Before discussing the metallurgy of the nickel and cobalt alloys, it is important that certain metallurgical terms are understood. First and foremost, it should be understood that an alloy is a mixture of metals, possibly containing small quantities of non-metals, such as carbon. The predominant metal in the alloy is known as the base.
A solid solution is an alloy in the solid state having a single atomic structure or phase. Second phases are possible when the combined levels of alloying additions to the base exceed their solubility limits. So, as with a liquid solution, there are natural limits to how much can be dissolved in a metallic material of a given atomic structure, and as with a liquid solution, the higher the temperature, the more can be dissolved. Fortunately, it is possible to create supersaturated solid solutions by heat treating materials at high temperatures where the solubilities are higher, then rapidly cooling the materials to room temperature, or at least below 500°C, where the diffusion of atoms (the main catalyst for microstructural change) is no longer appreciable. Holding alloys at high temperatures, to dissolve unwanted second phases in their microstructures, is known as solution annealing. Rapid cooling, to lock in the high temperature microstructure, is known as quenching, and is best performed in cool water.
The problem with such supersaturated materials is that they are prone to second phase precipitation during excursions above 500°C, when diffusion becomes appreciable. Such excursions are common during welding, for example.
Unfortunately, precipitates tend to nucleate and grow at microstructural imperfections, such as grain boundaries. These then become prone to preferential corrosion attack.
Not all second phase precipitates are detrimental. Those that precipitate homogeneously (i.e. throughout the microstructure, rather than just at the grain boundaries) can be used to strengthen materials. This is known as precipitation-hardening or age-hardening. The heat treatments used to induce such precipitates often involve multiple steps in the temperature range 500°C to 800°C.
The microstructures of wrought and cast alloys comprise numerous grains, within which the crystal structure is aligned in a certain direction. However, these grains can sub-divide under the action of mechanical stress or temperature by a process known as twinning, whereby bands of material within a grain can realign.
Grain boundaries (of irregular geometry) and twin boundaries (which are straight and parallel) are very important microstructural features, since they are preferred nucleation sites for second phase precipitates.
The major alloying elements determine the general behavior of a material. However, minor alloying elements are also important. Some minor elements are there to ensure successful melting and processing; some are used to fine-tune performance in specific environments. Others are added to induce hardening precipitates.
Except in the case of the few precipitation-hardenable nickel alloys designed to resist aqueous corrosion, strengths are determined largely by the major alloying elements. These provide solid solution strengthening. Large atoms such as molybdenum are particularly effective strengtheners.
To maximize the corrosion resistance of the nickel alloys, many are deliberately overalloyed and reliant upon the previously mentioned process of solution annealing and quenching to optimize their microstructures. Even those that are not overalloyed are prone to second phases, due to the presence of insoluble residuals, such as carbon.