Materials > Titanium
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When we looked at a jet engine, the fan section and compressor are mostly made out of titanium alloys. Titanium is a material with high strength but low density which makes it good for building big, strong and light things like fan blades. For aerospace the key to developing titanium alloys to such advanced levels stems from being able to produce a bi-modal crystal structure. In it’s basic form titanium has a hexagonal close packed crystal (HCP) structure known as α phase. When heated up to over 883ºC the atoms start to re-arrange themselves in a different way. The titanium atoms now bond together in a body centered cubic (BCC) structure, or β phase. This results in some grains now being HCP and others BCC hence a bi-modal microstructure.
If we look at the crystal structure or atomic arrangement we see big differences between the two types of titanium. Going back to the crystal structure we see α phase is HCP. If we look at HCP, we see that it is anisotropic or does not look the same from all directions. You could tell if I knocked it over, whereas you couldn’t with the BCC or β phase. From this, logically we can expect the material properties to not be the same in all directions which we can take advantage of.
To make the best material possible, what we do is change the chemistry, the amount of each phase, the size of the grains, and even line up all the α phase grains to point in the same direction. By doing this we get enhanced strength, ductility and low-cycle fatigue properties from the α phase. The β phase adds more low-cycle fatigue resistance to the material, but also resists fatigue crack growth. This combination of modifying phases and heat treating is how we design an optimize titanium alloys for use in modern jet engines.
Ti, 6% Al, 4% V
This material is majority titanium (Ti) with added aluminium (Al) and vanadium (V). Starting with basic titanium in α phase, when it is heated to 883ºC it transforms to β phase. To stabilize this new phase and keep it when we cool it back down, we add V. Adding Al stabilizes and reduces the weight of the α phase. As β phase is more ductile than α phase, our titanium alloy now has the advantage of the bi-modal microstructure and can withstand temperatures of up to 350ºC. Given it has both phases, Ti 64 is considered an α+β titanium alloy.
Ti, 6% Al, 2% Sn, 4% Zr, 6% Mo
Once we get past the early part of the compressor the temperature and stress increases beyond what Ti 64 can handle. Starting with pure Ti we again want both α and β phases. Adding the Al reduces the weight of the α phase whilst stabilizing it. The 6% Mo is added to stabilize the β phase. Sn and Zr are also stabilizing elements that encourage solid solution strengthening like in superalloys. Compared to Ti 64, Ti 6246 has less β phase. Because there is only a small amount of β phase, Ti 6246 is considered to be a near α titanium alloy This material can be heat treated to be made harder. The advantage of this material over Ti 64 is that it is stronger at higher temperatures (about 450°C) and therefore is used in the later section of the compressor.
Ti, 11% Sn, 5% Zr, 2.25% Al, 1% Mo, 0.2% Si
When we look at the composition of Ti 679 compared to Ti 6246 we almost the same composition. We see Sn and Zr as general stabilizing elements locking in the microstructure. For the α phase we see a small amount of Al, and for the β phase Mo. The biggest difference is the use of Si. When we add Si to titanium it creates a complex crystal structure which at grain boundaries prevents dislocations or holes moving about. This means the Si makes it more difficult for the material to break. This makes the material more capable of dealing with high temperatures seen at the later stages of the compressor. This material is also used by Rolls Royce plc.to make fan blades on certain engines.
Ti, 6% Al, 5% Zr, 0.5% Mo, 0.25% Si
Ti 685 has a similar composition to Ti 679 and is also used in the later stages of the compressor and for fan blades. The difference between these two materials can be seen in the composition with more Al and less Mo, meaning that Ti 685 would have a different ratio of α and β phase. The increased use of Al results in stabilized α phase, but also a lighter material. This material also uses the complex crystal structure Si creates to resist damage.
Ti, 5.8% Al, 4% Sn, 3.5% Zr, 0.7% Nb, 0.5% Mo, 0.35% Si, 0.06%C
Ti 834 was used previously to make discs alloy in the Trent 700 but is be superseded by more modern titanium alloys. This Ti alloy is considered near α phase, encouraged by 5.8% Al stabilizing the α. The combination of 0.7% Nb and 0.5% Mo means some β phase is present. Strengthening by solid solution (Sn, Zr) and via Si means this material has good strength up to 600°C, but given advances in nickel based superalloys this material is currently being phased out and replaced.