybdenum-rich boride; a similar boride has not been identified in cobalt alloys. Boron levels of typically 0.015 wt. % are used; however, additions of up to 0.1 wt. % have been employed to provide additional strengthening.
Significant improvements in the oxidation resistance of cobalt alloys have been achieved in the past two decades through the addition of the rare-earth elements yttrium and lanthanum in alloys such as cast FSX-418 and wrought HS-188, respectively. Surprisingly, additions of just 0.08-0.15 wt. % promote oxide scale adhesion and reduced oxidation kinetics, especially under thermal cycling conditions, and are particularly effective in stabilizing the Cr203 oxide and minimizing the formation of CoCr204 spinel and COO.
