Jet turbine blades from single crystals, how are they formed? I know about nothing about crystals,  although I do know a bit more about jet turbine engines, and I definitely know that you don't want the fan blade hitting the fan housing.
The reason given in the documentary I watched about the manufacture of jet turbine blades seemed plausible. It's a single crystal because you want to reduce the expansion due to heat and single crystals apparently can achieve that.
Could anybody  briefly  explain,  how single crystals avoid expansion due to heat as against multi crystal materials. 
Also, how are single crystals formed?
Wikipedia has a brief note Single Crystals

Another application of single crystal solids is in materials science in the production of high strength materials with low thermal creep, such as turbine blades. Here, the absence of grain boundaries actually gives a decrease in yield strength, but more importantly decreases the amount of creep which is critical for high temperature, close tolerance part applications.

 A: Okay so admittedly I know nothing about jet turbines, however I know a little something about crystals. The occupied lattice points of a single crystal are interacting with good overlap of atomic orbitals. If you have a d-block metal then you get the standard d-d and d-p interactions as predicted by tight binding and Hubbard models. 
Grain boundaries are the boundaries between single crystals. They arise due to the presence of some defect (insert long discussion of defects and their effects?). The attractive forces between grain boundaries are not as strong as the single crystal in general, they are often fracture points in a material. This is due to poor overlap of atomic orbitals. The greater the overlap of atomic orbitals the larger the covalent contribution to the bonding (general result from klopman-Salem equation). 
As you apply heat to the solid, the atoms displace from their lattice points. With strong forces, this displacement is small. With weaker forces you get a lot more "jiggling" going on. The more oscillation about the equilibrium conformation the more the solid expands. 
Therefore the more grain boundaries that are present, the more thermal expansion. I would imagine that is what is going on in this case, but as I say I don't know anything about jet turbines! 
Single crystals are grown in the lab. I remember waiting around for ages in my undergraduate organic chemistry labs waiting for my solution to crystallise out, then getting bored and crashing it! 
In general we have a small nanoparticle in solution. That will develop faces. Particles will deposit on the faces and grow the crystal according to the symmetry of the original particle. That's why we talk of the (111) face of a crystal etc. Different crystals have different symmetries and hence different structures. 
If you aren't patient when growing your crystals then you get powders! 
A: Single crystal jet turbine blades are grown in a substantially different way than most crystals.  The following article gives details of some of the R&D and large scale production process that is used.
http://www.tms.org/superalloys/10.7449/1980/superalloys_1980_205_214.pdf
A: I am not sure thermal expansion is much greater in polycrystals than in single crystals (http://nvlpubs.nist.gov/nistpubs/jres/14/Jresv14n5p523_A1b.pdf, Fig. 13). Wikipedia article that you quote says that single crystals have higher creep strength at high temperatures, so they can provide higher turbine efficiency.
