Turbine materials
Designers are constantly striving to improve the design of their turbine blades. One of the biggest enemies of the turbine is a phenomenon called creep. Creep is the tendency for the turbine blade to gradually elongate during its life, due to a combination of high temperature and the centrifugal load on the blade as it rotates at high speed.
The engine designer has to allow for creep when he designs a new engine. He will have to ensure that he allows enough clearance between the tip of the turbine blades and the turbine casing. The problem is that any clearance between the turbine blade and the turbine casing will allow some of the air to bypass the turbine without doing any work.
The designer has several weapons in controlling the turbine tip clearance but his main aim is to reduce the amount of creep that a blade will suffer so that he can set a smaller tip clearance.
One method of ensuring a small tip clearance is to coat the turbine casing with a soft "abradable" lining material. The turbine tip will have one or two raised lips, known as "knife edge" seals and these will rub into the soft lining, forming a good seal. If the blade stretches due to creep, it will wear deeper into the abradeable coating and still maintain a good seal.
The material and construction of the blade can have a large bearing on creep. Early turbine blades were made from steel but soon the engine makers started to use nickel alloys which are more suited to high temperatures.
In the previous page we saw how a modern turbine blade uses internal air passages to keep the blade to a manageable temperature. Another manufacturing technique that the designer can use is to control the grain structure of the blade.
The grain structure of a conventionally cast blade is like that of a polystyrene ceiling tile with lots of small particles. Each grain forms a bond with its neighbouring grains, and its these bonds which tend to weaken when subjected to high temperatures causing the blade to creep.
To overcome this, the directionally solidified blade was developed. When the directionally solidified is cast, it is cooled (or solidified) from one end only. This results in a grain structure that resembles the grain on a long plank of wood. There are less grain boundaries and the blade is less likely to creep.
The ideal solution would be to have only one grain with no boundaries and such technology does exist and is in use in the more recent engines. This type of blade is know as a single crystal blade because the blade has been grown from a single grain. With no grain boundaries, the blade has a high resistance to creep and can last two or three times longer than other blades.
The other recent development is in the area of ceramic turbine blades and turbine discs but at the moment, this technology is more suited to smaller, faster turbines.
