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Alloy

Nickel superalloy

A nickel alloy that keeps its strength at four fifths of its own melting point — the reason a jet engine works, and the only large use of rhenium there is.

Construction material

A jet engine's turbine blade sits in gas hotter than the alloy's own melting point, spinning fast enough to pull several tonnes of load through a part the size of a hand. That it survives at all is a materials achievement, and nickel superalloys are what it is made of.

The strength comes from a second phase, gamma prime, precipitated through the nickel as cubes a fraction of a micron across. Unlike almost every other alloy, its strength *rises* with temperature over much of the working range, which is why nothing else has displaced it in seventy years.

Processing

The blade is grown as a single crystal. Grain boundaries are where a metal creeps and cracks at temperature, so the casting is drawn slowly out of the furnace through a spiral selector that admits exactly one crystal orientation, and the finished part has no boundaries at all — the same reasoning as a silicon wafer, for a completely different reason.

It is then hollow, with cooling air bled from the compressor passing through internal passages and out through hundreds of laser-drilled holes to form a film over the surface. And it is coated: a thermal barrier of yttria-stabilised zirconia a few hundred microns thick, which insulates the metal from the gas.

Each of those three ideas buys perhaps a hundred degrees, and each hundred degrees is worth a measurable percentage of fuel burn.

Economic significance

Superalloys are the reason several obscure elements are mined. Rhenium has essentially no other use at scale, and the two or three per cent added to a blade alloy accounts for most of world consumption — from a metal recovered as a by-product of molybdenum roasting, in quantities of a few tens of tonnes a year.

Hafnium and ruthenium are in a similar position, and the whole family sits behind aviation, power generation and the industrial gas turbine. It is an unusually direct case of a handful of scarce elements determining what an industry can do.

Medium confidence Weak evidence

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.

is composed of

  • Nickel — element · 50–70% · the base, and what supplies the gamma prime phase the alloy's strength depends on
  • Chromium — element · 5–20% · for oxidation resistance, and reduced in the newest alloys because it competes with the elements that give strength
  • Cobalt — element · 5–15% · a substantial addition, and one of the reasons aviation and battery demand compete for the same metal
  • Aluminium — element · 3–7% · with titanium, the elements that form the strengthening phase — a few per cent, and the whole mechanism
  • Titanium — element · 0–5% · the other half of the strengthening phase
  • Tungsten — element · 0–10% · a heavy addition that stiffens the lattice against creep
  • Molybdenum — element · 0–6% · for the same reason as tungsten, at a third the density
  • Tantalum — element · 0–9% · in the single-crystal alloys, where it strengthens the strengthening phase itself
  • Rhenium — element · 0–6% · two or three per cent, and essentially the only use rhenium has — most of world consumption goes into turbine blades
  • Hafnium — element · 0–2% · a fraction of a per cent, at the grain boundaries of the alloys that still have grain boundaries
  • Ruthenium — element · 0–4% · in the newest generations, to stop the heavy elements segregating into phases that embrittle the blade

is used as

  • Structural engineering — application · the turbine blade carries its own centrifugal load at four fifths of its melting point, which is structural engineering under the hardest conditions anybody attempts

is produced by

  • Alloying and melting — process · vacuum melted, because the reactive elements in it would oxidise in air — and then cast as a single crystal rather than poured

is an alternative to

  • Stainless steel — alloy · in hot service, where stainless has lost most of its strength by 600 °C and a superalloy has not; the cost difference is a factor of ten and the temperature difference is what buys it

is used in

  • Aerospace manufacture — industry · the turbine hot section, which runs above the alloy's own melting point and only works because of internal cooling and ceramic coating
  • Energy generation — industry · gas turbine blades and vanes, in the same alloys and for the same reasons as aerospace
  • Turbine blade — object · cast as a single crystal, because at temperature and sustained load the failure mode is creep along grain boundaries — so the boundaries are removed entirely

is associated with

Sources

  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
  • Material World
    Our own writing

Questions this page answers

Where it comes from, and what it becomes

Follow Nickel superalloy back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Nickel superalloy → is composed of (with titanium, the elements that form the strengthening phase — a few per cent, and the whole mechanism) → Aluminium → is extracted from (via alumina — bauxite is refined by the Bayer process before smelting) → Bauxite → is sourced from (the residue of prolonged tropical weathering, which strips the silica and alkalis out of an aluminosilicate rock and leaves the aluminium behind — basalt is one common parent among several) → Basalt → is sourced from (partial melting of mantle peridotite is what basalt magma is — the residue left behind stays peridotite, depleted of what went into the melt) → Peridotite → is composed of (the mineral that defines the rock and gives it its name — a peridotite is olivine-dominant by definition) → Olivine
  • Nickel superalloy → is composed of (a substantial addition, and one of the reasons aviation and battery demand compete for the same metal) → Cobalt → is sourced from (much of world supply arrives as a by-product of copper mining rather than from cobalt-first operations) → Copper → is produced by (as blister copper, refined electrolytically afterwards) → Smelting → takes as input (as the element removed, not added) → Oxygen → is produced by (the largest output by tonnage, and the reason air separation units sit beside steelworks) → Air separation
  • Nickel superalloy → is composed of (for oxidation resistance, and reduced in the newest alloys because it competes with the elements that give strength) → Chromium → is produced by (as ferrochrome, reduced from chromite and added to steel without ever being separated as the pure metal) → Smelting → takes as input (as the element removed, not added) → Oxygen → is produced by (the largest output by tonnage, and the reason air separation units sit beside steelworks) → Air separation → takes as input (the feedstock, and an unusual one in being free, unlimited and available anywhere — the cost is entirely the energy to liquefy it) → Air
  • Nickel superalloy → is composed of (for the same reason as tungsten, at a third the density) → Molybdenum → is produced by (reduced from the oxide that roasting molybdenite produces) → Smelting → takes as input (as the element removed, not added) → Oxygen → is produced by (the largest output by tonnage, and the reason air separation units sit beside steelworks) → Air separation → takes as input (the feedstock, and an unusual one in being free, unlimited and available anywhere — the cost is entirely the energy to liquefy it) → Air
  • Nickel superalloy → is composed of (the base, and what supplies the gamma prime phase the alloy's strength depends on) → Nickel → is produced by (reduced from roasted pentlandite concentrate, with the platinum-group metals following into the refinery) → Smelting → takes as input (as the element removed, not added) → Oxygen → is produced by (the largest output by tonnage, and the reason air separation units sit beside steelworks) → Air separation → takes as input (the feedstock, and an unusual one in being free, unlimited and available anywhere — the cost is entirely the energy to liquefy it) → Air
  • Nickel superalloy → is composed of (the other half of the strengthening phase) → Titanium → is produced by (as sponge, which must then be crushed, melted and cast before it is usable metal) → Kroll process → takes as input (the inert atmosphere, without which the titanium would take oxygen from the air) → Argon → is produced by (drawn from an intermediate height in the column, between nitrogen and oxygen) → Air separation → takes as input (the feedstock, and an unusual one in being free, unlimited and available anywhere — the cost is entirely the energy to liquefy it) → Air

These are the most distinct paths back. Nickel superalloy can be traced through others besides.

Downstream — what it becomes

  • Nickel superalloy → is used in (cast as a single crystal, because at temperature and sustained load the failure mode is creep along grain boundaries — so the boundaries are removed entirely) → Turbine blade → is associated with (the jet engine created the superalloy industry, because no existing material survived the turbine inlet — and turbine temperature has risen about 500 °C since, almost entirely on the blade) → The wartime materials programmes complete chain
  • Nickel superalloy → is associated with (the jet engine created the industry, because no existing material survived the turbine inlet) → The wartime materials programmes complete chain
  • Nickel superalloy → is used as (the turbine blade carries its own centrifugal load at four fifths of its melting point, which is structural engineering under the hardest conditions anybody attempts) → Structural engineering
  • Nickel superalloy → is used in (the turbine hot section, which runs above the alloy's own melting point and only works because of internal cooling and ceramic coating) → Aerospace manufacture
  • Nickel superalloy → is used in (gas turbine blades and vanes, in the same alloys and for the same reasons as aerospace) → Energy generation
  • Nickel superalloy → is used in (cast as a single crystal, because at temperature and sustained load the failure mode is creep along grain boundaries — so the boundaries are removed entirely) → Turbine blade → is used in (and the capability is the casting yield and the coating rather than the alloy, whose composition is published — which is why jet engines are a three-company industry) → Aerospace manufacture