Gamma Prime (γ′): How It Keeps Nickel Superalloys Strong When Hot

What Is Gamma Prime?

Gamma prime is an ordered compound of nickel with aluminium and titanium, with the formula Ni₃(Al,Ti), that forms as tiny particles inside a nickel alloy. The surrounding metal is called γ (gamma): a face-centred cubic solid solution of nickel with chromium, cobalt, molybdenum and other elements.

In γ, atoms of different elements sit on the cube's lattice sites at random. In γ′, the arrangement is fixed: aluminium or titanium atoms take the corners of each cube and nickel atoms take the face centres. This ordered arrangement is called the L1₂ structure. Because its lattice spacing is very close to that of γ, small γ′ particles are coherent with the matrix, which means the rows of atoms run continuously from one phase into the other.

The model above shows Haynes 282, a wrought alloy hardened by γ′, with outlined regions standing in for the precipitates. In real metal the particles are measured in fractions of a micrometre, and there are enormous numbers of them.

How Does Gamma Prime Strengthen a Nickel Superalloy?

Gamma prime strengthens a superalloy by obstructing dislocations, the line defects whose movement lets a metal deform. A dislocation that glides easily through the disordered γ matrix runs into trouble when it meets an ordered γ′ particle.

• Cutting an ordered particle costs energy. A single dislocation passing through γ′ scrambles the ordered pattern behind it and leaves a fault called an antiphase boundary. Dislocations therefore have to travel in pairs, the second restoring the order the first disturbed, which takes a higher stress.
• Many fine particles leave short gaps. Because γ′ is coherent and has low interfacial energy, it can form as a dense population of small particles. Dislocations cannot easily bow around them either.
• Strength holds as temperature rises. Dislocations in γ′ tend to cross-slip into configurations that lock them in place. The result is that a γ′-hardened alloy's strength is much less sensitive to temperature than that of an ordinary alloy.

The share of γ′ in the alloy sets how far this goes. Alloys for lower-temperature parts may need only about a fifth of their volume as γ′. Modern cast turbine blade alloys carry typically more than 60%, so the γ′ particles are the bulk of the metal and the γ matrix forms thin channels between them.

Which Elements Form Gamma Prime?

Aluminium and titanium are the main gamma prime formers, with tantalum and niobium taking part and hafnium partitioning into the phase. Raising their combined content raises the amount of γ′ an alloy can form.

Each element does a slightly different job. Aluminium is the backbone of the phase and also builds the protective alumina scale on the surface. Titanium and tantalum substitute on the aluminium sites and strengthen the particles. Niobium does the same in some grades and, in larger amounts, forms a different phase altogether, described below. Cobalt, chromium, molybdenum, tungsten and rhenium sit mainly in the γ matrix, strengthening it and slowing the diffusion that lets γ′ particles grow.

The chart shows the pattern clearly. Cast blade alloys such as René N5 and Inconel 713C carry around 6% aluminium. Wrought grades such as Nimonic 80A carry less than 2%, because too much γ′ makes an alloy very hard to hot work. Inconel 625 relies on molybdenum and niobium in solid solution, not on γ′, and has no aluminium of note in its nominal composition.

Which Grades Rely on Gamma Prime?

Most cast turbine alloys and a family of wrought high-temperature alloys rely on gamma prime. Grouping them by their nominal γ′-forming content gives a useful map of the superalloy scrap market.

The figures are sums of the nominal values on each grade page and show the trend only; specifications allow ranges. The difference between a γ′ alloy and its solid-solution relative can be small on paper. Nimonic 75 and Nimonic 80A are both nickel with about 20% chromium, but 80A adds titanium and aluminium and becomes age-hardenable.

The same split separates Monel 400 from Monel K-500: the K-500 adds aluminium and titanium, ages to a much higher strength and is a different grade in the scrap trade.

What About Gamma Double Prime in Inconel 718?

Gamma double prime, γ″, is a related phase, Ni₃Nb, that strengthens Inconel 718 and Inconel 706 more than γ′ does. It forms where niobium is high and aluminium and titanium are modest, which is why 718 carries about 5% niobium.

γ″ has a body-centred tetragonal structure rather than the cubic L1₂ of γ′, and it precipitates far more slowly. That slowness is useful. A 718 weld or casting can cool through the ageing range without hardening on the way, so it is far less prone to the cracking that troubles high-γ′ alloys after welding. It is a large part of why 718 is so widely used for discs, shafts, casings and fasteners. The trade-off is temperature: γ″ is less stable than γ′ at the top of the range, so the hottest parts use γ′ alloys. The differences between the two niobium-hardened grades are covered in Inconel 718 vs 706.

How Does Heat Treatment Control Gamma Prime?

Heat treatment controls gamma prime in two stages: a solution treatment dissolves it, and one or more ageing treatments precipitate it again at the size and spacing the designer wants.

The same alloy can therefore be soft or hard depending on its last heat treatment. Solution-annealed Nimonic 80A bar machines and forms quite differently from the aged condition. This is also why material specifications, such as the AMS aerospace specifications, define both the chemistry and the heat-treated condition of a product.

What Happens to Gamma Prime in Service?

In long service at high temperature, gamma prime slowly coarsens, and under stress it can change shape. These changes are a large part of why hot-section parts have a finite life.

• Coarsening. Small particles dissolve and larger ones grow, so the spacing between them widens and the strengthening effect falls.
• Rafting. In single-crystal blade alloys under stress, the cuboidal particles can join into plates, known as rafts, oriented relative to the load.
• Surface depletion. Oxidation consumes aluminium near the surface, leaving a thin zone with little γ′ under the scale or coating.

Overhaul shops can sometimes restore the structure with a rejuvenation heat treatment. When parts are retired instead, they become service revert.

Why Does Gamma Prime Matter for Superalloy Scrap and Revert?

Gamma prime matters for superalloy scrap because the elements that form it are a large part of what identifies a superalloy grade, and they are among the hardest for a handheld analyser to read. The microstructure itself does not matter to a melter: remelting dissolves it.

Identification

Aluminium and titanium separate many superalloys from each other and from solid-solution grades. Handheld XRF reads heavy elements well but aluminium poorly, and its titanium readings at low levels need care, so a lot of γ′ alloy scrap is confirmed by spark OES or laboratory analysis, as explained in XRF, OES or lab analysis. On the analyser screen, a reading of nickel with about 20% chromium and no molybdenum could be Nimonic 75 or Nimonic 80A. Titanium helps, at about 0.4% against about 2.25% nominal, but the aluminium in 80A is what confirms it; how handheld XRF sorts nickel alloys shows where the method's limits lie. Tantalum and hafnium, which also sit in γ′, are the elements that most often decide which cast alloy a blade is.

Reactive Elements and Remelting

Aluminium, titanium and hafnium oxidise readily. That is why γ′ superalloys are melted under vacuum, and why revert usually runs a little low in them and has to be topped up with fresh additions. How superalloy revert returns to the melt explains the loop.

Mixing

Aluminium and titanium are unwanted in many solid-solution grades. A few γ′ alloy offcuts in a lot of 625 can push a melt outside its limits, and γ′ alloys mixed together become neither grade. Keep each one apart.

Condition Does Not Change Melt Value

Whether a bar was supplied solution-treated or fully aged changes its hardness, not its chemistry. For a melter, aged and annealed material of the same grade are the same raw material, so there is no need to sort scrap by temper. Sort it by grade.

Rhenium and Single-Crystal Generations

Single-crystal blade alloys are often grouped into generations, largely by rhenium content. First-generation alloys carry none; second-generation alloys such as René N5 carry about 3%; third-generation alloys carry about 6%. Rhenium strengthens the γ matrix and slows γ′ coarsening at the highest temperatures, but it is scarce, so rhenium-bearing blades must never be mixed with rhenium-free material. Sorting blades by alloy is covered in selling turbine blade scrap, and the superalloy scrap hub lists the cast and wrought grades.

If you hold γ′ alloy scrap, the practical rules follow from the metallurgy: one grade per container, aluminium and titanium proved by a method that can read them, and records that tie each piece to its alloy. When a lot is ready, send the grade, form, weight and evidence and we'll review your enquiry.

FAQ