johnny comelately wrote: ↑
Thu Mar 29, 2018 11:26 pm
I'm trying to understand the merits of different crystalline structures as they apply to crankshaft applications.
for example, grain boundary crack initiated problems are reduced by single crystal steels.
Yes but you get a whole host of other problems back for single crystals.
The advantage of single crystals is that they are very strong. But since the crystal planes are running through the whole workpiece you are susceptible to cleavage along a crystal plane. sC’s are therefore inherently more brittle than their grainy counterparts.
In SC you have no grains sliding against each other, but in single crystals you can get the whole material to slip over a crystal plane so they are not immune to creep.
One advantage of singly crystals is that you do not suffer from grain growth, since you only have one grain. So for high temperatures that can be an advantage. For Polycristalline materials you have to play some tricks like putting an inert material along the grain boundaries (grain pinning). Problem is that the best materials like thorium oxide are being banned.
A real advantage can be the heat conductivity, like in turbine blades. Having no grain boundaries and secondary phases really helps here.
Of course there are single crystals which have their specialist uses, sapphire for bearrings, watches barcode scanners etc (scratch resistance). Or CaF for optical windows.
But overall I seldomly come across a large mechanical application where the advantages of single crystals outweigh the problems with brittleness. Alloys where you have the freedom of controlling microstructure are usually a lot more versatile.