The transformation temperature of pure cobalt is 417°C. Alloying elements such as nickel, iron, and carbon (within its limited soluble range) are known as FCC stabilizers, and suppress the transformation temperature. Chromium, molybdenum, and tungsten, on the other hand, are HCP stabilizers and increase the transformation temperature.
In reality, the transformation is extremely sluggish, and not easily brought about by either heating or cooling. Indeed, after solidification from the molten state (or after solution annealing and quenching, in the case of wrought products), cobalt and cobalt alloys (with elevated transformation temperatures) normally exhibit metastable FCC structures at room temperature. However, partial transformation to HCP is easily induced by cold-work (i.e. plastic deformation at room temperature).
The transformation of cobalt and cobalt alloys under the action of mechanical stresses is believed to progress by the creation of wide stacking faults (the FCC form of the materials having very low stacking fault energies) and by subsequent coalescence. Extensive micro-twinning is also observed in plastically-deformed, cobalt-based alloys.
Chromium provides the same benefits to cobalt as it does to nickel, i.e. it is key to the formation of protective films/scales in both corrosive fluids and high-temperature gases. Moreover, it influences the driving force for structural change in cobalt and its alloys, which in turn affects their mechanical and wear behavior.
The primary role of nickel (if present) in the cobalt-based is to stabilize the FCC form. This negatively impacts wear performance, but provides many benefits, especially ease of wrought processing (at sufficiently high nickel contents).
Molybdenum and tungsten are both strong, solid-solution strengthening agents in cobalt-based alloys. They also result in higher transformation temperatures, which increase resistance to those forms of wear that involve a micro-fatigue component (such as metal-to-metal sliding and cavitation erosion). Molybdenum is used in those cobalt alloys developed primarily for resistance to aqueous corrosion and wear. Tungsten is used in those wrought cobalt alloys developed for high temperature use and those cast (and weld overlay), high-carbon alloys developed primarily for wear resistance in hostile environments.
In the cast (and weld overlay) cobalt alloys with relatively high carbon contents (i.e. from 0.5 to 3.5 wt.%), chromium, molybdenum, and tungsten also encourage the formation of carbides within the microstructure. These carbides (chromium-rich M7C3 and M23C6, and molybdenum/tungsten-rich M6C) are very beneficial under low stress (two-body) abrasion conditions.
As with the nickel-based alloys, iron can be used to reduce cost, particularly if it allows the use of ferro-compounds or iron-contaminated scraps in the charge materials during melting. However, it can also be used as an alternate FCC stabilizer (rather than nickel), to decrease the transformation temperature and make the alloys more amenable to wrought processing and fabrication.
The solubility of carbon in cobalt is higher than that in nickel; thus, there is less need to minimize carbon in wrought, corrosion- and wear-resistant, cobalt-based alloys. Furthermore, carbon is an important minor addition to the wrought, high temperature alloys (both of cobalt and nickel), and a major addition to those cast and weld overlay, cobalt alloys developed primarily for resistance to wear. Its purpose in the high temperature alloys is for strengthening, through the formation of sparsely dispersed carbides. Its purpose in the wear-resistant alloys is to generate high volume fractions of carbide in their microstructures, to increase their cutting and deformation resistance.