Alloying and melting
Dissolving one metal into another while molten — the oldest deliberate materials engineering there is, and still how every alloy is made.
An alloy is a solution. Melt a metal, dissolve another element into it, and let it freeze: the second element takes up positions in the first one's crystal lattice, and the resulting metal is not a mixture of two things but a single material with properties neither parent had.
What the addition does is get in the way. A dislocation moving through a pure metal is what lets it deform, and a foreign atom of a different size obstructs that movement. This is why almost every structural metal in use is an alloy, and why the amounts involved are often small — a percent of carbon changes iron more than most people expect of a percent of anything.
Processing
The difficulties are practical rather than conceptual. The elements must be mutually soluble at the temperature used, and stay so on cooling, or the alloy separates into phases as it freezes. Volatile additions boil off — zinc boils below copper's melting point, which is why brass is made by adding zinc to molten copper under conditions that keep it in.
Much modern alloying is done under vacuum or an inert atmosphere, because the reactive elements that improve an alloy most are also the ones that would rather combine with the air.
History
Alloying is the first deliberate materials engineering in the record. Bronze is not a metal anybody found; it is a metal somebody made, by combining two ores or two metals that behave quite differently alone, and getting something harder than either.
That step — the recognition that materials can be designed rather than only discovered — is arguably a larger event than any particular alloy, and it happened long before there was any theory to explain why it worked.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
produces
- Brass — alloy · zinc dissolved into molten copper, under conditions that stop the zinc boiling off
- Bronze — alloy · tin into copper — the first alloy anybody made deliberately
- Stainless steel — alloy · chromium into steel, at the proportion where the oxide film becomes continuous
- Solder — alloy · melted and cast into wire or paste; the alloy is simple and the composition control is not
- Duralumin — alloy · melted and cast, then solution treated and quenched — the strength comes from the heat treatment rather than from the melting
- Cupronickel — alloy · straightforward to make, the two metals being completely soluble in one another at every ratio
- Pewter — alloy · cast in reusable moulds at under 230 °C, which is low enough for a workshop without a serious furnace — much of why it spread
- Nichrome — alloy · melted and drawn to wire, the drawing being the harder half
- Neodymium magnet — alloy · melted, cast, milled to single-crystal powder, pressed in a magnetic field and sintered — the alignment step is what makes it a magnet rather than an alloy
- Samarium–cobalt magnet — alloy · by the same sintering route, and needing no plating afterwards because unlike neodymium it does not corrode
- Nickel superalloy — alloy · vacuum melted, because the reactive elements in it would oxidise in air — and then cast as a single crystal rather than poured
- 6000 series aluminium alloy — alloy · and the composition window is narrow: the magnesium-to-silicon ratio decides whether the precipitate forms usefully
- Ti-6Al-4V — alloy · melted under vacuum or inert gas, because molten titanium reacts with essentially every crucible material and with air
- Nitinol — alloy · melted under vacuum, because oxygen and carbon form inclusions that shift the transformation temperature as well as weakening the alloy
- 5000 series aluminium alloy — alloy · magnesium into aluminium, and it must be melted under cover because magnesium burns
- 7000 series aluminium alloy — alloy · zinc and magnesium into aluminium, then solution treated and aged — the strength comes from the heat treatment, not from the melt
- Tool steel — alloy · carbon and carbide formers into iron, and the carbide is the working part
- High-speed steel — alloy · tungsten, molybdenum, chromium and vanadium into steel, in quantities that keep it hard at the red heat a cutting edge reaches
- Weathering steel — alloy · copper, chromium, nickel and phosphorus at around a per cent between them, which is what makes the rust adherent instead of flaking
- Invar — alloy · thirty-six per cent nickel into iron, at a composition where thermal expansion very nearly cancels
- Cobalt-chromium alloy — alloy · chromium into cobalt, vacuum melted because the alloy is used in bodies and inclusions are failure sites
- Sterling silver — alloy · seven and a half per cent copper into silver, which is the standard because pure silver is too soft to hold a shape
- Zinc die-casting alloy — alloy · aluminium, magnesium and copper into zinc, to a composition tight enough that the name is an acronym of the four ingredients
- Babbitt metal — alloy · tin, antimony and copper, arranged so hard crystals sit in a soft matrix that wears away around them and holds the oil
- Electrical steel — alloy · silicon into iron, and about three per cent is where the rolling mill stops tolerating it
- Wootz steel — alloy · in a sealed crucible, iron and a carbon source held for hours — which is what made it a cast steel a thousand years before anyone else had one
- Amalgam — alloy · the one alloy made without heat: mercury dissolves the other metal at room temperature, which is exactly why it was used in fillings and exactly why it stopped being
takes as input
succeeded
- Smelting — process · smelting produces the metal; alloying combines metals into something neither was
Sources
- Material WorldOur own writing