NICKEL ALLOYS

Nickel holds a privileged place in the universe of metals, not only for its versatility as an alloying element but also for the exceptional quality of the properties it imparts to the alloys in which it participates. It is, without exaggeration, one of the most valuable elements in the entire Periodic Table, and possibly the most beneficial in terms of metallurgical compatibility. Its ability to mix with almost all metals is so remarkable that it only encounters resistance from some post-transition elements and silver (Ag), with which it cannot form alloys even under extreme conditions of pressure and temperature. This exception, far from diminishing its merit, confirms the rule: nickel is the great reconciler of metallic chemistry.

Nickel's affinity for transition metals is so high that it allows for solubility indices ranging from 1% to 99%, something that does not occur with other metallic systems. For example, copper (Cu), although widely used, presents clear limitations regarding the amount of zinc (Zn) or tin (Sn) it can dissolve by mass. Nickel, on the other hand, does not suffer from these restrictions: it can form solid solutions in virtually any proportion with elements like iron (Fe), cobalt (Co), manganese (Mn), or molybdenum (Mo), surpassing even the two great pillars of the first transition series, iron and copper, in terms of compatibility and performance.

Unlike iron, nickel has the capacity to dissolve aluminum (Al) and silicon (Si) in significant volumes, which greatly expands its field of application. Furthermore, it can incorporate elements such as zirconium (Zr), hafnium (Hf), and titanium (Ti), whose integration into ferrous alloys is much more complex. The possibility of forming compounds with titanium, in particular, opens the door to materials with theoretically superlative properties, although difficult to obtain by other means. But nickel's potential does not stop there: it also alloys with refractory metals such as tungsten (W) and tantalum (Ta), with the platinum group metals (PGM), with copper itself — despite its reputation for poor compatibility — and even with gold (Au), which is particularly surprising given the nobility and chemical stability of the latter.

However, what truly distinguishes nickel is not just its ability to form alloys with almost all metals, but the quality of these alloys. They exhibit an exceptional combination of mechanical properties, corrosion resistance, and stability at extreme temperatures. Nickel improves everything it touches: it hardens, stabilizes, and strengthens. It is therefore not surprising that its price is high and its demand, constant and growing. In this context, governments of countries with significant nickel reserves should seriously consider the strategic value of this resource. Like all metals, it is non-renewable, and its massive export — often in the form of hundreds or thousands of tons destined for the armaments industry — can compromise a nation's technological and military sovereignty. In hypothetical conflict scenarios, nickel scarcity could pose an insurmountable obstacle for any country or alliance that depends on it, regardless of the ethical or political considerations surrounding such a conflict. After all, history — that material so malleable, so ductile like the metals studied here — reminds us that strategic resources have always been the silent engine of great human decisions.

Within the universe of nickel alloys, two compounds stand out with special relevance, having revolutionized thermal measurement technology: Alumel and Nisil. Both are the result of the interaction of nickel (Ni) with elements such as aluminum (Al), manganese (Mn), and silicon (Si), and are primarily used in the manufacture of electrical thermocouples, essential devices for precise temperature measurement in industrial, scientific, and aerospace environments.

Alumel is an alloy typically composed of 2% aluminum, 2% manganese, and 1% silicon, in a nickel matrix that acts as the structural and functional base. This combination imparts magnetic properties similar to pure nickel, which is useful in applications where magnetic response can be leveraged or must be considered. Its thermal stability, oxidation resistance, and ability to maintain a constant electrical signal under temperature variations make it an ideal component for the negative leg of certain types of thermocouples, such as Type K.

On the other hand, Nisil — an acronym derived from “Nickel” and “Silicon” — presents an alternative composition in which silicon replaces the percentages of aluminum and manganese present in Alumel. This modification not only alters the internal structure of the alloy but also modifies its physical properties: Nisil is non-magnetic, making it especially useful in environments where electromagnetic interference must be minimized. Its electrical behavior is stable, its corrosion resistance is high, and its compatibility with nickel makes it the perfect partner to form the positive leg of Type N thermocouples, designed to offer greater precision and durability in extreme conditions.

Both alloys, although similar in appearance and purpose, represent two distinct approaches in materials engineering: one magnetic and traditional, the other non-magnetic and more modern. Their existence and use demonstrate how small variations in chemical composition can lead to radically different properties, adapted to specific needs within the vast field of applied metallurgy.

Nitinol, an acronym for Nickel Titanium Naval Ordnance Laboratory, is one of the most unique and revolutionary alloys in modern metallurgy. Registered as a trademark, this alloy is composed mainly of nickel (Ni) and titanium (Ti), in proportions that usually range around 60% nickel and 40% titanium, although the original formulation proposed by American researcher William Buehler was equitable: a 1:1 ratio, meaning 50% of each metal. The origin of this alloy was not accidental or academic, but military: Buehler sought a material that offered superior resistance to both high temperatures and impact, and proposed the combination of these two elements hoping to obtain a more robust and functional compound. What he discovered far exceeded his expectations.

When testing the mixture, Buehler observed two absolutely unusual properties in the context of metals: shape memory and superelasticity. The first, also known as plastic memory, refers to Nitinol's ability to recover its original shape after being deformed, provided sufficient heat is applied. For example, if an originally flat Nitinol sheet is bent, it can return to its initial state by heating, as if "remembering" its primitive geometry. This phenomenon, which seems almost magical, is based on a reversible phase transformation between the martensitic and austenitic crystal structures, and its complete explanation requires extensive technical treatment.

The second outstanding property of Nitinol is its superelasticity, which allows it to stretch up to ten times more than a conventional metal without fracturing or permanent deformation. This characteristic should not be confused with superplasticity, nor with malleability or ductility, as it responds to a completely different mechanism. Nitinol not only deforms reversibly, but it does so with an energy efficiency that makes it an ideal material for dynamic and demanding applications.

To these exceptional properties is added a third virtue: its high corrosion resistance. The high titanium content — a minimum of 40% — favors the spontaneous formation of a surface film of titanium dioxide (TiO₂) , known as rutile, which acts as a protective barrier against aggressive chemical agents. This resistance, combined with its elasticity and durability, has allowed its use in the medical field, especially in the manufacture of stents — devices inserted into coronary arteries or other pathways of the circulatory system to keep them open in patients with arterial obstructions, often caused by high-fat diets or chronic cardiovascular pathologies. The Nitinol stent, thanks to its controlled expansion capacity and shape memory, facilitates blood flow without the need for invasive surgical intervention.

It is worth noting that nickel and titanium exhibit exceptional metallurgical compatibility, being able to form stable alloys in proportions ranging from 90:10 to 10:90, something impossible with iron (Fe), whose interaction with titanium is limited and problematic. This compositional versatility makes Nitinol a unique alloy, not only for its physical properties but also for its adaptability to multiple environments and technical demands.

Nickel (Ni) and chromium (Cr) alloys constitute one of the most emblematic and versatile families within modern metallurgy. Their popularity is not due to chance, but to an exceptional combination of mechanical, thermal, and chemical properties that make them reference materials in both the electrical industry and high-tech sectors. The original alloy, known as Nichrome, is essentially a mixture of nickel and chromium, with or without additives, and represents the starting point of an evolution that has led to registered variants such as Brightray and Nimonic. Although all share the Ni–Cr base, they should not be confused with each other or with the more complex superalloys that also include these elements.

Nichrome, in its classic Ni₈₀Cr₂₀ formulation, was patented in 1906 by Albert Leroy Marsh, and since then has been primarily used in electrical applications, especially in the manufacture of resistive wires. Its use in electronics is more limited, though not nonexistent. Despite its excellent mechanical properties and corrosion resistance, its main value lies in its electrical behavior under extreme conditions. The composition can vary slightly through the incorporation of third elements such as iron (Fe), cobalt (Co), aluminum (Al), or titanium (Ti), with the aim of reducing costs or modifying specific properties. In these cases, the percentage of nickel is usually reduced, given its high cost, while the chromium content is maintained or adjusted according to technical needs. Although chromium is expensive, nickel is even more so, and in large-scale industrial applications — such as aeronautical manufacturing — this difference translates into a considerable economic impact.

During the 20th century, other companies introduced their own versions of Ni–Cr alloys, such as Brightray and Nimonic. The first is so similar to Nichrome that it is difficult to distinguish them without detailed chemical analysis, while the second replaces iron with cobalt in its most common grades, which substantially modifies its thermal and mechanical behavior. These brands also offer variants with discrete percentages of elements such as vanadium (V), molybdenum (Mo), tungsten (W), and yttrium oxide (Y₂O₃), each with specific functions in improving resistance, stability, or oxidation behavior.

The properties that make these alloys so valuable are numerous. Their chemical resistance to highly corrosive media is outstanding, and even more remarkably: they maintain this resistance even at temperatures above 1000 °C, something that not even the most advanced super-duplex stainless steels can guarantee. The high proportion of nickel imparts exceptional ductility and malleability to the alloy, allowing it to be commercialized in the form of coiled wires, something unthinkable with pure chromium, which is extremely hard and brittle. The minimum chromium content (≈20%) ensures effective protection against oxygen oxidation even under extreme thermal conditions, reaching temperatures up to 1050 °C without significant loss of structural integrity.

Regarding electrical conductivity, although chromium is a poor conductor and nickel is only moderately acceptable, the combination of both is sufficient for resistive applications, especially in contexts where resistance to heating by voltage difference is more important than absolute conductivity. Furthermore, these alloys exhibit excellent resistance to thermal deformation, maintaining their shape and functionality even under intense thermal cycles. They do not embrittle at low temperatures thanks to the gamma crystal structure of iron — austenite — induced by the high nickel content, making them suitable for cryogenic environments.

A lesser-known but crucial property is their resistance to crepitation, a phenomenon describing wear by oxidation under high speed and temperature conditions, such as those occurring at the nose of a missile or rocket breaking the sound barrier. Although Nichrome is not used in these parts — titanium is usually sufficient — it does find application in internal components of engines, turbines, shafts, and rotors of high-speed aircraft, where revolutions per minute (RPM) generate extreme heat. In these cases, Nichrome can be replaced by more complex superalloys that include molybdenum, tungsten, and other elements, but its historical and functional role remains relevant.

Typical uses of Nichrome, Brightray, and Nimonic include high thermal and oxidative resistance conductive wires, exhaust and seat valves, cylinder heads, pistons, turbines, rotor blades, combustion chambers, and other components subjected to extreme conditions. Despite their excellent mechanical properties, their main application remains the manufacture of resistive wires in the electrical industry, where their performance far exceeds that of other materials.

It should be emphasized that there is no single type of Nichrome. Although the oldest and still valid grade is Ni₈₀Cr₂₀, there are variants with lower nickel content, compensated with up to 20% iron, such as Ni₆₀Fe₂₀Cr₂₀, which maintains similar properties at a lower cost. Brightray, though chemically similar, belongs to a different brand, and Nimonic is distinguished by its use of cobalt instead of iron. All these alloys can incorporate additional elements in discrete proportions, adapting to specific needs without losing their essence as high-performance materials.

Nickel (Ni) and iron (Fe) alloys constitute a singular family within metallurgy, not only for their physical and magnetic properties but also for the type of applications they have come to cover. It is worth clarifying from the outset that, although some of these mixtures contain high percentages of iron, they should not be confused with steels. Steel, by definition, requires the presence of carbon (C) in specific proportions, and in these alloys, this element is usually absent, precisely to preserve certain properties that carbon would negatively alter.

Unlike Ni–Cr or Ni–Cu alloys, which are oriented towards conventional structural or electrical applications, Ni–Fe and Ni–Fe–Co alloys were developed for very different purposes, focused on magnetic behavior and dimensional stability. Among the most notable are Mu-metal, Permalloy, and Supermalloy, all characterized by very high magnetic permeability, especially after specific heat treatments. This property, which measures the ease with which a material responds to an induced magnetic field, reaches values in these alloys ranging from several thousands to hundreds of thousands of units, making them elite ferromagnetic materials.

Pure iron exhibits the highest magnetic permeability among known elements, followed by cobalt (Co) and nickel. Gadolinium (Gd), under special conditions, also shows ferromagnetic behavior. Ni–Fe alloys exploit this synergy to create materials that "absorb" magnetic fields, acting as protective shields in sensitive electronic devices. Hence their use in transformers, proximity sensors, hard drives, magnetic resonance imaging equipment, and other systems where electromagnetic interference must be precisely controlled.

An alloy that deserves special mention is Invar, whose fame is not so much due to its magnetism as to its dimensional stability. With an extremely low coefficient of thermal expansion, Invar maintains its dimensions even under temperature variations, making it ideal for precision instruments such as clocks, seismographs, or components of old engines. Its behavior resembles that of pure tungsten (W), but with superior malleability.

Curiously, iron and nickel appear forming natural alloys in meteorites, suggesting a sidereal affinity between the two. They share many characteristics: they are magnetic, tough, relatively hard, have high melting points, similar oxidation states, and practically identical atomic radii. The main difference lies in their chemical behavior: iron has a greater affinity for oxygen and oxidizes easily, while nickel, although also siderophilic according to Goldschmidt's classification, tends to form complex sulfides with elements such as copper (Cu).

Even in proportions as low as 2%, nickel significantly improves the toughness of iron or steel, making it more malleable and ductile. As the nickel content increases, these properties are accentuated, allowing the manufacture of long and strong wires, although many of these mixtures are not considered recognized alloys, but rather functional compositions that can be reproduced with relatively simple means.

The historical development of these alloys also deserves attention. Mu-metal was created in 1923 by Britons Willoughby Smith and Henry Garnett, while Permalloy was patented in 1914 by Gustav Waldemar Elmen, a Swedish-American scientist working for the Bell Company, linked to the legacy of Alexander Graham Bell. Although Elmen was Swedish by birth, his work was carried out in the United States, which allows his discovery to be considered essentially American. Just as the discovery of America belongs to those who financed and made it possible, beyond the nationality of the navigator, the history of science also deserves a fair and contextualized reading.

Invar, for its part, was developed in 1896 by Charles Guillaume, Swiss by birth, which explains its association with precision instruments, given the watchmaking prestige of his country. The name "Invar" comes from its most outstanding property: dimensional invariability against heat. The same applies to Permalloy — a contraction of "Permeability alloy" — and to Supermalloy, which incorporates molybdenum (Mo) to achieve even higher levels of ferromagnetism.

Beyond their specific applications, all these alloys exhibit excellent mechanical properties, especially with regard to toughness and impact resistance. They are materials that, although not always visible in conventional structures, underpin a large part of modern technology from their invisible foundations.

Nickel (Ni) and copper (Cu) alloys hold a privileged place in modern industry, not only for their technical properties but also for their everyday presence in common objects such as coins. Although in these cases the nickel content rarely exceeds 25%, early contact with this type of alloy has contributed to their popularity among the general public. In the technical field, however, their value lies in something much deeper: corrosion resistance that far surpasses that of many other metallic combinations.

What defines Ni–Cu alloys is not solely their toughness, malleability, or ductility — properties which, of course, they possess to a remarkable degree — but their ability to resist corrosive media, especially those of a reducing nature. This resistance is due to the synergy between nickel's tolerance to alkalis and copper's tolerance to reducing bases. The result is an alloy that remains stable in the presence of water vapor, alkaline solutions, halogenated acids like hydrochloric (HCl) and even hydrofluoric (HF), and, even more impressively: in seawater, where it behaves almost inertly even long-term. In this context, nickel and copper stand as the two transition metals of period 4 with the highest resistance in saline media and dilute acids like sulfuric (H₂SO₄), which acts as a reducing agent in low concentrations and an oxidizing agent in high concentrations.

However, these alloys present two significant disadvantages: their high cost — being two of the most expensive base metals — and their vulnerability to aggressive oxidizing media, such as nitric acid (HNO₃) or concentrated sulfuric acid. This limitation does not diminish their virtues but does delimit their applications.

It is essential to distinguish between alloys where nickel acts as the base with copper addition, and those where copper is the main metal with nickel addition. The former belong to the nickel family, while the latter — such as cupronickel or so-called "German silver" — are classified within copper alloys. Although they may seem similar, they do not share the same market or applications. Ni–Cu alloys are designed for demanding environments, while Cu–Ni alloys are more oriented towards decorative or low-structural-demand uses.

The most representative alloy of the Ni–Cu family is Monel, whose minimum nickel content is around 63%, which unequivocally defines it as a nickel alloy. The oldest and still current grade is Monel 400, with an approximate composition of Ni⁶³Cu³², supplemented by 5% iron (Fe) and manganese (Mn). Iron acts as a hardener, while manganese contributes to grain refinement and the fixation of sulfur (S) during casting, preventing its migration and improving the quality of the final product, in a process similar to what occurs in steel manufacturing.

Other grades of Monel incorporate aluminum (Al) and titanium (Ti) as hardeners, albeit in proportions so low that they do not significantly affect corrosion resistance. In fact, the most resistant Monel is one that maintains the relative purity of the Ni–Cu mixture, without significant additions of third elements. The introduction of metals like gold (Au) or platinum (Pt) could improve certain properties, but their use is restricted to jewelry, not industrial applications.

Despite its virtues, Monel is not used in costume jewelry or decorative applications. Its color is unattractive, its machining is complex, its cost is high, and its surface tends to form a verdigris patina — a mixture of nickel and copper sulfates — which gives it an undesirable blue-green hue in aesthetic contexts. However, in industrial environments, especially those exposed to saltwater, steam, or aggressive chemical agents, Monel remains irreplaceable.

Monel, an alloy of nickel (Ni) and copper (Cu) with a typical Ni content of ≥ 63%, represents one of the most effective solutions in corrosive environments, especially in the presence of saltwater. Its resistance in aqueous media, both fresh and saline, even when boiling, places it above many elite alloys, including stainless steel and titanium. Unlike the latter, which depend on a passive oxide layer maintained by the presence of dissolved oxygen, Monel does not require such protection. Its resistance is intrinsic, not dependent on surface reactions, making it an ideal choice for sealed marine environments, where conventional steels and alloys suffer persistent attack from chloride ions (Cl⁻). Furthermore, the copper content, close to 28%, helps inhibit the proliferation of microorganisms on the surface, which reinforces its suitability for prolonged underwater applications.

Monel's resistance extends to caustic and reducing media, including acids such as hydrochloric (HCl), dilute sulfuric (H₂SO₄), and hydrofluoric (HF), all known for their aggressiveness towards stainless steels, titanium (Ti), tantalum (Ta), and other high-performance alloys. In these environments, Monel maintains its structural integrity without the need for coatings or additional treatments.

Regarding its thermal behavior, Monel retains a gamma crystal structure — austenitic — even at cryogenic temperatures, ensuring dimensional stability and mechanical resistance under extreme conditions. This structure, shared by other nickel and copper alloys, allows for a combination of moderate toughness, high ductility, and excellent malleability, facilitating its cold forming and weldability without complex processes.

From a production standpoint, Monel is relatively easy to manufacture. It requires less energy and fewer steps than other more sophisticated alloys, making it an attractive option for industrial applications that demand efficiency without sacrificing performance. Its ease of molding, working, and welding makes it especially useful in structural components, valves, pipes, heat exchange systems, and elements exposed to aggressive environments.

However, it is not without its drawbacks. The main disadvantage of Monel is its cost: both nickel and copper are high-value base metals, which significantly increases the alloy's price, especially in large-scale applications. Additionally, its performance in oxidizing media is limited. Acids like nitric (HNO₃) or concentrated sulfuric acid rapidly attack its surface, reducing its lifespan in these environments. Finally, its rigidity is lower than that of nickel-cobalt superalloys, elite steels, or alloys reinforced with elements such as molybdenum (Mo) or tungsten (W), which restricts its use in applications requiring high structural strength under load.

Despite these limitations, Monel remains one of the most valued alloys in marine, chemical, and cryogenic engineering, where its natural resistance and ease of manufacture make it a reference material.

Monel, an alloy of Nickel (≈ 65%) and Copper (≈ 30%), with traces of Fe, Mn, C, and Si, is distinguished by its exceptional corrosion resistance in saline, acidic, and reducing environments. This property makes it a material of choice in the petrochemical industry, especially for transporting highly corrosive substances such as elemental Fluorine (F₂) and hydrofluoric acid (HF), where conventional stainless steels fail due to insufficient passivation or localized attack. It is also used in desalination plants, where continuous exposure to NaCl(aq) and humid environments demands superior durability, and in boat propellers, where it surpasses stainless steel in longevity and resistance to seawater.

In structures without high mechanical demands, Monel is used for components exposed to aggressive agents, such as pipes for corrosive liquids or long-term storage tanks for chemical substances. Its ornamental use and in cutlery were common in the past, although Ni²⁺ allergy led to its withdrawal from domestic applications. In the aerospace sector, its presence has decreased in favor of more advanced Nickel superalloys, although it is still found in secondary components. Interestingly, US Marine Corps identification tags ("dog tags") are made of Monel due to its ability to resist decomposition in marine environments, allowing identification even in extreme post-mortem conditions.

In the musical field, Monel has established itself as an alternative to Brass in the manufacture of wind instruments such as trumpets and tubas. Its higher density contributes to a richer and deeper sound. Although it presents an opaque white surface with a slight tendency to sulfation, this is mitigated by authentic silver baths, which also improve its aesthetics. The maintenance of these instruments is simple and less demanding than that of Brass, whose golden appearance, though attractive, does not compensate for its inferior chemical resistance to humid or acidic environments.

Monel, like all high-nickel content alloys, has a high market cost, especially considering its significant proportion of Copper, which acts as the main alloying element. This combination of noble elements not only increases its production cost but also positions it as a specialized material, reserved for applications where its chemical resistance amply justifies the economic outlay.

Despite sharing a Nickel base with other advanced alloys, Monel and other Nickel-Copper mixtures are not classified within the group of Nickel superalloys. The latter, such as Inconel®, Hastelloy®, Waspaloy®, or René®, are designed to operate under extreme conditions of temperature and mechanical stress, such as those found in aircraft turbines or nuclear reactors. Monel, in contrast, lacks structural stability in such thermal environments, which excludes it from this category. Although all these alloys share a Nickel matrix, the differences in composition and thermal behavior mark a clear boundary between them, both in properties and applications.

It is important not to confuse Monel with Cupronickel, despite their visual similarity. Typical Monel has an approximate composition of Ni⁶³Cu²⁸, while the most common Cupronickel is formulated as Cu⁷⁵Ni²⁵. Both exhibit an opaque white color, easily polishable but with little shine, and share notable corrosion resistance. However, their mechanical properties, density, and response to chemical agents differ sufficiently to justify their technical distinction. This differentiation is essential in industrial contexts where material choice can compromise safety, durability, or system performance.

The name is not pretentious: these are truly superlative alloys, with unparalleled properties to date in both mechanical aspects and corrosion resistance. However, when speaking of a "Superalloy", one usually thinks, correctly, of a mixture of metals dissolved in a Nickel or Nickel-Cobalt base, expressly developed to perform a task under extreme conditions. What will be detailed below far exceeds the performance of any alloy mentioned so far and any other that follows: we are facing the elite of the elite, the zenith of engineering as far as metallic materials are concerned. No other family of alloys combines mechanical virtues and resistance to heat and corrosion (especially oxidation) in the way these do. I have patiently awaited the right moment, which has finally arrived, to share the knowledge I have acquired about these alloys over the years.

To begin, let's define concepts. Why "Superalloy"? As I said at the beginning, it might seem pretentious, on the part of the manufacturer or seller, to label with such a name something that is, beyond the difference in chemical composition, a mixture of various industrial-grade metals and to a lesser extent metalloids or non-metals (such as Silicon and Carbon, respectively) that, bonded in the appropriate percentages, result in a product of superior performance.

So superior are these alloys in more than one aspect that it is not surprising their prices are so high, and the applications for which they have been designed are very specific. Indeed, it is more interesting and fun to see the properties they possess than the uses to which they are put. Later you will see why I say this.

Although there are iron-based and cobalt-based superalloys, it is the nickel-based ones that even today are justly considered the best among the best. I find it difficult to classify a Super-Austenitic or Super-Duplex Stainless Steel as a superalloy, as it is not. Nor are those where the base is Cobalt with enormous amounts of Chromium, because although the hardness is superior, the overall performance is not, and in direct comparison with properly named Nickel-based superalloys, they clearly lose out.

A superalloy is, therefore, any alloy specifically designed to perform tasks under extreme temperature conditions. In reality, there is no official rule to determine what is or is not a "superalloy"; the notion or "unwritten rule" is that the term should only be applied to those that combine three main factors: good mechanical properties, good corrosion resistance, and most importantly and definitively, good heat resistance, which translates into good mechanical and chemical resistance at high temperatures. What does this mean?

You see, many metals (in fact, most transition metals, except curiously Iron and Manganese) show very high resistance to chemical attack, but this applies only up to a certain point; when we raise the temperature enough, the resistance vanishes and the metal begins to be attacked.

When chemical attack is not a problem and is already resolved, we encounter mechanical problems: indeed, the metals most resistant to corrosion are often too soft, and vice versa. It is as if nature itself were telling you that you cannot have both things at once.

The problem, of course, is that you need both, if not at an excellent level in each separately, then at a very high level in both. To understand each other, think if you could develop an alloy as tough (strong) and hard as Steel, which at the same time was chemically inert like a precious metal. We have Stainless Steel, which is very noble, yes, but its properties have quite narrow limits. You would be surprised to know that the enormous number of grades of Stainless Steels we are familiar with, and even more, those considered elite that have even been used in body implants, have a lower tolerance threshold for the mixture of corrosion and high temperature than you might think (this does not mean they are not good for medical use).

Most expensive alloys that you could buy in tonnage might seem very good, if not excellent, until you compare them with superalloys. Do not notice emotion in me, feeling does not carry me: when I say they are much better, it is because they are. But better in what? Why are they even used?

Like all artificial/synthetic materials that man has developed since the dawn of time, superalloys are the answer to a need: "something" was required that was strong, resistant to extreme heat, and oxidation. This "request" came from the aeronautical industry. It seems like a child's wish to the genie in the lamp, no less, because as I said before, there seems to be a universal law for all things, not only of the soul as it is always expressed, but also in the material: those metals that have an advantage in one thing will lose points in another, so there is none that is perfect by itself, far from it. The most complete metal is Titanium; it has everything: low density, resistance to corrosion and high temperatures, rigid yet ductile and malleable... despite which, it is not suitable for certain uses. It is here, and in many other deficits, where the superalloy family enters fully, like an elephant in a lampshade shop.

These are combinations of gamma phase crystal structure, austenitic, which, unlike "luxury" Stainless Steel and also expensive alloys like Cobalt-Chromium (e.g., Stellite, Vitallium), are entirely based on Nickel as the base metal for dissolution, and contain significant mass percentages of third metals such as Molybdenum, Tungsten, Titanium, Ruthenium, Rhenium, and on special, very special occasions, Iridium (a noble metal whose price has even exceeded that of Gold).

If in Super-Austenitic Steels (the best and most expensive along with Super-Duplex) the base was Iron with substantially high contents of Chromium, Nickel, and Molybdenum (among other elements present in small doses), in Superalloys, Nickel has completely replaced Iron; it is no longer steel, and the percentages of Chromium and Molybdenum have skyrocketed. As if this were not enough, the contents of Cobalt, Titanium, Rhenium, Tungsten, Ruthenium, and Aluminum are also high enough (even if they do not exceed 4% by mass) to have significant effects on the alloy's response. It is the ultimate triumph of the human being over the material world. It is the zenith, the highest point of the science responsible for taming metal.

Unlike Monel, which is the main Nickel-Copper alloy with a relatively simple composition (9 out of 10 Nickel-Copper alloys are Monel or imitations), Superalloys have up to six (or more) registered trademarks from companies specifically dedicated to manufacturing the alloys, or in some cases, directly manufacturing the part instead of selling them as ingots or bars. Some of their main clients? NASA, USAF (the US air force, currently the most powerful in the world), Formula 1 (the premier category of motor sports), et cetera, all clients of very specific manufacturers. These organizations do not hide: Waspaloy, Incoloy, Inconel, René, Hastelloy... these are the names you should know. The uses will follow, but since you cannot put the cart before the horse, let's first look at the main and common characteristics of all these brands.

As always when I summarize several similar alloys, I take into consideration the common values of all and do not elaborate on any in particular, since to begin with, there are several grades of Hastelloy, Incoloy, et cetera (there are several grades among all brands) that compete with each other, sometimes being products of the same manufacturer, which is of no further importance to consider unless you are interested in manufacturing a world-famous prototype, in which case I will feel flattered that you read my work, although the merits are not mine, I only publicize what others have done.

What, in my opinion, is the key to understanding the sum of virtues in these alloys, correctly called "Superalloys" with solidly backed reasons, is the content of metals that each one carries, with slight variations.

The typical Superalloy is a mixture where Iron has been replaced by Nickel, and the percentages of alloying metals have increased significantly.

The 16% Chromium, 12% Nickel, and 2.5% Molybdenum of AISI 316, the "jewelry" steel par excellence, pales in comparison to the typical content of a Hastelloy with 22% Chromium and 20% Molybdenum, where Nickel goes from a typical 12% to over 60%, and Iron is reduced to a minimum or directly replaced by Cobalt. The rest is distributed among metals such as Titanium, Tungsten, Niobium, Ruthenium, Rhenium, Iridium, and Cobalt itself, although it is possible to find Iron with or without Cobalt.

Some grades contain one metal and not another, other grades contain all the metals already cited; in any case, the presence of the Chromium-Molybdenum mixture as the main alloying element in an alloy where Nickel is not an alloying element but the base, constitutes the true "core" of a typical Superalloy.

Chromium is a relatively abundant metal; in fact, although it is not cheap (compared to Iron, any metal seems expensive), it can be said to be the most affordable on the list I just gave, except for Iron, Manganese, and Aluminum, which are very abundant. The problem with Chromium is that it is very brittle, and although it can be used pure in pipes for conducting highly corrosive gases or liquids, its great brittleness makes it a nightmare in, say, an accidental event where it is impacted, not to mention that it cannot be welded or hammered, hence it is produced in pure form by sintering. The same applies to Molybdenum and Tungsten. Metals with such high melting points are hard, impractical (cannot be worked like steel). Nickel is the answer to this problem: it is a soft metal, moderately tough, ductile, and malleable. When mixed with Chromium and Molybdenum in high doses, Nickel allows the final alloy to retain its good mechanical properties, perhaps improved by the increased hardness and rigidity provided by the percentages of Chromium and Molybdenum, which are expressly added for a main reason: to protect the alloy from corrosion at high temperatures.

Note that I have emphasized "at high temperatures" several times because, although a mixture of 12% Chromium in normal steel is sufficient to consider it "stainless" at least in fresh water, alcohols, blood, oils, et cetera, exposed to temperatures of more than 400°C it begins not only to lose its mechanical integrity (it begins to expand and soften) but also to lose the protective oxide layer provided by Chromium. In any case, if 400°C seems high to you, imagine 600, 700°C.

What happens with the maximum admitted temperature service point is that it is often misinterpreted with the melting point of the alloy. That is, some alloys that have lower melting points than others are less "heat resistant" since they begin to expand earlier, deforming. Heat resistance IS NOT the melting point of the alloy, but rather measures up to what temperature they can be used. In my house, and surely in yours, the stainless steel piece that reaches the highest temperature might be the steam pots, but in an absolutely different context where temperatures exceed 500°C, normal metal is unable to remain stable, and depending on the case, begins to be vulnerable to chemical attack. Superalloys must, therefore, be capable not only of resisting corrosion but also mechanical deformation.

Extreme heat causes a lot of damage to most metals as it acts negatively on two fronts: the first and most obvious is that it softens the mass, expanding it. When it cools, it is already deformed. Imagine what would happen if, for example, a precision instrument like a needle (or needle-shaped) were deformed: it would be the end of the device. Sometimes, it can lead to tragedies: when a fundamental part is subjected to critical temperatures that deform it, this can result in an accident, for example, when a brake overheats, or an aircraft engine is forced enough, finally yielding to heat and deforming.

The second problem related to extreme heat is none other than corrosion. In metals that are easily corroded, an increase in temperature accelerates corrosion. In those that are not easily corroded, heat allows them to suffer the consequences. That is why whenever you read something like "resistant to hydrochloric acid" (for example), you should ask, "yes, but to what limit?"

The upper limit of superalloys is always, on average, the highest among all metallic alloys. It is inferior to ceramics, which can be used even up to more than 1000°C (which explains why they are used as a replacement for traditional metal) only that said high-performance ceramics are brittle: however tough they may be (e.g., Silicon Nitride), they are still very, very far from the characteristic toughness of metals and their combinations.

Nickel is used and not another metal because despite being expensive, it is still a base metal. It resembles Copper, although it is harder and more rigid, it is still malleable and ductile. Unlike Copper, which only alloys with some metals, Nickel perfectly dissolves most agents that are added to it during melting. Unlike what happens with refractory metal alloys or ultra-hard alloys, Nickel superalloys are vacuum-melted in a single piece, that is, as true fusion during casting, not through processes that are more reminiscent of cementation than proper smelting.

Molybdenum increases corrosion resistance in saline environments, to such an extent that Superalloys can be used in oxygen-free seawater for prolonged periods of time because the passivating oxide layer of the piece is a perfect mixture of Chromium, Molybdenum, Titanium, Tungsten, et cetera oxides: keep in mind that although Chromium and Molybdenum are primarily responsible for corrosion resistance and hardening, they are not the only ones. Most Superalloys have very complex chemical compositions, including most industrial-grade metals and in exceptional cases, those considered noble or semi-noble, such as Ruthenium and Rhenium. These metals are extremely expensive (Ruthenium is used as a hardener for Platinum in some jewelry alloys), so their use is limited. Molybdenum, which is typically present in 20% by mass, is already an expensive metal, imagine the others.

In exchange for the high price paid, the properties are extraordinary:

A very interesting identity trait of these alloys is that they contain elements that you would not normally find in any other alloy with commercial use, such as Ruthenium, Rhenium, Zirconium (metal), Hafnium, and on very, very rare occasions, Iridium. All these additions are marginal in their content, but consider that if the piece weighs 3 kg and the Ruthenium percentage is 6%, this is equivalent to 180 grams of the raw metal. It's not that Ruthenium is a big deal, but it is sold by the gram (i.e., it is a metal with market value) very scarce and expensive. Frankly, I couldn't give a figure because they fluctuate a lot, although I would like to. I'll just tell you that it can match or exceed Silver.

In the case of Iridium, it is even more "bleeding" as it is truly precious (its price, similar to Gold, proves it), so it is used in just the right amount and only when absolutely indispensable. Keep in mind that sometimes manufacturers prefer to add more of another element to imitate the effect of a more beneficial one that would be more expensive for them. Common logic.

Nickel-based superalloys are distinguished by an exceptional combination of physical-chemical properties that make them irreplaceable in critical applications. Their corrosion resistance is outstanding in practically all media, both oxidizing and reducing, including concentrated alkalis even when hot, fresh and marine water, aggressive vapors, and solutions of ferric chlorides (FeCl₃) and cupric chlorides (CuCl₂), among others. This ability to resist highly reactive environments is maintained even at temperatures close to 1000°C, which represents an extraordinary thermal threshold. Although the P.R.E.N (Pitting Resistance Equivalent Number) index is not formally applied to these alloys, a theoretical estimate would yield values greater than 70, well above the pitting immunity threshold set at 40 for special stainless steels. This is without considering the presence of elements such as Copper (Cu), Titanium (Ti), Niobium (Nb), Molybdenum (Mo), or Tungsten (W), which further reinforce their resistance to chlorides and hydrogen sulfide (H₂S), typical of the petrochemical industry.

In addition to their chemical resistance, these alloys exhibit remarkable immunity to structural degradation phenomena such as crepitation, stress corrosion cracking, or notch fracture induced by abrasive oxidation at high temperatures. Regarding their mechanical properties, they stand out for their toughness, moderate hardness (superior to austenitic steels), ductility, and malleability. They can be formed into extremely thin sheets or very thin wires without compromising their rigidity, which translates into outstanding dimensional stability, even under extreme thermal loads. This unalterability against heat is, in fact, one of their most critical virtues in aerospace, nuclear, or high-demand industrial environments.

The coefficient of thermal expansion of these alloys is low, meaning their thermal expansion is minimal, ensuring the geometric integrity of the parts even in intense thermal cycles. Although primarily marketed for their extreme heat resistance, they also retain their properties at temperatures below 0°C, making them suitable for cryogenic applications, such as in liquefied gas storage systems or satellite components.

It should be clarified, despite the sometimes negative press Nickel receives, that these alloys are completely safe from an environmental perspective. There is no scientific evidence to indicate them as pollutants or dangerous under normal use conditions, and their durability precisely contributes to reducing the ecological impact from frequent material replacement.

Finally, due to their crystal structure and high content of elements such as Chromium (Cr) and Molybdenum (Mo), these alloys exhibit no magnetism under normal conditions. This property, seemingly secondary, becomes highly relevant in environments where operations are conducted near electromagnetic wave sources or in systems sensitive to magnetic interference, such as medical equipment, precision sensors, or high-frequency electronic components.

Nickel, Cobalt, or Iron superalloys, originally developed to meet extreme thermal and mechanical demands, find their two main fields of application in the aeronautical industry and in highly corrosive environments. The first of these uses, and undoubtedly the most emblematic, is what motivated their creation: the manufacture of components subjected to intense stresses, repeated impacts, and sudden thermal variations that preclude the use of ceramic materials. The most representative example is the blades or vanes of rotors in aviation engines, both in suction turbines and cooling ducts. Although aircraft fuselages are usually composed of Aluminum (Al), Titanium (Ti), or Magnesium (Mg), in the areas where combustion occurs —the "burning heart" of the engine— superalloys are almost exclusively used, capable of maintaining their structural integrity at temperatures exceeding 1000°C.

These parts, rotating at thousands of revolutions per minute (RPM) during prolonged flights, must withstand not only the heat generated by friction and combustion but also the dynamic air pressure at altitudes above 20,000 feet. Their function, comparable in very simplified terms to that of an inverted fan, consists of sucking in air to generate vacuum and thrust, allowing the ascent and propulsion of large tonnage aircraft. If conventional materials were used, the size and power of the engines would be drastically limited. This reality is especially evident in jets, fighters, and bombers, where thermal efficiency and mechanical resistance are vital. Although the example has military connotations, it is still true that many of the most cutting-edge technological advances are born precisely at the intersection of military and aerospace engineering.

The second major use of superalloys is based on their extraordinary corrosion resistance, even in aggressive liquid or gaseous media. They are used in chemical plant piping, ducts for corrosive substances such as halogenated acids or sulfurous compounds, hermetic chambers, and in the treatment of radioactive waste in nuclear power plants. It should be noted that those superalloys with high Cobalt (Co) content may show some reactivity in nuclear environments, which limits their use in specific applications. Although they could be used in medicine —for example, in implants or surgical instruments exposed to thermal sterilization— their high cost makes them unviable in this context. In contrast, more accessible alloys such as AISI 316 austenitic stainless steel offer acceptable performance, although clearly inferior in terms of chemical and thermal resistance.