Alloy

• 1. Introduction

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  An alloy can be made simply to spend less money. Mixing gold with copper stretches a precious metal further, and the result still looks and behaves like gold. That small economic trick hints at something larger. Almost every metal you touch in commercial life has been deliberately impured. The vast majority of metals used commercially are alloyed, not left pure. A metal that is normally soft can become strong. A metal that rusts can be made to resist corrosion. Inside an alloy, the atoms hold together by metallic bonding, not the covalent bonds of ordinary chemical compounds. So how does adding a foreign element to a pure metal make it harder, tougher, or more stubborn against rust? Why does steel sometimes turn soft and sometimes turn brittle from nothing more than how fast it cools? And how did people start blending metals thousands of years before anyone understood what was happening inside the crystal? The answers run from a meteorite forged into a knife to a chemist quenching aluminium for machine-gun cartridges.

• 2. Strength From Impurity

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  Aluminium on its own is very soft and malleable, and so is copper. Mix the two and the resulting aluminium-copper alloy has far greater strength than either parent. This is the central paradox of alloying. Combining two weak metals can produce a strong one. The mechanical properties of an alloy are often quite different from those of its individual constituents. Adding a small amount of non-metallic carbon to iron trades the metal's great ductility for the much greater strength of steel. Steel earns its place as one of the most useful and common alloys partly because heat treatment can alter it so dramatically. The recipe can be tuned for different jobs. Adding chromium to steel improves its resistance to corrosion, creating stainless steel. Adding silicon instead changes its electrical characteristics, producing silicon steel. The reasons reach down to atomic size. When a foreign element joins a metal, differences in atom size create internal stresses in the crystal lattice, and those stresses often enhance the metal's properties. Larger atoms press on their neighbors with a compressive force, smaller atoms pull with a tensile force, and both help the alloy resist deformation. Not every property climbs. The electrical and thermal conductivity of alloys is usually lower than that of the pure metals. Density and reactivity may barely move, while tensile strength, ductility, and shear strength can shift sharply. Even tiny additions can swing behavior, a sensitivity that semiconducting ferromagnetic alloys reveal when impurities change their properties.

• 3. Substitution And Interstice

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  Two mechanisms can build an alloy when a molten metal meets another substance, and the relative size of the atoms decides which one happens. The first is atom exchange. When the atoms are similar in size, some of the atoms in the metallic crystals are simply swapped out for atoms of the other element. Bronze and brass are substitutional alloys of exactly this kind. In bronze, some copper atoms are replaced by tin, and in brass some copper atoms are replaced by zinc. The interstitial mechanism works differently. Here one atom is much smaller than the other and cannot stand in for it inside the crystal. Instead the small atoms slip into the gaps, the interstitial sites, between the larger atoms of the base metal's matrix. Steel is the classic example, because the very small carbon atoms tuck into the interstices of the iron lattice. Some alloys use both tricks at once. In stainless steel the carbon atoms sit in the interstices while some iron atoms are substituted by nickel and chromium. These structural differences feed into how an alloy is classified. Alloys can be substitutional or interstitial, homogeneous or heterogeneous, or intermetallic, and an alloy may be a single-phase solid solution or a mixture of metallic phases forming a microstructure of different crystals.

• 4. The Limits Of Mixing

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  Like oil and water, a molten metal will not always mix with another element. Lithium, magnesium, and silver are almost completely insoluble in pure iron. Even when two elements do dissolve, each usually has a saturation point beyond which no more can be added. Iron can stably hold a maximum of 6.67 percent carbon, at which point it forms a compound called cementite. Solubility in the liquid state does not guarantee solubility in the solid. As an alloy cools, the rules can change. If the metals stay soluble when solid, the alloy freezes into a solid solution, a homogeneous structure of identical crystals called a phase. If the constituents become insoluble as it cools, they separate into two or more types of crystal, producing a heterogeneous microstructure of different phases. Timing complicates this further. In some alloys the insoluble elements do not separate until after crystallization. Cooled very quickly, they first freeze as a single homogeneous phase, but one supersaturated with the secondary element. Over time the trapped atoms migrate out of the crystal lattice, settling into a more stable arrangement and forming a second phase that reinforces the crystals from within. Melting behaves strangely too. Unlike pure metals, most alloys have no single melting point but a melting range, a slush of solid and liquid together. The temperature where melting begins is the solidus, and the point where it finishes is the liquidus. Certain proportions, called eutectic or peritectic compositions, give an alloy a single low melting point with no slushy transition at all.

• 5. Fire, Gas, And Solid Diffusion

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  Heating a base metal beyond its melting point and dissolving other elements into the molten liquid is the oldest and most common way to alloy. It can work even when the added element melts at a far higher temperature than the base. Titanium shows how delicate this gets. In its liquid state titanium is a powerful solvent able to dissolve most metals, but it also greedily absorbs oxygen and burns in the presence of nitrogen. To avoid contamination it must be melted in vacuum induction-heating and special water-cooled copper crucibles. Carbon defeated the ancients entirely. Its melting point is so high, and reachable only under high atmospheric pressure, that no early civilization could combine it with iron as a liquid. The workaround was to alloy without full melting. Interstitial alloying can be done with a constituent in gaseous form, as in a blast furnace making pig iron, or in nitriding and carbonitriding and other case hardening, or in the cementation process that makes blister steel. It can also be done entirely in the solid state, mixing elements through solid-state diffusion. Ancient pattern welding, shear steel, and crucible steel all relied on this slow migration of atoms through solids. Oxygen lurks as a constant threat. Present in the air, it readily combines with most metals to form oxides, especially at the high temperatures of alloying, and can eventually cause a finished component to fail. Metalworkers fight back with fluxes, chemical additives, and other methods of extractive metallurgy to strip out excess impurities such as sulfur, which combines with iron to form brittle iron sulfide and weak spots in steel.

• 6. The Heat Treatment Dance

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  Iron inside steel changes the arrangement of its crystal atoms at a certain temperature, usually 1500 degrees Fahrenheit or more depending on carbon content. This shift, a form of allotropy, opens the lattice so the smaller carbon atoms can enter. Dissolved into the iron, the carbon forms a single homogeneous crystalline phase called austenite. What happens next depends entirely on cooling speed. Cool the steel slowly and the carbon diffuses back out, precipitating into iron carbide, the compound Fe3C, in the spaces between the pure iron crystals. The steel becomes heterogeneous, a mix of the carbide phase called cementite and pure iron ferrite, and the result is rather soft. Cool it quickly and the carbon has no time to escape. Trapped in the iron crystals, it forces a diffusionless martensite transformation. The crystal structure strains to reach its low-temperature state, leaving the steel very hard but much less ductile and more brittle. Most heat-treatable alloys behave in the opposite way from steel. They are precipitation hardening alloys, and they depend on diffusion rather than its prevention. Heated into solution and quenched, they turn softer than normal, then harden as they age, with solutes slowly precipitating into intermetallic phases that are hard to tell from the base metal. Where steel splits into separate crystal phases, these alloys form different phases within the same crystal, appearing homogeneous in structure yet behaving as something harder and more brittle. Not every metal responds equally. Nearly all can be softened by annealing, which recrystallizes the metal and repairs defects, but far fewer can be hardened by controlled heating and cooling, and none respond to it quite like steel.

• 7. Wilm's Overnight Surprise

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  In 1906, Alfred Wilm was searching for a way to harden aluminium alloys so they could be used in machine-gun cartridge cases. He knew aluminium-copper alloys were heat-treatable to some degree. So he tried quenching a ternary alloy of aluminium, copper, and added magnesium. At first the results disappointed him. When Wilm retested the alloy the next day, he found it had grown harder simply by sitting at room temperature, far exceeding what he expected. He had stumbled onto age hardening, though no explanation for the phenomenon arrived until 1919. The material became known as duralumin, one of the first age-hardening alloys ever used. Its combination of high strength and low weight made it valuable wherever weight mattered. Duralumin became the primary building material for the first Zeppelins, and many similar alloys soon followed. The same qualities later carried these alloys into the construction of modern aircraft.

• 8. From Meteorites To Stainless

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  Meteoric iron, a naturally occurring alloy of nickel and iron, was the first alloy humans ever used. No metallurgy was needed to make it. The metal arrived ready-made as the main constituent of iron meteorites, rare and valuable and hard to work. People forged it from a red heat into tools, weapons, and nails, cold-hammered it into knives and arrowheads, and often used it as anvils. Smelting opened the next chapter. Around 10,000 years ago in the highlands of Anatolia, in modern Turkey, humans learned to smelt copper and tin from ore. Early bronze paired copper with arsenic, which could poison the metalworkers, until around 2500 BC people began alloying tin and copper into a far harder bronze. Tin was rare, found mostly in Great Britain. In the Middle East, copper was alloyed with zinc to make brass, and Chinese smiths of the Qin dynasty around 200 BC even combined a hard bronze head with a softer bronze tang on their arrowheads to resist both dulling and breaking. Iron followed its own long road. The first smelting of iron began in Anatolia around 1800 BC through the bloomery process, yielding soft but ductile wrought iron. Steel, an alloy of iron and around 1 percent carbon, was always a byproduct of that process, and the trick of hardening it by heat treatment had been known since 1100 BC. China was producing brittle pig iron as early as 1200 BC. Quality came slowly and secretively. In 1740, Benjamin Huntsman began melting blister steel in a crucible to even out its carbon, creating the first mass production of tool steel. In 1858, Henry Bessemer blew hot air through liquid pig iron to burn off carbon, enabling the first large-scale manufacture of steel. The steel trade guarded its methods so fiercely that the people of Sheffield in England barred visitors from town to deter industrial espionage. Almost no metallurgical knowledge of steel existed until 1860. The breakthroughs then came fast. In 1882, Robert Hadfield produced a steel containing around 12 percent manganese, called mangalloy, the first commercially viable alloy steel, and went on to create silicon steel. Robert Forester Mushet found that adding tungsten gave steel a hard edge that held at high temperatures, making the first high-speed steel. In 1912, the Krupp Ironworks in Germany added 21 percent chromium and 7 percent nickel to make the first stainless steel. Its descendant, 304 grade stainless steel, sometimes called 18/8, is roughly 74 percent iron, 18 percent chromium, and 8 percent nickel, and it sits in kitchen drawers today as pans, knives, and forks that refuse to rust.