Hi Rex, yes, single crystal blades are used in turbine engines. Mostly for the hot section where the temps are extreme. Most of the baldes are nickel and other very high temp oxidation resistant materials.
On your specific question, the grain boundaries are where it is easier for fatigue cracks to initiate. Fatigue cracks grow as transgranular, so the crack surface is flat typically a semi-elliptical shape. Fatigue cracks are the most common as they are a result of cyclic loading under the yield strength. Almost all cracks you find on your racecar or anything that has cyclic loading will be a fatigue crack. The key is to keep the stress level low enough the crack does not initiate or propagate, called fatigue limit. Not all materials have fatigue limit, but steel does and aluminum does not. This is why alum connecting rods have a set life, they will develop a crack eventually. Another key to reduce fatigue is limit the stress concentration. Smooth fillets as opposed to sharp radius, gussets vs unsupported corners on tubing, etc.
One thing that is a bit unique to the single crystal is that they can orient the structure for optimum result in one direction. Most product you buy is rolled and has best properties in the longitudinal (think length) direction. The long transverse (think width) is a bit less and the short-transverse (think thickness) is the worst. Always try to orient stress in longitudinal or long-transverse if possible.
Kind of contrary to the single crystal is that very small grain size is generally better than large grain size. Smaller grain size is beneficial to most properties. This is why heat treating you need to watch for excess time at temp, it causes grain growth. You can;t really control grain size in what you buy however. Sorry for the rambling reply.
Terry,
Great job! Looking forward to your up coming articles. I really haven't thought about Body Centered Crystals and Face Centered Crystals since I was taking metallurgy from H.P.Davis back in 1964!!! Again "Great Job"!
A little side story (can't get away from these with old men!) I did some work for a company that made (makes) very sophisticated casting machines that control the rate of poor and also the rate of cooling. The big product that is made with these machines are titanium compressor blades for jet engines. Using their method they can cast a complete compressor blade, almost net size, that is one crystal of titanium. As I understand the actual physical properties, tensil and yeild strength are not very affected but the fatigue life is very significantly extended, no inter crystal surfaces. Any comments?
Rex