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A nickel-based alloy is an alloy in which nickel is the element the structure is built on, and it is the structure that decides the class. The base element is not chosen for its own properties so much as for the fact that it will hold a large number of other elements in solution without changing its crystal form. That is what makes the family so broad, and it is also why the family cannot be described by a single list of properties.
One distinction has to be settled before the categories, because it is the source of most of the confusion on the subject. A nickel-based alloy is wider than a nickel-based high-temperature alloy. The high-temperature alloys, also called superalloys, are a class defined by the service temperature: the published definition is an alloy that can be used above about 0.7 of its absolute melting temperature, where creep and oxidation resistance are the governing design criteria. Superalloys are built on an iron base, a cobalt base, or a nickel base, so the nickel base is one option inside that class, and the class is not the whole of nickel metallurgy.
Pure nickel and the nickel-copper alloys are nickel-based alloys that are not superalloys. The precision alloys, of which the soft magnetic grades are the most familiar, sit outside the temperature class as well. So does the shape memory family. Treating the two terms as synonyms makes the categories below impossible to state accurately, and the sections after this one keep them separate.
The matrix is what carries the name. If the continuous phase in the finished material is nickel, the alloy is nickel-based, whatever the other elements happen to be and whatever the application turns out to be. The composition range is therefore enormous, because the family covers everything from a commercially pure nickel at 99.6 % to a precipitation hardened superalloy with a dozen deliberate additions.
| The base of the matrix | What that base contributes | Where it is chosen |
|---|---|---|
| Iron | Cheap and workable, and adequate to moderate temperatures. The matrix can transform, which puts a ceiling on how much alloying it will tolerate before the properties become unstable | Large structural parts where cost matters more than the top of the temperature range |
| Cobalt | A high melting point and good resistance to thermal fatigue, with a matrix that resists the softening that precipitation hardened alloys suffer at the top of their range | Hot sections that must survive thermal cycling, and wear-facing work |
| Nickel | A matrix that does not change crystal form from room temperature to melting, which allows a large amount of alloying to be added without the structure becoming unstable. Published guidance for aeroengine work puts nickel ahead of the other two bases for that duty | The widest range of duties in the family, from corrosion service through to the hottest rotating parts |
The structural point that everything else follows from is simple to state. Nickel is face-centred cubic, and it stays face-centred cubic. There is no allotropic transformation between room temperature and the melting point, so heating a nickel alloy does not change the arrangement of its lattice.
Iron does transform. That difference is why a nickel base tolerates a chemistry that an iron base cannot: elements can be added in large amounts to a matrix that is not going to rearrange underneath them. The consequence for a buyer is the reason a single alloy family can span corrosion duty, electrical duty, and high-temperature duty without the metallurgy becoming contradictory.
The limit on how much can be added is not a fixed number. It is reached when the alloy begins to throw out phases that were never intended, and the ones to avoid are well known. They are brittle in themselves, and they remove from the matrix the very elements that were added to strengthen it, so the damage is doubled.
| The phase that forms | Its character | Why it is unwelcome |
|---|---|---|
| Sigma | A topologically close-packed phase. Its atoms keep the stacking sequence of a close-packed metal, but they are not close packed | Described in the published literature as the most deleterious of the group. Brittle, and it strips chromium and molybdenum out of the matrix |
| Mu | Another topologically close-packed phase, forming as plates | The plates appear as needles in section and reduce ductility and creep rupture life |
| Laves | A topologically close-packed phase associated with niobium and similar additions | Same mechanism of harm: brittle in place, and it depletes the matrix |
| Carbides at the boundary | Not always harmful. MC and related carbides pin the grain boundaries and reduce sliding along them | Useful at the boundary, harmful when they form as a continuous film or coarsen in service |
There is no simple rule for the concentration at which any of these appears. The published guidance is that the composition limits have to be read from a phase diagram for the particular system, or measured, which is why an alloy specification cannot be written by adding up the beneficial elements and stopping.
Strengthening means making it harder for a dislocation to move through the lattice. Nickel alloys are strengthened in two ways, and most commercial grades use both.
An atom of a different size sitting in the nickel lattice distorts it locally and makes the lattice harder to slide. The published list of elements that act this way in both the matrix and the precipitate includes cobalt, iron, chromium, niobium, tantalum, molybdenum, tungsten, vanadium, titanium and aluminium. The effect is real but modest, and it holds at every temperature up to the point where the atoms themselves start to diffuse.
The second mechanism is what gives the family its high-temperature character, and it depends on a structural coincidence. Adding aluminium or titanium to a nickel base produces a two-phase microstructure: a matrix, written γ, with particles of an intermetallic phase, written γ′, dispersed through it. The two lattices are similar in size and line up cell for cell, a relationship the published description calls cube-cube, and the particle is therefore coherent with the matrix while it is small.
That coincidence produces three separate benefits at once. The interface between the two phases has a low energy, so the particles resist coarsening, which is a structural stability that matters at temperature. The particle is atomically ordered, and that order obstructs the passage of dislocations. And because coarsening is driven by reducing interfacial energy, a coherent interface slows the very process that would otherwise soften the alloy.
| The matrix, γ | The precipitate, γ′ | |
|---|---|---|
| Lattice | Face-centred cubic, described as cubic-F | Primitive cubic, described as cubic-P |
| Arrangement of atoms | A solid solution with a random distribution | Nickel atoms at the face centres, aluminium or titanium atoms at the cube corners |
| Composition | Not fixed, which is the point of a matrix | Ni3Al, Ni3Ti or Ni3(Al,Ti). Not strictly stoichiometric, because vacancies or substituted atoms shift it off the ideal ratio |
| What else dissolves in it | Most of the deliberate additions | Niobium, hafnium and tantalum partition preferentially into the precipitate |
| Its relation to the other | Forms the continuous phase | Cube-cube orientation, so the cell edges of the two are exactly parallel. Coherent while the particle is small |
| What it does for strength | Carries the ductility | Carries the high-temperature strength, and is the reason the strength of these alloys is relatively insensitive to temperature |
The aluminium to titanium ratio is a control, not a detail. It sets the size and the sign of the mismatch between the two lattices, and a negative mismatch, which is to say a precipitate with the smaller cell, promotes the formation of layered structures under load. The published account notes that this can reduce the creep rate.
A third group of additions does no strengthening inside the grains at all. Boron and zirconium segregate to the grain boundaries, and the reduction in boundary energy that follows is associated with better creep strength and better ductility when failure would otherwise happen by grains separating. Carbide forming elements behave in a related way: the carbides tend to precipitate at the boundaries and reduce sliding along them.
| The role | The elements | What the element is doing |
|---|---|---|
| Oxidation resistance | Chromium, and aluminium | Forming a protective oxide scale. Small additions of yttrium help that scale stay attached to the metal underneath |
| Solid solution strengthening | Cobalt, iron, chromium, niobium, tantalum, molybdenum, tungsten, vanadium, titanium, aluminium | Distorting the lattice in both the matrix and the precipitate. Most of these act in both |
| Precipitate forming | Aluminium and titanium; niobium or vanadium in the grades that use the other precipitate | Building the ordered phase that carries the high-temperature strength |
| Grain boundary strengthening | Boron and zirconium | Segregating to the boundaries and lowering their energy |
| Carbide forming | Carbon together with chromium, molybdenum, tungsten, niobium, tantalum, titanium and hafnium | Precipitating at the boundaries and reducing sliding, provided the carbide stays discrete |
The proportion of the ordered phase in the microstructure can be set by the heat treatment, and the published figures show how far the requirement moves with the job. This is the single most useful number on the page for a buyer, because it explains why two alloys with similar compositions are supplied in completely different tempers.
| The duty | Published fraction of the ordered phase | What that means for the supply |
|---|---|---|
| Turbine blades for aeroengines, running above 1,000 °C | Typically above 0.6 | The microstructure is dominated by the precipitate. These parts are cast instead of welded into assemblies, and the high precipitate fraction is why |
| Service at about 750 °C where welding is used to fabricate the part | About 0.2 | Only a fifth of the microstructure needs to be the ordered phase. A lower fraction leaves enough ductility for the part to be welded, which the highly precipitated grades do not reliably allow |
The same source gives the temperature at which that mechanism begins to fail. Strength in these alloys is remarkably insensitive to temperature up to a point, and the published figure for the limit is about 600 °C, beyond which thermal activation is enough for dislocations to get past the obstacles. That is the reason the fraction rises so sharply for the hottest applications, and not because the alloy is simply made harder.
The categories below are organised by the service the alloy is bought for, which is how the material is ordered. Each one is a different set of elements doing a different job, and the temperature column is the part that is most often misstated.
| The category | What defines it | The elements that carry it | Temperature character |
|---|---|---|---|
| Heat resistant | Strength and a protective oxide scale retained at temperature, with creep resistance as the governing criterion | Chromium and aluminium for the scale, aluminium and titanium for the precipitate, the refractory metals in solution | The class that gives the family its high-temperature reputation. The published definition of a superalloy is service above about 0.7 of the absolute melting temperature |
| Corrosion resistant | Resistance to a chemical medium, which is a different problem and does not need high strength | Copper, chromium and molybdenum, in proportions set by the medium | Commercially pure nickel sits at one end of this group, and the nickel-molybdenum grades at the other. None of them is a high-temperature alloy |
| Wear resistant | Hardness and resistance to material loss at a contact, usually at temperature as well | Chromium, molybdenum and tungsten, with carbides for the hard phase | The carbides do the work. The matrix holds them in place, which is why the grade is chosen as much for what it will not lose as for its strength |
| Precision | A precisely controlled physical response instead of a mechanical one. The property being controlled is whichever of magnetic permeability, electrical resistance or thermal expansion the application turns on | The soft magnetic grades run at around 80 % nickel, which is what gives high permeability with low coercivity | Read the property, not the temperature. These grades are not turbine materials, and a temperature near 1,000 °C belongs to the heat resistant category and not to this one |
| Shape memory | A reversible change of crystal structure that returns the part to a remembered shape | Nickel and titanium in approximately equal atomic proportions | The useful property is the transformation temperature, which is set by the ratio of the two elements and is chosen for the application |
No, and the difference matters when a specification is written. A superalloy is defined by its service temperature, above roughly 0.7 of its absolute melting temperature, and can be built on an iron base, a cobalt base, or a nickel base. A nickel-based alloy is defined by its matrix: the continuous phase is nickel. Commercially pure nickel, the nickel-copper alloys, the soft magnetic precision grades and the shape memory alloys are all nickel-based alloys, and none of them is a superalloy.
Because the matrix does not change crystal form. Nickel stays face-centred cubic from room temperature to its melting point, with no allotropic transformation in between, so a large amount of alloying can be added without the structure rearranging underneath it. Iron transforms, which puts a ceiling on the chemistry an iron base will carry before the properties become unstable.
The matrix, written γ, is a face-centred cubic solid solution with a random distribution of atoms, and it is the continuous phase. The precipitate, written γ′, is an intermetallic compound of the form Ni3Al or Ni3(Al,Ti) with a primitive cubic lattice, in which nickel sits at the face centres and aluminium or titanium at the corners. The two lattices line up cell for cell, which is called a cube-cube relationship and makes the particle coherent with the matrix while it is small.
Three effects that reinforce each other. The interface between the two phases has low energy because the lattices match, so the particles resist coarsening and the structure stays stable. The particle is atomically ordered, and that order obstructs the movement of dislocations. And because coarsening is driven by reducing interfacial energy, the coherent interface slows the process that would otherwise soften the alloy. The published data also shows strength holding nearly flat with temperature to about 600 °C before thermal activation takes over.
No. The limit is reached when the alloy starts to form phases that were not intended, and the topologically close-packed phases are the ones to watch. Sigma is described in the published literature as the most deleterious of them. These phases are brittle in themselves, and they also remove chromium, molybdenum and similar elements from the matrix, so the alloy loses the strengthening it was carrying as well as gaining a brittle constituent. There is no simple rule for the limit, which has to be read from the phase diagram for the system or measured.
It is a nickel-based alloy chosen for a physical response instead of a mechanical one. The soft magnetic grades at around 80 % nickel are the clearest example: the property being bought is high permeability with low coercivity, which makes the material useful as a core instead of as a structural part. That is a different design requirement from creep resistance, and the two categories are specified in different ways.
A nickel-based alloy is one in which nickel forms the continuous phase, and it is that matrix, not the application, that puts an alloy in the family. Nickel is chosen as the base because it stays face-centred cubic from room temperature to melting, which lets the structure accept more alloying than an iron base will take. Strength comes from elements held in solution and from an ordered precipitate, γ′, that lines up cell for cell with the matrix and is coherent with it. Grain boundary additions and carbides work at the boundaries. The useful fraction of that precipitate ranges from about 0.2 where the part has to be welded at around 750 °C to above 0.6 in blades that run hotter than 1,000 °C.
Nickel-based material is supplied from Changzhou, Jiangsu across the categories above: pure nickel and nickel-copper grades, the nickel-chromium and nickel-chromium-iron families, the corrosion resistant nickel-molybdenum grades, the resistance and heating alloys, and the precision grades, in bar, wire, strip, plate and tube. The plant holds ISO 9001 certification and is audited by SGS. Send the service medium or the service temperature and the form required, and the category follows from that.
The figures and structural descriptions quoted are published values and are not acceptance limits. Precipitation fractions and the limits on alloying depend on the individual chemistry, the heat treatment and the section size, and no general rule replaces a phase diagram for the system concerned. Acceptance of a delivered material rests on the mill test certificate for the heat supplied.
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