COBALT ALLOYS

Cobalt-chromium alloys constitute the most representative group among those in which cobalt acts as the base metal. Their use is reserved for applications requiring high resistance to wear, high temperatures, and thermal deformation, especially in contexts where material expansion or loss of mechanical strength under heat are intolerable. The most emblematic and commercially widespread brand of this family is Stellite®, developed by Elwood Haynes at the beginning of the 20th century and registered by the Kennametal Stellite Company. Its origin is linked to the need for an alloy with high corrosion resistance, leveraging the affinity for oxygen of its two main components: cobalt and chromium.

Over time, the original formula was enriched with new elements that improve its hardness, chemical resistance, or a combination of both, turning it into a superalloy. Within this group, Stellite® stands out for its ability to maintain its properties in the face of sudden temperature changes, making it a preferred option for extreme environments. Its appearance is a bright metallic white, similar to rhodium or platinum, and it retains its luster with remarkable persistence. However, its intrinsic hardness makes it brittle and difficult to machine, which is why it is usually vacuum cast to avoid contamination by impurities during the manufacturing process.

The cost of this alloy is considerably higher than any grade of stainless steel, mainly due to its high cobalt content. Nevertheless, its resistance in oxidizing media is outstanding, and when alloyed with molybdenum—in proportions not exceeding 10% by mass—it becomes biocompatible, which has allowed its use in medicine for decades, especially in bone implants. In this field, it competes with titanium alloys and ceramics like zirconia (ZrO₂), offering greater hardness and, therefore, superior wear resistance. Although its density is more than double that of titanium, which represents a disadvantage in terms of weight, its ease of manufacture and mechanical behavior compensate for this limitation in many clinical applications.

Regarding corrosion resistance, the traditional cobalt-chromium alloy without molybdenum is completely inert to oxidizing solutions such as nitric, tartaric, concentrated sulfuric, and phosphoric acids, which reinforce its passive layer of chromium and cobalt oxides. However, it suffers in reducing media and from halogen attack at elevated temperatures, so it is not recommended in marine environments or in contact with acids such as hydrochloric, dilute sulfuric, or hydrofluoric. Its resistance to alkalis is moderate. Although its cost is more accessible than variants with molybdenum, it remains high compared to other conventional alloys.

The addition of molybdenum—usually between 5% and 7% by mass—significantly improves resistance to reducing acids, increasing tolerance to seawater even beyond 316L stainless steel, known for its performance in saline environments. This improvement is more notable when the carbon content is low, as the formation of intergranular carbides, although useful for increasing hardness, can compromise chemical resistance. Additionally, molybdenum increases the toughness and final density of the alloy.

Versions with tungsten seek similar effects to those of molybdenum, but with a focus on the formation of tungsten carbides, which are more thermally stable and robust than those generated by molybdenum or chromium. These variants are used in applications where extreme hardness is a priority, such as cutting tools or components subjected to intense friction. Grades combining chromium and molybdenum are biocompatible and do not cause allergic reactions in the human body in the short or medium term. Some modifications of this alloy have become popular in contemporary alternative jewelry, even surpassing the prices of tungsten carbide and titanium, as is the case with BioBlu27.

The hardness of these alloys is among the highest in the metallic spectrum, although it is not their cold hardness that justifies their use, but their ability to retain it at high temperatures. In this aspect, they surpass certain grades of high-speed steel, which, although tougher due to being iron-based, lose their mechanical properties when exposed to high temperatures generated by friction, as occurs in milling cutters, lathes, and drilling parts. Cobalt-chromium alloys with high carbon content—up to 5% by mass—are designed to operate under these conditions without dangerous alterations to their internal microstructure. Unlike tempered steel, which can lose its temper if a critical temperature is reached, weakening and losing toughness, the intergranular carbides in the cobalt/chromium matrix are extremely stable. This stability allows the alloy, initially brittle, to acquire some ductility under heat, retaining its hardness and prolonging its useful life without being affected by corrosion.

The role of each metal in the alloy is fundamental. Cobalt, among base metals, is the one that best responds to high temperatures: it becomes more ductile, retains its hardness, and surpasses iron in corrosion resistance. Although nickel alloys are also used in thermal applications, they do not achieve the hardness of cobalt. This metal associates well with refractory elements such as chromium, molybdenum, and tungsten, as well as with members of its own subgroup like nickel and iron. Unlike these, cobalt does not form carbides at any temperature, but rather acts as a matrix for the carbides generated by the other elements. Small amounts of vanadium, titanium, and tantalum are also used as dopants, especially in the mechanical field, where carbon is indispensable for carbide formation. In jewelry, the inclusion of carbon is minimal, while in medical grades it is kept low, though sufficient to confer hardness and wear resistance without compromising biocompatibility.

Since the dawn of the 20th century, cobalt has been used as a base for manufacturing hard alloys that are corrosion-resistant and capable of maintaining their mechanical properties under extreme conditions. What many do not know is that these alloys preceded stainless steel in everyday use, at a time when table cutlery was still made of silver, alpaca (nickel silver), or even pewter. The cobalt-chromium combination, commercialized under the name Stellite® for over four decades, represents the most recognized formulation of this family. However, there are variants such as Vitallium, which, although less known, share similar properties and have been equally relevant in demanding applications.

These alloys are distinguished by their extreme hardness, combined with remarkable toughness and outstanding corrosion resistance. Although their price far exceeds that of the best stainless steel, their performance in critical conditions amply justifies the investment. Vitallium, in particular, has been used in fields such as medicine, mechanical engineering, and the aerospace industry, where material reliability is a priority.

In addition to chromium, other metals that are often part of these alloys include molybdenum and tungsten—all belonging to the same group of transition elements—whose presence enhances the formation of stable and resistant carbides. Lesser proportions of elements such as silicon, manganese, iron, and nickel may also be found, acting as structural modifiers or thermal stabilizers. The carbon content in these alloys is high, reaching up to 6% by mass in special formulations. This high concentration does not generate cobalt carbides—as this metal does not form carbides by itself—but rather cobalt acts as a matrix, that is, as a structural support for the intergranular carbides generated by the other metals when combining with carbon during the casting process.

The result is a metallic compound of extraordinary hardness, capable of resisting intense mechanical stresses without fracturing. Despite their cold hardness, these alloys acquire sufficient ductility when heated, allowing them to withstand elevated temperatures without compromising their structural integrity. Even when the piece reaches an incandescent state—"red hot"—the alloy retains its toughness, preventing fracture and allowing its use in environments where other materials would succumb to thermal stress. This combination of hardness, toughness, and thermal resistance makes Vitallium and its variants benchmark materials for applications where the compromise between performance and durability is non-negotiable.

Cobalt superalloys, like their nickel-based counterparts, have been developed to operate under extreme temperature and corrosion conditions, maintaining their hardness and toughness even when other materials fail. These alloys represent the pinnacle of contemporary metallurgical engineering, and their performance surpasses even that of elite high-speed steels—such as AISIM50—originally designed for engines and heavy-duty bearings in the aeronautical industry. Cobalt superalloys' ability to retain their mechanical properties at temperatures exceeding 1000°C makes them strategic materials for critical applications.

The chemical composition of these alloys is highly complex, and their manufacture requires advanced, precisely controlled processes carried out in specialized facilities. They are alloyed with elements such as chromium, molybdenum, titanium, rhenium, and ruthenium, especially in formulations designed for highly demanding aerospace components such as missile warheads, rotors, turbine blades, and aircraft engines. Due to the sensitivity of these processes and the strategic nature of the resulting products, many of these superalloys are manufactured exclusively in certain regions of the United States and the European Union, while Russia and China have developed their own versions tailored to their industrial and military needs.

There are three major families of superalloys: cobalt-based, nickel-based, and hybrids that combine both metals in similar proportions. Nickel superalloys are more resistant to corrosion and can operate effectively in sub-zero temperatures, making them ideal for cryogenic or marine environments. However, their wear resistance is inferior to that of cobalt superalloys. Iconic examples of this family include Hastelloy, Inconel, and Renèe, widely used in the aerospace, chemical, and energy industries. Their popularity is due, in part, to their greater accessibility and versatile properties.

On the other hand, cobalt superalloys stand out for their ability to withstand sudden temperature changes without compromising their structural integrity. This characteristic is especially relevant in applications where thermal rise is not gradual, such as in combustion, propulsion, or intensive friction systems. Although they do not respond well to sub-zero temperatures—due to their non-austenitic nature—their performance in extreme thermal environments is unsurpassed. Therefore, they are used in sectors such as the military, cruise aviation, aerospace, Formula 1, high-precision medicine, and racing vehicle manufacturing.

Hybrid cobalt-nickel superalloys exist, but their use is limited. In most cases, nickel-based alloys contain small amounts of cobalt, and vice versa, except in parts that must be exposed to radiation, where the presence of certain elements may be restricted for safety reasons. In any case, both families of superalloys are extremely expensive, and their production is reserved for a small number of companies with the technological and logistical capacity to meet the required standards. This exclusivity explains, in part, the high cost of systems such as commercial and military aircraft, where each component must respond precisely to demands that leave no room for error.