Materials > Nickel
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In aerospace nickel is mostly used in the form of superalloys. The basic crystal structure of nickel alloys is face centered cubic (FCC) with atoms in random locations. This is called austenite or γ phase. Starting with this we now want to cause precipitates to form. We do this by adding aluminium (Al) and/or titanium (Ti). What happens is that instead of having any atom at any location in the FCC structure, the Al or Ti take the positions on the corners and the Ni sits in the middle of the faces. This rearrangement means the newly formed precipitates have an ordered structure and therefore have different properties. This new phase is called γ’ phase. The following animation shows the difference between the two phases and what a precipitate and superalloy looks like.
Although we have γ and γ’, there is another phase called γ”. Basically when we have Nb or V, instead of Al or Ti taking the corner positions in crystal structure Nb or V does instead. Because these atoms have a different arrangement of electrons the crystal structure elongates. So thinking about our FCC γ’ phase, if we change Ti or Al for Nb or V, then stretch it in one direction so it is rectangular rather than square, then we have the body centered tetragonal (BCT) γ” phase.
Polycrystal Nickel Superalloys
Ni, 19% Cr, 13% Co, 4% Mo, 4% Ti, 1.4% Al
Waspaloy is one of the original nickel based superalloys. This material uses precipitation hardening forming two phases within one grain of material. The Ni, Ti and Al form the basis of the γ and γ’ phases which make it a superalloy. To stop this material from corroding, we add Cr to help out the Al. Although this material uses precipitates to prevent movement of holes/dislocations in the material, it also uses solid solution strengthening. By adding Cr, Co and Mo, this introduces atomic arrangements and bonds that also make it difficult for dislocations/holes to form, join together and move about. This material is capable of handling temperatures up to 650°C when the part is rotating at high speeds.
Ni, 10% Cr, 15% Co, 3% Mo, 4.7% Ti, 5.5% Al, 0.9% V
This is another precipitation hardened nickel based superalloy. When we compare it to Waspaloy we see it has more Ti and Al, meaning it is lighter. It also means we can form more γ’ phase which makes the material less sensitive to changes in temperature. The Cr and Al once again provide corrosion resistance. Solid solution strengthening is also achieved by adding Cr, Co and Mo. Inconel 100 also can have 0.18% C. This carbon with the help of Cr and Mo form carbides which is different phase, that sits on the grain boundaries. By having these carbides on the grain boundaries it can also thwart holes/dislocations from causing damage. This is how this material is capable of dealing with temperatures just over 1000°C.
50-55% Ni, 17-21% Cr, 2.8-3.3% Mo, 1% Co, 4.75-5.5% Nb, 0.65-1.15% Al, 0.3% Ti, balance% Fe
From the composition of Inconel 718 we can tell it is different to Inconel 100. This material is mainly Ni with very little Ti and Al. We see Cr, Mo and Co giving us some solid solution strengthening but how do we make precipitates? By adding Nb, instead of making γ’ phase out of Ni, Ti and Al, the atoms now arrange themselves so we now make γ” phase out of Ni and Nb. So when we look at it Nb and V line themselves up as γ” phase, Ti and Al prefer to hang out as γ’ phase, whereas Cr, Co, Mo and Fe like to stay in the γ phase. Inconel 718 operates in temperatures up to 700°C but also can be easily welded meaning making complex parts out of this material is possible.
Ni, 16% Cr, 14-15.5% Co, 3% Mo, 1-1.5% W, 5% Ti, 2.5% Al
So far our superalloys have been primarily made super thanks to precipitates with a little help from solid solution strengthening. Udimet 720 takes a different approach focusing on solid solution strengthening i.e. long and/or short range ordering and creating internal resistance to dislocation/hole movement. Udimet 720 does this through using lots more Cr and Co with a dash of W. Because we still add a bit of Ti and Al we still make precipitates but in this case solid solution strengthening is how Udimet 720 becomes a superalloy.
Ni, 18-21% Cr, 2% Co, 1.8-2.7% Ti, 1-1.8% Al, 1% Si, 3% Fe, 1% Mn
When we look at Nimonic 80A we see the classic ingrediants to make γ’ phase precipitates. With a good amount of Ti and Al we have what we need to make γ’ phase and therefore the major strengthening mechanism of this superalloy is precipitate hardening. Having lots of Cr means good corrosion resistance. Adding in some Si, Fe with the Cr creates some solid solution strengthening for both γ and γ’ phases meaning it can handle up to 815°C.
Ni, 14-15.7% Cr, 18-22% Co, 4.5-5.5% Mo, 0.9-1.5% Ti, 4.5-4.9 Al, 1% Si, 1% Fe, 1% Mn
Looking at these two Nimonics, we see a big difference between 80A and 105. Nimonic 105 has Ti and Al to form precipitates of γ’ phase. It also has a lot of Cr, Co and Mo to stabilize the γ phase and solid solution strengthen the material. Nimonic 105 focuses on both superalloy mechanisms and by doing so is able to handle temperatures up to 950°C!
Ni, 20% Cr, 20% Co, 6% Mo, 1.9-2.4% Ti
Nimonic C263 with essentially no Al. This means as a superalloy it relies on solely on solid solution strengthening with lots of Cr, Co and Mo. This material can handle up to 816°C and has high strength, corrosion resistance, good formability and high temperature ductility in welded structures.
Ni, 13.5-17% Cr, 14-20% Co, 3.8-5.5% Mo, 3.4-5% Ti, 2.5-4% Al, 0-3% Ta, 0-0.4% Hf
Patented by Rolls Royce plc., RR1000 is a solid solution strengthened nickel based superalloy. This superalloy also has precipitates with the Ti and Al forming γ and γ’ phase. When forming the γ’ sometimes we deliberately heat treat it to get two different sized blobs of γ’ phase. Having a large and a small population γ’ phase globs in the material can tackle different types of dislocations/holes. The Hf, Cr, and Mo also can spark the growth of carbides on the grain boundaries.
Ni, 20-24% Cr, 0-5% Co, 1-3% Mo, 13-15% W, 0.2-0.5% Al, 0-3% Fe, 0.5% Si, 0.3-1% Mn, 0.05-0.15% C
Haynes 230 can be used in temperatures up to 1150°C! This superalloy is solid solution strengthened by Mo and W. In addition to this Cr rich carbides form adding more strength at high temperatures. This material can handle carburizing and nitriding environments which makes it perfect for the most extreme part of a jet engine, the combustor.
Single Crystal Nickel Superalloys
Single crystal superalloys are classified into generations. Having moved on from the very first generation of nickel based super alloy single crystals, the second and third generations success stems from using rhenium (Re). Second generation single crystal superlalloys use about 3% Re, and the third generation about 6%. The problem with Re is that it is heavy, rare and very expensive. This means single crystal superalloys are only used in the most extreme parts of a jet engine.
The other problem with a single crystal is orientation. If we have a polycrystal, it means we have lots of little crystals pointing in different directions and so we get mostly isotropic materials (isotropic= roughly the same in all directions). This means it doesn’t matter which way I cut my part out of the larger bit of material. Single crystals are tricky as they are almost a perfect arrangement of atoms. This means if I pull it at a slightly different angle the material won’t necessarily have the same properties (it will be anisotropic). Think about going to the gym, you are able to do an overhead press at one weight (vertical to your body), and then bench press at a different weight (horizontal to your body). This is because you flex different muscles, and for a material it is just flexing different atomic bonds. This makes it more complicated when making parts out of single crystals as we need to make sure we are using the best orientation for the properties we need.
Ni, 8% Cr, 5% Co, 0.6% Mo, 6% Ta, 8% W, 5.6% Al, 1% Ti, 0.1% Hf
CMSX-3 is a 1st generation single crystal nickel based superalloy with the obvious lack of Re. The Mo, Ta and W, although in small amounts, are very good at solid solution strengthening. The Ta also strongly support the growth of γ’ phase and makes it stiffer, so this material also precipitate strengthens. Countering the weight of these heavy elements, there is a good amount of Ti and Al in CMSX-3 which is important as this is still quite a heavy material. When we look at what temperature CMSX-3 can handle i.e. 1120°C, it is almost 200°C more than polycrystal superalloys proving single crystals can really take the heat!
Ni, 6.5% Cr, 9.6% Co, 0.6% Mo, 6.5% Ta, 6.4% W, 3% Re, 5.6% Al, 1% Ti, 0.1% Hf
Moving onto 2nd generation nickel single crystal superalloys, CMSX-4 has a complex chemistry. The 2nd generation of nickel single crystal superalloys introduces Re, reduces W, an overall increase in solid solution strengthening elements Ta+W+Re+Mo, a drop on Cr, and an increase in Co. Having a γ phase matrix and γ’ phase precipitates, aided by solid solution strengthening of each phase CMSX-4 is capable of handling stresses at a temperature of at least 1150°C. This improvement over CMSX-3 in properties is counter balanced by the increase in cost of the material especially due to the Re and so in many cases if the cheaper CMSX-3 can be used, it will be used.
RR3010 or CMSX-10+
Ni, 1.7% Cr, 3.1% Co, 0.5% Mo, 8.5% Ta, 5.5% W, 0.1% Nb, 5.9% Al, 0.1% Ti, 6.8% Re
RR3010 or CMSX-10+ is a third generation nickel single crystal superalloy with quite a lot of Re at 6.8% wt. Also when we go up a generation we see an overall increase in solid solution strengthening elements Ta+W+Re+Mo. Also we have a small amount of Nb. We loose more Cr but still maintain good at corrosion and oxidation resistance without it. Now this very expensive material is capable of handling stresses at temperatures over 1200°C!!!