MK940
23rd Feb 2003, 10:41
Just wondering about turbine blades and temperatures they are exposed to. I have heard of ceramic coated and of course full ceramic blades, however I have read about Single Crystal Blade(SCB) on the B777 engines(not sure which manufacturer).
Can anyone please explain what these SCB are and all the goodies about them. Obviously they can stand up to higher temps, however what materials are they made of and so on....
Many Thanks.....;)
Without going into too much detail. Metals have a type of crystal structure (definition of a metal) usually many crystals in a piece and this can be clearly seen in polished and etched specimens. A common example is old brass doorknobs polished by palms and etched by sweat. The reason etching shows the grains is that the grain boundaries often have residual stresses and impurities tend to congregate there so etching will dissolve the metal at a different rate at the boundary compared the the middle of a grain. One of the methods of getting the desired properties is fiddling with the grain size.
In the case of materials for high temperature the impurities at the grain boundaries can weaken the structure by in the worst case melting or by less dramatic means. For turbine blades the first step (30 yrs ago) was carefully controlling the composition,casting and cooling process in a vacuum to give a few long crystals the full length of the blade to avoid crystal boundaries across the direction of the main stress - Directionally Solidified Blades. Further development enabled the turbine blades to be cast as a single crystal.
The exact compostions tend to be secret but nickel/cobalt is often used as a base with all sorts of exotic additions eg hafnium, ytterium and rhenium
Keith.Williams.
24th Feb 2003, 18:36
The principal problems affecting turbine blades are melting and creep. The concept of melting is fairly obvious, but the phenomenon of creep and how single crystal blades resist it are less so.
In the solid state a turbine blade will be far too strong to deform manually. But when melted, the same material can be stirred with very little effort. The transformation from stong solid to fluid is not instantaneous, but occurs very gradually. When a metal is heated, its tensile strength gradually decreases, making it much easier to deform.
In the case of a rotating turbine, this reduction in tensile strength enables the centrifugal stresses to gradually stretch the blades. This stretching process is not simply a matter of thermal expansion and contraction, but is permanent. The overall effect is that the blades gradually extend until they either rub against the casing or are pulled apart.
This gradual extension is called creep, and it takes place at the atomic level where the tensile stresses cause individual atoms to be moved relative to their neighbours. The structure of metals takes the form of a regular geometric patern, the shape of which depends upon the type of metal. The easiest to visualise is the cubic structure which takes the form of a three dimensional square grid, with atoms at each intersection.
Movement of atoms within such a grid requires very high tensile forces, but is much easier in any areas where the structure is faulty. Such faults occur at the boundaries of individual grains of the metal.
The concept of grains can be visualised by imagining what happens when molten metal is allowed to cool and solidify. Tiny particles of solid form and grow to become small sections of the crystal structure. But these individual sections of structure can be oriented in any direction, Some will be oriented horizontally and vertically while others will be tilted at different angles in all three dimensions.
As these sections grow they eventually meet. But because of their different orientations, they cannot knit together to form a continuous structure. The result of this process is a series of grains of ordered structure, joined together by boundaries of disrupted structure. The disruption caused by the mismatching at the boundaries of these grains produces weak areas, where the creeping process can be readily achieved at quite low stress levels.
Single crystal turbine blades are made by ensuring that solidification is permitted to start at only a single point. By carefully controlling the coolling process, this single point is made to grow until it fills the entire blade. Becuse such blades have no internal garin boundaries, creep can occur only within crystal structure. This requires much greater stresses, so the creep process is much slower. The overall effect is blades with a much greater creep life.