Edax wrote: ↑
Fri Mar 30, 2018 12:24 am
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.
Edax, I'd be interested in your thoughts about this:
Neil Glover, chief of materials, Rolls-Royce
The single-crystal structure isn’t intended to cope with temperature, however; it’s to make the blades resistant to the huge mechanical loads that result from their rotational speed. “Every single blade extracts power from the gas stream equivalent to a Formula One car engine,” Glover said. “And the centrifugal force on them is equivalent to the weight of a double-decker bus.”
Normally, metals are composed of many crystals – ordered structures of atoms arranged in a regular lattice, which form naturally as the metal cools from a molten state. These crystals are typically of the order of tens of microns in size, positioned in many orientations. At high temperatures and under strain, the crystals can slide against each other, and impurities can diffuse along the boundaries between the grains. This is known as creep, and it badly affected early turbine blades, which were forged from steel and later nickel bars.
The first stage in development was to get rid of any grain boundaries at right angles to the centrifugal loading, which led to the development of blades that were cast so the metal crystals all ran from top to bottom. Later, this was optimised further by casting single crystals, with no grain boundaries at all. It’s a highly complex process: not only must the blades be cast with the internal cooling channels already in place, but the crystals are not homogeneous. Rather, zones of different composition and crystallographic structure exist within the blade.